International Symposium On Exotic Nuclei: Peterhof, Russia, July 5-12, 2004

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International Symposium on

Peterhof, Russia

July 5 -12, 2004

Editors

Yu. E. Penionzhkevich E. A. Cherepanov Flerov Laboratoly of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Russia

N E W JERSEY * L O N D O N

-

vp World Scientific SINGAPORE * BElJlNG * S H A N G H A I * HONG KONG * TAIPEI

CHENh

Published by

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British Library Cataloguing-in-PublicationData A catalogue record for this book is available from the British Library.

Cover design and color photos compilation by E. Cherepanov and Yu. Penionzhkevich Symposium logotype idea by V . Bashevoy, design by E. Cherepanov Photos by V. Maslov, V . Schetinkina, E. Cherepanov, S. Hofmann, H. Geissel, G. Miinzenberg, H. Feldmeier, W. Trzaska, R. Kalpakchieva, E. Ryabov, A. Svirikhin, A. Yakushev, D. Kamanin, A. Yeremin Cover background photo by V . Maslov

EXOTIC NUCLEI (EXON2004) Proceedings of the International Symposium Copyright 0 2005 by World Scientific Publishing Co. Re. Ltd. All rights reserved. This book, or parts thereoJ may not be reproduced in any form or by any means, electronic or mechanical, includingphotocopying,recording or any informationstorage and retrieval system now known or to be invented, without written permissionfrom the Publisher.

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Preface The International Symposium on Exotic Nuclei (EXON-2004), held in July 2004 in Peterhof (Russia), was dedicated to the problems of producing and investigating nuclei far from the line of stability. The Symposium was organized jointly by four scientific centers, where this field is actively developed - The Flerov Laboratory of Nuclear Reactions JINR (Dubna), RIKEN (Japan), GANIL (France) and GSI (Germany). According to this the Symposium had four cochairmen: Profs. Yu.Ts.Oganessian, T.Motobayashi, D.Goutte and G.Muenzenberg. The main goal of the Symposium was to discuss the latest results on the production and study of the properties of the lightest to heaviest nuclei, as well as the plans for fkture joint investigations in the field of exotic nuclei. The results were summarized during a round table discussion. The topics of the Symposium could be conditionally divided into: Methods of Production of Light Exotic Nuclei and Study of their Properties Superheavy Elements. Synthisis and Properties Nuclear Fission Mechanisms of Nuclear Reactions Induced by Heavy Ions 0 Rare Processes, Decay and Nuclear Structure 0 Experimental Set-Ups and Future Projects Radioactive Ion Beams Production and Research Programmes Public Relations The big interest in the synthesis of new isotopes of the light elements could explain the numerous reports on this topic. In principle, in the region of neutron-rich isotopes of light elements the nucleon dripline has been reached. Here search is carried out of quasi-stationary states in super neutron-rich systems. Of great interest from this point of view are the super neutron-rich isotopes of hydrogen (4H, 5H, 6H, 7H), helium (9He, "He), as well as the multineutron systems (the tetraneutron). Considerable success in this direction has been achieved at FLNR, JINR. Using 6He and t beams information was obtained on resonance states in 4H and 5H. In a joint FLNR - RIKEN experiment the superheavy isotopes of hydrogen (7H) and helium ('He) were studied. Search for the tetraneutron was carried out in experiments at GANIL. In these experiments the breakup of the neutron-rich *He and I4Be nuclei was used. Twelve events were registered in a neutron detector that could be interpreted as bound states in the system of 4 neutrons. In the d(8He,6Li)4n-reaction, studied at GANIL, a resonance at 2.5 MeV was observed and interpreted as corresponding to the tetraneutron. However, these preliminary results need to be checked with higher statistics.

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Principally new information was obtained for the neutron-rich nuclei close to the magic neutron numbers N = 20 and N = 28. It turned out that, what concerns their stability, the magicity of these nuclei was of no importance. New magic numbers N = 6, 16 and 32 were discovered. Moreover, the presence of two shapes - spherical and deformed - was found to exist close to the magic numbers. In that, a significant role belongs to deformation. Many theoretical and experimental studies have been dedicated to these properties. Direct experiments to measure the deformation of nuclei close to the N = 20 shell (32Mg) were carried out. They confirmed the presence of deformation in this nucleus. For the first time a method, based on the nuclear magnetic resonance (NMR), was used. Experiments of precision y-spectroscopy with radioactive ion beams using the detectors EXOGAM, AGATA and EUROGAM were also performed. Nuclear masses in the region of N = 20-28 have also been measured. The results of all these experiments have shown that we are dealing with a new region of nuclei ("island of inversion"). Exotic nuclei manifest their unusual properties also in reactions with other nuclei. Recently new interesting results were obtained in experiments aimed at the measurement of excitation functions of total reaction cross sections, fusionfission and evaporation residue reactions. Such reactions were investigated using radioactive beams, such as 6He, 'He, "F, 'Be, etc. In some papers an increase in the cross section at energies close to the barrier was reported. On the contrary, at high energies in the case of weakly bound nuclei, a significant decrease in the total reaction cross section was observed and interpreted as due to the breakup of the interacting nuclei. In the last few years the experiments on the synthesis of new transfennium elements and the study of their properties have been really intensive. The search for new superheavy elements is being done in four leading laboratories - the Flerov Laboratory of Nuclear Reactions at JINR (Dubna), the National Center GANIL (France), GSI (Germany) and RIKEN (Japan). The latest results, obtained in Dubna for the isotopes 282,2831 12, 286-2891 14, 287'288 115, 290,291,293 116 and 294118 in fusion reactions of 48Ca ions with targets of 23'U, 2423244Pu, 243Amand 249Cf,respectively, were presented at the Symposium. These results convincingly have demonstrated the influence of the neutron shell at N = 184 on the stability of superheavy nuclei. The experimental programme at GSI is aimed at the synthesis of isotopes of elements 113 and 115 in reactions with a uranium target. The experiments there are performed with the new setup SHIPTRAP for direct measuring of the mass of the isotopes. The investigations in the field have been joined by the Japanese physicists from RIKEN. Here the gas-filled separator GARIS was used and the isotope 277112 was produced and identified, and correspondingly the properties of the products of its a-decay 269H~, 265Sgand 261Rfwere determined. New experiments to synthesize heavier

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elements have been planned. As a result, recently in the reaction '09Bi + "Zn an isotope of element 113 (*"l13) was produced. The attempts of the French group at GANIL to synthesize element 114 allowed to obtain only the upper limit for the reaction cross section. In this way the experiments on the synthesis of superheavy elements are a significant part of the scientific programmes of the leading physics centers in the world. An important step in the further understanding of the properties of the superheavy elements are the experiments aimed at the study of the structure of transfermium nuclei. The development of the technique of y-ray spectroscopy made it possible to start measurements of the y-transitions in the isotopes of elements 101, 102 and 103. These experiments are performed at the University of Jyvaskyla (Finland), GANIL (France) and FLNR JINR (Dubna). An important advance in the chemical identification of the superheavy elements and the determination of their place in Mendeleev's periodic system was achieved by the radiochemists from Dubna, the Technical University at Munich and GSI, who performed the f i s t experiments to study the chemical properties of elements 105. This important direction in the investigation of the properties of superheavy elements will be continued by collaborations of physicists and chemists from the leading scientific centers. Nowadays unique techniques for the synthesis and investigation of the properties of exotic nuclei are being developed together with the realization of large accelerator projects for radioactive ion beams. The creation of the new radioactive ion beam factories needs big investments and they can be built only with the support of international collaborations. The new accelerator complex NUSTAR at GSI, which includes new experimental setups for precise investigations of nuclear structure, is built within the frame of international collaborations. The Japanese physicists have achieved great success in building the new accelerator complex at RIKEN, which will be used for the production of intense beams of stable and radioactive nuclei that will in turn be used for the investigation of the structure of exotic nuclei. This project is expected to start operating in 2010. The accelerator complex at GANIL for the production of radioactive ion beams is also being developed. At the SPIRAL complex at GANIL, the first experiments using intense beams of light radioactive nuclei have been started. First interesting results concerning the structure of exotic nuclei have already been obtained. The characteristics of reactions induced by such beams have also been studied. Now the SPIRAL-2 project is developed - it is supposed to produce the first beams of heavy radioactive fission-fragment beams. In 2004 the first experiments using 6He beams with intensity of 10' pps were started at the accelerator complex DRIBs in Dubna. Work is being done on the second stage DRIBs-2, where fission fragments, produced in photofission, will be accelerated. Photofission will be used also at ALTO (Orsay, France) -the first fission fragment beams are expected at the end of 2005. Interesting projects

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for the production of radioactive ion beams are available also in Italy (Catania), Brazil (Sao Paulo), China (Lanzhou) and some other large scientific centers. Thus, by 2008-2012 a series of new accelerator complexes, supplied with unique experimental setups, will be operating and will make it possible to make a new significant step in the synthesis and investigation of new exotic states of nuclear matter. At the Symposium, reports on the organization of scientific collaborations were also given. The scientific programme of the Symposium was supplemented by a rich cultural programme. The participants could get acquainted with the cultural and historical monuments of Saint Petersburg and its surroundings, could have a gorgeous sightseeing tour during the boat trip to the island of Valaam on the Ladoga Lake. The very broad range of participants and their informal contacts helped to create new collaborations for fbture investigations in this now, intensively developing field of nuclear physics - the physics of exotic nuclei. As suggested by the participants, the next Symposium will be held again in Russia in 2006. Financially the Symposium was supported by JINR, GANIL, RIKEN and GSI, as well by Russian Foundation for Basic Research.

Yu. Penionzhkevich Vice-chairman of EXON 2004

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Co-chairmen: Yu. Oganessian ( JINR) G. Munzenberg (GSI) T. Motobayashi (RIKEN) D. Goutte (GANIL)

Local Organizing Committee: Yu. Penionzhkevich (Vice-chairman) E. Cherepanov (Scientific secretary) S. Dmitriev, T. Donskova, K. Gridnev, S. Lukyanov 0. Semchenkova

Organized by: Joint Institute for Nuclear Research (Dubna) IG

RIKZN

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Gesellschaft h r Schwerionenforschung(Darmstadt) The Institute of Physical and Chemical Research (Wako)

Grand Accelerateur National .IA zi #- R d’Ions h Lourds (Caen) Supported by: Russian Foundation for Fundamental Research

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CONTENTS PREFACE

V

EXON-2004 SYMPOSIUM

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METHODS OF PRODUCTION OF LIGHT EXOTIC NUCLEI

1

AND STUDY OF THEIR PROPERTIES Advance of Experimental Studies of Exotic Nuclei A. A. Korsheninnikov

3

Masses, Half-Lives, Spins and Decays of Exotic Nuclides G. Audi, C. Thibault, A. H. Wapstra, J. Blachot and 0.Bersillon

11

Recent MISTRAL Mass Measurements of Magnesium 29-33 and Lithium 11 C. Thibault, G. Audi, C. Bachelet, C. Gaulard, C. Guenaut, D. Lunney, M. de Saint Simon, N. Vieira, F. Herfurth and the Isolde Collaboration

17

New Mass Measurements at the Neutron Drip-Line H. Savajols, 3.Jurado, W. Mittig, C. E. Demonchy, L. Giot, A. Khouaja, S. Pita, M. Rousseau, P. Roussel-Chomaz, A. C. C. Villari, D. Baiborodin, Z. Dlouhy, J. Mrazek, S. Lukyanov, Y. Penionzhkevich, A. Gillibert, N. A. Orr, M Chartier, A. Lkpine-Szily and W. Catford

23

Search for Tetraneutrons in the Breakup of 8He V. Bouchat, F. M. Marques, F. Hanappe, J. C. Angdique, C. Angulo, N. Ashwood, M J. G. Borge, W. Catford, N. Clarke, N. Curtis, P. Descouvemont, A4 Freer, V. Kinnard, M Labiche, P. Mcewan, T. Materna, T. Nilsson, A. Ninane, G. Normand, N. A. Orr, S. Pain, E. V. Prokhorova, L. Stuttge and C. Timis

29

Study of 4-Neutron System Using the 8He(D,6Li)Reaction E. Rich, S. Fortier, D. Beaumel, E. Becheva, Y. Blumenfeld, F. Delaunq N. Frascaria, S. Gales, J. Guillot, F. Hammache, E. Khan, V. Lima, B. Pothet, J-A. Scarpaci, 0.Sorlin, E. Tryggestad, R. Wolski, A. Gillibert, V. Lapoux, L. Nalpas, A. Obertelli, E. C. Pollacco, F. Skaza, P. Roussel-Chomaz, A. Fomichev, S. V. Stepantsov and D. Santonocito

36

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Experimental Study of the Hydrogen Isotopes Beyond the Drip-Line 4,5H S. I. Sidorchuk, M. S. Golovkov, L. V. Grigorenko, A. S. Fomichev, Yu. Ts. Oganessian, A. M. Rodin, R. S. Slepnev, S. V. Stepantsov, G. M Ter-Akopian, R. Wolski, A. A. Korsheninnikov, E. Yu. Nikolskii, A. A. Yukhimchuk, V. V. Perevozchikov and Yu. I. Vinogradov

45

Observation of a T = 3/2 Isobaric Analogue State of 'H in 'He S. V. Stepantsov, A. S. Fomichev, M. S. Golovkov, A. M. Rodin, S. I. Sidorchuk, R. S. Slepnev, G. M Ter-Akopian, M. L. Chelnokov, V. A. Gorshkov, Yu. Ts. Oganessian, A. A. Korsheninnikov. E. Yu. Nikolskii and R. Wolski

52

Decoupled Proton-Neutron Distributions in I6C Z. Elekes, Zs. Dombrddi, Zs. Fiilop, A. Krasznahorkay, M. Csatlos, L. Csige, Z. Gacsi, J. Gulyas, H. Baba, H. Kinugawa, A. Saito, N. Fukuda, T. Minemura, T. Motobayashi, S. Takeuchi, I. Tanihata,Y. Yanagisawa, K. Yoshida, N. Iwasa, P. Thirolf; S. Kubono, M. Kurokawa, X. Liu, S. Michimasa, S. Shimoura and A. Ozawa

58

y Ray Spectroscopy of 25,26,27F nuclei

64

Z. Elekes, Zs. Dombrddi, Zs. Fiilop, A. Saito, H. Baba, K. Demichi, T. Gomi, H. Hasegawa, S. Kanno, S. Kawai, K. Kurita, Y. Matsuyama, H. K. Sakai, E. Takeshita, Y. Togano, K. Yamada, N. Aoi, M. Ishihara, T. Kishida, T. Kubo, T. Minemura, T. Motobayashi, S. Takeuchi, Y. Yanagisawa, K. Yoneda, J. Gibelin, N. Imai, H. Iwasaki, S. Michimasa, M. Notani, T. K. Ohnishi, H. J. Ong, H. Sakurai, S. Shimoura, M. Tamaki, S. Ota and A. Ozawa Precision Experiments with Exotic Atoms and Relativistic Exotic Nuclei at GSI H. Geissel

68

New Structure Problems in Carbon and Neon Isotopes H. Sagawa, X . R. Zhou, X Z. Zhang and T. Suzuki

78

Nuclear-Moment Measurements of Light Neutron-Rich Nuclei H. Ueno, A. Yoshimi, H. Watanabe, T. Haseyama, Y. Kobayashi, M. Ishihara, K. Asahi, D. Kameda, H. Miyoshi, K. Shimada, G. Kato, S. Emori, G. Kijima, M. Tsukui and H. Ogawa

84

...

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Direct Mass Measurements of Short-Lived Neutron-Rich Fission Fragments at the FRS-ESR Facility at GSI M Matoi, Yu. Novikov, X . Beckert, P. Beller, F. Bosch, D. Boutin, B. Franczak, B. Franzke, H. Geissel, M. Hausmann, E. Kaza, 0.Klepper, H.-J. Kluge, C. Kozhuharov, K. L. Kratz, S. Litvinov, Yu. Litvinov, G. Miinzenberg, F. Nolden, T. Ohtsubo, A. N. Ostrowski, 2. Patyk, B. Pfeifer, M Portillo, H. Schatz, C. Scheidenberger, J. Stadlmann, M. Steck, D. Vieira, G. Vorobjev, H. Weick, M Winkler, H. Wollnik and T. Yamaguchi

90

Direct Mass Measurements of Neutron-Rich Nuclides from Zinc-70 Fragmentation M Mato:, Yu. Novikov, F. Attallah, K. Beckert, P. Beller, F. Bosch, D. Boutin, B. Franczak, B. Franzke, H. Geissel, M. Hausmann, M. Hellstrom, E. Kaza, 0.Klepper, H. J. Kluge, C. Kozhuharov, K. L. Kratz, Yu. Litvinov, G. Miinzenberg, F. Nolden, T. Ohtsubo, Z. Patyk, B. Pfeiffer, H. Schatz, C. Scheidenberger, J. Stadlmann, M Steck, G. Vorobjev, H. Weick, M. Winkler, H. Wollnik and T. Yamaguchi

96

Neutron-Rich Nuclei Studied in High-Energy Coulomb Breakup of Secondary Beams H. Emling

100

New Measurements of Reaction Cross Sections and Reduced Strong Absorption Radii of Neutron-Rich Exotic Nuclei in the Vicinity of Closed Shells N = 20 and N = 28 A. Khouaja, A. C. C. Villari, M Benjelloun, D. Hirata, H. Savajols, W. Mittig, P. Roussel-Chomaz, N. A. Orr, S. Pita, C. E. Demonchy, L. Giot, M Chartier, A. Gillibert, D. Baiborodin, Y. Penionzhkevich, W. N. Catford, A. Lkpine-Szily and Z. Dlouhy

108

Sub-Barrier Fusion with Exotic Nuclei N. Alamanos, F. Auger, N. Keeley, V. Lapoux, K. Rusek and A. Pakou

121

Study of Fusion Reactions Induced by Weakly Bound Nuclei A. A. Hassan, S. M Lukyanov, R. Kalpakchieva, Yu. E. Penionzhkevich, R. Astabatyan, I. Vinsour, Z. Dlouhy, A. A. Kulko, S. Lobastov, J. Mrazek, E. Markaryan, V. Maslov, N. K. Skobelev and Yu. G. Sobolev

128

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Reaction Cross Section for the Interaction of 4,6Heand 7Li at 5-30 AMev with Silicon Yu. G. Sobolev, V. Yu. Ugryumov, R. Kalpakchieva, A. A. Kulko, V. F. Kushniruk, I. V. Kuznetsov, S. P. Lobastov, S. M. Lukyanov, V. A. Maslov, Yu. E. Penionzhkevich, N. K. Skobelev, A. Bialkowski, A. Budzanowski, I. Skwirchinska, A. Kugler, K. A. Kuterbekov, T. K. Zholdybqev, W. H. Trzaska and S. V. Khlebnikov

133

Structure of Light Exotic Nuclei in Fermionic Molecular Dynamics H. Feldmeier, T. NefSand R. Roth

136

Effective Interaction Generated by the Pauli Exclusion Principle in Collisions of the Light Neutron-Rich Nuclei G. F. Filippov and Yu. A . Lashko

142

Shell Model Representation with Antibound States for Analysing Exotic Nuclei T. Vertse, R. Id Betan, R. J. Liotta and N. Sandulescu

148

SUPERHEAVY ELEMENTS. SYNTHESIS AND PROPERTIES

155

Heavy Element Research at SHIP S. Hofmann, D. Ackermann, W. Barth, L. Dahl, F. P. Hessberger, B. Kindler, B. Lommel, R. Mann, G. Miinzenberg, K. Tinschert, U. Ratzinger and A . Schempp

157

Synthesis and Decay Properties of Superheavy Nuclei Yu. Ts. Oganessian, V. K. Utvonkov, Yu. V. Lobanov, F. Sh. Abdullin, A. N. Polyakov, I. V. Shirokovsky, Yu. S. Tsyganov, G. G. Gulbekian, S. L. Bogomolov, B. N. Gikal, A. N. Mezentsev, S. Iliev, V. G. Subbotin, A. M Sukhov, A. A . Voinov, G. V. Buklanov, K. Subotic, V. I. Zagrebaev, M. G. Itkis, J. B. Patin, K. J. Moody, J. F. Wild, M. A. Stoyer, N. J. Stoyer, D. A. Shaughnessy, J. M. Kenneally, P. A. Wilkand R. W. Lougheed

168

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Search for Super-Heavy Elements at GANIL Ch. Stodel, R. Anne, G. Auger, B. Bouriquet, J. M. Casandjian, R. Cee, G. de France, F. de Oliveira Santos, R. de Tourreil, A. Khouaja, A. Pkghaire, M G. Saint-Laurent, A. C. C. Villari, J. P. Wieleczko, N. Amar, S. Grkvy, J. Peter, R. Dayras, A. Drouart, A. Gillibert, Ch. Theisen, A. Chatillon, E. Clkment, K. Lojek, Z. Sosin, A. Wieloch, K. Hauschild, F. Hannachi, A. Lopez-Martens, L. Stuttge, F. P. Hessberger, S. Hojinann, R. Lichtenthaler and F. Hanappe

180

Decay of an Isotope 277112 Produced by "'Pb + 70ZnReaction K. Morita

188

Evidence of Z = 120 Compound Nucleus Formation from Lifetime Measurements in the 238U+ Ni Reaction at 6.62 Mev/Nucleon A. Drouart, J. L. Charvet, R. Dayras, L. Nalpas, C. Volant, C. Stodel, A. Chbihi, C. Escano Rodriguez, J. D. Frankland, M. Morjean, M. Chevallier, D. Dauvergne, R. Kirsch, P. Lautesse, C. Ray, E. Testa, D. Jacquet and M. Laget

192

Spectroscopy of the Odd Transfermium "'Md and '"Lr Nuclei Using y, Electron and a Spectroscopy A. Chatillon, Ch. Theisen, E. Bouchez, E. Cle'rnent,R. Dayras, A. Drouart, A. Gorgen, A. Hiirstel, W. Korten, Y. Le Coz, C. Sirnenel, J. Wilson, S. Eeckhaudt, T. Grahn, P. T. Greenlees, P. Jones, R. Julin, S. Juutinen, H. Kettunen, M Leino, A-P. Leppanen, V. Maanselka, P. Nieminen, J. Pakarinen, J. Perkowski, P. Rahkila, J. Saren, C. Scholey, J. Uusitalo, K. Van de Vel, G. Auger, B. Bouriquet, J. M Casandjian, R. Cee, G. de France, R. de Tourreil, M. G. St Laurent, Ch. Stodel, A. Villari, M Rejmund, N. Amzal, J. E. Bastin, P. A . Butler, R-D. Herzberg, P. J. C. Ikin, G. D. Jones, A. Pritchard, S. Gre'vy, K. Hauschild, A. Korichi. A. Lopez-Martens, F. P. Hessberger, S. M. Lukyanov, Yu. E. Penionzhkevich, Yu. G. Sobolev, 0.Dorvaux, B. Gall, F. Khalfallah and M Rousseau

198

Gamma Spectroscopy of Transfermium Elements at the VASSILISSA Setup A . V. Yerernin,A . V. Belozerov, M. L. Chelnokov, V. I. Chepigin, V. A. Gorshkov, A. P. Kabachenko, 0.N. Malyshev, Yu. Ts. Oganessian, A. G. Popeko, R. N. Sagaidak, A. V. Shutov, A. I. Svirikhin, Ch. Briancon, K. Hauschild, A. Korichi, A. Lopez-Martens and 0.Dorvaux

206

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Fullerene-Structure in Superheavies, Nuclei Containing Antimatter and Cold Compression K Greiner and T. J. Biirvenich

2 12

Unified Consideration of Deep-Inelastic, Quasi-Fission and Fusion-Fission Phenomena V. Zagrebaev and W. Greiner

233

Theoretical Predictions of Excitation Functions for Synthesis of the Superheavy Elements Y. Abe, B. Bouriquet and G. Kosenko

24 1

Effect of Non-Axiality on the Fission-Barrier Height of Heaviest Nuclei A. Sobiczewski and I. Muntian

249

Dynamics of the Neck Formation and its Effect on the Fusion Probability T. Wada, A. Fukushima and M. Ohta

255

A Microscopic Description of a-Decay Chains of Z = 115,118 M. Gupta, A. Bhagwat and Y. K. Gambhir

26 1

Systematics of Superheavy Nuclei: Microscopic Description Y. K. Gambhir, A. Bhagwat and M Gupta

265

Shell Correction Effects in Quasi-Fission Reactions Leading to the Synthesis of Superheavy Elements E. Cherepanov

27 1

Nucleosynthesis and Search for Superheavy Elements in Nature: The Possible Scenario G. N. Goncharov

279

Results of the Experiment for Chemical Identification of Db as a Decay Product of Element 115 S. N. Dmitriev, Yu. Ts. Oganessian, V. K. Utyonkov, S. V. Shishkin, A. V. Yeremin, Yu. V. Lobanov, Yu. S. Tsyganov, V. I. Chepygin, E. A. Sokol, G. K. Vostokin, N. V. Aksenov, M. Hussonnois, M G. Itkis, H. W Gaggeler, D. Shumann, H. Bruchertseifer, R. Eichler, D. A . Shaughnessy, P. A. Wilk, J. M Kennealb, M. A. Stoyer and J. F. Wild

285

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Chemistry of SHE: What Allows us to Judge the Bulk Properties of Compounds from the Behavior of Single Molecules? I. ZvLira

295

Approach to First Experiments with Elements 114 and Results of Test Experiments A. Yakushev, A. Turler, B. Wierczinski, W. Briichle, E. Jager, M Schadel and E. Schimpf

301

Dependence of the Production of Heavy-Element Nuclei from Complete-Fusion Neutron Evaporation Reactions A. Yakushev, S. Reitmeier, A. Tiirler, B. Wierczinski, W. Briichle, E. Jager, M Schadel, E. Schimpj: A . Popeko, R. Sagaidak, A. Shutov and A. Yeremin

305

Theoretical Predictions for the Chemical Identification of Superheavy Elements. Role of Relativistic Effects V. Pershina and T. Bastug

309

NUCLEAR FISSION

315

Neutron Emission in Fission and Quasi-Fission I. Itkis, A . A. Bogatchev, A . Yu. Chizhov, M G. Itkis, f.Kliman, G. N. Knyazheva, N. A. Kondratiev, E. A4 Kozulin, I. V. Korzyukov, L. Krupa, Yu. Ts. Oganessian, I. V. Pokrovski, E. V. Prokhorova, R. N. Sagaidak, V. M Voskressenski, A. Ya. Rusanov, L. Corradi, A . M Stefanini, M Trotta, S. Beghini, G. Montagnoli, F. Scarlassara, G. Chubarian, F. Hanappe, T. Materna, 0.Dowaux, N. Rowley, L. Stuttge and G. Giardina.

317

Shell Effect Manifestation in Mass-Energy Distributions of Fission and Quasi-Fission Fragments of Nuclei with Z = 102-122 E. V. Prokhorova, A. A. Bogachev, I. M. Itkis, M G. Itkis, M Jandel, . I Kliman, G. N. Knyazheva, N. A. Kondratiev, E. M Kozulin, L. Krupa, Yu. Ts. Oganessian, I. V. Pokrovsky, A. Ya. Rusanov, V. M. Voskresenski, V. Bouchat, F. Hanappe, T. Materna, 0.Dowaux, N. Rowley, C. Schmitt, L. Stuttge and G. Giardina

325

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The Influence of Entrance Channel Properties on Quasifission G. N.Knyazheva, A. Yu. Chizhov, M G. Itkis, N. A. Kondratiev, E. M Kozulin, R. N. Sagaidak, V. M Voskressensb, B. R. Behera, L. Corradi, E. Fioretto, A. Gadea, A. Latina, A. M Stefanini, S. Szilner, S. Beghini, G. Montagnoli, F. Scarlassara, M Trotta, V. A. Rubchenya, V. G. Lyapin and W. H. Trzaska

333

Yields of Correlated Fragment Pairs in the Reaction 208Pb('80,F)Obtained 339 in y-y-y Coincidence Method A. Bogachev, 0.Dorvaux, E. Kozulin, L. Krupa, A. Astier, M G. Porquet, I. Deloncle, D. Curien, G. Duchene, B. J. P. Gall, F. Hanappe, M G. Itkis, F. Khelfallah, R. Lucas, I. Piqueras, M Rousseau, M Meyer, N.Redon, 0.Stezowski and L. Stuttge Neutron and Prompt Gamma Ray Emission in the Proton Induced Fission of 239Npand 243Amand Spontaneous Fission of *'*Cf L. Krupa, G. N. Kniajeva, J. Kliman, A. A. Bogatchev, G. M Chubarian, 0.Dorvaux, I. M Itkis, M. G.Itkis, S. Khlebnikov, N. A. Kondratiev, E. M Kozulin, V. Lyapin, T. Materna, W. Rubchenia, I. V. Pokrovsky, W. Trzaska, D. Vakhtin and V. M Voskressensky

343

Experimental Confirmation of the Collinear Cluster Tripartition of the *j2CfNucleus Yu. V. Pyatkov, D. V. Kamanin, A. A. Alexandrov, I. A. Alexandrova, E. A. Kuznetsova, S. V. Mitrofanov, Yu. E. Penionzhkevich, E. A. Sokol, V. G. Tishchenko, A. N. Tjukavkin, B. V. Florko, W. Trzaska, S. R. Yamaletdinov, V. G. Lyapin, S. V. Khlebnikov and Yu. V. Ryabov

35 1

Shape Coexistence, Triaxiality, Chiral Bands in Neutron-Rich Nuclei and Hot Fission Mode J. H. Hamilton, A. V. Ramawa, J. K. Hwang, S. J. Zhu, Y. X.Luo, J. 0.Rasmussen, P. M Gore, E. F. Jones, D. Fong, K. Li, C. J. Beyer, L. Chatuwedi, R. Q. Xu, L. M. Yang, Z. Jiang, Z. Zhang, S. D. Xiou, X . Q. Zhang, G. M Ter-Akopian, A. V. Daniel, Yu. Ts. Oganessian, V. Dimitrov, S. Frauendorf; A. Gelberg, J. Kormicki, J. Gilat, I. Y. Lee, P. Fallon, W. C. M. A,, J. D. Cole, M. W. Drigert, M A. Stover, T. N.Ginter, S. C. Wu and R. Donangelo

357

x1x

New Data on the Ternary Fission OF 252Cffiom the Gamma-Ray Spectroscopy G. S. Popeko, A. V. Daniel, A. S. Fomichev, A. M. Rodin, Yu. Ts. Oganessian, G. M. Ter-Akopian, M. Jandel, L. Krupa, J. Kliman, J. H. Hamilton, A . V. Ramayya, J. Kormicki, J . K. Hwang, D. Fong, P. Gore, J. D. Cole, J. 0.Rasmussen, S. C. Wu, I. Y. Lee,.M A. Stover, W. Greiner and R. Donangelo

365

Delayed Fission of Heavy Nuclei N. K. Skobelev

369

Angular Distributions and Asymmetries of Fragments and Ternary Particles in Low Energy Fission F. Gonnenwein, M Mutterer, P. Jesinger, A. Gagarski, G. Petrov, G. Danilyan, S. Khlebnikov, G. Tiourine, W. Trzaska, J. Von Kalben, A. Kotzle, K. Schmidt, 0. Zimmer and V. Nesvishevski

3 74

NUCLEAR REACTIONS

383

Breakup Processes in the Systems 6Li, ‘7F+208Pb C. Signorini, A4 Mazzocco, T. Glodariu, F. Soramel, A. de Francesco, G. Inglima, M La Commara, D. Pierroutsakou, M. Romoli, M Sandoli and E. Vardaci

385

‘‘AbnormalNuclear Dispersion” in Heavy Ion Scattering: Manifestation of Exotic Nuclear Excitation? A. A. Ogloblin, A. S. Demyanova, Yu. A. Glukhov, S. A. Goncharov and W. Trzaska

392

Study of Rainbow Scattering in I6O + 14C System A. S. Dernyanovu, Yu. A. Glukhov, A. A. Ogloblin, W. Trzaska, H. G. Bohlen, W. Von Oertzen, S. A. Goncharov, A. Izadpanakh, V. A. Maslov, Yu. E. Penionzhkevich, Yu. G. Sobolev, S. V. Khlebnikov and G. P. Tyurin

400

Elastic and Inelastic Scattering of 6LI on I2C at 63 MeV V. A. Maslov, R. A. Astabatyan, A. S. Denikin, A. A. Hassan, R. Kalpakchieva, I. V. Kuznetsov, S. P. Lobastov, S. M Lukyanov, E. R. Markaryan, L. Mikhailov, Yu. E. Penionzhkevich, N. K. Skobelev, Yu. G. Sobolev, V. Yu. Uggwrnov, J. Vincour and T. K. Zhol4baev

404

xx

Interaction of 4He-Particles with Stable Nuclei and Effective Nucleon-Nucleon Forces K. A. Kuterbekov, T. K. Zholdybayev, A. Mukhambetzhan, I. N. Kukhtina and Yu. E. Penionzhkevich

408

Energy Dependence of Total Reaction Cross Sections for Interaction of 4He with the Nuclei 28Siat Energies from the Coulomb Barrier to 200 MeV K. A. Kuterbekov, T. K. Zholdybayev, K. B. Basybekov, Yu. E. Penionzhkevich, I. N. Kukhtina, Yu. G. Sobolev, V. Yu. UgTumov, L. I. Slyusarenko and V. V. Tokarevsky

414

Time-Dependent Quantum Analysis of Neutron Transfer in Heavy Ion Fusion Reactions V. V. Samarin and V. I. Zagrebaev

420

Semiclassical Analysis of Many-Nucleon Removal Reactions in the '80(35MaV/u) + '"Ta System V. P. Aleshin, A. G. Artukh, G. Kaminski, S. A. Klygin, Yu. M Sereda, Yu. G. Teterev and A. N. Vorontsov

424

LISE++ Development: Application to Projectile Fission at Relativistic Energies 0.B. Tarasov

428

Complex Nuclear-Structure Phenomena Revealed from the Nuclide Production in Fragmentation Reactions M. V. Ricciardi, K.-H. Schmidt, A . KeliC, P. Napolitani, 0. Yordanov, A. V. Ignatyuk and F. Rejmund

432

Spallation Reactions with Neutron-Rich and Neutron-Poor Nuclei Th. Aoust and J. Cugnon

439

Spin Modes in Nuclei and Neutrino Nucleus Reactions T. Suzuki

445

Comparative Analysis of the '78m2Hf Yield in Reactions with Different Projectiles S. A. Karamian

45 1

xxi

RARE PROCESSES, DECAY AND NUCLEAR STRUCTURE

465

Shell Structure of Exotic Nuclei and Nuclear Force T. Otsuka, T. Suzuki, R. Fujimoto, T. Matsuo, D. Abe, H. Grawe and Y. Akaishi

467

Studies of Fine Structure Decay in Proton Emitters at the Holifield Radioactive Ion Beam Facility J. Batchelder, M. Tantawy, C, R. Bingham, R. K. Grzywacz, C. Mazzocchi, C. J. Gross, K. P. Rykaczewski, C.-H. Yu, D. J. Fong, J. H. Hamilton, W. Krolas, A. V. Ramayya, T. N. Ginter, A. Stolz, A. Piechaczek, E. F. Zganjar, J. A. Winger, M. Karny and K. Hagino

479

Nuclear Model of Binding Alpha-Particles K. A. Gridnev, S. Yu. Torilov, V. G. Kartavenko, W. Greiner, D. K. Gridnev and J. Hamilton

485

Structure of the Heavy Ca Isotopes and Effective Interaction in the SD-FP Shell F. Mare'chal, F. Perrot, Ph. Dessagne, J. C. Angklique, G. Ban, P. Baumann, F. Benrachi, C. Borcea, A. Buta, E. Caurier, S. Courtin, S. Grkvy, C. Jollet, F. R. Lecolley, E. Lie'nard, G. Le Scornet, Ch. Mikhe, F. Negoita, F. Nowacki, N. A. Orr, E. Poirier, M Ramdhane and I. Stefan

489

Statical and Statistical Properties of Heated Rotating Nuclei in the Temperature-Dependent Finite-Range Model E. G. Ryabov and G. D. Adeev

495

Nucleus-Nucleus Potentials from Deep Sub-Barrier Fusion and Their Relation to Cluster Radioactivity R. N. Sagaidak, S. P. Tretyakova, A. A. Ogloblin, S. V. Khlebnikov and W Trzaska

499

Decay Schemes of Nuclei Far from Stability I. N. Izosimov

503

Beta Decay of Odd-Mass As-Ge Isotopes in the Interacting Boson-Femion Model L. Zufj, N. Yoshida and S. Brant

511

xxii

On Analysis of Data of Radioactive Decays Under Conditions of Poor Statistics and Small Observation Time V. B. Zlokazov

515

Coplanar Ternary Decay of Hyper-Deformed Nuclei of Mass A = 56 W. Von Oertzen, B. Gebauer, C. Schulz, S. Thummerer, H. G. Bohlen, Tz. Kokalova, C. Beck, M Rousseau, P. Papka, G. Efimov, D. Kamanin and G. de Angelis

520

Nuclear Moment Measurements of Spin-Aligned Isomeric Fragments J. M. Daugas, G. Bklier, M. Girod, H. Goutte, V. Mkot, 0.Roig, I. Matea, G. Georgiev, M. Lewitowicz, F. de Oliveira Santos, M Ham, L. T. Baby, G. Goldring, G. Nqens, D. Borremans, P. Himpe, R. Astabatyan, S. Lukyanov, Yu. E. Penionzhkevich, D. L. Balabanski and M. Sawicka

524

Unexpected Rapid Variations in Odd-Even Level Staggering in Gamma-Vibrational Bands E. F. Jones, P. M. Gore, J. H. Hamilton, A. V. R a m w a , X. Q. Zhang, J. K. Hwang, Y. X Luo, J. Kormicki, K. Li, S. J. Zhu, W. C. Ma, I, Y. Lee, J. 0. Rasmussen, P. Fallon, M. Stover, J. D. Cole, A. V. Daniel, G. M Ter-Akopian, Yu. Ts. Oganessian, R. Donangelo and J. B. Gupta

530

Matter Radii of Proton Rich Ga, Ge, As, Se and Br Nuclei A . Lkpine-Szily, G. F. Lima, R. Lichtenthaler, A. C. C. Villari, W. Mittig, M Chartier and N. A. Orr

536

Shell Model Treatment of Neutron-Rich Nuclei Near 78Ni A. F. Lisetskiy, B. A. Brown, M. Horoi andH. Grawe

542

Particle-Number Projection in the t = 1 Neutron-Proton Pairing N. H. Allal, M. Fellah, M. R. Oudih and N. Benhamouda

548

553 Laser Spectroscopy of Transuranium Elements Yu. P. Gangrsb, D. V. Karaivanov, K. P. Marinova, B. N. Markov, Yu. E. Penionzhkevich and S. G. Zemlyunoi

xxiii

Spectroscopy at N = 28 New Evidences of Deformation S. Grkv, J. C. Angklique, F. R. Lecolley, J. L. Lecouey, E. Lienard, N. A. Orr, J. Peter, S. Pietri, I. Stefan, C, Borcea, A. Buta, F. Negoita, D. Pantelica P. Baumann, G. Canchel, S. Courtin, P. Dessagne, A. Knipper, G. Lhersonneau, F. Marechal, C. Miehe, E. Poirier, Yu. E. Penionzhkevich, S. Lukianov, J. M Daugas, F. de Oliveira, M Lewitowicz, M Stanoiu, C. Stodel, D. Guillemaud Mueller, F. Pougheon, 0.Sorlin, W. Catford, C. Tirnis, Z. Dlouhy, J. Mrazek, K.-L. Kratz and B. Pfeiffer

559

EXPERIMENTAL SET-UPS AND FUTURE PROJECTS

565

A Plutonium Ceramic Target for Masha P. A . Wilk, D. A. Shaughnessy, K. J. MOO^, J. M Kenneally, J. F. Wild, M. A. Stoyer, J. B. Patin, R. W. Lougheed, B. B. Ebbinghaus, R. L. Landingham, Yu. Ts. Oganessian, A. V. Yeremin and S. N. Dmitriev

567

The Detection System of the Dubna Gas-Filled Recoil Separator V. G. Subbotin, Yu. S. Tsyganov, A. M. Sukhov, S. N. Iliev, A. N. Polyakov and A. A. Voinov

571

Neutron Detector at the Focal Plane of the Setup VASSILISSA A . I. Svirikhin, A. V. Belozerov, M. L. Chelnokov, V. I. Chepigin, V. A. Gorshkov, A. P. Kabachenko, 0.N. Malyshev, A. G. Popeko, R. N. Sagaidak, A. V. Shutov, E. A. Sokol andA. V. Yeremin

575

Multichannel Electronic Module for COMBAS Separator A. G. Artukh, S. A. Klygin, Yu. M Sereda, Yu. G. Teterev, A. N. Vorontsov, G. Kaminski, A. Budzanowski, J. Szmider, N. I. Zamiatin, D. A. Smolin, N. V. Gorbunov, A. A. Povtoreiko and P. G. Litovchenko

579

New Lines of Research with the Magnex Large-Acceptance Spectrometer F. Cappuzzello, A. Cunsolo, A. Foti, A. Lazzaro, S. E. A. Orrigo, J. S. Winfield, C. Nociforo and H. Lenske

582

xxiv

The Modified Mini-FOBOS Setup D. V. Kamanin, A. A. Alexandrov, I. A . Alexandrova, S. V. Denisov, E. A. Kuznetsova, S. V. Mitrofanov, V. G. Tishchenko, A. N. Tyukavkin, I. P.Tsurin, Yu. E. Penionzhkevich, E. A . Sokol, Yu. V. Pyatkov, S. V. Khlebnikov, T.E. Kuzrnina, Yu. V. Ryabov and S. R. Yamaletdinov

588

Gas Feeding System Supplying the U-400m Cyclotron Ion Source with Hydrogen Isotopes A. A. Yukhimchuk, V. V. Antilopov, V. A. Apasov, Yu. I. Vinogradov, A. N. Golubkov, Ye. V. Gornostaev, S. K. Grishechkin, A. M. Demin, S. V. Zlatoustovski, V. G. Levtsov, A. V. Kuryakin, I. N. Mulkov, R. K. Musyaev, V. I. Pustovoi, V. V. Bekhterev, S. L. Bogomolov, G. G. Gulbekian, A. A. Yefiemov, A. Zelenak, M Leporis, V. N. Loginov, Yu. Ts. Oganessian, S. V. Pashchenko, A. M Rodin, Yu. I. Smirnov, G. M. Ter-Akopian and N. Yu. Yazvitski

592

RADIOACTIVE BEAMS. PRODUCTION AND RESEARCH PROGRAMMES

603

Status of the SPIRAL2 Project at GANIL Remy Anne, for the SPIRAL2 Group

605

Nustar: Nuclear Structure Research Within Fair Objectives and Organisation G. Miinzenberg

614

Perspective of RI-Beam Based Research at RIKEN T. Motobayushi

622

UCx Target Design for the Spiral 2 Project and the ALTO Project 0.Bujeat, F. Azaiez, C. Bourgeois, M. Cheikh Mahmed, H. Croizet, M. Ducourtieux, S. Essabaa, R. F$, S. Franchoo, F. Ibruhim, C. Lau, F. Leblunc, H. Lefort, M Mirea, C. Phan Viet, Jc. Potier, B. Roussiere, J. Suuvage, D. Verney, F. Pougheon, G. Gaubert, Y. Huguet, N. Lecesne, P. Lecomte, R. Leroy, F. Pellemoine, M. G. Saint-Laurent, F. NizeT, D. Ridikas and R. V. Ribas

630

Present Status of the KEK-JAERI Joint RNB Project

636

H. Miyatake

xxv

Dubna Cyclotrons - Status and Plans G. G. Gulbekyan, B. N. Gikal, S. L. Bogomolov, S. N. Dmitriev, M G. Itkis, V. V. Kalagin, Yu. Ts. Oganessian and V. A. Sokolov

643

Radioactive Ion Beams in Brazil (RIBRAS) R. Lichtenthaler, A. Lkpine-Szily, V. Guimariies, C. Perego, V. Placco, 0. Carnargo Jr., R. Denke, P. h? de Faria, E. A. Benjamim, R. Y. R. Kuramoto, N. Added, G. F. Lima, M S. Hussein, J.KoIata and A. Arazi

65 1

The Slow RI-Beam Facility at RIKEN RIBF M. Wada

656

First Radioactive Beams at the Excyt Facility M Menna, G. Cuttone, M Re, F. Chines, G. Messina, A. Amato, L. Calabretta, F. Cappuzzello, L. Celona, L. Cosentino, P. Finocchiaro, S. Gammino, D. Garuj?,S. Passarello, G. Raia, D.Rijiuggiato, A. Rovelli, G. Schillari and V. Scuderi

662

Isotopes Production from UCx Target for the SPES Project A. Andrighetto

668

PUBLIC RELATIONS

677

The Russian - German Co-operation at GSI an Example of Success and Friendship H. Zeittrager

679

JINR: International Scientific Centre Bringing Nations Together V. M. Zhabitsky

690

JINR University Centre S. Ivanova

698

Public Awareness of Nuclear SCIENCE IN Europe A. Kugler

704

xxvi

Heavy Metals Atmospheric Deposition Study in Poznan Using the Moss Technique Z. Blaszczak, I. Ciszewska, M. V. Frontasyeva and 0.A. CuIicov

709

List of Participants

715

Author Index

729

xxvii

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3

ADVANCE O F EXPERIMENTAL STUDIES OF EXOTIC NUCLEI

A. A. KORSHENINNIKOV R I K E N , Hirosawa 2-1, Wako, Saitama 551-0198, Japan On leave from the Icurchatov Institute, 123182 Moscow, Russia A review of recent experimental studies of exotic nuclei is presented. During last years, many interesting experiments were performed in this field. Some of t h e topics stand out in comparison with other experiments as being the most hot or sometimes problematic, and such selected topics are reviewed.

1. Introduction

Studies of exotic nuclei were stimulated by an experimental discovery' of abnormal radius of I1Li. This event finally changed the modern nuclear physics and nowadays a great number of investigations are devoted to studies of exotic nuclei in a vicinity of both neutron and proton drip lines. During last years, a lot of experiments were performed for studies of exotic nuclei, and to review all the experiments would be practically impossible. The present review is concentrated upon selected subjects which seem to be especially hot and sometimes problematic. Namely, the following topics will be discussed: - investigations of the 16C nucleus structure; - a two-proton halo in 17Ne; - recently discovered two-proton radioactivity; - a long standing problem of spectroscopy of 4H; - a controversial situation in spectroscopic studies of 5H; - a search for superheavy hydrogen 7H; - a situation in spectroscopic studies of excited states in 7He. At last, experimental facilities of new generation will be briefly reviewed. 2. S t r u c t u r e of 16C

The first experiments' with exotic nuclei were performed using secondary beams and a simple transmission method for measurements of interaction

4

cross sections. Now the transmission method still remains very effective. Using this method, the RIKEN group has recently performed a systematical study2l3 of many nuclei, from heliums to magnesiums as well as chlorines and argons, in order to estimate radii of the nuclei. Measurements4 of longitudinal momentum distributions of fragments from the projectile fragmentation were also performed. Here the 16C nucleus attracts attention. Though this nucleus seems t o be not so exotic as, e. g., heavier carbon isotopes, a peculiar situation takes place already in the case of "C. Analyzing the reaction cross sections, the authors of Ref.3 found that two valence neutrons in 16C are dominated by the s-wave component (- loo%), like in "C. On the other hand, the longitudinal momentum distributions provide completely different result. Both the "C and 14C momentum distributions from fragmentation of 16C are consistent with small spectroscopic factor for the s-wave motion (- 30%)4. This puzzle remains to be solved. In particular, additional theoretical developments are needed t o analyze the experimental data. Another interesting finding4 for 16C is that the two-neutron removal cross section has occurred to be larger than the one-neutron removal cross section. Recently one more intriguing result on 16Cwas reported in Ref.'. Using a shadow method, the authors measured a life time of the first 2+ state in 16C and studied the electric quadrupole transition from the 2+ state t o the ground O+ state. The transition strength was found to be anomalously small, about order of magnitude smaller than B ( E 2 ) for other nuclei. Such an unprecedented hindrance for I6C should be related to an anomalous nuclear structure and further investigations are needed here.

3. Proton halo in 17Ne

Now let us turn from the neutron rich nuclei to the proton rich side. Here recently interesting experiment6 was devoted to a study of I7Ne. Eight years ago this nucleus was predicted7 to be a candidate for a two proton halo. The authors of Ref.6 measured the longitudinal momentum distribution of "0 from fragmentation of 17Ne. This distribution has occurred to be very narrow corresponding617 to the distance between each proton and the " 0 core more than 4 fm, while the rms radius of 150is 2.5 fm only. It indicates the two proton halo. Note that so far a one-proton halo was discussed in 'B and 27P,but there has been no evidence for a two-proton halo. At the same time, looking at quantitative details, one can find some problems. The experimental data were analyzed using simple Glauber-

5 type models and a spectroscopic factor for the two valence protons being in s-orbital was found6 to be about 90%. A level scheme of 16F shows two levels with the valence proton in sorbital and two levels with the proton in d-orbital. These levels are not much separated from each other, and one can expect a strong configuration mixing for the two valence protons in 17Ne. Indeed, the most recent calculations' of 17Ne give a weight of 39.8% for the s-wave and 55.6% for the d-wave. Thus, there are alternative versions of the 17Ne structure, the s-wave weight of 90% and 40% ', and additional developments are needed at this point. In particular, it can be useful to take into account a transition from the 17Ne ground state to a continuum populated in its fragmentation. Another interesting aspect related to 17Ne was recently reported in Ref.g. A reaction rate for 150(2p,y)17Ne process was theoretically investigated at temperatures of astrophysical interest. It turns out that the direct capture of two protons on 150leading to the first excited state of 17Ne can increase the reaction rate by many orders of magnitude. It is important, that a sequential two-proton capture is not possible for this state of I7Ne, because resonances in 16F are situated at higher energies being very narrow. The direct two-proton capture represents an inverse process to a two-proton radioactivity.

-

-

4. Two-proton radioactivity

Two-proton radioactivity was predicted in the early sixties". Simultaneous or direct emission of two protons was experimentally investigated, e. g., for the cases of short-lived resonances of 6Be ' l , l 2 and l2O 13. Two-proton decay of long-lived state of 45Fe, that is the two-proton radioactivity, was recently observed for the first time in two experiment^'^?'^. Recently a theoretical three-body approach16 was developed to calculate widths for the two-proton radioactivity. It is important, because a traditional R-matrix approach is not an appropriate tool for the direct two-proton decays. In further experiments it would be interesting to investigate momentum correlations of the two protons emitted from 45Fe. At first sight such a task could be considered as to distinguish between two different mechanisms: (i) a three-body decay being understood as a three-body phase-space; (ii) escape of "the 2He resonance" producing strongly correlated two protons. However, in reality both these mechanisms should produce very smooth and not-distinctive distributions. Two protons in a singlet state (in reality

6

they form not a resonance, but a virtual state) give a distribution different from that of two neutrons. Relative energy distribution of two neutrons in the singlet state shows a peak at 70 keV. While for two protons, due to Coulomb repulsion, the corresponding peak has a maximum at the energy one order of magnitude higher, in addition this peak is much broader. Since the decay energy of 45Fe is 1.1MeV only, the relative energy distribution of the two protons attracting each other in the singlet state should be very smooth, without sharp correlations. Nevertheless, proton correlations can exist being reflection of the 45Fe configuration and it is a very interesting subject for further experiments.

-

-

5. Correlations in three-body decay of 12C*(1+) An example of extremely pronounced correlations in a direct three-body decay is given by the recent paper17. The authors studied a decay of l+-state in 12C at E* = 12. 71 MeV into three alpha-particles. The spin-parity of the decaying state strictly excludes a decay via narrow ground state 8Be(0+). Nevertheless, very distinct correlations between alpha-particles were observed. An appearance of these correlations is determined by angular momenta between the particles. In this particular case angular momenta between each two alpha-particles are dominated by L = 2. Correlations of similar nature can appear in a direct two-proton decay also. Note also that since originally the R-matrix approach was not intended for description of direct three-particle decays (e.g., see page 335 in 18), additional theoretical developments are needed here. The first step was done in Ref.lg where an expansion of the three-body decay amplitude in K-harmonics was introduced. This approach is good, when there is no Coulomb interaction, and it plays a role of the first approximation, when particles have charges. However, if one wants to obtain more precise results, the long-range Coulomb interaction makes difficulties in the K-harmonics treatment, and additional theoretical developments are needed at this point. 6. Spectroscopy of 4H

Now let us turn towards very light nuclei. Here 4H represents a longstanding problem. Since sixties many experiments were performed to investigate 4H. They provided the 4H resonance energies relative to the t n threshold, which vary from 1.7 to 8 MeV. The 4H resonance widths measured in various papers vary from I' l MeV to 4.7 MeV. In such a situation. the most reliable information on 4H can be extracted from the

+

N

7

isobaric analog states in 4Li, which were studied much better in the p-t3He scattering. In Ref.”, a charge-symmetric reflection of the 4Li R-matrix parameters was applied, and energies of 3.19 and 3.5 MeV, respectively, were deduced for two lowest 2- and 1- states in 4H. Broad widths were found for these states, 5.42 and 6.43 MeV for the 2- and 1- states, respectively. Two other states were found” at higher energies: 0- state at 5.27 MeV with I? = 8.92 MeV and 1- state at 6.02 MeV with I? = 12.99 MeV. Recently, experimental attempts of direct observation of 4H were continued21i22.In Ref.21, 4H was investigated in fragmentation of 6He and a peak at about 1.5 MeV relative to the t n threshold was observed. This peak differs from the above mentioned parameters of 4H obtained from 4Li and an additional clarification is needed here. The second experiment22was devoted to a study of 4H in two reaclions, ’ H ( ~ , P ) ~and H 3H(t,d)4H. Analysis of the data was performed with very careful consideration of competing processes. The results obtained for 4H in the two reactions are well consistent one with another and show a peak at about 3 MeV above the t + n threshold. Finally, the following parameters of 4H were extracted: E,.,, = 3.05f0.19 MeV and r = 5.14k1.38 MeV. They are in good agreement with the above mentioned parameters of 4H(2-) known from 4Li.

+

7. Studies of 5H The very neutron rich nucleus 5H was the object for research over more than 40 years. However, until recently the existence of 5H resonance remained unclear. Very recently several experiments23~24~21~25~z6~27 were performed to study 5H in various reactions. Here the experimental situation is quite complicated and the main question is related to a position of the 1/2+ ground state in 5H. These recent experiments can be subdivided into three groups: some e ~ p e r i r n e n t s argue ~ ~ J that ~ ~ the ~ ~ ’H ~ ~ground ~ state is located at 2 MeV above the t + 2n threshold, another measurementz1 was interpreted as an observation of the 5H ground state at 3 MeV, while in Ref.25 a broad structure with a center of gravity at 5.5 MeV was reported. To clarify the situation, additional experimental studies are needed, and correlational measurements are very promising. For example, in the most recent Ref.27, the 3H(t,p)5H reaction was investigated. It was observed, that events corresponding to the 5H energies above 2.5 MeV show a very pronounced angular correlation. This correlation shows a polynomial character with L, = 2 corresponding to excited states in 5H with spin-parity 5/2+

-

-

-

8

and 3/2+. The 1/2+ ground state in 5H was interpreted in this paperz7 to be consistent with the first o b ~ e r v a t i o nof~ 5Hg.s. ~ at 1.7 MeV. Note also that in the recent paperz6 the isobaric analog state of 'H was observed in 'He for the first time. It was identified using the three-body 3He+n+n and t + p + n decays of 5He* produced in the 2H(6He,t)reaction.

-

8. Search for 7H

Due to the known systematics for helium isotopes, where 8He having two more neutrons than 6He is more bound relative to the separation of two neutrons than 6He as well as 7He is less unbound than 'He, the results for 5H allow one to speculate that 7H may exist as a state in a vicinity of the t 4n decay threshold. Being close to the threshold, 7H could be an especially interesting system. It should undergo the unique five particle decay into t 4n channel and its width may be very narrow. Experimental search for unstable 7H presents a difficult task, and this resonance has never been observed. In the recent paper28, an experimental search for 7H was performed in the proton pickup reaction, P ( ~ H ~ , ' H ~ )by~ H detecting , two protons from decay of the virtual state 'He. Obtained results show a very rapid increase of the 7H spectrum starting from the t + 4n threshold. It provides a strong indication on the possible existence of 7H state near the threshold and further experimental studies of 7H seem to be promising.

+

+

9. Spectroscopy of excited states in 7He

As we known, nuclei have excited states. There is a famous exception from this rule, the absence of excited states in 3He and 3H. Another kind of exception was 7He. The ground state of 7He is a well established resonance that decays into n+6He and has spin-parity 3/2-. 7He was investigated for 30 years in many reactions with stable beams, and no excited states were found. As a result, 7He began to be considered as a nucleus which may not have excited states. Not long ago the situation has changed. In Ref.", the ~ ( ' H e , d ) ~ H e reaction was investigated and an excited state in 7He was observed at E* 2.9 MeV. This state decays mainly into 3nf4He. Most likely, this excited state has a spin-parity 5/2- and its structure represents a neutron in the P112state coupled to the 6He core which itself is in the excited 2+ state. In the next experiment3', a fragmentation of 8He on a carbon target was investigated, and low-lying resonances in 7He were st,udied in n+'He

-

9

coincidences. The obtained spectrum shows the 7He ground state, but also a higher energy tail which was interpreted in Ref.30 as an 1/2- excited state, that is a spin-orbit partner of the 3/2- ground state of 7He. Surprising was very low excitation energy of the 1/2- excited state, E* 0.6 MeV. However, such a l/2- state in 7He at low excitation energy was not confirmed in the next experiment31. In this paper, the 9Be(15N,17F)7He reaction was investigated and the 7He excited state at the excitation energy of E* 2.95 MeV was observed in agreement with the observation of Ref.”. Also evidence of a broad excited state at higher excitation energy of E* 5.8 MeV was obtained 31. The most recent paper3’ was devoted to a study of isobaric analog states of 7He in 7Li. In this experiment, the resonant yield of neutrons from the ‘He(p, n)6Li(OS; T = 1) reaction was measured in coincidences with y rays from the decay of the O + ; T = 1 state in ‘Li. The measured excitation function shows a strong peak of 7Li corresponding to the isobaric analog state of the 7He ground state 3/2-. On the other hand, the authors concluded that they conclusively showed that the low-lying l/2- state reported for 7He in Ref.30 does not exist. If so, a question arises, where is the l / 2 - spin-orbit partner of the 7He ground state? So, further experimental studies of excited states in 7He are needed. During last years other experiments devoted to investigations of 7He were performed in several laboratories over the world, and one can expect new interesting publications.

-

-

10. Paths in future

Further progress in experimental studies of exotic nuclei is evidently connected with an upgrade of experimental facilities. Two examples can illustrate projects of new generation, the Radioactive Ion Beam Factory in RIKEN and the International Accelerator Facility in GSI. In the RI Beam Factory, a cascade of a K520-MeV fixed-frequency ring cyclotron, a K980-MeV intermediate-stage ring cyclotron and a K2500MeV superconducting ring cyclotron will serve as a post-accelerator for the existing cyclotron. This new system will be able to boost the beam’s energy up to 400 MeV/A for light ions and 350 MeV/A for very heavy ions. The beam intensity will be higher than l p microA. New fragment separator BigRIPS will be installed to generate secondary beams. Experiments using the RI Beam Factory will be started in 2007. Another ambitious project is planed in GSI, the International Accel-

10

erator Facility. The existing GSI accelerators, a linear accelerator and a synchrotron, will serve as injector for the new facility. A heart of new complex will be a double-ring synchrotron. It will be connected to a high-energy storage ring, new fragment separator SuperFRS, a collector ring, and new experimental storage ring. The new facility will provide 15 times higher energy and 100 times higher intensity of primary beams, while intensities of secondary beams will be 10 000 times higher than at present GSI’s facility. References 1. I. Tanihata et al., Phys. Rev. Lett. 55, 2676 (1985). 2. A. Ozawa et al., Nucl. Phys. A691, 599 (2001). 3. T . Zheng et al., Nucl. Phys. A709, 103 (2002). 4. T. Yamaguchi et al., Nucl. Phys. A724, 3 (2003). 5. N. Imai et al., Phys. Rev. Lett. 92, 062501 (2004). 6. R. Kanungo et al., Phys. Lett. B571, 21 (2003). 7. M.V. Zhukov and I. J. Thompson, Phys. Rev. C52, 3505 (1995). 8. L.V. Grigorenko et al., Nucl. Phys. A713, 372 (2003). 9. L.V. Grigorenko and M.V. Zhukov, Phys. Rev. C (2004) in print. 10. V.I. Goldansky, Nucl. Phys. 19, 482 (1960). 11. O.V. Bochkarev et al., Nucl. Phys. A505, 215 (1989). 12. O.V. Bochkarev e t al., Sou. J . Nucl. Phys. 55, 955 (1992). 13. R.A. Kryger et al., Phys. Rev. Lett. 74, 860 (1995). 14. J . Giovinazzo et al., Phys. Rev. Lett. 89, 102501 (2002). 15. M. Pfutzner et al., Eur. Phys. J . A14, 279 (2002). 16. L.V. Grigorenko et al., Phys. Rev. C64, 054002 (2001). 17. H.O.U. Fynbo et al., Phys. Rev. Lett. 91, 082502 (2003). 18. A.M. Lane and R.G. Thomas, Rev. Mod. Phys. 3 0 , 257 (1958). 19. B.V. Danilin et al., Y a d . Fiz. [Sou. J . Nucl. Phys.146, 427 (1987). 20. D.R. Tilley et al., Nucl. Phys. A541, 1 (1992). 21. M. Meister et al., Nucl. Phys. A 723, 13 (2003). 22. S.I. Sidorchuk et al., P h y s . Lett. B594, 54 (2004). 23. A.A. Korsheninnikov et al., Phys. Rev. Lett. 87, 092501 (2001). 24. M.S. Golovkov et al., Phys. Lett. B566, 70 (2003). 25. M.G. Gornov et al., JETP Lett. 77, 344 (2003). 26. S.V. Stepantsov et al., under preparation. 27. M.S. Golovkov et al., Phys. Rev. Lett. (2004) in print. 28. A.A. Korsheninnikov et al., Phys. Rev. Lett. 90, 082501 (2003). 29. A.A. Korsheninnikov et al., Phys. Rev. Lett. 82, 3581 (1999). 30. M. Meister et al., Phys. Rev. Lett. 88, 102501 (2002). 31. H.G. Bohlen et al., Phys. Rev. C64, 024312 (2001). 32. G.V. Rogachev et al., Phys. Rev. Lett. 92, 232502 (2004).

11

MASSES, HALF-LIVES, SPINS AND DECAYS OF EXOTIC NUCLIDES

GEORGES AUDI AND CATHERINE THIBAULT Centre d e Spectrome'trie Nucle'aire et d e Spectrome'trie d e Masse, CSNSM, IN2P3-CNRS & UPS, Bitiment 108, F-91405 Orsay Campus, fiance AALDERT H. WAPSTRA National Institute of Nuclear Physics and High-Energy Physics, NIKHEF, PO Box 41882, 1009DB Amsterdam, The Netherlands JEAN BLACHOT AND OLIVIER BERSILLQN Service de Physique Nuclkaire, CEA, B.P. 12, F-91680 Bruytres-le-Chitel, fiance In December 2003, a full collection of atomic masses, AME2003, was published, based on an evaluation of experimental masses. While evaluating the atomic masses it was found essential to create the NUBASEevaluation. We finally succeeded in having AMEand NUBASEcc-ordinated and published for the first time together.

1. The evaluations of nuclear data

The data considered by the Nuclear Structure and Decay Data network (the NSDD) covers most of the 'static' nuclear data ': half-lives, spins and parities of excited and ground-state levels; the relative position (excitation energies) of these levels; their decay modes and the relative intensities of these decays; the transition probabilities from one level to another and the level width; the deformations; the magnetic moments;. . . The NSDDevaluation for each nuclide is not dependent, at first order, on the properties of a neighboring nuclide, except when there is a decay relation with that neighbor. Such evaluation, conducted nuclide by nuclide, is called a 'vertical' evaluation. On the opposite side, the evaluation of data for energy relations between nuclides is more complex due to numerous links that overdetermine the system and exhibit sometimes inconsistencies among data. This ensemble of energy relations is taken into account in the 'horizontal' structure of the

12

Atomic Mass Evaluation (AME). By ‘horizontal’ one means that a unique nuclear property is being considered across the whole chart of nuclides, here the ground-state masses l.

2. The matter of isomers and the NUBASE evaluation At the interface between the NSDDand the AME, one is faced with the problem of identifying - in some difficult cases - which state is the groundstate. The isomer matter is important in the AME,since a mistreatment can have important consequences on the ground-state masses. When an isomer decays by an internal transition, there is no ambiguity and the assignment as well as the excitation energy is given by the NSDDevaluators. However, when a connection to the ground-state cannot be obtained, most often a decay energy to (and sometimes from) a different nuclide can be measured (generally with less precision). In the latter case one enters the domain of the AME,where combination of the energy relations of the two long-lived levels to their daughters (or to their parents) with the masses of the latter, allows to derive the masses of both states, thus an excitation energy (and, in general, an ordering). Up to the 1993 mass table, the AMEwas not concerned with all known cases of isomerism, but only in those that were relevant to the determination of the ground-state masses. In 1992 it was decided, after discussion with the NSDD evaluators, to include all isomers for which the excitation energy “is not derived from y-transition energy measurements (7-rays and conversion electron transitions), and also those for which the precision in y-transitions is not decidedly better than that of particle decay or reaction energies leading to them” However, differences in isomer assignment between the NSDDand the AME evaluations cannot be all removed at once, since the renewal of all A-chains in NSDDcan take several years. In the meantime also, new experiments can yield information that could change some assignments. Here a ‘horizontal’ evaluation should help. The isomer matter was one of the main reasons for setting up in 1993 the NUBASEcollaboration leading to a thorough examination and evaluation of those ground-state and isomeric properties that can help in identifying which state is the ground-state and which states are involved in a mass measurement 3 . NUBASEappears thus as a ‘horizontal’ database for several nuclear properties: masses, excitation energies of isomers, half-lives, spins and parities, decay modes and their intensities. Applications extend

’.

13

from the AMEto nuclear reactors, waste management, astrophysical nucleosynthesis, and to preparation of nuclear physics experiments. Setting up NUBASEallowed in several cases to predict the existence of an unknown ground-state from trends of isomers in neighboring nuclides, whereas only one long-lived state was reported. A typical example is 161Re, for which NUBASE'97 predicted a (1/2+#) proton emitting state below an observed 14 ms a-decaying high-spin state. (Everywhere in AME and NUBASEthe symbol # is used to flag values estimated from trends in systematics.) Since then, the 370 ps, 1/2+, proton emitting state was reported with a mass 124 keV below the 14 ms state. For the latter a spin 11/2- was also assigned '. Similarly, the 11/2- bandhead level discovered in lZ7Pr is almost certainly an excited isomer. We estimate for this isomer, from systematical trends, an excitation energy of 600(200)# keV and a half-life of approximatively 50# ms. In some cases the value determined by the AMEfor the isomeric excitation energy allows no decision as to which of the two isomers is the groundstate. This is particularly the case when the uncertainty on the excitation energy is large compared to that energy, e.g.: E m ( 8 2 A ~ )250 = f 200 keV; Em(134Sb)= 80 f 110 keV; Em(154Pm)=120 f 120 keV. Three main cases may occur. In the first case, there is no indication from the trends in J" systematics of neighboring nuclides with same parities in N and 2, and no preference for ground-state or excited state can be derived from nuclear structure data. Then the adopted ordering, as a general rule, is such that the obtained value for Em is positive. In the three examples above, 82As will then have its (5-) state located a t 2505200 keV above the (l+); in 134Sb the (7-) will be 803~110keV above (0-); and lS4Pm's spin (3,4) isomer 120f120 keV above the (0,l) ground-state. In the second case, one level could be preferred as ground-state from consideration of the trends of systematics in J". Then, the NUBASEevaluators accept the ordering given by these trends, even if it may yield a (slightly) negative value for the excitation energy, like in lo8Rh (high spin state at -603~110 keV). Such trends in systematics are still more useful for odd-A nuclides, for which isomeric excitation energies of isotopes (if N is even) or, similarly, isotones follow usually a systematic course. This allows to derive estimates both for the relative position and for the excitation energies where they are not known. Finally, there are cases where data exist on the order of the isomers, e.g. if one of them is known to decay into the other one, or if the Gallagher-Moszkowski rule for relative positions of combinations points strongly to one of the two as being the ground-state. Then the negative

14

part, if any, of the distribution of probability has to be rejected. 3. The evaluation of atomic masses (AME)

Generally a mass measurement can be obtained either by establishing an energy relation between the mass we want to determine and a well known mass, this energy relation is then expressed in electron-volts (eV); or obtained as an inertial mass from its movement characteristics in an electromagnetic field, the mass is then expressed in ‘unified atomic mass’ (u) (or its sub-unit, pu), since it is obtained as a ratio of masses. The mass unit is defined, since 1960, by 1u = M(I2C)/12, one twelfth of the mass of one free atom of Carbon-12 in its atomic and nuclear groundstates. The choice of the volt in the energy unit (the electronvolt) is not evident. In the AME,it appeared that not the international volt V should be used, but the volt Vgo * as maintained in standard laboratories. The latter is defined by adopting a value for the constant ( 2 e / h ) in the relation between frequency and voltage in the Josephson effect. This choice results from an analysis showing that all precision measurements of reaction and decay energies are calibrated in such a way that they can be more accurately expressed in the standard volt. Also, the precision of the conversion factor between mass units and standard volts Vgo is more accurate than that between it and international volts V: 1u = 931 494.009 0 f0.007 1 keVgo 1u = 931 494.013 f 0.037

keV

The evaluation of masses share with most other evaluations many procedures. However, the very special character in the treatment of data in the mass evaluation is that all measurements of masses are relative measurements. Each experimental datum sets a relation in energy or mass among two (in a few cases, more) nuclides. It can be therefore represented by one link among these two nuclides. The ensemble of these links generates a highly entangled network l . Here lies the very challenge to extract values of masses from the experiments. The counterpart is that the overdetermined data system will allow cross-checks and studies of the consistencies within this system. One might for example examine all data for which the adjusted values deviate importantly from the input ones. This might help to locate erroneous pieces of information. One could also examine a group of data in one experiment and check if the errors assigned to them in the experimental paper were not underestimated.

15

The other help to the evaluator will be the property of regularity of the surface of masses. Experimentally, it has been observed that: the surface of masses varies very smoothly with N and 2, however these variations are very rapid. They are only interrupted in places by sharp cusps or large depressions associated with important changes in nuclear structure: shell or sub-shell closures (sharp cusps), shape transitions (spherical-deformed, prolate-oblate: depressions on the mass surface), and the so-called ‘Wigner’ cusp along the N = 2 line. This observed regularity of the mass sheets in all places where no change in the physics of the nucleus are known to exist, can be considered as ONE OF THE BASIC PROPERTIES of the mass surface. Thus, dependable estimates of unknown, poorly known or questionable masses can be obtained by extrapolation from well-known mass values on the same sheet. Any coherent deviation from regularity, in a region ( N ,2 ) of some extent, could be considered as an indication that some new physical property is being discovered. However, if one single mass violates the systematic trends, then one may seriously question the correctness of the related datum. There might be, for example, some undetected systematica contribution to the reported result of the experiment measuring this mass. To be complete, it should be said that REGULARITY is not the only property used to make estimates: all available experimental information is taken into account. In particular, knowledge of stability or instability against particle emission, or limits on proton or alpha emission, yield upper or lower limits on the separation energies. 4. The tables

In December 2003, we succeeded in having published TOGETHER the and the NUBASEevaluation 3 , which “Atomic Mass Evaluation” AME have the same “horizontal” structure and basic interconnections at the level of isomers. The long delay between the 1995 and the 2003 AME’Swas due to the urgency in having the first NUBASE evaluation completed. The NUBASE evaluation was thus published for the first time in September 1997 4 , but loyll

aSystematic errors are those due to instrumental drifts or instrumental fluctuations, that are beyond control and are not accounted for in the error budget. They might show up in the calibration process, or when the measurement is repeated under different experimental conditions. The experimentalist adds then quadratically a systematic error to the statistical and the calibration ones, in such a way as to have consistency of his data. If not completely accounted for or not seen in that experiment, they can still be observed by the mass evaluators when considering the mass adjustment as a whole.

16

in order t o have consistency between the two tables, it was decided then that the masses in NUBASE’97 should be exactly those from AME’95. This time, the AME2003 and NUBASE2003 are completely ‘synchronized’. Full content of the two evaluations is accessible on-line at the web site of the Atomic Mass Data Center (AMDC)l2 through the World Wide Web. One will find at the AMDC,not only the published material, but also extra figures, tables, documentation, and more specially the ASCII files for the AME2003 and the NUBASE2003 tables, for use with computer programs. T h e contents of NUBASEcan be displayed with a PC-program called “NUCLEUS”13, and also by a Java program JVNUBASEl4 through the World Wide Web, both distributed by the AMDC.

References 1. G. Audi, “A Lecture on the evaluation of atomic masses”, arXiv http://arxiv.org/abs/nucl-ex/0302020 2. G. Audi and A.H. Wapstra, Nucl. Phys. A 595 (1995) 409. 3. G. Audi, 0.Bersillon, J. Blachot and A.H. Wapstra, Nucl. Phys. A 7 2 9

(2003)3. 4. G. Audi, 0.Bersillon, J. Blachot and A.H. Wapstra, Nucl. Phys. A 624 (1997) 1. 5. R.J. Irvine, C.N. Davids, P.J. Woods, D.J. Blumenthal, L.T. Brown, L.F. Conticchio, T. Davinson, D.J. Henderson, J.A. Mackenzie, H.T. Penttila, D. Seweryniak and W.B. Walters, Phys. Rev. C 55 (1997) 1621. 6. T. Morek, K.Starosta, Ch. Droste, D. Fossan, G. Lane, J. Sears, J. Smith and P. Vaska, Eur. Phys. Journal A 3 (1998)99. 7. C.J. Gallagher, Jr. and S.A. Moszkowski, Phys. Rew.111 (1958) 1282. 8. G. Audi, A.H. Wapstra and M. Dedieu, Nucl. Phys. A 565 (1993) 193. 9. E.R. Cohen and A.H. Wapstra, Nucl. Instrum. Meth. 2 1 1 (1983) 153 10. A.H. Wapstra, G.Audi and C. Thibault, Nucl. Phys. A 7 2 9 (2003) 129. 11. G. Audi, A.H. Wapstra and C. Thibault, Nucl. Phys. A 7 2 9 (2003)337. 12. The NUBASEand the AME files in the electronic distribution can be retrieved from the Atomic Mass Data Center through the Web at http://www.nndc.bnl.gov/amdc/. 13. B.Potet, J. Duflo and G. Audi, Proc. Znt. Conf. on Exotic Nuclei and Atomic Masses (ENAM’%), Arles, June 1995,p. 151; htt p://www.nndc.bnl.gov/amdc/nucleus/arlnucleus.ps 14. E. Durand, Report CSNSM 97-09, July 1997; http://www.nndc.bnl.gov/amdc/nucleus/stg-durand.doc

17

RECENT MISTRAL MASS MEASUREMENTS OF MAGNESIUM 29-33 AND LITHIUM 11

c . THIBAULT, G . AUDI, c . BACHELET, c . GAULARD, c . GUENAUT, D. LUNNEY, M. DE SAINT SIMON, N. VIEIRA CSNSM, IN2P3- CNRS and Universite' Paris Sud, B2t. lO4-lO8, F-91405 Orsay-campus, France E-mail: [email protected]

F. HERFURTH AND T H E ISOLDE COLLABORATION CERN, EP Division, Geneva 1211, Switzerland High precision mass measurements have been performed on the very neutron-rich 2g-33Mg and I'Li nuclides using the MISTRALradio-frequency mass spectrometer, especially suited to very short-lived nuclides. This method, combined with has allowed to sigthe powerful tool of resonant laser ionization at CERN-ISOLDE nificantly reduce the uncertainty in mass of these nuclides. An accuracy of 5 to 7x10-' could be achieved for the weakly produced 33Mg (T,12=90.5 ms) and "Li (T1/2=8.75 ms). All these nuclei were selected for their special interest: the isotopic chain 2g-33Mg crosses the N = 20 line which is a shell closure for heavier elements, but not for light ones as sodium and magnesium, while the borromean nuclide "Li is known as a halo nucleus in which two neutrons stray very far from the 'Li core.

1. Introduction

Using the MISTRALspectrometer at ISOLDE at CERN,which is especially suited for nuclei with half lives less than 100 ms, it was possible to largely improve the accuracy of masses in two particularly interesting physics cases. The first one is the study of the collapse of the N=20 shell closure when the number of protons is decreasing down to 11 and 12. In this context, the masses of 29-33Mg with N=17-21 could be remeasured. The second topic is the study of 'lLi which has a half life of 8.75 ms. This nucleus is known to have its two last neutrons in a halo very far from its 'Li core. The two-

18 neutron binding energy is one of the key parameters for the models which calculate the halo radius. Furthermore, “Li is a “borromean” nucleus in which the substructures l0Li and n-n are not bound. This feature implies that the three-body interactions very probably play an important role. 2. The Mistral spectrometer

MISTRALis a transmission radio-frequency ~ p e c t r o m e t e r [ ~in~ which ~,~~~] the mass is measured via the determination of the cyclotron frequency fc. The ions are injected into an homogeneous magnetic field B in which they rotate twice according to an helicoidal trajectory before being ejected. f c is related to the mass m and charge q of the ion by:

fc =

qB

In order to determine the cyclotron frequency, the ion beam kinetic energy is modulated after one and three half-turns. As a result, the beam can only pass through the exit slit of the spectrometer if the two modulations cancel, ie. if the excitation radio-frequency f R F is such that

Scanning the transmission as a function of the radio-frequency thus provides the value of f R F and consequently that of fc. The unaccurately known value of the magnetic field B is eliminated thanks to the comparison with a well known reference mass m, provided by an auxiliary source of MISTRALso that

mzfC,,= mrfc,r

(3)

where rn, is the mass to measure. The reference and unknown masses are alternately measured to eliminate the magnetic field fluctuations. This procedure requires to also alternate all the voltages of the injection and ejection lines of MISTRALaccording to the relation m,V, = m,V,. However Eq. ( 3 ) does not exactly hold because both the trajectories are not strictly identical and the magnetic field is not perfectly homogeneous. A calibration using neighbouring well known masses produced by ISOLDE has to be done. The observed relative deviation from the 1995 Atomic Mass evaluation (AME95)15]may be fitted by a linear law as a function of

(vz - vr)/(Vx-t Vr)[61.

19 3. Magnesium measurements The magnesium isotopes were produced by ISOLDEat CERNin the fragmentation of uranium by the PS-booster 1 GeV protons. They were singly ionized by Resonant Ionization Laser Ion Source (RILIS) which is a very selective and efficient method. However, by switching off the RILIS lasers, it was possible to do also measurements on sodium isotopes. The chosen reference mass was the nitrogen molecule 14N-14N. The calibrant masses were 23Na, and 24-26,28Mg. The calibration law is shown Figure l(1eft). The result^[^^^] after corrections taking into account this calibration are shown on Figure l(right). It may be seen that the agreement with the

i

10-'units

40 ref='*N''N

f

20

?/ndf=9.5/10 slope =-445. f 20. shift = 1.2 f 2.8

2

-80 -100

%g

%g %Ig %4g YNa

-120

0

0.05

0.1

0.15

0.2

0.25

2(vx-v,>/(vx+v,>

-200

I

-300&

"

29

' " " " "

30

31

"

32

A . >

1

33 I

Figure 1. (left): linear calibration law fitted for 23Na, and 24p26,28Mgmeasured using the l4N-I4N molecule as reference; (right): measured masses of 29-33M9. The AME95 precision is given by the two full lines

AME95 recommended values is rather good for 30131,32Mgwhile nearly 2 CT deviations are observed for 2gMg and 33Mg. However, looking at individual ~neasurements[~], it appears that the main disagreement comes from the Los Alamos time-of-flight TOFI measurements made in 1991['] as already observed for the sodium mass measurement^[^]. The accuracy is improved by a factor 2 to 7 as compared to AME95. The interesting point is that 32,33Mg(N=20,21) are found more bound than previously accepted by respectively 120 and 260 keV. This reinforces even further the collapse of the N=20 shell closure.

20 4. L i t h i u m measurements

Lithium and beryllium isotopes were produced by ISOLDE at CERNin the fragmentation of uranium by the PS-booster 1 GeV protons. They were singly ionized either by surface ionization (lithium), or by RILIS (beryllium). The lithium and beryllium isobars are separated by MISTRAL.Anyway, when the RILIS lasers are switched off, a pure lithium ion beam is obtained. When the lasers are on, the beryllium isotopes are much more produced than their lithium isobars so that a lithium contamination is totally excluded. The intensity of the "Li beam delivered by ISOLDEwas -1000 ions/pulse. Examples of a "Li recorded peak and of its time dependance following the proton pulse are shown in Figure 2. It is clear that there is no background around the peak, nor before the proton pulse. By fitting an exponential to the time dependance of llLi, which is controlled by its short half-life, the adjusted value is Tl/2 = 13.64 x Ln2 = 9.5(1.2) ms, which is quite compatible with the accepted value of 8.75 ms.

....................... . .. ............. .

Figure 2. (left): example of a scan of llLi counting rate versus radio-frequency; (right): example of time dependance of the llLi counting rate. The proton pulse is delivered at time t=260ms

The reference masses were 10illB. The calibrant masses were 'Li and 9,10Be. The fitted calibration law is shown in Figure 3. Using this calibration, new mass values were determined for llBe and llLi[lo,lll. The ''Be mass is in perfect agreement with previous ones with an accuracy of 4 keV, slightly better than the 6 keV of AME95. Concerning llLi, the accuracy is 5 keV, 7 times better than the most accurate previous measurement[12].

21

Relative Mass Jump MISTRAL-ISOLDE Figure 3. Linear calibration law fitted for 9Li, and 9,10Bemeasured using the lo,l1B as references. The mass measurements of "Li with "B as a reference are also indicated. It appears that they deviate strongly from AME95. The "Be measurements which fall on the calibration line are not shown in order to avoid a too tangled figure.

-

However, as suggested by Figure 3, the AME95 value is far away, at 3a. The new value agrees with time-of-flight measurements by Wouters et u L . [ ~ ~ ] and with a reaction Q-value by Kobayashi et ~ l . [ ~The ~ ]disagreement . with AME95 comes from the Q-value measured by Young et aJ.Il'1 with a quoted uncertainty of 35 keV. The reason of this discrepancy is not clear. In our case, the recorded peak and time dependance (Fig. 2) clearly demonstrate the purity of llLi. Furthermore, our llBe value agrees perfectly with previous accurate measurements, and no difference is observed between 'Li and 'Be, which were both used as calibrants. Our result only changes the "Li mass value by some lop6. However, since the two-neutron binding energy S2n is very small, it is increased by as much as 25%. The new value better agrees with the Nuclear Field Theory predictions based on a Wood-Saxon potential and a particle-vibration coupling[15]. On another hand, our Szn value allowed Yamashita et al.[ls] to reproduce the experimental radius of "Li with a realistic energy for the required virtual excited state of ''Li.

22 5 . Conclusion

During the last years, MISTRALhas allowed to measure the masses of light nuclei with half-lives as short as 9 ms with an accuracy of about 5 x This accuracy provided a better knowledge of the binding energy around the N=20 magic number. In the case of halo nuclei which have very low neutron binding energies, the example of "Li shows that an accuracy around 5 x lop7 is required to finally get a few percent accuracy on S, or S,,, which is valuable for theoretical models. Another very good physics case would be 14Be (T1/2=4.35 ms) which is also a borromean nucleus with a two-neutron halo. However the available intensity at ISOLDEis only 10 ions/s, much below the present sensitivity of MISTRAL.Current improvements of MISTRALtend to increase its sensitivity by a factor 10' to lo3. They are based on the development of a cooling d e v i ~ e [ ' ~ >which ' ~ ] will substantially reduce the emittance of the ISOLDEbeam to better adapt it to the small acceptance of MISTRAL.

-

References 1. A. COCet al., Nucl. Instr. and Meth. A271,512 (1988). 2. M. de Saint Simon et al., Phys. Scripta T59,406 (1995). 3. M.D. Lunney et al., Hyp. Int. 99,105 (1996). 4. D.Lunney et al., Hyp. Int. 132,299 (2001). 5. G. Audi and A.H. Wapstra, Nucl. Phys. A595,409 (1995). 6. C. Gaulard et al., to be submitted. 7. D. Lunney et al., submitted to Eur. Phys. J . A. 8. X.G. Zhou et al., Phys. Lett. B260,285 (1991). 9. D. Lunney et al., Phys. Rev. C64,054311 (2001). 10. C. Bachelet, PhD thesis, Universite' ParisXI, (2004). 11. C . Bachelet et al., Letter to be submitted. 12. B.Young et al., Phys. Rev. Lett. 71,4124 (1993). 13. J. Wouters et al., 2. Phys. A331,229 (1988). 14. T.Kobayashi et al., K E K Preprint 91-22,1 (1991)http://ccdb3fs.kek.jp/cgibin/img- index?199127022 15. R.A. Broglia et al., INPC2001, Proc. in A I P 610 , 746 (2002). 16. M.T. Yamashita et al., Nucl. Phys. A735,40 (2004),and private communication. 17. M. Sewtz et al., ECARRTB(september 2OO4), Proc. in Nucl. Instr. Meth. B , t o be published (2005).

23

NEW MASS MEASUREMENTS AT THE NEUTRON DRIP-LINE *

H. SAVAJOLS, B. JURADO, W. MITTIG, C.E. DEMONCHY, L. GIOT, A. KHOUAJA, S. PITA, M. ROUSSEAU, P. ROUSSEL-CHOMAZ AND A.C.C. VILLARI+ GANIL, BP 55027 F-14075 Caen Cedex 5, h n c e D. BAIBORODIN, Z. DLOUHY AND J.MRAZEK Nucl. Phys. Ins. ASCR 25068 Rez, Czech Republic S. LUKYANOV AND Y. PENIONZHKEVICH FLNR, JINR Dubna P.O.Box 79 101000 Moscow, Russia

A. GILLIBERT CEA/DSM/DAPNIA/SPHN, CEN Saclay F-91191 Gif-sur Yvette, fiance N. ORR LPC - ISMRA and University of Caen F-6704 Caen, fiance M. CHARTIER University of Liverpool Dept. of Physics, Liverpool L69 7ZE, UK A. LEPINE-SZILY University of Sdo Paulo IFUSP C.P. 66318 05915-970 Sdo Paulo, Brazil W. CATFORD University of Surrey Nuclear Physics Dept., Guilford, GU27XH, UK

'This work was particularly supported by the INTAS-00-00463, by the russian foundation for fundamental research (RFFR) and PICS (IN2P3) n01171. tACCV acknowledge his partial support by the U.S. Department of Energy, Office of Nuclear Physics, under contract W-31-109-ENG-38.

24 A mass measurement experiment by a time of flight method with the SPEG spectrometer at GANIL has been performed to investigate the N = 20 and N = 28 shell closure far from stability. The masses of more than 23 neutron-rich nuclei in the range A = 23-48 have been measured. The region covered was motivated by the study of shell structure and of shape coexistence in the region of closed shells N = 20 and N =28. The evolution of the two neutron separation energies and the Shell Correction Energy have been studied as a function of the neutron number. The results thus obtained provide a means of identifying, in exotic nuclei, new nuclear structure effects that are well illustrated by the changes of the conventional magic structure from those of stable nuclei.

1. Introduction

One of the fundamental questions, which emerge from the study of nuclei far from stability, concerns the persistence of the magic character of certain configurations of protons and neutrons. In this context, the measurement of masses (or binding energy) of nuclei far from stability is of fundamental interest for our understanding of nuclear structure. Their knowledge over a broad range of the nuclear chart is an excellent and severe test of nuclear models. This is why considerable experimental and theoretical efforts have been and axe invested in this domain. In this contribution, we present new mass measurements for neutron rich nuclei in the region defined by (5 < N < 28 ; 7 < Z < 18) obtained with the spectrometer SPEG at GANIL. These data correspond to the most exotic nuclei presently attainable in this region and provide first indications of new regions of deformation or shell closures very far from stability. 2. Results of mass measurements

From this experiment and its subsequent analysis, the masses of 23 neutronrich nuclei have been measured. Details of the analysis can be either found in already published paper 132. The separation energy of the 2 last neutrons corresponding to a deriv+ tive of the nass surface, Szn, derived from the current and previous me+ surements are displayed in figure 1. A more direct way to see shell effects on nuclear masses is to subtract from the mass excesses the contribution of the macroscopic properties of the nuclei. Here we have used the finite range liquid drop model of 3 . The difference - the microscopic or Shell Correction Energy (SCE) - is plotted in figure 2 (left) for Z=14 to Z=20 isotopes and figure 2 (right) for Z=8 to Z=13 isotopes. As can be seen for both observables, i.e. experimental Szn and SCE, the Ca isotopes (Z=20) show the typical behavior of the filling of shells with the two shell closures at N =

25

Neutron number

Figure 1. Experimental Szn values as a function of the neutron number N in the region of N=20 and N=28 shell closures .

20 and N = 28; sharp decrease of the Szn at N=20 and a slow decrease of Szn as the 1f7/2 shell is filled and SCE minima at N=20 and N=28. In the rest of the article, the standard behavior represented by the Ca chain will be taken as reference. 2.1. The N=28 region

Contrary to the Ar and K isotopes, both Szn and SCE values of the Cl, S, P and Si isotopic chains differ around N=28 from the standard behavior represented by the Ca chain. A discontinuity in the SZn slope when filling the u l f7/2 shell (from N=20 to N=28) is strongly pronounced for the C1, S and P isotopes. This trend is attenuated for the Si and A1 chains. This overbinding, already observed in our previous mass measurement experiment 4 , but with large uncertainties, was attributed to deformed ground state configurations. The observation, in the same experiment, of a low excited isomeric state in 43S 4 , confirmed the analysis of the masses and constituted the first shape coexistence in that region. More detailed informations have been obtained for these nuclei by other experimental probes, i.e. Coulomb exciand in beam gamma spectroscopy tation measurement for the S isotopes experiment 7 ; both conclude for deformed ground state configurations. Beyond N=28, the isotopic P and S isotopic chains show a clear increase of the S Z n , this is an indication for the vanishing of this shell closure for these very neutron-rich nuclei. The standard behavior represented by the 516

26

5

i0

15

20

25

30

Number of neutrons

Figure 2. Shell corrections as defined in the text of the mass of Si, P, S, C1, Ar and Ca isotopes (right) and of 0 , F, Ne, Na, Mg, A1 and Ca isotopes (left).

Ca chain s e e m to reappear slowly when moving to the chains of C1 and Ar. In order to determine the origin of the increase of S2, for 44Pand 45S, the present results should be compared with model calculations. For the neutron-rich nucleus 42Si, the protons confined in the ~ d 5 / 2orbital (Z=14 sub-shell gap) and the N=28 gap together, could favor spherical configuration. Indeed, our result for the mass excess of 42Si is around 3 MeV smaller than the extrapolation of the mass table '. This indicates that this nucleus is much more bound than what one would obtain if the Si isotopic chain would follow the standard trend of the Ca chain. This could possibly be an indication of the strong deformation of 42Si. From the theoretical point of view different theoretical approach exit. On one hand, shell model calculations performed by Retamosa et al. ', indicate that the 42Si has the characteristics of a double magic nucleus such as 48Ca. More recently, the interaction has been adjusted to reproduce single particle states in 35Si lo and the shell gap l f 7 / 2 - 2pp3/2 is steadily reduced from its initial value of 2 MeV at Z=20 until almost zero at Z=8. Therefore the closed-shell configuration becomes vulnerable and at some point it becomes energetically favorable to promote neutrons across the gap, recovering the cost in single-particle energies by the gain in neutron

27 proton quadrupole correlation energy. Moreover, those calculations lead to a deformed 43S ground state with spin 3/2- while the spherical single hole state 7/2- would be the first-excited state, in good agreement with experimental data. On the other hand, the calculations performed by Lalazissis et al. l1 (relativistic Hartree Bogoliubov) predict the breaking of the N=28 shell gap below 48Ca with a large deformed configuration for 42Si.

2.2. The N=&O region

The value obtained for the 23N mass excess is considerably smaller (1.5MeV ) than the extrapolation of the mass evaluation 2003 value given by Audi et al. which is obtained assuming a regular behaviour of Szn. This indicates that the 23N with 16 neutrons is more bound than expected. This might be an indication for the existence of a shell closure at N=16 for very neutron rich nuclei. Also the value for the mass excess that we obtain for the 240with 16 neutrons is lower than the value given in the mass table, which again indicates that the 240is more bound than what was thought previously. For the Ne and the Na isotopic chains the twoneutron separation energies decrease much more steeply after N=16 than after N=20, which is again an indication for the existence of the shell closure N=16 for this very neutron-rich nuclei. Moreover, the absence of the steep decrease after N=20 for the Mg and A1 chains confirms the vanishing of this spherical shell closure. Shell closure N=20 starts to reappear for the less neutron-rich isotopes of the A1 and Si chains. Beyond N=20, for the Ne, Na and Mg isotopes, a rapid decrease of Sz, indicates that those isotopes may become unbound rapidly with respect to the neutron emissions. If we add one proton in the r d g / 2 from the oxygen configuration, the picture for the fluorine isotopes changes drastically. The Sz, values decrease continuously to almost zero for the "F. Only strong shell effects could bound the last known fluorine, 31F. The shell correction energies from figure 2 show nicely the shell effect evolution in that region and confirm the previous discussion on the experimental Sz,. If we start from the 0 isotopes, we clearly observe two N=8 and N=16 minima with a rather high, 5MeV, difference in magnitude between them (0with N>16 do not exist as bound nuclei). The gap at N=16 still persists when we add a proton in the r d g p shell, but the amplitude decreases smoothly up to 29Al. Moreover, the SCE confirm the vanishing of the shell closure at N=20, SCE are maximized ar, N=20 for the F, Ne, Na and Mg isotopes.

28 More recent shell model calculations l 3 interpret this disappearance of the magic number N=20 by the inversion of the order of the shells due to the dependence of the neutron-proton interaction on the combination of their spin in the nucleus (nucleon-nucleon spin-isospin Vcr interaction). In that region, the basic mechanism of this change is the strongly attractive interaction between spin-orbit partners 7rd512 and u d 3 p . As Z increases from 8 to 14, valence protons are added into the ~ d 5 1 2orbit. Due t o the strong attraction between a proton in ~ d 5 l 2and a neutron in ~ d 3 1 2as , more / ~ , in ~ d 3 is/ more ~ strongly bound. The protons are put in ~ d ~a neutron magic number N=20 should be therefore replaced by N=16 for the nuclei in this region very far from stability, and that this phenomenon should occur over all the chart of the nuclei. In particular, the non-observance of ' * O , a doubly magic nucleus in theory, could also be explained by this modification of its shell structure.

3. Conclusions

-

The direct time-of-flight method with SPEG is the only method for mea10-50. suring masses up to the neutron drip-line in the mass region A The experimental shell corrections and the two neutron separation energies have been calculated. The results thus obtained provide a means of identifying new nuclear structure effects that are well illustrated by this work in the N=16, N=20 and N=28 region.

References 1. F.Sarazin, Thesis GANIL T 99 03 (1999). 2. H.Savajols, Hyperfine Interactions B132: 245-254, 2001. 3. P.Moller and J.R.Nix, At. Data and Nucl. Data Tables 59,(1995) 185 4. F.Sarazin et al., Phys. Rev. Lett. 84, (2000) 5062. 5. H.Scheit et al., Phys. Rev. Lett. 77,(1996) 3967. 6. T.Glasmacher et al., Phys. Lett. B395,(1997) 163. 7. D.Sohler et al., Phys. Rev. C 66,(2002) 054302. 8. G.Audi et al., Nuclear Physics A 729,2003 3. 9. J.Retamosa et al., Phys. Rev. C 55, (1997) 1266. 10. S.Nummela et al., Phys. Rev. C 63,(2001) 044316. 11. G.A.Lalazissis et al., Phys. Rev. C 60,(1999) 01431. 12. E.Caurier, F.Nowacki, and A.Poves, EPJ A 15,(2002) 145. 13. T.Otsuka et al., Phys. Rev. Lett. 87,(2001) 082502-1.

29

SEARCH FOR TETRANEUTRONS IN THE BREAKUP OF *HE

I

BOUCHAT VIRGINIE Universite' Libre de Bruxelles, CP226, B-10.50 Bruxelles, Belgium

F.M. MARQUES2, F. HANAPPE', J.C. ANGELIQUE2, C. ANGUL03, N. ASHWOOD4, M.J.G. BORGE', W. CATFORD6, N. CLARKE4, N. CURTIS4, P. DESCOWEMONT', M. FREER4, V. KINNARD', M. LABICHE', P. MCEWAN4, T. MATERNA', T. NILSSON', A. NINANE3, G. NORMAND2, N.A. O M 2 ,S. PAIN6, E.V. PROKHOR0VAIs9,L. STUTTGE'', C. TIMIS6 Laboratoire de Physique Corpusculaire. IN2P3-CNRS, ISMRa et Universite' de Caen, F-14050 Caen cedex, France Centre de recherche du Cyclotron, UC L, B-1348 Louvain-La-Neuve, Belgium School of Physics and Astronomy, University of Birmingham, BIS 2TT, U.K. Instituto de Estructura de la Materia, CSIC, E-28006Madrid, Spain Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, U.K. ' Department of Physics, University of Paisley, Paisley PA I 2BB, Scotland Experimentell Fysik, Chalmers Tekniska Hogskola, S-412 96 Goteborg, Sweden ' Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia lo Institut de Recherche Subatomique. IN2P3-CNRS. Universitt Louis Pasteur, BP 28, F-6703 7 Strasbourg cedex, France

'

'

'

A new approach to the production and detection of neutron clusters was recently proposed by the DEMON-CHARISSA collaboration. Its application to the breakup of 14Be into " b e and 4 neutrons revealed 6 events that exhibited the characteristics of a 4n state, either bound or resonant. We have applied this technique to the breakup of another system that could be seen as a core plus a 4n cluster, 'He. Preliminary results present 18 events in the 4He+4n channel exhibiting the characteristics of a 4n state: an alpha particle in coincidence with the high-energy recoil of a proton in the liquid scintillator of DEMON. The simulations in progress will be discussed in order to clarify the origin of these events. We have also determined the energy surfaces by a microscopic theoretical approach using the Generator Coordinate Method in a four-centre model where the neutrons are located at the apexes of a tetrahedral configuration.

1.

INTRODUCTION

In a recent study on the structure of I4Be, 6 events exhibit the characteristics of multi-neutron clusters liberated during the break-up of I4Be on a Carbon target and most probably in the channel 'oBe+4n [I], The goal of this work is then to verify this results by applying the same analysis procedures on 'He. Indeed, we know today that 'He is probably a four-neutron halo and not a three body system (6He+n+n), as 6He is too weakly bound to act as a core and the different light neutron rich-nuclei properties of this nucleus are better described

30 by four neutrons around the a particles [2,3]. The 8He, like the I4Be and the "B become then good candidates to observe multineutrons because of their particularly low binding energy of the 4 last neutrons around the @particles (S4n = 3,l MeV, 5 MeV and 2 MeV, respectively) and their ability to form neutron drops inside this neutron-rich isotopes close to particle emission threshold 141. This paper presents also a theoretical calculation on the tetraneutron. Today, all the approaches realised on the multineutrons have shown the non-existence of a bound state [5,6].

2.

EXPERIMENTAL TECHNIQUE

The experiment was run at GANIL with a 15 MeV/N 'He secondary beam delivered by SPIRAL. The principle to produce and to detect the neutron clusters is the same that the one applied for the I4Be and can be found elsewhere [l]. The charged fragments produced during the break-up are identified thanks to the particle identification (PID) distribution obtained via the energy loss in 2 Silicium strip detectors (AEsi) and residual energy (Ecsl) coming from the telescope CHARISSA 171. The PID spectrum (Fig.2) exhlbits peaks corresponding to isotope of 6He and 4He. To detect the possible multineutrons, we use 93 modules DEMON placed outside the vacuum chamber following four walls located at distance of 2,O to 5.5 meters and we impose that only one module must be fired in coincidence with a charged fragment [8,9]. Two signals are extracted from each firing module: the time-of-flight and the light output. In fact, to isolate multineutrons from simple neutrons also created during the 8He break-up, we consider the ratio E, on En where En [MeV/N] is the energy by nucleon coming from the time-of-flight of the incident particle and E, is the energy of the recoiling proton in the liquid scintillator obtained by the light output [1,10,11]. In general, just a part of the incident neutron energy is loss during the scattering with the hydrogen proton and E,(n) is less than En(n). But in the case of multineutrons, E,(4n) can be greater than E,(4n). Simulations have already been realised for the I4Be with a Monte Carlo Code, called MENATE [1,12,13] and have showed a flat distribution until 1 with an exponential decrease to 1,4 coming from the resolution of the DEMON detectors. However, the ratio is greater than 1,4 for the multineutrons. The neutrons and the y coming from the break-up are separated by a standard pulse-shape discrimination 1141. At low energies, the background events arising from light charged particles, random cosmic rays represent also a potential contaminant for the EdEn distribution and can be eliminated by fixing a threshold at 12 MeV [l].The energy E, is obtained via a calibration between the

31

integrate charge Qtot and the light output which is then converted into proton energy [11,15]. The previous calibrations realised for the 14Be with a simple parabolic fit between y-ray sources at low energy and cosmic-ray muons at high energy (open symbols in Fig. 2) presented saturation effects at very high light output [l]. To avoid this problem, we have first decreased, the bias of the DeMoN photomultipliers [la] but many spectra owned always this bad effects (Fig.]). The second possibility to avoid this saturation problem is to increase the number of calibration points between the sources and the cosmic ray thanks to a new method developed by G. Normand (171 (star symbol in Fig. 2) and to use a more complicated fit obtained by the addition of a 3 degree polynomial and an exponential curve. Then, the upper limit in neutron energy introduced for the 14 Be [ l ] is no more necessary for own analyse. 0

4 y-roy sources

+ cosmic (MeVee)

Figure 1. The light output calibration for a DEMON module. The round symbols and the solid line are associated to a parabolic fit on y-sources and cosmic-ray muons in MeVee. The dashed line correspond to the more complicated fit, defined in the box, on y sources (square symbol) and the new calibration points (star symbol) in MeVep.

"

L [MeVeel & CMeVepl

3.

RESULT & DISCUSSION

We can now draw the same two-dimensional plot EdEn versus PID (Fig.2) which had allowed to isolate the 6 events for the I4Be [I]. Here, in the region where multineutron events may be expected to appear (EIJE,> 1,4), we observe, actually, 18 events ranging from 1,41 to 3.44 around the region centred on the 4 He peak against only one event around the component 6He. The majority of background is eliminated by fixing a lower threshold at 12 MeV in the neutron energy, but an other possible source to find events upper to the limit 1,4 is the 2 neutrons pile-up which arrive when 2 neutrons are detected in the same module. The contribution of more than 2 neutrons pile-up is almost impossible because of the low intrinsic neutron detection efficiency. Experimentally, to determine the probability of pile-up, we apply the same method used by M. Marques [I] and we find as result: Pzn(6He) 1x 10-5 and Pln(4He) 1,48x

-

-

32 E

Figure 2. Scatter plot and the projection onto both axes of the particle identijication PID vs Ep/En for C t H e , X+n). The dashed lines represents the limits around the 'He peak and the solid line correspond to EJEn =1,4. On right, the zooinfiom the solid box present in the EJEn vs PIDplot.

EnMin = 12 MeV

-+

1

D ioa

-?

0

0

150

D

a so

1w 80

IL 60

0

40

-5a

20

-100

a

-150

-20 -200 0

I

3

2

0

10000 2(XIDO

-40

1.2

1.4

1.6

1.8

E,/L

But, to take into account the contribution of neutrons coming from the Carbon target and the limits placed on the energy En,we have used simulations. The neutrons from the break-up of *He are described by an average multiplicity and a Gaussian momentum distribution with a measured width 8 [MeV/c]. The neutrons coming from the Carbon target are also simulated by using an average multiplicity, an energy distribution of the form e-En/Eo, and a Lorentzian angular distribution with a half-width r1,2 PI. The different parameters chosen for the simulations are collected in table 1. If we look at the E, /Endistributions obtained by the simulations (Fig3), all the events greater than 1,4 are only pileup events. Table 1 Parametersfor the simulations cfig.3) andprobabiltty deduced from the simulations

Frag 'He 4He

(M) ( H ~ )

fW V c I

1,48

31

(M) (c) 0,9

2.96

39

2,4

EOIMW 7 4

PI

13

29

P2,,x10-5 1,08f 1 4,94+ 1

The results in this table allow to say that the target nucleus contribution for 4He is larger and exhibits a smaller slope. The neutrons detected in coincidence with 4He present also a broader momentum distribution and a higher multiplicity. We can say finally that only maximum 2 events upper the limit 1,4 for the fragment 4He come from pile-up, against one for the 6He, and it is not

33

enough to explain the presence of the 18 events. Other verifications on the pileup must be yet realised. 5i

2

-c

2

=

140w

12900 1WM)

MK)o

fa00 4000

2000

i-8 BOO0

woo 4000 MOO 0

0

40

2

4

0

2

Figure 3 . On the upperpanels, datafrom the reactions CfHe,4He+n) and on the lowerpanels. data from the reaction CfHe.6He+n).For each reaction, we can observe from lefr to right: neutron energv, multiplicity, distribution of neutron angle and E$E# distribution. The open symbols are the data and the solid lines or black symbols are the simulations.

A third explanation which is not investigated here, could allow to understand the presence of these 18 events. It is the possibility that the tetraneutron could exist as a low-lying resonance. This idea was introduced for the I4Be and had showed the increase of the probability that 2 neutrons go through the same module [HI. An other experiment to find the tetraneutron via the a-particle transfer reaction (d,6Li) suggest a peak corresponding to a resonant state[l9]. 4.

THEORETICAL TECHNIQUE

The goal of the theoretical aspect is to determine the surface energy of the tetraneutron ground-state using the energy variation principle. The tetraneutron is described thanks to a four-centre model using the GCM (Generatrice Coordinates Method). This model was already applied on light exotic nuclei 120) and had shown good results. Each neutron of the 4n is then placed on the apex of a tetrahedral configuration where R, is the length of the sides and Rh denoted the height of the tetrahedron. R, and Rh are both the

34

generatrice coordinates (Fig.4). The most important point of the variationnal approach is then the choice of trial functions. To form the 4n state, we have used an antisymmetric SDWF which respect the Pauli exclusion principle. They are build from the single particle states I&, thanks the equation: @SKS

(R, R, >O) = 9 2 A 3

[9"(R,(0))9"(R, (0))9" (R,(0))9, ( R m l

(1)

where index 0 refers to the unrotated system, A is the antisymmetrization operator and $c,,, is a center-of-masse gaussian function introduced to avoid spurious c.m. effects. The wave function of each neutron ($n) are klly ) a space wave determined by the product of spin-isospin wave function ( x ~ and function which describe the motion of a single neutron in a 1s harmonic oscillator orbit centred at the apex of the tetrahedron Ri where i = 1,.. .,4 and b=1,36 fm is the oscillator parameter.

Our variationnal method requires the evaluation of a microscopic Hamiltonian H between eigenstates of total angular momentum J and parity n. Therefore, we restore the space rotation and the space reflexion symmetries by standard projection techniques [21].

Figure 5 . Leff: energy surface using the Minnesota potential as a function of R, and Rh. The numbers correspond to the value of the curve energy [MeV]. Right-top: zoom of the valley.

Now, we must make the choice of the interaction to use in our calculation. The realistic nucleon-nucleon interaction contain central-, tensor-, and spin-orbit forces The only state considered in this paper is S=O and K, = 0 and the spin-

35 orbit or the tensor forces between spin-zero states are vanished by the WignerEckart theorem. For the central potential, we have used the two-body Minnesota potential owing only one adjustable parameter [22]. We select then a set of generatrice coordinates and look at the one that gives the lowest expectation value for the energy. The first plot is obtained varying R, and R,, from 0,5 to 10 finby steps of 0,5 fin.The energy decrease rapidly until about 3 fm and does not present binding energy for the ground state. This shows that, theoretically, the tetraneutron is unbound. However, the Fig. 5 presents a little valley which could characterise a low-lying resonance in the four-body continuum. A second plot was realised to observe more precisely this region and has shown that there was no minimum in the valley. Actually, the results of the theoretical calculations suggest that the tetraneutron must be unbound and should not exist as a resonance. Therefore, if experimentally, the observation of a tetraneutron was confirmed, a revision concerning the existing models of nuclear forces should be necessary. If not, then a different interpretation of this 18 events around 4He should be looked for.

References 1. F.M. Marques et al, Phys. Rev. C65,044006 (2002). 2. Y. Iwata et al, Phys. Rev. C62, 0643 11 (2000). 3. Y. Ogawa, K. Yabana, and Y . Suzuki, Nucl. Phys. A543,722 (1992). 4. Y. Kanada et al, Phys. Rev. C60,064304 (1999). 5 . N.K. Timofeyuk, Private Communication. 6 . A.V. Belozyorov et al, Nucl. Phys. A477,13 1 (1988). 7. J.L. Lecouey, PhD Thesis, Universite de Caen, September 2000. 8. M. Labiche et al, Phys. Rev. Lett. 86, 600 (2001). 9. F.M. Marques et al, Nucl. Instr. And Meth. A 450, 109 (2000). 10. J. Wang et al, Nucl. Instr. And Meth. A397 (1997), 380. 11. S. Mouatassim, PhD Thesis, Universite Louis Pasteur de Strasbourg (1994). 12. P. Desesquelles, Nucl. Inst. And. Meth. A307, 366 (1991). 13. P. Desesquelles, Nucl. Phys. A 604, 183 (1996). 14. I. Tilquin et al, Nucl. Instr. And Meth. A365,446 (1995). 15. R.A. Cecil et al, Nucl. Inst. And. Meth. A161 (1979), 439. 16. F.M. Marques, F. Hanappe et al, GANIL proposal (January 2002). 17. G. Normand, PhD Thesis, Universite de Caen, Octobre 2004. 18. F.M. Marques, Tours Symposium On Nuclear Physics V, 169 (2003). 19. E. Rich et al, EXON 2004 Symposium, Peterhof. 20. M. Dufour & P. Descouvemont, Nucl. Phys. A 605, 160-172 (1996). 21. D. Brink, Proc. Int. School Enrico Fermi 36, Varenna, 1965 22.I.Reichstein & Y.C. Tang, Nucl. Phys. A158, 529 (1970).

36

STUDY OF 4-NEUTRON SYSTEM USING THE 8HE(D,6LI) REACTION E.RICH, S.FORTIER, D.BEAUMEL, E.BECHEVA, Y .BLUMENFELD, F.DELAUNAY, N.FRASCARIA, S.GALES, J.GUILLOT, F.HAMMACHE, E.KHAN, V.LIMA, B.POTHET, J-A.SCARPAC1, O.SORLIN, E.TRYGGESTAD Institut de Physique NuclPaire d 'Orsay, IN2P3-CNRS F-91406 Orsay Cedex,France R. WOLSKI The Henryk Niewodniczahski Institute of Nuclear Physics PAN, Krakdw, Poland A.GILLIBERT, V.LAPOUX, L.NALPAS, A.OBERTELL1, E.C.POLLACC0, F.SKAZA DSWDAPNIA, CEA-Sacluy F-91191, France P.ROUSSEL-CHOMAZ GANIL, CEA-IN2P3 I3000 Cuen, France A.FOMICHEV, S.V.STEPANTSOV JINR Dubnu, Russia

D. SANTONOCITO INFN, LNS Cutunia, Italy

The "He(d,hLi)4n reaction at 15.8MeV/A incident energy was studied at GANIL-SPIRAL in order to investigate the eventual existence of bound or resonant states in the four neutron system (tetraneutron 4n). The 4n missing mass spectrum was deduced from kinetic energies and emission angles of 'Li ejectiles, measured by the Silicon array MUST. Neutrons emitted in coincidence were also detected in plastic detectors. A resonant-like structure is observed at -2.5MeV above 4n threshold in both single and coincident spectra. Its identification with an unbound "tetraneutron" must still be confirmed in a forthcoming experiment with higher statistics.

37 1.

Introduction

The debate about the possible existence of neutral nuclei has a long history that may be traced back to the early 1960’s. The list of experiments and calculations performed in these decades is given in ref [l]. Numerous attempts to produce 4nsystems have been undertaken. But, in spite of all efforts, forty years later, there is an overall consensus that no convincing evidence of a bound or resonant multineutron has been found. Theoretical calculations as well as previous experimental results from fission, spallation and double charge exchange reactions have concluded against the existence of bound multineutron clusters [2]. However, the recent availability of intense neutron-rich radioactive beams provides the opportunity to reinvestigate this long-standing problem in a highly selective way. The existence of a bound 4-neutron system has been recently suggested by experimental results on I4Be breakup, six events being compatible with an interpretation in terms of a bound state [3]. If confirmed, the observation of “Tetraneutron” would require an important revision of the existing models of nuclear forces. Several theoretical works have been conducted, and the most recent published by S.Pieper [4] stresses the huge impact that a bound tetraneutron would have on our present knowledge on nuclear 2 and 3-body forces. However, despite their sophistication, such calculations cannot give any definite conclusion regarding the existence of the 4n as a resonance. Pieper does suggest that a resonance some 2MeV above threshold would not be incompatible with his calculations. This paper reports a recent experimental search for tetraneutron via the aparticle transfer reaction ‘He(d,6Li)4n in inverse kinematics at the GANILSPIRAL facility. The predominant structure of the *He ground state is usually viewed as an almost inert a-particle core, surrounded by a skin of four neutrons. The 4-neutron system can thus be released by transferring the a-particle core from the *He projectile to the deuteron target nucleus. The c.m. energy spectrum of the 4n system can be directly determined by measuring kinetic energies and emission angles of ejectiles. It is to be stressed that such a missing mass measurement can provide evidence for an eventual bound “Tetraneutron” as well as for resonances superimposed above the continuum of the 4n system on the same footing. Due to the predominant a-d structure of the 6Li ground state, the (d,6Li) reaction has been widely used for studies of a-particle clustering in stable nuclei, with typical cross-sections -0.1-lmb/sr at forward c.m. angles. Thus, the major advantage of the present experimental approach is that it should have a larger cross-section (-mb) and lower background than former investigations via double charge

38

exchange reactions which had very low cross section (-1 -1 Onb). 2.

Experimental technique

The *He beam accelerated by SPIRAL-GANIL at 15.8 MeV/A incident energy impinged a Cd2 target of thickness 1.lmg.cm-2 placed in the center of the SPEG reaction chamber. A scheme of the experimental set-up is shown on Fig1 . The position and incident angle of 'He on target was determined event-byevent by means of two beam tracking detectors (multi-wire chambers CATS [5]) placed upstream. Charged particles emitted between 7 and 40 degrees (c.m) were detected by the Silicon detector array MUST [6] placed at 200mm from the target position. It consists of eight 60mmx60mm, 300pm thick, double sided stripped Silicon detectors backed by 3mm thick Si(Li) detectors. The XY localization given by the 60x60 strips with an accuracy of Imm, coupled with information from beam detectors, allows to determine particle emission angle with 0.3deg accuracy. Energy deposited in the strip detectors is measured independently both on X and Y strips, providing a good check of energy calibrations and reducing eventual background. Identification of particles is deduced from AE-E and time of flight measurements.

Si

MWPCl

Fig. 1 : Experimental Setup.

According to kinematics, the 6Li ejectiles from the 8He(d,6Li)4n reaction are emitted in a cone with maximum angle 25 deg (lab) covered by the four central

39

telescopes. In the case of Tetraneutrons with c.m. energy close to threshold, 'jLi emitted at c.m. angles 50deg with kinetic energy 22-40 MeV stop in the strip detectors. As E-TOF identification does not allow to discriminate 'jLi from 'jHe an 'Li ejectiles, additional AE-E particle identification was obtained by means of 50mmx50mm, 70 pm thick Silicon detectors placed as the first stage of the central telescopes. An additional constraint on the spectrum was provided by detecting neutrons emitted in coincidence . A set of four 160mmx160mm plastic detectors with thickness 90mm was placed behind MUST at 600mm from the target position, in order to cover with maximum geometrical efficiency the emission angle of an eventual tetraneutron at threshold energy. Neutrons and gammas were separated by time of flight measurement. The interaction processes of tetraneutrons with plastic being unknown, the overall efficiency of our set-up has been estimated as function of the 4n c.m. energy considering the sequential decay of the system of four neutrons. Monte-Carlo simulations were performed using a modified version of code SEMILI, including the neutron efficiency calculations of ref [7]. It amounts to 70% for E(4n)=2MeV in the c.m. angular range 7-50deg for single 'jLi events and 35% for coincident events. 3.

Calibration runs with '*C beam

The determination of the 4n missing mass spectrum relies both on precise kinetic energy and emission angle measurements. The simulation program quoted above also predicts experimental c.m. energy resolution as hnction of detector energy resolution, and accuracy on angle determination. The effect of an eventual error on the kinetic energy and/or angle calibration can give rise to substantial additional broadening of peaks. Thus special care was taken to achieve good energy calibration and position measurement of the detectors, in order to avoid distortions of the reconstructed 4n spectrum. The relative geometrical positions of MUST stripped Silicon modules and second beam tracking detector were measured precisely, so as to get -0.3deg accuracy on emission angle. The energy calibration of Si detectors was performed using a-particles from a mixed source of 233U,239Pu,24'Am(E=4.8 19, 5.149, 5.479MeV) and a source of 2'oPb(T=10h)providing calibration points at higher energy (E=6.063, 8.785MeV). Incident kinetic energy as well as measured 6Li kinetic energies were further corrected for energy losses in target and dead layers of the detectors. Data extraction and procedure for missing mass determination were crosschecked using calibration runs with the same experimental set-up and a '*C3+ beam at 15.8MeV/A incident energy. Several transfer reactions with the

40

deuteron target could then be studied by measuring different types of light particles emitted. Missing mass spectra of final nuclei were extracted using the same method as for extraction of 4n c.m. energy spectrum. Peaks could be observed, corresponding to well known ground and excited states of "C, "B, 1°B and 8Be, populated respectively by the (d,t), (d,3He), (dp) and (d,6Li) reactions in inverse kinematics. This last reaction I2C(d,6Li)'Be is of special interest, as it implies 6Li ejectiles with kinetic energies in the same range as those from the 8He(d,6Li)4n reaction, thus providing an excellent check of both experimental data and its analysis. The energy spectrum of 8Be shown in Fig2. corresponds to 6Li kinetic energies between 24 and 40 MeV. One clearly observes the known ground state of unbound 8Be, located at 0.09 MeV above threshold and its first excited state at 3 MeV. These states are superimposed on a background, which could be due to the low-energy tail of Carbon breakup in two 6Li. The 0.6 MeV measured energy resolution for the ground state with almost negligible natural width (=6.8keV) is consistent with simulation, giving confidence in the present method for missing mass measurement.

100

ao

60

40

20

" n

-10

-5

0

5

F,*cm(We) in MeV

10

15 F R i c h FYflN7nfl4

Fig 2 : 'Be missing mass spectrum deduced from kinetic energy and angle of 6Li produced by the I2C(d,'Li)'Be reaction

41

3.1. Results

The excitation energy spectrum of the 4-neutron system in the center of mass, deduced from the 6Li total energy and scattering angle, is shown on Fig3a. Negative energies correspond to an eventual bound 4n, and positive energies to the continuum. This spectrum corresponds to data obtained with 4.7.109 ‘He incident on the Cdz target. In order to estimate the contribution of background due to the presence of ‘*C nuclei in the Cdz target, data were also accumulated using a pure Carbon target. No structure was observed in the spectrum. However the low statistics obtained in -1day accumulation time (a factor of 5 lower than for Cd,) did not allow a channel by channel background subtraction. Alternatively, the shape of background due to the Carbon in the target was calculated using the same angular cuts as in the case of Cdz and simple assumptions for kinetic energy and angular distributions of 6Li (flat and isotropic distributions). Carbon background is shown in Fig3a (dashed line), with appropriate normalization using the total number of 6Li observed with Carbon target. Note that counts observed below 4n threshold where an eventual bound tetraneutron would be expected, may be attributed to Carbon background, in the limit of statistics. In a second step of the analysis, the experimental spectrum was compared with phase space calculations filtered by the experimental acceptance, shown in Fig3b. The dashed curve represents the result of a 5-body phase space calculation (i.e 4 non interacting neutrons + 6Li in the exit channel). Other phase space calculations were also performed, taking final state interaction between two neutrons into account. Results corresponding to two correlated pairs of neutrons (nn-nn) in the exit channel are shown as plain line in Fig3b. Both curves are normalized to the high energy part of the experimental spectrum, where the decreasing slope only results from kinematical cuts. It is to be noted that we added the background (due to the contribution of ”C in the target) to these curves. One observes that the 4n spectrum above 5 MeV is nicely reproduced by the (6Li-nn-nn) calculation. This is an important result, which emphasizes the importance of nn-nn correlation in the structure of ‘He ground state. Another interesting feature of this spectrum is the presence of two structures not reproduced by these phase space calculations: one resonant-like structure at about 2.5 MeV above threshold and another one at about 1 MeV below the threshold, in the “bound tetraneutron” region. However, large statistical uncertainty on Carbon background to be subtracted in the singles spectrum does not allow to firmly conclude. A useful check can be provided by considering

42

data in coincidence with neutrons. This coincidence spectrum should help to select the reaction channel of interest. Coincidence of 6Li ejectiles with neutrons detected in the plastic blocks placed behind MUST allows strong reduction of 'ZC(d,6Li) contribution, as demonstrated by the absence of counts in the coincident spectrum accumulated with the Carbon target. Fig3c displays the 4n missing mass spectrum in coincidence with at least one neutron hitting plastic detectors. As in Fig3b, plain and dashed lines represent the results of (6Li-nn-nn) and ('jLi-n-n-n-n) phase space calculations in case of coincidence with one neutron or more. Normalization is the same as the one adopted in Fig3b, as the simulation program can predict the efficiency of our set-up for detecting the four neutrons emitted in the *He(d,6Li)4n reaction. The good agreement of (6Li-nn-nn) phase space predictions with the coincident spectrum above 5 MeV strongly confirms that observed events are due to this particular reaction.

35

30

E

s

B

g3

25

20

8

Y5 l5

8

1 0

5

0

-5

0

5

1 0

1 5

20

2z

E"cm(4n) in MeV Fig.3 : a) 4n-system excitation energy spectrum in the center of mass obtained with *He(d,"Li) reaction and Background due to the C in the Cd2 target

43 EtotpLi) :24-50 MeV

L a b Angle (%i) :7-20 d 30

25

20

1

5

1

0

5

0

-5

0

1

0

1

5

E"cm(4n) in MeV

Fig.3 b) Comparison with phase space calculations.

On the other hand, no counts are observed in the coincident spectrum of Fig3c, corresponding to the peak observed with -20 counts in Fig3 at -lMeV below threshold. If such a structure originates from the formation of the hypothetical bound tetraneutron, a coincidenthingles ratio of -30% would have been expected, if one assumes that detection efficiency for tetraneutron is the same as for four neutrons emitted simultaneously.

Etotpl. i ) : 24-50 M c V L s b A n g l e PLi} :7 - 2 0 d f

E*crn(-ln) in M e V

Fig.3 : c). Same spectrum, in coincidence with neutrons

44

The absence of counts in the coincident spectrum precludes this interpretation. As mentioned above, a likely origin of these counts in the negative c.m.energy region is 'Li emitted in reactions on "C nuclei in the Cdz target. Corresponding to the resonant-like structure with -30 counts observed at 2.5MeV in the singles spectrum (Fig3b), about ten counts are observed in the coincident spectrum of Fig3c. below E(4n)=5MeV, which cannot be reproduced by phase-space calculations. Here the coincidenthingles counting ratio is consistent with the Monte-Carlo predictions quoted above. In the limits of the present statistics, the present results could be consistent with the existence of a resonance in the four-neutron system. In summary, these results show the validity of the present experimental approach using the a-transfer reaction sHe(d,6Li) in inverse kinematics for the study of the 4 neutron system. The continuum spectrum above 5 MeV c.m. energy can be reproduced by phase space calculations involving emission of two pairs of correlated neutrons after a-particle stripping of 'He interacting with deuteron. Further, -in spite of rather low statistics- a resonant-like structure has been observed in the 4n missing mass spectrum at 2.5 MeV above threshold. If confirmed, the existence of tetraneutron as a resonance in the 4n system should bring strong new constraint on our understanding of nucleon-nucleon interactions. It is hopped that data with higher statistics will be obtained in a forthcoming experiment at GANIL-SPIRAL in September 2004.

Acknowledgement The partial support by the In2p3-Poland accord (02-160) is acknowledged. We wish to acknowledge the technical staff of Ganil for efficient running of the accelerator. J.F Libin, P. Gangnant and M. Vilmay are also kindly thanked for their assistance.

References [ l]D.R.Tilley, H.R. Weller, G.M.Hale, Nucl. Phys. A541, 1 (1992) [2]J.E.Ungar, R.D.McKeown, D.F.Geesaman , PhysLetters B, Vo1.144. [3]F.M.Marques et al., Phys. Rev. C65,044006 (2002) [4] S.Pieper,Phys.Rev.Let.90,25250 l(2003) [S]S.Ottini-Hustache et. al., Nucl.Instr.and Meth in Phys.Res. A43 1 (1999) 476 [G]Y.Blumenfeld et al., N. Instr. Meth. A421,479(1999) [7]R.A.Cecil,B.M.Anderson and R.Madey, Nucl.lnstr.Meth 161(1979)439

45

EXPERIMENTAL STUDY OF THE HYDROGEN ISOTOPES BEYOND THE DRIP-LINE 435H S.I. SIDORCHUK, M.S. GOLOVKOV, L.V. GRIGORENKO, A.S. FOMICHEV, Yu.Ts. OGANESSIAN, A.M. RODIN, R.S. SLEPNEV, S.V. STEPANTSOV, G.M. TER-AKOPIAN, R. WOLSKI Flerov Laboratory of Nuclear Reactions, JINR, Dubna, 141980 Russia

A.A. KORSHENINNIKOV, E.Yu. NIKOLSKII RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan A.A. YUKHIMCHUK, V.V. PEREVOZCHIKOV, Yu.1. VINOGRADOV Russian Nuclear Federal enter - All-Russian Research Institute of Experimental Physics, Sarov, Nizhni Novgorod Region, 6071 90 Russia

The nucleon unstable hydrogen isotopes 4.SHwere studied in the transfer reactions, respectively, 2H(t,p)4H and 'H(t,p)'H at the triton beam energy of 58 MeV. The experimental data show a peak in the 4H missing mass spectrum corresponding to the resonance parameters Er.,=3.05iO.19 MeV and r,,,=5.14*1.38 MeV. The pole of Smatrix for 4H, Eu=1.99i0.37 MeV and r0=2.85*0.3 MeV has been extracted. The complete kinematical reconstruction of energy and angular correlations among the decay fragments of 'H allowed to identify the broad structure in the missing mass spectrum of 'H above 2.5 MeV as a mixture of 3/2+ and 5/2' states. There is also an evidence for the ground 1/2+state of 'H at about 2 MeV.

1. Introduction The present work is dedicated to the study of nucleon unstable hydrogen isotopes 4,5Hin transfer reactions. The 40-years history of investigations of these nuclei has not up to now resulted in a clear insight of their properties. Even in the case of 4H, the most simple two-body nuclear system, the data obtained in different experiments give rise a lot of questions. Particularly, there is no a satisfactory agreement between the resonance parameters of 4Hobtained with the use of the phase-shift analysis of elastic scattering [1,2] and those obtained in reactions of other kinds (see for example [2,3] and references therein). One can assume that the large width of the states of 4H makes difficulties for the correct selection of the resonance peak from a sum of concomitant processes. The study of 'H is naturally much more complicated task and different experiments yield very contradictory data. One can say that each work dedicated to the study of 'H,

46

either experimental [4,5] or theoretical [6,7], gives new values of the resonance energy and width, as a rule very different from those obtained in other experiments. From the experimental point of view these measurements are complicated due to the small cross-section of the 'H production and a large number of particles and interplays in the output channel. In our work we studied the nuclei 435H respectively in the one- and twoneutron transfer reactions 'H(~,P)~H and 3H(t,p)5H.The experiments were carried out in Dubna (FLNR, JMR). The primary triton beam of the energy of 58 MeV was delivered from the U400M cyclotron. The typical beam intensity on the plane of the physical target made about 3.10' s-I. The separator ACCULINNA [8] was used for the beam forming and transportation to the physical target. The liquid deuterium and tritium targets [9] of the thickness of 2.10" cm-' were irradiated in these experiments. 2.

Hydrogen-4

While studying the reaction t + d -, t + p + n we detected coincidences of the recoil proton and one from two particles originated from the 4Hdecay (p-t or pn). Charged reaction products were detected with the use of two telescopes consisted of a set of silicon detectors. The telescope intended for the proton detection (Tp) covered lab angles from 19" to 31". The tritons outgoing in the angular range 12" - 41" were detected with the use of the second telescope (TJ. To detect neutrons the neutron wall (1.7x1.7 m') assembled of 41 DEMON modules [lo] was installed behind the Trtelescope at the distance 2.5 m from the target. The detection of two from three particles in the output channel p + t + n provided the condition of kinematically complete measurement. So the angle and the energy of the third, unobserved particle could be calculated. In Fig. l(a) the momentum distribution of unobserved neutron in the plane of the reaction 'H ( ~ , P ) ~isHshown. The neutron momenta are calculated in the CMS of 4H (point 1 in Fig.1 (a)). Arrows indicate the directions of p and 4H in the CMS of the p-t-n system. Areas indicated as 2 and 3 in Fig. l(a) approximately correspond to neutrons having the energy E, ~studying . the three-body decay modes 5He+3He+n + n and 5He+t + p + n we for the first time identified the isobaric analog of the 5H ground state resonance in 5He formed in the 2H(6He,t)5Hereaction. This

57

T=3/2 state is located at the 5He excitation energy 22.1f0.3 MeV and has the width 2.5f0.3 MeV. Within the experimental uncertainty, the observed energy of the T=3/2 5He resonance is close to the position which follows from estimates made on the basis of the measured position 2.2*0.4 MeV of the 5H ground state resonance. Our data show that the cross sections of the lp/ln-transfers from the a-core of the 6 H e nucleus to deuteron resulting in the formation of the T=3/2 states of 5H/5He are close to each other in their values. In the strict sense of the isospin selection rule, the cross section ratio of the two reactions leading to the 5H ground state and to the lowest T=3/2 state in 5He should equal 3. However, isospin mixing and/or reaction dynamics could be the origins of the approximate equality obtained for these cross sections. As a whole, revealing the T=3/2 isobaric analog state in 5He with the energy and width given above presents an additional argument in favor of the conclusions drawn in ref^.^!^ about the 5H ground state resonance. Acknowledgments Partial support of the work by the Russian Basic Research Foundation (grant No. 02-02-16550) and by the INTAS grant No. 03-51-4496 is acknowledged. References 1. D. V. Aleksandrov et al., in Proceedings of the International Conference on

Exotic Nuclei and Atomic Masses, ( E N A M 9 5 ) , Arles, France, 1995 (Editions Frontiers, Gif-sur-Yvette, France, 1995), 329. 2. M. G. Gornov et al., J E T P Lett. 77 , 344 (2003). 3 . A. A. Korsheninnikov et al., Phys. Rev. Lett. 87, 092501 (2001). 4. M. Golovkov et al., Phys. Lett. 566B, 70 (2003). 5. M. Meister et al., Phys. Rev. Lett. 91, 162504 (2003). 6. J. J. Bevelacque, Nucl. Phys. 157B, 126 (1981). 7. N. A. F. M. Poppelier, L. D. Wood, and P. W. M. Glaudemans, Phys. Lett. 157B, 120 (1985). 8. N. B. Shul’gina et al., Phys. Rev. C 6 2 , 014312 (2002). 9. P. Descouvemont and A. Kharbach, Phys. Rev. C 6 3 , 027001 (2001). 10. R. L. McGrath, J. Cerny, and S. W. Cosper, Phys. Rev. 165, 1126 (1968). 11. D. R. Tilley et al., Nucl. Phys. A708, 3 (2002). 12. A. M. Rodin et al., Nucl. Inst. Meth. B204, 114 (2003). 13. S. N. Ershov et al., Phys. Rev. C (to be published).

58

DECOUPLED PROTON-NEUTRON DISTRIBUTIONS IN 1 6 c*

z. ELEKES, zs. DOMBRADI, zs. FULOP, A. KRASZNAHORKAY, M. CSATLOS, L. CSIGE, z. GACSI, J. GULYAS Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, Debrecen, H-4001, Hungary

H. BABA, H. KINUGAWA, A. SAITO Rikkyo University, 3 Nishi-Ikebukuro, Toshima, Tokyo 171, Japan N. FUKUDA, T. MINEMURA, T. MOTOBAYASHI, S. TAKEUCHI, I. TANIHATA, Y. YANAGISAWA, K. YOSHIDA The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

N. IWASA Tohoku University, Sendai, Miyagi 9808578, Japan P. THIROLF Ludwig-Maximilians- Universitat Munchen, 0-85748 Garching, Germany S. KUBONO, M. KUROKAWA, X. LIU, S. MICHIMASA, S. SHIMOURA University of Tokyo, Tokyo 1130033, Japan A. OZAWA Tsukuba University, Tennoudai 1-1-1, Tsukuba-shi, Ibaraki 305-8571, Japan

*The present work was partly supported by OTKA T38404, T42733. Z.E. thanks the JSPS fellowship program in Japan.

59

1. Introduction

Due to the strong proton-neutron interaction, the proton and neutron distributions are strongly correlated in nuclei. However, a large difference in radial distributions have already been found in halo nuclei which raises the possibility of the difference between proton and neutron deformations. The deformations can be characterized by the transition matrix elements. Determination of the proton and neutron contributions to the excited states can yield information on the nature of the ground state proton and neutron deformations Recent calculations suggest different deformations for proton and neutron distributions in carbon nuclei, as well. The proton density is expected to be oblate, while the neutron densities vary with neutron numbers and a prolate shape is predicted for “C. Our aim was to search for these possible different proton and neutron contributions to excitation of the first 2: state in “C. For this purpose, we applied the Coulomb-nuclear interference method

’.

334.

2. Experimental The experiment was carried out at RIKEN Accelerator Research Facility. A 100 A.MeV energy primary beam of l80with 400 pnA intensity hit a 9Be production target of 9.0 mm thickness. The reaction products were momentum and mass analyzed by the RIPS fragment separator. A secondary beam of 16Cat 52.7 A.MeV with an intensity of 4 ~ particles/s 0 ~ was transmitted to a ’08Pb target of 50 mg/cm2 thickness with an enrichment of 99.5%. The secondary beam was isotopically pure and the momentum resolution of the separator was set to 0.1%. The scattered beam particles were detected by a plastic scintillator hodoscope of 1x1 m2 active area placed at 5 m downstream of the target. The particle identification was performed applying standard AE-E and AE-time-of-flight methods. For tracking the beam, four parallel plate avalanche counters (PPAC’s) with 100x100 cm2 were used. The uncertainty of the position determination of the PPAC’s was about 0.2 mm at 1 0 . The angular resolution was found to be 0.28” at 10 value. The information on the inelastic scattering cross section was deduced by detecting the deexciting y-rays from the 2: state of 16Cin coincidence with the 16Cparticles. They were detected by an array of 68 NaI(T1) scintillator detectors.

60

3. Analysis and results

The total inelastic cross section populating the first 2+ state corrected for the acceptance of the setup was determined at 30.5 f 2.8 mb using the information on absolute full-energy-photopeak-efficiency for the NaI(T1)array calculated by the GEANT code and checked by source measurements. Individual y-ray spectra were made applying 0.2 ‘-bin gates on the events from 1.4 O to 4.4 O scattering angle. The differential cross sections for the scattering angle bins were calculated taking into account the solid angles covered. The resulting angular distribution of the inelastically scattered nuclei is shown in Fig. 1. The vertical error bars reflect the statistical and systematic errors originating from background fitting. The horizontal ones represent the angular bins. The minimum from the Coulomb-nuclear interference can be seen at about 2.6”. For the analysis of the angular distribution, where both Coulomb and nuclear effects contribute to the reaction, we used the coupled channel code ECIS95 ’. The optical model parameters were taken from an experiment where I7O was scattered from ’08Pb at 84 A.MeV laboratory energy 6. The standard collective form factors were applied. In this case, the ECIS calculations require two free parameters for the coupling potential: the “Coulomb deformation” PC and the “matter deformation” ,& characterizing respectively the Coulomb and nuclear interactions between the target and projectile. Finally, “deformation-lengths” (6 = OR) were determined using the mass and charge deformations multiplied by the mass and charge radius parameters applied in the ECIS calculations. A minimization x2 analysis was carried out to find the appropriate ratio of the deformationlengths. Two minima was clearly observed: a local one at 6 ~ / 6 ~ = 0 . 6and 7 an absolute one at a value of 3.1 with reduced ~ ~ ~’. 1 . 6 In Fig. 1, the bold solid and dashed curves correspond to the results at the two minima. The calculated angular distributions are smoothed by Gaussian functions according to the experimental angular uncertainty of 0.28’. The x2 analysis thus results in the ratio of the deformation-lengths hM/bc=3.1 f0.5.By normalizing the curve to the data, the absolute values of the Coulomb and matter deformation-lengths are obtained respectively as 6 ~ = 1 . 3f 0.15 fm and 6 ~ = 0 . 4 2& 0.05 fm. The errors include the systematic ones from background fitting, selection of optical potential set and uncertainty in the efficiency of the y ray setup.

61

2.5 -I

n

L

ii-

,.

,,,, -

E,,,=52.7

AMeV

$ ,\

< 2

r

L

U

0 1.5 u n

0

c- 1 U

\ 73

0.5 0

Figure 1. Differential cross sections for the inelastic scattering exciting the 2; state in lSC. The bold solid line represent the best fit with ECIS calculation which was smoothed by the angular resolution of the experimental setup (thin solid line: Coulomb part, dotted line: nuclear part). Dashed line is plotted with SM/& = 0.67 local minimum value.

4. Discussion and conclusions

This result shows that the mass deformation in I6C is much larger than the Coulomb one. This extraordinary feature becomes more distinct when the proton and neutron transition strengths are extracted from the b~ and bc values derived. The role of the neutron and proton excitations in the 2f state can be characterized by the neutron and proton transition matrix elements Mn and Mp which are determined as ':

62

It should be noted that Mp is directly related to B(E2;2; -+ O+) as B(E2;2; O')=M;. In the formula above, N and Z respectively denote the neutron and proton numbers of the nucleus, while b, and b, are the sensitivity parameters of the inelastic scattering to the proton and neutron deformations. Based on the ratio of proton-neutron interaction to proton-proton or neutron-neutron interaction in this energy domain, the ratio b,/b, was obtained at 0.85. The ratio of the neutron and proton transition matrix elements determined from eq. (1) is M,/Mp=7.6f1.7. This value can be compared to the value of M,/Mp=N/Z=1.67 that would be expected for a purely isoscalar transition. Our present value, 7.6 f 1.7 represents the largest deviation from the LLnormal" isoscalar case observed so far. Taking into account the absolute cross section determined, the value of the proton and neutron deformation-lengths of 6, = 0.42 f 0.05 fm and 6, = 1.9 f 0.2 fm can be deduced. The proton deformation corresponds to an electromagnetic transition probability of B(E2)=0.28 f 0.06 Weisskopf units, while the neutron deformation-length represents a value slightly higher than the single particle one. Our B(E2) value being significantly less than 1 W.U. is consistent with the result of a recent direct lifetime measurement, which gives B(E2)=0.26&0.05 8, and is the smallest transition probability ever observed. This unprecedently small charge transition probability is the reason for the large value of the M,/Mp ratio. This fact is in contrast to the case of 18i200 isotopes where the increased MJMP ratio is interpreted as a result of enhanced neutron contribution. Indeed, the charge transition strength in 18,200 isotopes is a factor of 30 stronger than the present value found for "C. In a simple shell-model picture allowing for only two valence neutrons above a closed 14C core, the amount of core polarization can be estimated as -+

Mn/Mp = (1+ d")/dP

(2)

where d is the core polarization per valence neutron, which is connected to the effective charge as en = d" and e , = 1 d p . Applying the usual assumption of d" = d p , the neutron effective charge is en = 0.15e, much smaller than usual in the shell-model calculations in the region (en = 0.5). This small value obtained for the effective charge in 16C can be considered as an indication of decoupling of the valence neutrons from the 14C core. Regarding our initial aim prompted by the AMD predictions in 2 , our experimental results clearly show that a very small proton deformation, or

+

63

more strictly a very small amount of proton excitation, contributes to the 2: state in 16C. The excitation is dominated by neutron transitions. This is consistent with a picture drawn from the results of 2 , where the first 2+ state is collectively excited due mainly to the prolate neutron deformation. Since the symmetry axis of the oblate proton distribution is perpendicular to that of the neutrons, the proton deformation effective to the 2+ excitation is reduced from the initial one. The extracted 6p value is very close to the theoretical one in l o . However, the neutron deformation-length 6, is largely underestimated. A recent shell-model calculation l1 predicts a hard and stable proton closed shell. They contribute only a little to the excitation from the ground to the first 2+ state. As a result, the proton transition matrix element is suppressed compared to the neutron one. In this model, the ground state is dominated by the ~(1.5112)~ configuration, and the first 2+ state is largely composed of the quadrupole excitation of a neutron from the 1sl/2to Qd5/2 state. This also indicates the decoupling of the valence neutrons. The calculated very large ratio of shell-model-space matrix elements A,/A, might be the consequence of the same phenomenon that gives the experimental Mn/Mp=7.6 f 1.7 ratio. Despite the above consistent pictures, there remain discrepancies between the experimental and theoretical transition probabilities. The AMD calculation in failed to reproduce the neutron transition probability as mentioned above. The shell-model l1 somewhat overestimates the experimental transition probabilities. These discrepancies call for further theoretical studies to fully understand the observed transition probabilities. References A.M. Bernstein, et al., Phys. Rev. Lett. 42,425 (1979). Y . Kanada-En’yo, H. Horiuchi, Phys. Rev. C55, 2860 (1997). W. Bruckner, et al., Phys. Rev. Lett. 30, 57 (1973). S.H. Fricke, et al., Nucl. Phys. A500,399 (1989). J. Raynal, Phys. Rev. C23, 2571 (1981). J. Barrette, et al., Phys. Lett. B209, 182 (1988). Z. Elekes, et al., Phys. Lett. B586, 34 (2004). N. Imai, et al., Phys. Rev. Lett. 92, 062501 (2004). J.K. Jewell, et al., Phys. Lett. B454, 181 (1999). 10. Y . Kanada-En’yo, private communication based on 11. R. Fujimoto, PhD. Thesis. University of Tokyo (2003). 1. 2. 3. 4. 5. 6. 7. 8. 9.

64

y RAY SPECTROSCOPY OF 25,26727F NUCLEI *

z. ELEKES, zs. DOMBRADI, zs. FULOP Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, Debrecen, H-4001, Hungary

A. SAITO, H. BABA, K. DEMICHI, T. GOMI, H. HASEGAWA, S. KANNO, S. KAWAI, K. KURITA, Y. MATSUYAMA, H.K. SAKAI, E. TAKESHITA, Y. TOGANO, K. YAMADA Rikkyo University, 3 Nishi-Ikebukuro, Toshima, Tokyo 171, Japan N. AOI, M. ISHIHARA, T. KISHIDA, T. KUBO, T. MINEMURA, T. MOTOBAYASHI, S. TAKEUCHI, Y.YANAGISAWA, K. YONEDA The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan J . GIBELIN Institut de Physique Nucle'aire, 15 rue Georges Clemenceau 91406 Orsay, fiance N. IMAI, H. IWASAKI, S. MICHIMASA, M. NOTANI, T.K. OHNISHI,

H.J. ONG, H. SAKURAI, S. SHIMOURA, M. TAMAKI University of Tokyo, Tokyo 1130039, Japan S. OTA Kyoto University, Kyoto 606-8501, Japan A. OZAWA Tsukuba University, Tennoudai 1-1-1, Tsukuba-shi, Ibamki 305-8571, Japan

*The present work was partly supported by the Grant-in-Aid for Scientific Research (No. 1520417) by the Ministry of Education, Culture, Sports, Science and Technology and by OTKA T38404, T42733 and T46901. Z.E. thanks the JSPS fellowship program in Japan.

65

1. Introduction An intriguing problem is that the dripline of fluorine isotopes is located a t least 6 neutrons farther than that of the oxygen isotopes. Whatever mechanism makes 31F particle-bound; its traces should be visible in other fluorine nuclei, too. For instance, in 27F the psd shell model ', which can account for the properties of light fluorine nuclei, predicts the first excited state with spin 1/2+ a t 2.0 MeV energy, much higher than the neutron separation energy of 1.3(4) MeV. On the other hand, some shell breaking or dripline effects (continuum coupling, enhanced pairing) acting in 31F can lower the energy of this state below the separation energy. According to recent Monte-Carlo shell model calculations it is enough to allow for the possibility of neutron cross shell excitations to have a bound excited state in 27F '. To explore the traces of the mechanism which is expected to be responsible for the existence of 31F in lighter fluorine nuclei, and to gain more information on its properties we have searched for bound excited states in 27F. 2. Experimental

We have applied the (p,p') reaction in combination with y-ray spectroscopy, which allows us t o employ thick targets and low intensity radioactive beams. The experiment was carried out at the IUKEN Accelerator Research Facility. A 94 MeV/nucleon energy primary beam of 40Ar with 60 pnA intensity hit an '*'Ta production target of 0.5 cm thickness. The reaction products were momentum and mass analyzed by the RIPS fragment separator. The secondary beam included neutron-rich 0, F, Ne and Na nuclei with A / Z x 3. The total intensity was about 100 pps, while the fraction of individual isotopes varied in the range of l - l O % having a 27Fintensity of 4 pps on average. The identification of incident beam species was performed event by event by means of energy loss, time-of-flight (TOF) and magnetic rigidity ( B p ) . The 27F particles could be fully separated from other nuclei. The secondary beam was transmitted to a liquid hydrogen target of 30 mm diameter a t F3. The average areal density of the hydrogen cooled down to 22 K was 210 mg/cm2. The mean energy of 27Fisotopes was calculated a t 39.6 MeV/nucleon. The position of the incident particles was determined by two PPACs placed a t F3 upstream of the target. The scattered particles were detected and identified by a PPAC and a silicon telescope located about 80 cm downstream of the target. The telescope consisted of three layers of Si with thicknesses of 0.5, 0.5 and 1 mm. Each

66

layer was made of a 2x2 matrix of detectors. The Z identification was performed by TOF-energy loss method where the TOF was taken between the secondary target and the PPAC. Isotope separation was carried out among the different fluorine isotopes by use of the AE-E method. To detect the de-exciting y rays emitted by the inelastically scattered nuclei the DALI2 setup including 146 NaI(T1) scintillator detectors surrounded the target. 25

20

15

10

5

0 0

500

1WO

1500

ZOO0

2500

Ey ( k W

0

5W

$003

1500

Ey (keV)

2000

2500

0

500

1wO

1500

2000

2500

Ey ( k W

Doppler-corrected spectra of y rays emerging from 1H(27F,27F) (a), 1H(27F,26F) (b) and 1H(27F,25F) (c) reactions. The solid line is the final fit includ-

Figure 1.

ing the spectrum curves from GEANT4 simulation and additional smooth polynomial backgrounds plotted as separate dotted lines for each nucleus. The insets in gray boxes show the p s d shell model predictions.

3. Analysis and results Fig. 1 plots the Doppler-corrected y ray spectra for 27F(a), 26F (b) and 25F (c) nuclei. First, the positions of the peak candidates (500, 750, 1200 keV for 27F,470, 660, 1300 for 26F,730, 1000, 1350, 1750 keV for 25F)and their uncertainties were determined by fitting the spectra with Gaussian functions and smooth exponential backgrounds. After the peak positions have been determined they were fed into the detector simulation software GEANT and the resultant response curves plus smooth polynomial backgrounds were used to analyze the experimental spectra in terms of the significance of the peaks by taking the 2 0 level as a criterion. According to this, there are two significant peaks at 727(22) and 1753(53) keV in the 25Fspectrum (Fig. 1 (c)). In the 26F spectrum (Fig. 1 (b)), two peaks were found at 468(17) and 665(12) keV. The 27Fspectrum (Fig. 1 (a)) also shows two peaks at 504(15) and 777(19) keV.

67 4. Discussion and conclusions

The experimental data can be compared with the predictions of the shell model calculations. The sd shell model predicts the ground state of 25F to be 5/2+, followed by a 1/2+ state at 911 keV and a 3/2+ one at 3373 keV. In 26F, the members of the ~d5/2~d3/2 multiplet give the lowest energy states starting with the 1+ ground state and followed by the 2+ at 681, the 3+ at 1604 and the 4+ state at 353 keV. In 27F, a 5/2+ ground state is expected with the 1/2+ state as the first excited state at 1997 keV as mentioned earlier. Comparing the experimental data with these predictions, it is clearly seen that the energy of the 727 keV y ray of 25F and that of the 665 keV one in 26Fis fairly close to the predicted energies of the 1/2+ state in 25F,and the 2+ state in 26F,respectively, and can be assigned to the decay of these states. On the other hand, both levels of 27F and the second excited states of appear at too low energies independently whether they are constructed by placing the y rays parallel or in cascade. Extending the model space to the s d p f shells 4 , a lowering to 1.1 MeV of the 1/2+ excited state is calculated in 27F. Although an excited state with a similar energy can be constructed by placing the two y rays of 27F on top of each other, on the basis of the expected decay properties, a state directly feeding the ground state is a more probable candidate for the spin 1/2 state. In spite of the -300 keV energy difference, the 777 keV transition may be a reasonable candidate for the decay of the 1/2+ state of the sdpf shell model prediction. Thus, by allowing for breakdown of the N=20 neutron shell closure, half of the experimental results, namely the existence of the y ray peaks with 700 keV energy in all the 25726927F nuclei, may be explained '. The large energy deviation between at least one of the predicted and observed excited states suggests that these states intrude from a configuration outside of the model space, or the predicted energies strongly deviate from the reality due to some additional correlations not included in the models. 25126F

References 1. E.K. Warburton and B.A. Brown, Phys. Rev. C46, 923 (1992). 2. Y . Utsuno, et al., Phys. Rev. C64, 011301(R) (2001). 3. B. A. Brown, http ://www .nscl. msu. edu/ brown/sde. htm 4. Y . Utsuno, et al., Phys. Rev. C60, 054315 (1999). 5. Z. ELekes, et al., Phys. Lett. B599,17 (2004).

68

PRECISION EXPERIMENTS WITH EXOTIC ATOMS AND RELATIVISTIC EXOTIC NUCLEI AT GSI

H. GEISSEL Gesellschaft fur Schwerioaenforschuag, Planckstrasse 1, 0-64291 Darmstadt, Germany E-mail: H. [email protected] Precision experiments with secondary beams of exotic nuclei separated in flight require special experimental methods to compensate for the inevitable large phase space volume which is populated in the production reaction. In this contribution several key experiments are presented where the projectile fragment separator FRS has been used as a high resolution magnetic spectrometer to perform precise momentum measurements. Progress of mass and lifetime measurements with stored fragments demonstrates the research potential of the unique combination of the FRS and the storage-cooler ring ESR. Pioneer experiments have also been performed with heavy exotic atoms. The spectroscopy of deeply bound pionic states in various lead and tin isotopes has been done with precise momentum measurements at the FRS. The investigations of the in-medium mass modification of pions inside nuclear matter can be widely extended with the corresponding reaction in reverse kinematics with stored radioactive beams.

1. Introduction Studies of both exotic nuclei and exotic atoms have been significantly contributed to the basic understanding of matter. However, it remains still a great experimental challenge to create and study both species in the laboratory. Powerful accelerator facilities and novel experimental tools are required for this research. Exotic nuclei are characterized by a short half-life, a large protonneutron asymmetry and a decreasing binding energy. These features cause new "exotic" properties compared to isotopes near the valley of beta stability. Recent studies with exotic nuclei close to the driplines reveal new decay modes, e.g. the two-proton radioactivity, different shell closures and magic numbers and extend the studies to very dilute nuclear matter distributions. This modern nuclear structure research has been opened up by

69

new powerful secondary nuclear beam facilities. Recently large efforts have been devoted to experiments using radioactive nuclei at energies above the Coulomb barrier’. When in ordinary atoms bound electrons are replaced by other negative particles like muons, pions or antiprotons a so-called exotic atom is created. Exotic atoms represent a unique laboratory to study basic properties of matter Deeply bound pionic states in heavy atoms have been experimentally discovered only recently at the FRS ’. The strong nuclear interaction cause in conventional slowing-down experiments that the pions are absorbed before they can deexcite via characteristic x-ray emission.

’.

2. Production and separation of exotic nuclei with the FRS

Exotic nuclei can be produced over a wide energy range via various nuclear reactions. Research with relativistic exotic nuclei at GSI use projectile fragmentation and fission in flight to produce the interesting species. In experiments studying fission of 238U projectiles at the FRS 4 , it became clear that projectile fission at relativistic velocities is a prolific source of very neutron-rich nuclei of medium mass 5. More than 120 new neutronrich isotopes were discovered in spite of the meager projectile intensity of 107/s. However, due to the relatively large amount of kinetic energy isotropically released in the fission reaction, the products populate a large phase space. For in-flight facilities the kinematics of the production reaction determines the quality of the secondary nuclear beams. Although relativistic energies cause a strong kinematical forward focusing, still the phase-space population is relatively large for projectile fragments and even larger for fission fragments. This statement is illustrated in reference by the calculated momentum and angular spread of tin fragments created in projectile fragmentation and fission over a large energy range. The nuclei of interest are separated in flight with the fragment separator FRS 4 . The FRS is a universal four-stage magnetic in-flight separator providing spatially separated monoisotopic fragment beams of all elements up to the heaviest projectiles in the large energy range of 100 to 1500 MeV/u. The separation is based on a two-fold magnetic rigidity analysis in front and behind a thick layer of solid matter (BpAE-Bp method). Besides the main task of fragment separation in flight, the FRS represents also a versatile high-resolution magnetic spectrometer with 4 independent magnetic dipole stages including quadrupole and sextupole magnets

70

for focussing and corrections of image aberrations. The spectrometer performance of the FRS has been successfully applied in many precision experiments in the field of atomic, nuclear and applied physics with relativistic ion beams. Precise momentum measurements with the FRS have been employed to study also the atomic interaction of relativistic heavy ions with matter. The results have contributed to improve the basic theoretical description and lead to new applications of relativistic ions penetrating through matter 7. A spin-off from these experiments is also the new separation method to combine an in-flight separator with a gas-filled stopping cell 5 . Although the momentum distribution of projectile fragments due to their creation process have been known since long ago ',lo, systematic high-resolution measurements (,@ resolution FZ 5 x with the FRS demonstrate new features l1 for fragments which are far from the projectile in the N-Z plane. The mean velocities first decrease with increasing mass difference between the projectile and the fragment in agreement with ref. ',lo but run through a minimum and reach for very large mass losses even faster velocities than the primary projectile. This observation can be interpreted as a response of the spectator to the participant blast and provides new information on the equation of state of nuclear matter. In another series of experiments on the production of projectile fragments the measured velocity distribution for products created in charge pick-up reactions at 1 GeV/u have been studied. The results demonstrate clearly the different contributions from quasi-elastic scattering and the A-resonance excitation 12

3. Studies of Dripline Nuclei via Longitudinal M o m e n t u m Measurements Nuclei like llBe and 'lLi consist of a core surrounded by a very extended valence neutron density. This dilute density distribution, the halo, is a result of loosely bound nucleons close to the continuum and quantum mechanical tunneling beyond the short range of the nuclear force l3>l4.In the following, it will be shown that precise longitudinal momentum measurements of the core fragments after nucleon removal in relativistic collisions are a powerful spectroscopic method. Measuring the momentum distribution in coincidence with the gamma rays emitted from the deexcited core fragment provides via state selectivity the corresponding form factors. The investigation of nuclear structure via precise momentum measure-

71

ments after secondary reactions is complicated by the large momentum spread of the incident fragment due to the primary creation process. This problem has been solved by using the FRS operated as an energy-loss spectrometer 4,15. In this way, the momentum distribution induced from secondary reactions can be precisely measured without deterioration from the large incident phase space. Exclusive momentum measurements of secondary reaction products represent an ideal spectroscopic tool to study the population and correlations of nucleons in exotic nuclei 16. A clear advantage of knock-out reactions at several hundred MeV/u is the rather simple reaction mechanism dominated by single nucleon collisions. As an example for this category of experiments characteristic results for 8B and 230are presented in ref. l79I8. The proton halo in the 8B nucleus was experimentally discovered via a narrow momentum distribution of 7Be secondary fragments after one-proton removal reaction and by a strongly enlarged cross section 15. These two signatures were observed in different target materials. A follow-up experiment based on exclusive momentum measurements in coincidence with y rays has provided new information on the removal from the pure ground and excited states. The coincidence measurements reveal that 13% core excitation is involved in the knock-out reaction with a carbon target 17, see fig. 1. The experimental results have guided the theoretical descriptions such that ground state wave function of the halo proton in sB can now be well described with the three body cluster model and the mean field QRPA theory 19. Both theories can reproduce the measured momentum distribution well. It is also interesting to note that the experimental and theoretical spectroscopic factors of 90% occupancy agree over a large energy range from 80 to 1440 MeV/u 16. Momentum measurements analog to the ‘B case have recently been performed with oxygen isotopes up to the dripline. This experiment 18clarified the 230 ground state structure providing a spin and parity for the removed neutron of I”=1/2+.

4. Experiments with stored exotic nuclei

The combination of the in-flight separator FRS and the cooler storagering ESR 2o provides unique experimental conditions with bare and fewelectron ions for all elements up to uranium. The disadvantage of the large phase space of projectile fragments is overcome for stored particles by two methods. First the momentum and angular spread can be strongly reduced by electron cooling. Since the cooling process in the present ESR takes

72

FWHM = 95 L 5 MeVIc

-2+

E,

-150

(MeV)

-100

-50

0

50

100

150

P ,,(MeV/c)

Figure 1. Measured momentum spectra of 7Be fragments after one proton knockout reaction in a carbon target (right panel). Coincidence measurements with y radiation (left panel) provide spectroscopic factors and the ground state angular momentum population 17

a few seconds this time restricts the access of short-lived fragments. The stochastic precooling can further reduce the cooling time for nuclei in a narrow mass-over-charge window, see below. For short-lived nuclei the ring will be set to its transition energy (isochronous mode). In this mode the revolution time is independent of the velocity spread and the phase space compression with cooling is not required. The accuracy and resolution of time-resolved Schottky Mass Spectrometry (SMS) have been substantially improved as demonstrated in fig. 2 22,21. Tracing the isotope peaks in the time-resolved Schottky spectra down to a single stored ion, ground or isomeric states can be assigned even for very small excitation energies which cannot be resolved under the condition when both states are simultaneously populated. Additionally, time-resolved SMS provides an inherent check for the correct isotope identification because the observed decay characteristics have to agree with the assignment deduced from reference masses. The mass accuracy achieved is presently 30 keV (standard deviation) which represents an improvement of a factor of about three compared to our previous results The isochronous mass spectrometry (IMS) was applied for the first time in a larger range of isotopes with unknown masses produced with uranium fission in flight 26. The mass resolution achieved was 2 x lo5 (FWHM) and the accuracy about 200-500 keV. The analysis of the data is still in progress and represents a special challenge due to missing reliable reference masses in this area and secondly the ESR acceptance of fragments exceeds 23124.

73

3

4

.

,

.

.

.

.

.

,

Lead Isotopes

25

=

20

Z

15

5

10

---

5

-

0

-200

0

zw 4w Bw 8w Mass difference I keV

1wo

175

180

185

190

195

2W

205

HFB(BS-2) HFBWLI) RMF (NU) M X X U

210

215

220

Figure 2. Measured mass spectra analyzed from single 143Sm ions in the ground and isomeric states. A resolving power of 2 x lo6 (FWHM) has been achieved by tracing down to single particlesz2. The origin of the horizontal axis represents the corresponding value of the Atomic Mass Evaluation 2 5 . The achieved experimental accuracy is 30 keV which is 1-2 orders of magnitude smaller than the deviations observed in the comparison with modern microscopic theories. A representative comparison is illustrated for our measured lead isotopes (right panel).

the range of the good isochronous condition.

4.1. Lifetime measurements Stored exotic nuclei circulating in the ESR offer unique perspectives for decay spectroscopy The half-lives of the stored nuclei can be measured by detecting the daughter nuclides using the difference of their magnetic rigidity (Bp) compared to the mother nucleus. If the Bp difference is less than 2.5 % both nuclei orbit in the storage ring and can be observed in the same Schottky spectrum. For larger Bp differences the daughter species leave the closed orbit and can be detected after a dispersive magnetic dipole stage of the ESR lattice. The possibility to investigate bare nuclei allows measurements of decay properties under the conditions of hot stellar plasmas, i.e., for the first time bound and continuum p- decays have been simultaneously measured in the laboratory. The measurements of bound-state beta decay (Pb) was pioneered at the ESR with incident stable projectiles 29*30. Recently, we have also started the spectroscopy of stored bare radioactive beams. As an example to demonstrate the power and potential of this novel spectroscopic tool we present the decay measurements of 207Tlfragments in an isomeric state21y31. Bare 207Tlfragments were separated in flight with the FRS and injected into the ESR. Since the lifetime of the isomeric state for the neutral 207mTlatom is only 1.33 s, it is too short to be recorded with standard Schottky spectroscopy. However, 27128.

74

as demonstrated in fig. 3 the combination of stochastic precooling 32 and electron cooling yields access to the spectroscopy of hot fragments with lifetimes in the second range. In the measured Schottky spectrum the ground and isomeric states of '07Tl and the bound-state beta daughter '07Pbg1+ are observed. The half-life of 207mT181+ was determined from the evolution in time of the area of the corresponding peak in the Schottky spectrum, see fig. 3. Our experimental value for bare 207mTlfragments in the rest frame is 1.49f0.11 s which is in excellent agreement with the calculated prolongation (1.52 s) due to the complete suppression of the internal conversion decay branch. 1

200 1

0 0 - 0 9

900

20.

7

1000

.

1lW

1200

frequency / I

I

0

I

9

A

TIE l +

207

207pb81+

frequency / Hz

1300

Hz I

I

10

11

12

13

Time (s) Figure 3. Schottky frequency spectra of the decay of 207mT181+ fragments and the corresponding decay curve 21. The area of the peak in the Schottky spectra is proportional to the number of stored ions. The combination of stochastic and electron cooling yields access to short-lived isomers.

5. Deeply bound pionic states in heavy atoms Since many decades pionic atoms have been created by slowing down of pion beams ( T - ) in matter and subsequent capture in atoms of the stopping medium. The spectroscopy of pionic states was done by measuring the characteristic x-rays during the deexcitation process. For light atoms the corresponding binding energies could be measured down to the ground state in this way. However, for medium and heavy atoms the lowest pionic levels, 1s for Z>12 and 2p states for Z>30, could not be observed

75

in conventional experiments because of fast absorption due to the strong interaction. Theoretical predictions of the existence of narrow bound T states in heavy atoms and the possibility to transfer the T - in peripheral nuclear reactions renewed the experimental efforts According to calculations the repulsive s-wave part of the strong pion-nucleus interaction and the attractive Coulomb force form a potential pocket for bound pions with a narrow width of nuclear absorption. Halo-like pionic states should be possible near the surface of the nucleus. Many different experimental methods and facilities were employed without success till recently deeply bound pionic states have been discovered in lead and tin isotopes with the FRS operated as a high-resolution magnetic forward spectrometer '. The (d,3He) reaction at 500 and 600 MeV was applied to transfer under recoilfree conditions the pion. The basic process is n(d,3He)n-, i.e. the pionic states are created in the target atom after 1 neutron removal. The momentum distribution of the 3He reaction products recorded at the dispersive central focal plane of the FRS has provided the spectroscopic information of the occupied pionic states. The excitation spectra measured via (d,3He)reactions at 600 MeV with '08Pb and "'Pb target atoms are presented in the left and right panel . The 1s and 2p pionic peak structure are in fig. 4, respectively clearly observed below the threshold for quasifree production of pions at 139.6 MeV. Comparing the two experimental results the strong influence of the neutron hole states is characteristic. The smaller contribution of a much better separation of the the 3p112 hole in case of 2 0 5 P b @ ~allows 1s pionic state which represents therefore a more precise information for the spectroscopy of the 1s state. Particular the 1s state binding energy and width directly provides new unique information on the pion-nucleus interaction. The results have been evaluated in terms of the pion-nucleus potential and yield an increase of 27 MeV for the effective pion mass in the nuclear medium. In the subsequent experiments we have successfully investigated the isotope dependence of deeply bound pionic states in tin. The heavier tin isotopes are predestinated for the investigation of 1s states and thus the isovector strength of the pion-nucleus optical potential 37,38 which presents new information on the pion decay constant in nuclear medium. In summary, these experiments present the experimental discovery of deeply bound pionic states in medium and heavy atoms and new basic information on the pion-nucleus interaction, like the in-medium mass modification of the bound pions and reduction of the chiral order parameter, i.e. the partial restoration of chiral symmetry at finite nuclear density. In addition 33134.

35136

76

of these rich contributions it is also obvious that the spectroscopy of deeply bound pionic states can also contribute to explore the neutron density of exotic nuclei. A new generation of experiments can be performed in reverse kinematics with exotic nuclei stored in the ESR 39. The stored fragments interact then with an internal deuteron target and the measured 3He ions are recorded in coincidence with the A-1 fragments. The 3He ions had in the performed experiments about 360 MeV whereas in reverse geometry the energy is only 60 MeV and therefore allows to use a segmented semiconductor detector to measure the kinetic energy and the angle. The resolution for the spectroscopy could be improved with this setup. Another possibility would be the magnetic rigidity analysis of the A-1 fragment using the storage ring dipole magnets. Furthermore, a wide range of isospin variation will be possible in this new generation experiments for the spectroscopy of deeply bound pionic states. The main challenge will be to reach the required luminosity and this depends mainly on the thickness of the internal target.

Exdtatlon Energy [MeV]

Figure 4. Measured excitation spectra of deeply bound pionic states in the zosPb(d,3 H e ) (left panel 35) and in the zosPb(d,3H e ) (right panel 3 6 ) reactions. The different components are depicted by lines.

References 1. H. Geissel, G. Munzenberg, K . Riisager, Ann. Rev. Nucl. Part. Sci. 45, 163 (1995). 2. G. Langouche, H. de. Waard (Editors), Hyperfine Interactions 103, 1-422 (1996). 3. T. Yamazaki, R.S. Hayano, K. Itahashi, et al., 2. Phys. A355, 219 (1996). 4. H. Geissel, et al., Nucl. Instr. and Meth. B70, 247 (1992).

77 5. C. Engelmann, et al., 2.Phys. A352,351 (1995). 6. H. Geissel, et al., Nucl. Instr. and Meth. B204,71 (2003). 7. H. Geissel, H. Weick, C. Scheidenberger, R. Bimbot, D. Gardes, Nucl. Instr. and Meth. B195,3-54 (2002). 8. H. Geissel, et al., Nucl. Instr. and Meth. A282,247 (1989) and H. Geissel, C. Scheidenberger, Proc. International NUPECC Workshop on: Targets and Separators for Next Generation Fragmentation Facilities (Nov. 1998 at GSI). 9. A.S. Goldhaber, Phys. Lett. B53,306 (1974). 10. D.J. Morrissey, Phys. Rev. C39,460 (1989). 11. M.V. Ricciardi et al., Phys. Rev. Lett. 90,212302-1 (2003). 12. A. Kelic et al., GSI Preprint 2004-19,(2004) 13. I. Tanihata et al., Phys. Lett. B160,380 (1985). 14. P.G. Hansen, A.S. Jensen , B. Jonson, Ann. Rev. Nucl. Part. Sci. 45,591 (1995). 15. W. Schwab, et al., 2.Phys. A350, 283 (1995). 16. P.G. Hansen, J.A. Tostevin, Rev. Nucl. Part. Sci. 53, 219 (2003). 17. D. Cortina-Gil et al., Phys. Lett. B529,36 (2002). 18. D. Cortina-Gil et al., Phys. Rev. Lett. in print, (2004). 19. H. Lenske, F. Hofmann, C.M. Keil, Prog. Part. Nucl. Phys. 46,187 (2001). 20. B. Franzke, Nucl. Instr. and Meth. B24/25,18 (1987). 21. H. Geissel, Yu. Litvinov et al., Nucl. Phys. A in print (2004). 22. Yu.A. Litvinov, et al., Nucl. Phys. A734,473 (2003). 23. T. Radon, et al., Nucl. Phys. A677,75 (2000). 24. Yu.N. Novikov, et al., Nucl. Phys. A697,92 (2002). 25. G. Audi, et al., Nucl. Phys. A729,3 (2003). 26. M. Matos, et al., Contribution to this conference 27. H. Irnich, et al., Phys. Rev. Lett. 75,4182 (2003). 28. Yu.A. Litvinov, et al., Phys. Lett. B573,80 (2003). 29. M. Jung, et al., Phys. Rev. Lett. 69,2164 (1992). 30. F. Bosch, et al., Phys. Rev. Lett. 77,5190 (1992). 31. D. Boutin PHD in preparation. 32. F. Nolden, et al., Nucl. Instr. and Meth. A441,219 (2000). 33. E. Friedmann, G . Soff, J . Phys. Nucl. Phys. 11, L37 (1985). 34. H. Toki, T. Yamazaki, Phys. Lett. B213,129 (1988). 35. K. Itahashi et al., Phys. Rev. C62,025202 (2000). 36. H. Geissel et al., Phys. Rev. Lett. 88,122301-1 (2004). 37. Y. Umemoto, et al., Prog. Theo. Phys. 103,337 (2000). 38. K. Suzuki, et al., Phys. Rev. Lett. 92,072302 (2004). 39. K. Itahashi et al., Experimental proposal for the FRS-ESR facilities (2004).

78

NEW STRUCTURE PROBLEMS IN CARBON AND NEON ISOTOPES

H. SAGAWA: x. R. ZHOU +AND x. z. ZHANG 1 Center f o r Mathematical Sciences, University of A i z u A i m - Wakamatsu, Fukushima 965-8560, Japan E-mail: sagawaOu-aizu.ac.jp

TOSHIO SUZUKI Department of Physics, College of Humanities and Sciences, Nihon University Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan

Structure of Carbon and Neon isotopes are investigated by deformed Skyrme Hartree-Fock and shell model calculations. We point out that the quadrupole deformations of C-isotopes have a strong isotope dependence as a manifestation of nuclear Jahn-Teller effect. It is shown also that the quadrupole moments and the magnetic moments of the odd C isotopes depend clearly on assigned configurations, and their experimental data will be useful to determine the spin-parities and the deformations of the ground states of these nuclei. The electric quadrupole (E2) transitions in even C and Ne isotopes are also studied. The isotope dependence of the E2 transition strength is reproduced properly, although the calculated strength overestimates extremely small observed value in lSC.

1. DEFORMATION OF CARBON ISOTOPES

We investigate the neutron number dependence of deformation properties along the chain of C and Ne isotopes. For this purpose, we perform deformed HF+BCS calculations with a Skyrme interaction SGII. The axial symmetry is assumed for the HF deformed potential. The pairing interaction is taken to be a density dependent pairing interaction in BCS approximation. The pairing strength is taken to be -410 MeV.fm3 for both 'This work is supported in part by the Japanese Ministry of Education, Culture ,Sports, Science and Technology by Grant-in-AID for Scientfic Research under the program number (C(2)) 16540259. t o n leave from Department of Physics, Tsinghua University, Beijing, China $On leave from China Institute of Atomic Energy , Beijing, China

79

neutrons and protons '. A smooth energy cut-off is employed in the BCS calculations 2 . Fig. 1 shows the isotope dependence of the quadrupole deformation parameter ,& at the binding energy minima for even-mass C and Ne isotopes. The spin-orbit interaction of SGII interaction is reduced to be 60% of the original strength. The energy minimum in 12C appears at oblate deformation with p2 = -0.32. The energy minimum becomes spherical in 14C because of the neutron closed shell effect. For heavier C isotopes 16C and 18C, two minima appear both in the prolate and oblate sides. In 18C, the ground state has the largest deformation at ,&=0.36, while the local minimum appears at the oblate side at ,f32 -0.3. The deformation becomes oblate in 'OC and 22C. The HF calculations with the original spin-orbit strength gives a spherical shape for 22C. The similar trend is found also in Ne isotopes.

-

0.4

0.3

cl"

0.2

'

0.1

'

0.0

'

-0.1

'

-0.2

-0.3

-0.5 2

4

6

8

10 12 14 Neutron Number

16

18

20

22

Figure 1. Isotope dependence of deformations of C and Ne isotopes with SGII interaction. The points with error bars show the cases in which two deformation minima are found within the energy difference of O.1MeV.

A blocked deformed Skyrme HF+BCS calculation are performed for odd Carbon isotopes. The results shows that the ground state of 17C is prolate with J" = $+ while that of 19C is oblate with J" = ;+. In "C, the energy is almost degenerate with that of J" = with minimum of J" = almost the same oblate deformation ,& -0.36. These spin-parities of the

3'

-

:+

80

ground states are consistent with our shell model calculations '. The spin of the ground state of 17C has been assigned as 3/2+ in the magnetic moment measurement 3 . The spin of 19C is assigned as 1/2+ in the Coulomb breakup reactions while there is still controversial argument on the experimental assignment in Ref. 5 . 2. Q-MOMENTS, P-MOMENTS AND E2 TRANSITIONS

We study electromagnetic moments and transitions in C and Ne isotopes by the shell model calculations in comparison with the deformed HF results '. We first discuss on the Q-moments. The WBP interaction is adopted within the O f w space with the inclusion of effective charges. State dependent polarization charges are obtained by the microscopic particle vibration coupling model (Hartree-Fock Random Phase Approximation These polarization charges can be parameterized as Z N-Z ZN-Z +(c+d-epol/e = a- + b-

+

'.

A

A

A

A

with a=0.82, b=-0.25, c=0.12 and d=-0.36 to reproduce the calculated values for 12C and 16C in ref. *. Both the neutron (v) and proton (T) polarization charges decrease as the neutron excess increases. The Q-moments obtained by using these polarization charges are shown in Fig. 2 (a). Open circles denote results of the shell model calculations with the use of epol. Single-particle or -hole values with the use of epol are given by open triangles. The configurations for 'C and l l C are vp312 and and u p ; , 2 , respectively. The configurations for I7C and 19C are t'd;;2 (udg12), respectively, for the 5/2+ state. For the 3/2+ state of 17C, a case for a single particle configuration of ud3/2 is given. Filled triangles are obtained for vd:/2,1s1/2 configuration with the use of epol. The vdE12(J = and vd;/2((J = 2)1s1p are possible simple configurations for 17C and 19C, respectively, since the udE12 or ud;l,ls;l, configuration corresponding to the middle of the d5/2 shell results in the vanishing of the Q-moments. The Q-moments are given by ~ $ e ~ o l Q , p ( d 5 / 2 ) for 3/2+ and q = ~ e ~ o z Q s p ( dfor 5 / 25/2+ ) in the case of the ud$2(J = 2)1s1/2 configuration. Here, e:ol is the neutron polarization charge and Qsp(d5/2) is the single particle value of the Q-moment for d5/%.Note that the signs of the Q-moments for 17C and 19C are opposite. The shell model values of the Q-moments are obtained by the admixture among these configurations, and their magnitudes are usually enhanced compared to those of the simple configurations. Nevertheless, the difference

81 80

b'60

0 exp 0 HF(SGI1)

E

- 1

5 4 0

M

20 0

-20 -40

-80 -60

1'

L

A

9 11 J K 312- 312-

17 17 312+512'

19 19 312+512+

-5

J I I I I I I I I I I J

A 9

11 13 15 17 17 17 19 19 19

JR312- 3 R - 112- 112~/2i3/2i5Rf112i

312iS12f

Figure 2. Q moments and magnetic moments for the odd C isotopes ; (a) Open triangles denote Q moments of single-particle or -hole values, while filled triangles give results of the Od$?2 configuration. These values include the effects of the polarization charges, epol in Eq. (1). (b) Open circles denote the results of the magnetic g-factors of shell model calculations obtained with the use of WBP interaction and gzff = 0.9gs, while the H F results are obtained by using deformed HF wave functions and shown by open boxes. The filled circles are the experimental values taken from refs. 9,10,11,3,12.

of the signs between 17C and 19C can be understood from those of the simple configurations. Calculated values for the magnetic ( p ) moments are shown in Fig. 2 (b). Here, g,"ff/gf'ee = 0.9 is used for neutron. The values of the p moments are found to be sensitive to the configurations as in the case for the Qmoments, which is useful to find out the spin-parities and the deformations of the ground states of these nuclei. Let us now discuss on the E2 transitions in the even C and Ne isotopes. Calculated and experimental B(E2) values for the 2: -+ ,s:O transitions are shown in Fig. 3. The shell model values obtained with the use of epol are larger than the experimental values except for l0C, for which larger effective charges of ep = 1.5 and en = 0.8 are needed. For 12'"16C,the isotope dependence of the observed values 1 2 9 1 3 are well explained by that of epol, but their magnitudes are smaller than the calculation. In particular, the observed B(E2) value is quite small for I6C 13, which suggests some exotic structure yet unknown in the isotopes, for example, the shape coexistence of prolate and oblate deformations expected from the deformed HF calculations. It would be also interesting to find out if the B(E2) value increases

82

for 18C as the calculation predicts. This increase comes from that of the neutron contribution. 18 16

(a) C isotopes

• Exp. »---» WBPHO • » WBP HF+PV B—aMKHO

14 12

/

*?10

•' * ~\

PJ

ffl 8

s

6

//

/

\ ^^ \

^-

4 2 14 A

16

20

• Exp. —« WBP(HO) WBP(HF+PV model) —n MK(HO)

Figure 3. B(E2) values of the 2+ —> Og,s, transitions;(a) C isotopes and (b) Ne isotopes. Filled and open squares show the results of the shell model calculation with the use of the constant effective charges e^ = 1.3, en = 0.5 and the harmonic oscillator wave functions with b=1.64fm for C isotopes and b=1.83 fm for Ne isotopes. The WBP and MK interactions are used to calculate the shell model wave functions for the filled and open squares, respectively. Filled (WBP) and open (MK) diamonds are obtained with the use of HF wave functions and the isotope dependent polarization charges epoi given by Eq.( 1). Filled circles show experimental values 12.13.

3. SUMMARY We have studied the isotope dependence of deformation in C and Ne isotopes by using the deformed HF calculations with BCS pairing approximation. We found a clear isotope dependence of the deformation change

83

as a manifestation of nuclear Jahn-Teller effect. T h e configuration dependence of the Q-moments and p-moments in the odd C isotopes, which can be attributed to the deformation effects, is also pointed out by using the shell model wave functions. This dependence can be used to determine the spin-parities as well as the deformation properties of the ground states of the isotopes. The isotope dependence of the B(E2) values in even C a n d Ne isotopes is reproduced well by the calculations, while the experimental values are found to be smaller in 12“6C,in particular, in 16C where the observed B(E 2) value almost vanishes. This suggests an exotic structure of 16C still to be found out.

References 1. T. Suzuki, H. Sagawa, and K. Hagino, in the Proceedings of the International Symposium on “Fkontiers of Collective Motions (CM2002)”, (World Scientific 2003) p.236; H. Sagawa, T. Suzuki and K. Hagino, Nucl. Phys. A722, 183 (2003). 2. M. Bender, K. Rutz, P.-G. Reinhard, and J.A. Maruhn, Eur. Phys. J. A8, 59 (2000). 3. H. Ogawa et al., Euro. Phys. J. A13, 81 (2002). 4. D. Bazin et al., Phys. Rev. C57, 2156 (1998). T. Nakamura et al., Phys. Rev. Lett. 83, 1112 (1999). V. Maddalena et al., Phys. Rev. C63, 024613 (2001). 5. Rituparna Kanungo, I. Tanihata, Y. Ogawa, H. Toki and A. Ozawa, Nucl. Phys. A677, 171 (2000). 6. E. K. Warburton and B. A. Brown, Phys. Rev. C46, 923 (1992); OXBASH, the Oxford, Buenos-Aires, Michigan State, Shell Model Program, B. A. Brown et al., MSU Cyclotron Laboratory Report No. 524, 1986. 7. H. Sagawa, X. R. Zhou and X. Z. Zhang and T. Suzuki, Phys. Rev. C (2004) in press. 8. H. Sagawa and K. Asahi, Phys. Rev. C63, 064310 (2001). 9. K. Matsuta et al., Nucl. Phys. A588, 153c (1995). 10. P. Raghaven, Atomic Data Nucl. Data Tables 42, 189 (1989). 11. K . Asahi et al., AIP Conf. Proc. 570, 109 (2001). 12. S. Raman et al, Atomic Data Nucl. Data Tables 36, 1 (1987). 13. N. Imai et al., Phys. Rev. Lett. 92, 062501 (2004).

84

NUCLEAR-MOMENT MEASUREMENTS OF LIGHT NEUTRON-RICH NUCLEI

H. UENO, A. YOSHIMI,H. WATANABE: T. HASEYAMA, Y. KOBAYASHI, M. ISHIHARA RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan E-mail: [email protected] K. A S A H I ~ DKAMEDA, . H. MIYOSHI, K. SHIMADA, G. KATO, G. KIJIMA, M. TSUKUI Department of Physics, Tokyo Institute of Technology 2-12-1 Oh-okayama, Meguro, Tokyo 152-8551, Japan

s. EMORI,

H. OGAWA Photonics Research Institute, A I S T , Tsukuba, Ibaraki 305-8568, Japan

From the systematic study on the fragment-induced spin-polarization phenomena, it has been revealed that fragments can be spin-polarized as a function of the outgoing momentum and emission angle of fragments. Based on the P-NMR method with the spin-polarized radioactiveisotope beams, we have been conducting a series of experiments at RIKEN to measure magnetic moments and electric quadrupole moments of light unstable nuclei. So far the measurements have been made in the region of neutron-rich p shell nuclei. The obtained experimental nuclear moments have been shown quite effective in discussing the effect of neutron excess on their nuclear structure, where we discussed the deviation of magnetic moments from the Schmidt value and the isospin dependence of the effective charges. To extend the observation into the neutron-rich sd-shell nuclear region, we have recently measured ground-state magnetic moments of 30Al and 32Al.

1. Introduction The availability of recently developed spin-polarized radioactive-isotope beams (RIBS) offers us the opportunity of studying on the structures of *Present address: Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra ACT 0200, Australia t Also at RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

85

nuclei far from the stability line, through the measurement of electromagnetic moments In this method the spin-polarization is reduced through the projectile-fragmentation reaction. The mechanism of the fragmentinduced spin-polarization is essentially related to the fact that a portion of the projectile to be removed through the fragmentation process has nonvanishing angular momentum due to the internal motion of nucleons. From the general feature of the fragmentation reaction, essentially any fragments can be produced and polarized irrespective of their chemical properties. Up to now, 14 new p moments and 6 new Q moments have been determined. These systematic measurements provide us opportunity for investigating the effect of neutron and proton excesses on the nuclear structure microscopically. In this work we report on the topical results obtained in the p-shell neutron-rich nuclear region and the recent progress in the sd shell. 273.

2. Experimental Method

Figure 2 shows the arrangement of the RIKEN projectile-fragment separator RIPS for the production of spin-polarized RIBs. In order to have RIBs spin-polarized, the emission angle and the outgoing momentum are selected for the fragments. To attain the BL # 0 setting, which is essential for obtaining the non-vanishing spin polarization, a beam swinger installed upstream of the target is used. Also, an appropriate range of momentum is selected by a slit at the momentum-dispersive focal plane F1. The isotope separation is provided through the combined analysis of the magnetic rigidity and momentum loss. Then polarized fragments are introduced to the NMR apparatus located at the final focus. The NMR apparatus is also shown in Fig. 2. The spin-polarized fragments are implanted into the stopper located at the central part of the system. A static magnetic field is applied to the stopper in order to preserve spin polarization. Also, a radio-frequency oscillating field is applied perpendicular to the static field. &Rays emitted from the implanted fragments are detected by using plastic scintillator telescopes located above and below the stopper. We employ the P-NMR method to determine the magnetic moment. In this method, the nuclear magnetic resonance is detected through a change in the up/down asymmetry of the &ray angular distribution. The stopper material is chosen so as to provide long spin-relaxation time compared with the @decay life-time. As the NMR technique, the adiabatic fast passage method is employed. Thus, the frequency of the oscillating field is swept over a certain region of frequency, and when it is

86

PrlMIY beam

beam wingor magnet

from RAC

Figure 1. Arrangement of the FUKEN projectile fragment separator RIPS for the production of spin-polarized RIBS,and the schematic layout of the p-NMR apparatus.

swept across the Larmor frequency, the spin reversal takes place.

3. Nuclear moments of pshell neutron-rich nuclei 3.1. Magnetic moments The experimental magnetic moment of 17N has been found to show a substantial deviation from the Schmidt moment (pSchmjdt) 3. Most remarkably, the deviation is outward directed, while most of the experimental magnetic moments fall into the region between the Schmidt lines. Since the M1-type configuration mixing quenches p-moment, which gives oppositedirected contribution, only the configurations in which two sd-shell neutrons coupling to form 2+ is expected to shift the p-moment outward from PSchmidt. The observed amount of deviation, however, is larger than the shell-model predictions suggesting an enhanced contribution of the 2+ neutron configurations. Similar observation has been seen in the systematic magnetic-moment measurements of neutron-rich boron isotopes 3. Besides the above analyses, the obtained magnetic moment has been applied to the determination of the ground-state spin-parity of 17C. Although among theoretically suggested three candidates I " = 1 / 2 + , 3 / 2 + , and 5 / 2 + I T = 5 / 2 + is predicted for the ground state in the naive shell model, this possibility is excluded by the P-decay properties of 17C '. In the 536t7,

59617,

87

remaining two candidates, IK=1/2+ and 3/2+, the latter has been proposed in the nuclear-reaction studies and ,!?-delayed neutron spectroscopy. If the spin-parity of the 17C ground state is I"(17C)=l/2+, it is of a vs112 nature, whose g-factor should be close to that of neutron, g,=-3.83. If, on the other hand, In(17C) is 3/2+, the ground state would be dominated by two major configurations, $1=[(~d5/2)3]'/~+and $Z=[(VS~/Z) @ ( V d 5 / 2 )2+ 2 ] 3/2+ . The corresponding g-factor should take a value between g($1)=-0.77 and g($z)=-o.15. Compared with these values, the 1/2+ value is 5 times or larger. Owing to the large difference between the calculated g-values for the 1"=1/2+ and 3/2+ cases, the spin-parity should be reliably assigned from the determined experimental value of 1g(17C)1=0.5054(24).

3.2. Electric quadrupole moments Figure 2 shows the comparison of experimental Q-moments of boron isotopes with shell model predictions. Firstly, we compared the data with standard shell-mode calculations where constant effective charges e,=0.5 and e,=1.3 are taken. These values are commonly used for the sd-shell nuclei. The steep increase of Q for A=13 to 17 is in sharp contrast with the experimental observation that 70 I I the Q-moments are almost constant against A. In order to obtain agreement, we took an isospin dependence of the effective charges. According to Bohr and Mottelson, polarization charges, which 1lB 1 3 ~ 15p 1 7 ~ 0 10 are associated with the effective 4. , . , 0 charges, have isospin dependence. x=(N-Z) / A The result of the calculations l1 shows good agreement using the Figure 2. Comparison of the experimental same shell-model wave-functions, Q-moments of odd-mass boron isotopes and as shown in Fig. 2. This is the shell model calculations with the standard of constant effective charges en=0.5 first observation of isospin depen- values and e,=1.3 (solid lines) and those obtained dence of the E2 effective charges. taking into account the isospin dependence 516,

c

of the polarization charges (dashed lines).

4. sd-shell neutron-rich nuclei

Recently ground-state magnetic moments of 30A1(1"=3+, T = 5.19(9) sec) and 32Al(I"=l+, T = 48(6) ms) have been measured at RIKEN. In the

88

measurement, a beam of 30Al(32Al)was obtained from the fragmentation of 40Ar projectiles at an energy of E=95 A MeV on a 93Nbtarget of 0.13(0.37) g/cm2 thickness. In order to produce spin polarization, fragments emitted at angles 13~=1.3"-5.9~ are accepted by RIPS 4. Then the isotope-separated and spin-polarized 30Al(32Al)fragments were transported to the NMR apparatus and implanted into an (r-Al203 single crystal. The A1203 stopper is expected to provide longer spin-lattice relaxation time compared with the lifetime of 30Al, since the magnetic moment of '*A1, which has much longer lifetime T = 3.2 min, has been measured with this crystal 1 2 . The A1203 single crystal was kept under vacuum and oriented to the magic angle to the external static magnetic field. Thus, the eq&-line splitting does not appear. The A1203 stopper was cooled to a temperature of T < 100 K to preserve the spin polarization during the mean lifetimes. A static magnetic field BO -0.5 T was applied to the stopper in order to preserve the spin polarization 12. Experimental g-factors were derived by fitting the NMR spectra shown in Fig. 4 with a function representing expected NMR shapes in the AFP method. From the dip frequency, 1g(30A1)1=1.004(6)and 1g(32A1)1=1.959(17)have been determined, respectively. No correction was made for the experimental values here. To obtain the error assigned above to the experimental values, the width of the frequency sweep was quadratically added to the fitting error. The accuracy of the determination becomes much better by the additional calibr.ation measurements.

3800 3900 Frequency (kHr)

4000

7200

7400 7600 Frequency (kHr)

7800

Figure 3. The NMR spectra obtained for the ground states of (a) 30Al and (b) 32Al. The experimental points are accompanied by the statistical errors (vertical bars) and the widths of swept frequency (horizontal bars). The dotted curves show the result of the least-X2 fitting procedure.

89

5 . Summary Based on the P-NMR method with spin-polarized RTBs, nuclear moments of unstable nuclei have been measured in the light-mass region. The obtained experimental nuclear moments were shown quite effective in discussing the effect of neutron excess on their structure. From the p-moment measurement of the nitrogen and boron isotopes, enhanced contribution of the I" = 2+ neutron configurations has been suggested. In the case of Q-moment, our experimental data show large discrepancy from the shell-model predictions. By taking the isospin dependence of the effective charges, the observed Q-moments of boron isotopes could be explained. We also showed another application of p-moment measurements. Also as another application of the obtained magnetic moment, obtained p(17C) value, the groundstate spin-parity of 17C has been definitely assigned in quite a different approach from those taken by the recently performed nuclear-reaction studies and /3-delayed neutron spectroscopy. To extend the observation into the neutron-rich sd-shell nuclear region, we have recently measured groundstate magnetic moments of 30Al and 32Al. Further discussion will be given in the forthcoming paper.

References 1. K. Asahi et al., Phys. Lett. B251 (1990) 488; H. Okuno et al., Phys. Lett. B335 (1994) 29. 2. H. Okuno et al., Phys. Lett. B354 (1995) 41; H. Izumi et al., Phys. Lett. B366 (1995) 51; M.Schafer et al., Phys. Rev. C 57 (1998) 2205; H. Ogawa et al., Phys. Lett. B451 (1999) 11. 3. H. Ueno et al., Phys. Rev. C 53 (1996) 2142. 4. T. Kubo et al., Nucl. Instr. Meth. B70 (1992) 309. 5. B.A. Brown A. Etchegoyen and W.D.M. Rae, OXBASH, MSU Cyclotron Laboratory Report No. 524, (1986). 6. E.K. Warburton and B.A. Brown, Phys. Rev. C 46 (1992) 923. 7. D.J. Millener and D. Kurath, Nucl. Phys. A255 (1975) 315. 8. E.K. Warburton and D.J. Millener, Phys. Rev. C 39 (1989) 1120. 9. T. Baumann et al., Phys. Rev. Lett. 439 (1998) 256; D. Bazin et al., Phys. Rev. C 57 (1998) 2156; E. Sauvan et al., Phys. Lett. B491 (2000) 1; V. Maddelena et al., Phys. Rev. C 63 (2001) 024613. 10. P.L. Reeder et al., Phys. Rev. C 44 (1991) 1435; K.W. Scheller et al., Nucl. Phys. A582 (1995) 109. 11. H. Ogawa et al., Phys. Rev. C 67 064308 (2003). 12. H.-JStockmann et al., Hyp. Int. 4,(1978) 170.

90

DIRECT MASS MEASUREMENTS OF SHORT-LIVED NEUTRON-RICH FISSION FRAGMENTS AT THE FRS-ESR FACILITY AT GSI

M. MATOS'>*>+, YU. NOVIKOV2>3,K. BECKERT', P. BELLER', F. BOSCH', D. BOUTIN', B. FRANCZAK', B. FRANZKE', H. GEISSEL134, M. HAUSMANN5, E. KAZA', 0. KLEPPER', H.-J. KLUGE', c. KOZHUHAROV~,K.-L. K R A T Z ~, s. LITVINOV~,YU. LITVINOV~, G. MUNZENBERG1>7,F. NOLDEN', T. OHTSUBO', A. N. OSTROWSKI', z. PATYK'O, B. PFEIFFER~,M. P O R T I L L O ~ H. ~, S C H A T Z ~ ~ J ~ , C. SCHEIDENBERGERl, J. STADLMANN', M. STECK', D. VIEIRA5, G. VOROBJEV1i2, H. WEICK', M. WINKLER114, H. WOLLNIK4 AND T. YAMAGUCHII3 GSI, Darmstadt, Germany; 2PNPI, Gatchina, Russia; 3St. Petersburg State University, Russia; Justus Liebig University Giessen, Germany; Los Alamos National Laboratory, USA; Institute of Nuclear Chemistry, University Mainz, Germany €4 VISTARS; 'Institute of Physics, University Mainz, Germany; Niigata University, Japan; Faculty of Physics and Astronomy, University of Heidelberg, Germany; loINS, Warsaw, Poland; "NSCL, MSU, East Lansing, USA; l2Joint Institute for Nuclear Astrophysics; 13Saitama University, Japan

'

'

Masses of more than 280 neutron-rich isotopes have been observed by means of Isochronous Mass Spectrometry (IMS) and 41 mass values for exotic nuclides have been measured for the first time. Fission of 380-415 A.MeV uranium projectiles was used as a source of neutron-rich nuclei. They were separated in-flight and injected into the storage ring, which was operated in the isochronous mode as a high-resolution time-of-flight mass spectrometer. In this production run, a mass resolving power of 2 x lo5 was achieved. The uncertainties in this preliminary mass determination are between 140 and 400 keV.

1. Introduction

The neutron-rich region of nuclides has always attracted the attention of researchers because it is larger and less investigated (or even untouched *Part of the PhD thesis (http://geb.uni-giessen.de/geb/volltexte/200~/1582) tCurrent address: NSCL, MSU, East Lansing, USA

91

in some parts) compared to the neutron-deficient side. This large area of exotic nuclides can serve as an excellent testing ground for the model predictions of different nuclear properties. The knowledge of the properties of neutron-rich nuclei is essential for the simulation and understanding of the astrophysical r-process. The first key information about these exotic nuclei comes from their binding energies (i.e. masses) and their half-lives. Recent trends in the determination of nuclear masses were presented in the recent review article'. From this publication and from the table of known masses (see Atomic Mass Evaluation, AME2003') , one realizes that for neutron-rich nuclides the available information is scarce and often contradictory. For many observed nuclides the masses are still unknown. The information on the masses of heavy neutron-rich nuclides is drawn up mainly from indirect measurements by means of &decay spectroscopy. Precise mass measurements are provided by Penning-trap spectrometry. However, only a small fraction of the masses measured by this method belongs to neutron-rich nuclides3. In this work a first systematic direct mass mesurements of vast regions of neutron-rich nuclides by means of Isochronous Mass Spectrometry (IMS) is presented.

2. Experiment

The innovative method IMS of direct mass measurements for exotic nuclei was introduced at GS14t5. The IMS technique is based on TOF measurements in a storage ring that is operated in an isochronous mode. In this way first mass values were obtained for short-lived neutron-deficient nuclides6. After masses in a large area of neutron-deficient nuclides measured with Schottky Mass Spectrometry that is a complementary method in the program of mass measurements with stored exotic nuclei at GS17, the IMS technique, however, is especially suited for neutron-rich regions, where usually short half-lives restrict the available measuring time. Systematic investigations of neutron-rich nuclides were started with the mass measurements of medium-weight neutron-rich Zn-fragments' and were continued by the present work where a large surface of nuclides was covered by the products of uranium fissiong. The primary beam 238U in the energy range from 380 to 415 A.MeV and with an intensity of 1 . 5 ~ 1 0particles/spill ~ was provided by the GSI synchrotron facility SIS. The in-flight fragment separator FRS separated

92

the fission fragments that were produced in lg/cm2 beryllium targetlo. Bare and H-like ions were injected into the storage ring operated in the isochronous mode and their masses were measured by means of Isochronous Mass Spectrometry. The method is suitable for mass measurements of ions with half-lives down to the microsecond range. The peculiar property of IMS is that single ions revolving in the storage ring are detected.

3. Results

A revolution-time spectrum after multiple injections is shown in the lower part of Fig. 1. Mass values are derived via a calibration function obtained

a, C

I

600 -

Jz .-

500

-

2 100

z o 505

510

515

revolution time [ns] Figure 1. Revolution time spectrum (lower part). The analysis was restricted only to the isochronous part where the standard deviation of revolution times for one species is (T 5 M e V . These dependences for 4He, 6He and 'Li have a dissimilar behavior at low energies E < 20 M e V .

1. Inroduction

Extensive experimental studies of the total reaction cross section for the interaction of light radioactive projectiles with stable target nuclei have been performed during the last 20 years. The aim of these researches is to investigate the most important structural characteristic of the light nuclei lying near the drip line. Of particular interest are the nuclei with a neutron halo or a neutron excess in the peripheral area. The total reaction cross section for these nuclei turns out to be very sensitive to their geometric size and the nuclear matter density distribution, which makes it possible to establish their exotic structure [l], namely, their large radius of strong interaction and root-mean-square radius in comparison with the neighboring nuclei. It should be noted that an analysis of more informative elastic scattering data doesn't allow defining the unique parameters of the respective distributions of nuclear densities and the potentials of nucleus-nucleus interaction. For example, in [2] a good description of 6Li elastic scattering at 135 and 154 MeV was achieved in the framework of six models, using the nucleus-nucleus optical potential. However, the discrepancies of the calculated 0.values were very large within limits of about 1.7 + 2.2 and

134

1.65 - 2.0 barns, respectively. Evidently, measurement of aRwill give a possibility to essentially restrict the choice of testing models. 2. Experimental Results This paper contains new OR experimental data of 5-30 AMeV 6He and 7Li projectiles injecting into a silicon detector telescope which determined by measuring their energy deposition spectra. These measurements were made with the same set of detectors and electronics as in our previous experiments [3,4] with 4He projectiles. In comparison with our previous works, the method of particle production was changed. Whereas experiments [3,4] were carried out using elastically scattered a-particles, here we use secondary 6He beams produced from the fragmentation of 34 MeV/A 7Li beams on a 9Be target 1.3 mm thick. The secondary beam energy was changed using degraders containing hydrogen. The dipole magnet located after the degrader gave 2% of the beam energy spread.

1,6 8-

m

e b2

12 A

0

10

Phys. Rev C 54 (1996) 1700

20

30 40 WA, MeV

50

Figure I: Total reaction cross section plotted vs beam energy.

60

0

10

20

30

40

50

60

WA, MeV

Figure 2: Ratio of the experimental crR to the value calculated from the systematic for stable nuclei [8] plotted against beam energy.

Test measurement of the OR value in the reaction 4He+28Siat 20 MeV/A was also carried out; it was in agreement with the previous experiments. The values of the measured aRfor the 6He reaction on 2*Si are shown in Fig.1 as filled circles; for 4He - as filled squares; and for 7Li - as filled diamonds. The referenced data on the aRvalues are represented as open symbols: triangles [5] and circles [6] - 6He; squares [7] - 4He; diamonds [5] - 7Li. As one can see from Fig.1, the energy dependence CR(E) for the 4He+28Sireaction increases with energy up to 18 MeV/A then it slowly drops. The reaction 7Li+28Si is

135

characterized by a weak OR@) dependence, which is practically unchanged in the investigated region of energies. In the case of 6He, a slight increase is observed at 10 + 25 MeV/A. According to the well known Kox systematic [8] of the total reaction cross section for stable nuclei, the increase is about 180 mb. The ratio between the experimental OR values and the calculated ones oVis shown in Fig.2. This calculation takes into account the increasing of parameter rv due to the neutron excess of 6He [9]. The value rv was equal to 1.17 fin for 6Hewhereas for 4He and 'Li it was equal to 1.1 fm. 3. Conclusion

In conclusion it may be said that the energy dependence of the total reaction cross sections for the interaction of 6He and 7Li with %i in the energy ranges 5 + 30 MeV/A have been directly measured for the first time. A local slight irregularity in the behavior of the 6He energy dependence was observed in the framework of the Kox systematic. The obtained data require hrther experimental and theoretical investigations. References

1. I. Tanihata et al., Phys. Rev. Lett. 55,2676 (1985). 2.

M. El-Azab Farid and M.A. Hassanain, Nucl. Phys. A678,39 (2000).

3. V.Yu. Ugryumov et al., Nucl. Phys. A734, E53 (2004). 4.

V.Yu. Ugryumov et al., Phys. At. Nucl. 68, 16 (2005).

5.

R.E. Warner et al., Phys. Rev. C 54, 1700 (1996).

6.

Chen Zhiqiang et al., IMP & NLHIAL Annual Report. 2001.

7. A. Ingemarsson et al., Nucl. Phys. A676,3 (2000). 8.

S. Kox et al., Phys. Rev. C 35, 1678 (1987).

9. W. Mittig et al., Phys. Rev. Lett. 59, 1889 (1987).

136

STRUCTURE OF LIGHT EXOTIC NUCLEI IN FERMIONIC MOLECULAR DYNAMICS

H. FELDMEIER, T. NEFF Gesellschaft fur Schwerionenforschung, Planckstraj3e 1, 64891 Darmstadt, Germany R. ROTH Institut fur Kernphysik, T U Darmstadt, Schlossgartenstrafle 9, 64289 Darmstadt, Germany Helium and Beryllium isotopes are studied in the Fermionic Molecular Dynamics model. No a priori assumptions are made with respect to cluster structure or single-particle properties. An effective interaction based on the Argonne V18 interaction is used for all nuclei. Short-range central and tensor correlations are treated explicitly using a unitary correlation operator. Multiconfiguration calculations using the dipole and quadrupole moments as generator coordinates are able to describe the experimental binding energies and matter radii. The evolution of the cluster structure and the single-particle structure with increasing neutron number is discussed and predictions for yet unmeasured matter and charge radii are given.

1. Fermionic molecular dynamics

The A-body basis states in Fermionic Molecular Dynamics (FMD) parity and angular momentum projected Slater determinants Q )

I

10% I

of single particle states qi

= p;l;KI Q )

,

are

(1)

)

The single-particle wave functions are described by Gaussian wave packets that are localized in phase-space

137

In contrast to the AMD approach where the width parameter a is common to all wave packets FMD treats the width parameter as a complex variational parameter that can be different for each wave packet. Also the spins x ) of each wave packet are treated as variational parameters. A superposition of two Gaussian wave packets is used for each single-particle state qi ). The FMD many-particle state is determined by minimizing the intrinsic energy of the parity projected Slater determinant Q* ) = Q ) f Il Q )

I

I

I

I

I

I

with respect to all parameters of all single-particle states qi ) defined in Eq. (3). After the minimization the many-particle state is projected on angular momentum. The correlation energy obtained by the projection can be very large for the often deformed and clustered nuclei in the p-shell. We therefore improve this projection after variation procedure (PAV") by implementing a variation after projection (VAP) procedure in the sense of the generator coordinate method (GCM). We minimize the energy of the Slater determinants under additional constraints on collective variables like radius, dipole, quadrupole or octupole moments. The VAP minimum can then be found by minimizing the projected energies with respect to the constraints. A further improvement is achieved by diagonalizing the Hamiltonian in a set of many-body states. This allows to study also excited states. 2. Effective interaction For our calculations we use an effective interaction that is derived from the realistic Argonne V18 interaction by means of the Unitary Correlation Operator Method (UCOM) The correlated interaction includes the short-range central and tensor correlations induced by the repulsive core and the tensor force. The correlated interaction no longer connects to high momenta and can be used directly with the simple many-body states of a Hartree-Fock or FMD approach. The contributions of three-body correlations and genuine three-body forces are simulated by an additional two-body correction term. This correction term consists of a central momentum dependent part that is adjusted to fix the saturation properties by fitting to the binding energies and radii of 4He, l6O, 40Ca and a isospin dependent spin-orbit term that is fitted to the binding energies of 240, 34Si 334,5.

138

and 48Ca. The correction term used in this paper differs slightly from the one in 5, as l60and 40Ca are considered as tetrahedral a-cluster states that are about 5 MeV lower in energy after angular momentum projection than the spherical trial states. In total the correction term contributes about 15% to the potential energy.

3. Helium isotopes

I

Fig. 1 shows the one-body density of the intrinsic states Q ) obtained by minimizing the energy (4)of the Helium isotopes. In all nuclei a dipole deformation caused by a displacement of the neutrons against the a-core is found. In 6He the configuration with two neutrons on the same side of the core is preferred to configurations with the two neutrons located at opposite sides of the core. In 8He one approaches the p 3 / 2 neutron shell closure with an almost spherical neutron distribution but the displacement is still visible.

Figure 1. Intrinsic shapes of Helium isotopes corresponding to the variation after parity projection minima. Shown are cuts through the nucleon density calculated with the intrinsic state before parity projection. Densities are given in units of nuclear matter density po = 0.17fmP3.

In Fig. 3 the binding energies and matter radii obtained after angular momentum projection (PAV") are compared to the experimental binding energies and radii. To improve the many-body states we create additional configurations using the dipole moment as a generator coordinate, an example is shown in Fig. 2 for 6He. The multiconfiguration calculations, i.e. diagonalizing the Hamiltonian in the many-body space spanned by these configurations, reproduce the experimental binding energies and radii very well (see Fig. 3). Hence, the borromean nature of 6He and 8He is explained by a correlated neutron cloud that sways against the a-core as a quantum zero-point oscillation of a soft-dipole mode.

139

Figure 2.

Typical configurations used for Multiconfig calculations of 6He.

-22

-

3P A V

Binding energies

Multiconfis Experimer

-24 -26 0

r -28 -30 -32 3.5

-

Matter radii

3.0 2.5 2.0 1.5 He4

He5

He6

He7

He8

Figure 3. Binding energies and matter radii for the Helium isotopes. Results are given for the PAV" and the Multiconfig calculations. Experimental matter radii are taken from6.

4. Beryllium isotopes

In Fig. 4 the intrinsic shapes of the Beryllium isotopes are shown. For 7Be and 8Be pronounced cluster structures cam be seen and we find a strong similarity with Brink cluster wave-functions. In the heaver isotopes the a

Figure 4. Intrinsic shapes of Beryllium isotopes. In case of "Be the shape of the positive parity state is shown. 5

0 x m1

5

5

0

x

Ihl

5

5

0 x Ifml

5

140

-

-30

1 Binding energies

-40

-

-60

-

-70

-

-

PAV Multiconfig Experiment

sin

&&A3 + $ ! $

a

-

,A

34 -

Matter radii

&

32 30 -

2 28 I

-

26 -

24 -

3k

'y,

3&

22 -

I

Figure 5. Energies and matter radii of Beryllium Isotopes. So far multiconfiguration calculations have only been done for 7be, 8 Be and 9Be

cluster structure survives but is modified by the additional neutrons. In 9Be and l0Be the additional neutrons seem to occupy the p3/2 single-particle states. In experiment a parity inversion in "Be is observed. The d 5 / 2 single-particle states are coming down in energy and the 1/2+ groundstate is almost degenerate with the 1/2- excited state. In the FMD PAV" calculations we find the 1/2+ state to be 1.5 MeV higher in energy than the 1/2state (see Fig. 5). The matter radius of the 1/2+ state is in good agreement with the experimental value. It has to be checked whether the level ordering will change in the multiconfiguration calculation. For the heavier 12Be, 13Be and 14Be nuclei the d 5 / 2 and also the lsl/z single-particle states seem to be important. The general behaviour of 12Be being bound, 13Be being unbound and 14Be being bound again as well as the matter radii are well reproduced, see Table 2.

5. Prediction of radii Finally we summarize our predictions for the matter and charge radii in the following two tables. Matter radii are for point nucleons while the charge radii include the proton and neutron charge form factors.

141 Table 1. Matter and charge radii in fm for he isotopes

PAV"

Tmatter [fml Multiconfig

4He He

2.13

2.82

'He

2.28

2.49

7He

2.52

2.52

He

2.60

2.63

1.44

Table 2.

"Be 13Be 14Be

PAV"

1.57f0.04

1.65

2.48f0.03 2.52f0.03

Tcharge [fml Multiconfig

rmatter [fml Multiconfig

Exp'

1.85

2.06

1.93

2.02

1.97

1.98

1.97

1.99

2.31f0.02

2.26

2.44

2.33

2.56

2.35

2.43

PAV" 2.47

2.64 2.68 2.50

2.38f0.01

2.41

2.30f0.02

2.38

2.70

2.73f0.05

2.45

2.55

2.59f0.06

2.42

i'

states are used.

EXP

2.52

2.49

3.32 2.89

2.054fO.0147

rcharge [fm] Multiconfig

2.47

2.37

EXP 1.68

Matter and charge radii in fm for Be isotopes. For 11113Bethe

PAV" 7Be sBe 9Be l0Be "Be

Exp6

2.94f0.09

2.55

References 1. H. Feldrneier and J. Schnack, Rev. Mod. Phys. 72 (2000) 655. and references therein 2. Y. Kanada-En'yo and H. Horiuchi, Prog. Theor. Phys. Suppl. 142 (2001) 205 3. H. Feldmeier, T. Neff, R. Roth, J. Schnack, Nuc. Phys. A632 (1998) 61 4. T. Neff and H. Feldmeier, Nuc. Phys. A713 (2003) 311 5. R. Roth, T . Neff, H. Hergert, H. Feldmeier, Nuc. Phys. A745 (2004) 3 6. A. Ozawa, T. Suzuki and I. Tanihata, Nuc. Phys. A693 (2001) 32 7. L.-B. Wang, P. Mueller, K. Bailey, G. W. F. Drake, J. P. Greene, D. Henderson, R. J. Holt, R. V. F. Janssens, C. L. Jiang, Z.-T. Lu, T. P. O'Connor, R. C. Pardo, K. E. Rehm, J. P. Schiffer, and X. D. Tang, Phys. Rev. Lett. 93 (2004) 142501

142

EFFECTIVE INTERACTION GENERATED BY THE PAUL1 EXCLUSION PRINCIPLE IN COLLISIONS OF THE LIGHT NEUTRON-RICH NUCLEI

G. F. FILIPPOV AND YU. A. LASHKO Bogolyubov Institute for Theoretical Physics, 14b Metrolohichna Street, Kiev, 03143, Ukraine E-mail: 1ashkoQuniv.kiev.ua The role of the Pauli principle in the formation of both the discrete spectrum and multi-channel states of the two-cluster nuclear systems is studied in the Algebraic Version of the resonating-group method. The effective nucleus-nucleus potential induced by the kinetic energy operator modified by the Pauli principle is derived, and the range and strength of the latter potential are estimated for a number of binary cluster systems. Some features of the effective internuclear interaction are analyzed.

1. Introduction

The Pauli exclusion principle is known to significantly influence the interaction of composite particles (clusters). The existence of Pauli forbidden states leads to damping the wave function of relative motion of two nuclei in the interior region of internuclear distances. The so-called "partll~ Paula-forbadden states" also play an important role in two-cluster scattering affecting both the positions and widths of the resonances The influence of the Pauli principle on the structure of nucleus-nucleus potential was shown to be very sensitive to the choice of nucleon-nucleon interaction Hence, it is difficult to make general conclusions about the main features of the interaction between clusters arising from the exchange potentials. Meanwhile, study of the influence of the exchange effects on the kinetic energy can provide such information. But careful analysis of the effect on the physical observables of the cluster-cluster interaction derived from the kinetic-energy operator modified by the Pauli principle has not been performed yet. Such estimations allow one to judge when the contribution from the kinetic energy is essential. We assert that the latter 132.

'i3.

143

contribution is significant and, hence, the basis features of the antisymmetrization effects on the nucleus-nucleus interaction may be learned by studying the exchange effects on the kinetic energy operator exclusively. In the present paper, the role of the Pauli principle in the formation of both the discrete spectrum and multi-channel states of the two-cluster nuclear systems is studied on the basis of the microscopic approach - the Algebraic Version of the resonating-group method. The algorithm is outlined in rek4l5. The effective nucleus-nucleus potential induced by the kinetic energy operator modified by the Pauli principle is derived, and the range and strength of the latter potential are estimated for a number of binary cluster systems. Some features of the effective internuclear interaction are analyzed. An exact treatment of the antisymmetrization effects related to the kinetic energy exclusively is shown to result in either an effective repulsion of the clusters, or their effective attraction. Special attention is paid to the coupled-channel calculation of the 12Be nucleus (provided that 6He+6He and 4He+8He clusterings are taken into account).

2. Multi-channel binary cluster system 12Be

Antisymmetrization effects in a multi-channel binary cluster system are studied at an example of continuum states of 12Be that are able to decay through 6He+6He and 8He+4He channels. The resonance structure of the 12Be nucleus is still unclear and the problem is only likely to be resolved by a microscopic coupled-channel calculation To reveal the main features of the nucleus-nucleus interaction affected by the Pauli principle, we shall concentrate our attention on studying the exchange effects on the kinetic energy operator exclusively. The 6He clusters, as well as the 8He cluster, have open pshell, and, therefore, a possibility of excitation (to the 2+ resonance state) of these clusters should be properly taken into account. As a consequence, the basis of the Pauli-allowed states with the total orbital angular momentum L = 0 for the 12Be system corresponds to seven different channels, when the 6He+6He and the 8He+4He coupled cluster configurations are included. The Pauli principle manifests itself in making all the seven channels coupled at small intercluster distances. Important information about the multichannel continuous spectrum of the 12Be system is provided by its seven (according to the maximal number of the open channels) elastic scattering

'.

144

phase-shifts, presented in Fig. 1. Just above the corresponding threshold an elastic scattering phase-shift obeys the law 6 l ( ~= ) nr

+ constE1+1/2, n = 0 , I , . . . .

Here n is the number of forbidden states in the corresponding channel. The high-energy behavior of the phase shifts formed by the kinetic energy operator (with its exchange part) is in perfect agreement with Levinson's theorem. Moreover, the latter phase shifts are of the same order of magnitude as those obtained with the inclusion of the potential energy operator.

10

I

I

I

I

1

5

Figure 1. Elastic scattering phase-shifts b l ( E ) of the 12Be system formed by the kinetic energy operator modified by the Pauli principle. The values of the angular momentum 1 of the cluster relative motion are shown near the curves.

The energy behavior of the phase-shifts is compatible with the assumption on the existence of the resonance in the continuum of the "Be nucleus with energy Eres= 4.16 MeV and width I' 10 keV. Moreover, now even without any cluster-cluster potential generated by the nucleon-nucleon N

145

forces, a bound state of the 12Be nuclear system appears, with an energy EO = -50 keV.

3. Phase portraits

Analysis of collisions between light atomic nuclei is usually restricted to the calculation of the elastic scattering phases or the S-matrix elements; and the behavior of colliding nuclei at small distances between them is left beyond the scope of the studies. A great number of independent variables of the wave function binder creating a clear picture of the observable phenomena by traditional methods. Simple solution of the problem can be It is reduced to analyzing found in the Fock-Bargmann representation the cluster motion with the help of distribution function of the admissible phase trajectories defined in the phase space of momenta 77 and coordinates (phase portraits). Phase trajectories correspond to the continuum of the points at a given height above the (&q) plane. Realization probability of the phase trajectory is proportional to this height. Among phase trajectories there are both infinite (peculiar to classical mechanics) and finite ones (of quantum nature). Figure 2 represents the curves of equal probability (phase trajectories) for the Q-ascattering at three different values of energy of the relative motion of the clusters. Analyzing phase portraits of collisions between nuclei, it is possible to estimate the range of the influence of the Pauli principle r e f f on the interaction of composite particles as well as the strength of the Pauli barrier. Until energy E reaches some critical value, there exists the region of intercluster distances where phase trajectories are absent (see upper panel of Fig. 2). Furthermore, all infinite trajectories correspond to the repulsion due t o the Pauli effects. The distance between the centers of the two nuclear clusters where the antisymmetrization effects come into play is approximately equal to 4.65 fm for two a-particles, while for two l60nuclei r e f ?N 7.12 fm. In both cases, the range of the influence of the Pauli principle exceeds the root-mean-square radii of the nuclear clusters. The middle panel of Fig. 2 corresponds to the energy which is equal to the strength of the Pauli barrier (35 MeV for the a-(Ysystem and 350 MeV for the 160-160 system). At this energy the a-clusters can reach the region which is inaccessible in classical mechanics. Hence two kinds of infinite trajectories can be realized. Finally, the lower panel of Fig. 2 illustrates the behavior of the Qclusters in collision, when the energy of their relative motion exceeds the

',

lo-'' s) events 20

-$El0

I

3

I

1 3

f 2 1

100

200

300

WneUc energy ( Msv )

400

20

30 40 atomic number

50

,,-% --" 20

e m ( desms I

Figure 5 Energy, charge and angular distributions of the ssKr+166Er reaction products5 Dashed histogram in (a) is the contribution of fast events (see Fig 4b)

Two-dimensional energy-massy distribution of the fragments in the ssKr+166Er reaction at Ecm=464 MeV is shown in Fig. 4, and inclusive angular, charge and energy distributions of these fragments (with the energy loss more than 35 MeV) are shown in Fig. 5. Fast damping of kinetic energy in this reaction (more than 200 MeV during several units of s) should be especially underlined. Rather good agreement with experimental data of all the calculated DI reaction properties can be seen, which was never obtained before. Typical trajectory of the nuclear system in collision of 48Ca+248Cma t Ecm=203 MeV (zero impact parameter) is shown in Fig. 6. This trajectory leads the system to QF channel. After overcoming the Coulomb barrier the fragments become first very deformed, then the mass asymmetry gradually decreases and the system find itself in the quasi-fission valley with one of the fragments closed to the doubly magic nucleus 208Pb. After contact the nuclear system has almost zero kinetic energy up to scission, and the regions with higher potential energy are surmounted mainly due to fluctuations.

239

Excitation energy of the system (temperature) gradually increases (very sharply on descent stage to the scission point). In Fig. 7 the correlation of the total kinetic energy and mass distributions of the reaction products are shown (compare with Fig. l a ) along with inclusive mass distribution for the 48Ca+248Cmreaction. Note, that the cross section of CN formation here is only 0.1 mb.

5

F to.

=

28 24

20 18

4 0%

02

e

f

04

02

$08 08

P 04

Po

g 02

go

-180

EO

-100

E

$ 140

B

60

5 ” R

Figure 6.

20

(h)

One of the trajectories in collision of 48Ca+248Cm at E,,=203

MeV.

elongallon ( fnl )

mass number

Figure 7. (a) TKE-mass distribution in the MeV. (b) 48Ca+248Cm reaction at E,,=203 Trajectories corresponding t o DI (l), QF ( 2 and 3) and FF (4) processes. (c) Contributions of different processes t o the mass distribution of reaction products ( lo5 tested events).

4. Conclusion

For near-barrier collisions of heavy ions it is very important to perform a combined (unified) analysis of all strongly coupled channels: deep-inelastic

240

scattering, quasi-fission, fusion and regular fission. Now it becomes possible. A common potential energy surface is derived determining evolution of the nuclear system in all the channels, having appropriate value of the Coulomb barrier in the entrance channel and reasonable value of the fission barrier in the exit one. A common set of dynamic Langevin type equations is proposed for simultaneous description of DI and fusion-fission processes. For the first time a whole evolution of heavy nuclear system can be traced starting from approaching stage and ending in DI, QF, and/or FF channels. Satisfactory agreement of the first calculations with experimental data gives us hope not only to get rather accurate estimations of the probabilities for superheavy element formation but also to clarify much better the mechanisms of quasi-fission and fusion-fission processes and to determine the values of such fundamental characteristics of nuclear dynamics as nuclear viscosity and nucleon transfer rate. New experiments on near-barrier collisions of heavy nuclei with accurate and simultaneous detection of all significant reaction channels are needed for that.

References 1. Yu.Ts. Oganessian et al., Phys. Rev. C63, 011301 (2001); C69, 021601 (2004); C69,054607 (2004). 2. V.V. Volkov, Reactions of deep-inelastic transfer (Energoatomizdat, Moscow, 1984) (in Russian). 3. M.G. Itkis et al., in Fusion Dynamics at the Extremes, Eds. Yu.Ts. Oganessian and V.I. Zagrebaev (World Scientific, Singapore, 2001), p. 93. 4. V.I. Zagrebaev, Phys. Rev. C67, 061601 (2003). 5. A. Gobbi et al., in Proceedings of Int. School of Phys. ”Enrico Ferrni”, Course L X X V I I , Varenna, 1979 (North-Holl., 1981), p. 1. 6. J. Blocki et al., Ann. Phys. (N. Y.) 105,427 (1977). 7. U. Mosel, J. Maruhn and W. Greiner, Phys. Lett. B34, 587 (1971); J . Maruhn and W. Greiner, 2.Phys. 251,431 (1972). 8. V.I. Zagrebaev, Phys. Rev. C64, 034606 (2001); in Tours Symposium on Nucl. Phys. V, AIP Conf. Proc., 2004, 704,p. 31. 9 P. Moller et al., At. Data Nucl. Data Tables 59, 185 (1995). 10 W.D. Myers and W. Swiatecki, Ann. Phys. (N.Y.) 84,186 (1974). 11 G.F. Bertsch, 2.Phys. A289, 103 (1978); W. Casing and W. Norenberg, Nucl. Phys. A401, 467 (1983). 12 Y.Abe et al., Phys. Reports 275, 49 (1996); P. Frobrich and 1.1. Gonchar, Phys. Reports 292, 131 (1998). 13 F.G. Werner and J.A. Wheeler, unpublished; K.T.R. Davies, A.J. Sierk and J.R. Nix, Pys. Rev. C13, 2385 (1976). 14 W. Norenberg, Phys. Lett. B52, 289 (1974); L.G. Moretto and J.S. Sventek, Phys. Lett. B58, 26 (1975). 15 H.Risken, The Fokker Planck Equation (Springer, Berlin, 1984). 16 C.W. Gardiner, Stochastic Methods (Springer, Berlin, 1990).

241

THEORETICAL PREDICTIONS OF EXCITATION FUNCTIONS FOR SYNTHESIS OF THE SUPERHEAVY ELEMENTS

YASUHISA ABE Yukawa Institute for Theoretical Physics, Kyoto Univ. Kyoto 606-8502, Japan

BERTRAND BOURIQUET Department of Nuclear Physics, Research School of Physical Sciences a n d Engineering, Australian National Univ. Canberra, ACT 0200, Australia

GRIGORI KOSENKO Department of Physics, Omsk Univ. R U-644077 Omsk, Rissia The theoretical framework for reaction leading to residues of the superheavy elements is briefly reminded, which consists of the two-step model for fusion taking into account the fusion hindrance and of the statistical theory for survival probability taking into account the fragility of the superheavy nuclei. It is shown that the theory reproduces very well the systematic data on the excitation functions for the superheavy elements measured at GSI. In order to have the absolute magnitudes comparable to the measurements, we have to introduce a single scaling factor for the shell correction energies predicted by structure calculations. Predictions for the elements 2=113, 114 are also given, which are supposed to be useful for planning experiments.

1. Introduction

Theoretical predictions on a location of the double magic nucleus next to 'O*Pb have been made with various models of nuclear structure'. But the predictions are not unique still now. Nevertheless, it is commonly accepted that there exists the superheavy island, i.e., the region of stability far away from the known elements and isotpes in the nuclear chart. On the other hand, theory of heavy-ion reaction mechanisms which should help plannings of experiments has not yet been established, though many investigations have been d e ~ o t e d ' ~Thus, ~ ~ ~experiments ~~. have been made by extrapolations of the systematics of the available data. For theoretical predictions of

242

productions, we must have a reliable theoretical framework which enables us to calculate residue cross sections of the superhaevy element (SHEs), i.e., xn cross sections. Based on the compound nucleus theory of reactions6, the cross sections are given by the product of the fusion and the survival probabilities, P&ion and P,”,,, , as follows,

where J denotes the total angular momentum of the system, X the wave length divided by 2 . n corresponding to the incident energy E,,, and E* equal to Ecm plus Q-value of the fusion reaction. The probability PATv is a sum over xn reaction channels surviving against fission. From Eq. 1, we can immediately anticipate the experimental difficulty of the extremely small cross sections for SHES. Firstly, the P,”,,,, which is determined by the competition between neutron emission and the fission decay, is expected to be very small, because compound nuclei of SHEs formed by reactions are excited, especially in so-called hot fusion path and thereby their shell correction energies which effectively provide fission barriers are diminished dramatically. (Note that the macroscopic fission barriers are almost zero for SHEs, consistently with the fissility parameters close to 1.) Secondly, Piusion is also expected to be very small, and extremely small especially for so-called cold fusion path, which is experimentally known as the fusion hindrance7. It would be worth to mention here about a comparison between the hot and the cold fusion paths. The former is unfavored in P,”,,,, but favored in P;usion, while the latter is vice versa. Thus, in order to know which path is finally favorable for synthesis of the elements under consideration, careful studies should be made not only on P,!usion, but also J pSUTV

’

For fusion probabilities PfJzLsionr a believable theoretical framework is not yet available, though there are many attempts. It should be consistent with experimental observations, for example, should explain the fusion hindrance in SHE region. The present authors et al. have recently proposed a new model, i.e., a two-step model4>*,for fusion of massive systems which lead to SHEs. The necessity for the two steps is that the contact configuration or the pear-shaped configuration made by the sticking of the incident ions is located outside the conditional saddle point of the compound system. This means that even if the incident ions stick to each other after overcoming the Coulomb barrier, the system does not automatically fuse into the spherical compound nucleus, but must overcome the conditional saddle or

243

the ridge line. Thus, we have to take into account not only the probability for overcoming the Coulomb barrier, i.e., the sticking probability PZick, but also the probability for overcoming the conditional saddle point or the ridge line, i.e., the formation probability P,',,, of the spherical compound nucleus. Therefore, the fusion probability is given by the product of the two probabilities,

PfJzlsion(Ecm)= PsJtick (Ecrn)* PfJOTm(Ecrn). (2) In ordinary fusion reactions, PfJoTm is taken to be 1 and P2ick is transmission coefficient calculated by quantum mechanics or by WKB approximation with a suitable Coulomb barrier. Of course, one can refine by including coupled channels effects etc. For the formation probability, there would be several ways in taking into account the process of overcoming the saddlepoint. One is again to treat it as quantum-mechanical tunneling2, while another is t o treat it as successive nucleon transfers based on the assumption of the quasi-stable di-nucleus systems made of the incident targets and the projectiles3. Another simple treatment is to utilize the systematics of captured cross sections and to employ an approximated formula5 of P;oTm which has been developed recently by the present authors et al.839 The present authors et al. have proposed a possible realization of the two-step model. Firstly, up to the contact and the sticking of the projectile and the target nuclei, we employ the classical surface friction model (SFM) proposed by Gross and Kalinowski'', and secondly, for the formation phase of the spherical compound configuration starting from the pear-shaped sticked configuration, we employ a multi-dimensional Langevin equation like in fission of excited nuclei". Then, we propose the statistical connection method for the two processesg. By combination of the two-step model and the decay code KEWPIE12 which has recently developed for calculations for the survival probability, we will make reproductions of the experimental data so far available and predictions on excitations of zn reactions for unknown SHES. 2. Formation of the Sticked Configuration j Surface

Friction Model Extended with Fluctuation Since we make the extension so as to include the fluctuation, i.e., the random forces consistent with the frictional forced3, we have to solve many trajectories, starting with the same initial condition given at the infinite away. They distribute around the mean one. Therefore, physical quantities

244

at the contact point do not have a unique value, but distribute around the mean value. For example, the radial momentum has turned out to be of a Gaussian, which is considered to be due to the assumed Gaussian nature of the random forces. For SHES, for example, 48Ca+actinides (hot fusion path) and projectiles+ 208Pb(cold fusion path), it is found that the mean values are always equal to zero in a good accuracy. The results indicate that the relative motion is completely damped and reaches the thermal equilibrium with the heat bath of the intrinsic degrees of freedom at the contact point in the SFM. Naturally, the normalization gives the sticking probability as a function of E,,, which is smaller than 1 in low energies, very small especially just above the Coulomb barrier. This means that most of the trajectories are reflected back around the Coulomb barrier, and do not reach the contact point.

3. Formation of the Spherical Compound Nucleus

j

Shape

Transformation over the Saddle Like in dynamical studies of induced fission'', we employ a multidimensional Langevin equation for shape degrees of freedom. As the coordinates, we adopt the shape parameters of the two-center shell model14. The mass tensor of the collective motions is taken to be the hydrodynamical one with Werner-Wheeler approximation and the potential is calculated by the finite range liquid drop modelladded with the rotational energy of the system with the rigid-body moment of inertia. The temperature T J depends on the coordinates and the time, but is taken to be a constant for simplicity. The friction tensor is calculated with so-called wall-andwindow ( W W )formula15 without any modification. In order to obtain the formation probability, we, firstly, have to solve a Langevin equation with specified initial values. Since we have the random forces in the equation, we have to calculate many trajectories for a given set of the initial values. Some of them overcome the saddle point or the ridge line to reach the spherical configuration, but most of them creep down the potential slope to re-separation which could be interpreted as quasi-fission or DIC. Then, the ratio gives the formation probability for the specified set of initial values. Since initial values of the variables are of distributions as results of the approaching phase, the formation probability for a given incident energy and a total angular momentum is to be calculated by folding with the initial distributions of the variables (statistical connection). This affects

245

dramatically the formation probability, because the incident kinetic energy is almost completely damped in the approaching phase, and thereby, no incident momentum remains for overcoming the saddle or the ridge line. This means that the motion is nearly a diffusion, which results in a very small probability of overcoming the saddle, and thus in a very small formation probability P/o,,. In other words, the formation of the compound nucleus is strongly hindered. 4. Results,Comparisons with Experiments and Predictions

In order t o describe collective motions of the shape transformation from the pear-shaped configuration to the spherical compound nucleus configuration, at least three degrees of freedom are necessary. In the two-center parameterization, they are the distance between two mass centers z, the massasymmetry of two connected masses A1 and A2, a = (A1 - A2)/(A1+ Az), and the radius of neck which connects the two nuclear matters. The last one is described by so-called neck parameter E which is defined by the degree of smoothing of the edge of the two-center potential at the connection point. In view of very large ,BE,much larger than those of others in W - W formula, the motion of E is expected to be much slower than the others. Thus, we freeze its value as the first approximation. Of course, initial values of E should have a distribution as results of dynamics in the approaching phase, but we have not yet made enough detailed calculations of collision process. (We are now making an attempt to treat more properly16.) In the following, we take the constant to be 1.0 or 0.8 rather arbitrarily. It is worth to mention that even with only two degrees of freedom of the present approximation, we can describe two paths for fusion into the spherical shape along the elongation and the mass- asymmetry for fusion. The model with the di-nucleus system concept treats only the path along the latter degree of freedom without taking into account the former. Before proceeding to the productions of SHEs, we firstly look at fusion excitation functions, which are calculated according to the usual expression with the Pfusion,

Unfortunately, there are not much data on SHEs to compare. In Fig. 1 the data on 58Fe+ 208Pb17is compared with the calculated results, together with the fusion excitation functions predicted for several other systems. It

246

0

E, (MeV)

Figure 1. Calculated fusion excitation functions are presented for the systems of the cold fusion paths, together with the available data on the system 58Fe 208Pb.

+

is remarkable that the calculation without any adjustable parameter reproduces almost completely the measured excitation function. Probably this would be fortuitous. Inclusion of the neck motion would decrease the theoretical values, while the measured data might include components coming from so-called quasi-fission process. More careful studies are necessary both from theoretical and experimental sides. Fig. 2 shows comparisons of the calculated results of In excitation function with the measured onesla for the systems of the cold fusion path, where the shell correction energies are reduced by a scaling factor of 0.6 for the mass table by Moeller et al. 19. If we use the original values, the absolute values of the peaks are larger more than one order. But the shapes and the positions do not change at all. (The discrepancies in absolute values might be due to possible overestimates of the fusion probability.) Since the theory reproduces the data systematically with one scaling parameter, we apply it to unknown SHES, for Z=113 and 114. The predicted excitation functions are given in Fig. 3, which are expected to be helpful for planning experiments for their syntheses. For the hot fusion path20,we have applied the same model to result in fairly good reproductions of the available data 2 1 . In addition, we have

247 x 10

0.1

In E eriment GSI i z:::: {ln:Egeriment RI&N)

0. 0.0

Moller and Nix 0

x 10

0.3

B 0.2 B 0.1

z8

235 240 245 250 255 260 Ec,,, (MeV)

‘-7

8 10

b.15

@Ni + ’O%i

o.l 0.051

&

x 10

1,-

:;

,,,

j

, ,

240 245 250 255 260

‘-8

,

,,, ,

,;

254256258260262264266268270

Figure 2. Comparisons of the theory and the experiments are made on the excitation funcions of In residue cross section for the systems of the cold fusion path. x 10

-*

x 10

-*

e 0.1

0.1

0.05

0.05

‘-9

x 10

260

265

270 275 Em (MeV)

;:;V ” G e + ” ’ P b

260

265

270

275

Moller and Nix

0.1

Figure 3.

Theoretical predictions of the excitation functions for Z=113 and 114 systems.

248

made a prediction for Z=118 with the hot-fusion path. Now, the theoretical predictions are waiting for experimental confirmations. References 1. 1. A.T. Kruppa et al., Phys. Rev. C61, 034313 (ZOOO), R. Smolanczuk, Phys. Rev. C56, 812 (1999)), S. Cwiok et al., Nucl. Phys. A611, 211 (1996). 2. V.Yu. Denisov and S. Hofmann, Phys. Rev. C61, 034606 (2000). 3. G.G. Adamian et al., Nucl. Phys. A678, (2000). 4. C. Shen et al. Phys. Rev. C66, 061602R (2002), Y. Abe et al., Phys. Atomic Nuclei 66, 1057 (2003), Y. Abe et al., Acta Physica Polonica B34,2091 (2003), Y . Abe et al., Nucl. Phys. A734,168 (2004),

B. Bouriquet et al., to appear in Eur. Phys. J. A.. W.J. Swiatecki et al., Acta Physica Polonica B34, 2049 (2003). W. Hauser and H. Feshbach, Phys. Rev. 87, 366(1952). W.J. Swiatecki, Physica Scripta 24, 113 ( 1981). Y. Abe, Eur. Phys. J . A13, 143 (2002). Y. Abe et al., Phys. Rev. E61, 1125 (2000), Y . Abe et al., Prog. Theor. Phys. Suppl. 146, 104 (2002), D. Boilley et al., Eur. Phys. J . A18.627 (2003). 10. D.H.E. Gross and H. Kalinowski, Phys. Lett. 48B, 302 (1974). 11. Y. Abe et al., J. Phys. 47, C4-329 (1986), T. Wada et al., Phys. Rev. Lett. 70, 3538 (1993), Y. Abe et al.,Phys. Rep. 275,Nos. 2& 3 (1996). 12. B. Bouriquet et al., Comp. Phys. Comm. 159, 1. (2004). 13. G. Kosenko et al., J. Nucl. Radiochem. Sci. 3, 19 (2002). 14. A. Iwamoto et al., Prog. Theor. Phys. 55, 115 (1976), K. Sat0 et al., 2. Phys. A290, 145 (1979). 15. J. Blocki et al., Ann. Phys. 113, 330 (1978). 16. J. Bao et al., private communication. 17. M.G. Itkis et al., Int. Workshop on Fusion Dynamics at the Extremes, Dubna, 25-27 May, 2000 (World Scientific, 2001), p.93. 18. S. Hofmann and G. Muenzenberg, Rev. Mod. Phys. 72, 733 (2000), K. Morita et al., private communication. 19. P. Moeller et al., Atomic Data and Nuclear Data Tables 59, 185 (1995). 20. Yu. Oganessian et al., Phys. Rev. Lett. 83,3154 (1999); Nucl. Phys. A682, 108c (2001). 21. B. Bouriquet et al., under preparation.

5. 6. 7. 8. 9.

249

EFFECT OF NON-AXIALITY ON THE FISSION-BARRIER HEIGHT OF HEAVIEST NUCLEI

A. SOBICZEWSKI AND I. MUNTIAN Soltan Institute for Nuclear Studies, Hoia 69, PL-00-681 Warsaw, Poland E-mail: [email protected] Effect of non-axial shapes on the fission-barrier height BFt of heaviest nuclei is studied in a 4-dimensional deformation space. The energy of the nuclei is analyzed within a macroscopic-microscopic approach. Besides non-axiality of quadrupole shapes, also non-axiality of hexadecapole deformations is considered. It is found that for some nuclei the non-axiality lowers Bft by as much as about 2 MeV, while for other ones it does not influence this quantity at all.

1. Introduction

Heights of (static) fission barriers Bst are usually studied in the case of axial symmetry of a nucleus (e.g. [l-51). It is known, however, for a long time that the height BTt may be significantly reduced by non-axial deformations (e.g. [6]). Also more recent studies (e.g. [7-91) have demonstrated the importance of the effect of these deformations on the value of B!t. The objective of the present paper is to illustrate this effect, obtained in the analysis of BTt in a 4-dimensional deformation space. A study of Bft of heaviest nuclei is motivated by importance of this quantity in calculations of cross sections v for synthesis of these nuclei (e.g. [lo-151). A large sensitivity of u t o BFt stresses a need of possibly accurate calculations of BFt. For example, a change of BFt by 1 MeV may result in a change of u by about one order of magnitude or even more [16]. 2. Method of the calculations

Macroscopic-microscopic approach is used t o describe the potential energy of a nucleus. The Yukawa-plus-exponential model is taken for the macroscopic part of the energy and the Strutinski shell correction, based on the

250

Woods-Saxon single-particle potential, is used for its microscopic part. Details of the approach are specified in [17]. 2.1. Deformation space

A 4-dimensional deformation space is used. Besides a quadrupole nonaxiality, it also includes a hexadecapole non-axial shapes. The space is specified by the following expression for the nuclear radius R(B,q5)(in the intrinsic frame of reference) in terms of spherical harmonics

where y = yz is the Bohr quadrupole non-axiality parameter and the dependence of Ro on the deformation parameters is determined by the volumeconservation condition. The functions Y E ) are defined as:

The parametrization of the hexadecapole non-axial shapes is a particular cwe of that proposed in [18], with 7 4 = -27 in the notation of that paper. It does not introduce any additional parameter to describe non-axial hexadecapole deformations and assures that the hexadecapole shapes are axially symmetric with respect to 0.z axis at y = 0 and with respect to Oy axis at y = 60°, similarly as are the quadrupole shapes. The parametrization has been used earlier in [7,8],but in a smaller (3-dimensional) spaces. The hexadecapole tensor appearing in Eq. (1) may be obtained [19] as a product (ez x e 2 ) of the quadrupole tensor e2

= ( ~ 2 0a, z 2 ) ,

where

uzO= ,D2cosy,

a22

= flz sin y.

(3)

The shapes of the multipolarity X = 6 ( p 6 Y 6 0 ) are included to the analysis, to obtain barriers more realistic than those of [8].

3. Results Figure 1 shows a contour map of the potential energy of the nucleus z50Cf projected on the plane (pzcosy, pzsiny). The line y=O" corresponds to

251 the axially symmetric (with respect to Oz axis) prolate shapes, and the line y=60’ is corresponding to the axially symmetric (with respect to the Oy axis) oblate shapes of a nucleus. The line y=30° corresponds to shapes with maximal non-axiality. One can see that the saddle point in the case of axial symmetry (denoted by the symbol ”+”) has energy 3.5 MeV, while non-axiality shifts the saddle to the point denoted by the symbol ” x ” and decreases its energy to 1.8 MeV, i.e. by 1.7 MeV. (The energy is normalized so, that its macroscopic part is zero at spherical shape of a nucleus). This means that the reduction of the saddle-point energy (and, thus, of the barrier height Bst) of this nucleus by non-axiality is quite large. Also the deformation of the saddle point is significantly changed by non-axial shapes, from &, &, &, ys = (0.430, 0.070, 0.010, 0’) to (0.499, 0.030, 0.005, 15.7’).

04

03

.-

s v)

02

2 01

00 00

01

02

03

P,

04

05

06

07

CQS Y

Figure 1. Contour map of potential energy calculated for the nucleus 250Cf. Numbers at contour lines specify the value of the energy. Position of the saddle point is marked when axial symmetry of the nucleus is assumed, and by the symbol by the symbol ” x ” , when non-axiality is taken into account. Numbers in the parentheses specify the values of the energy at these points.

”+”,

Figure 2 illustrates shapes of the nucleus ’”Cf at the equilibrium and the saddle points. The illustration is done by showing cross sections of ) (s,y) planes. One can see that the shapes at the its surface by ( 2 , ~and equilibrium and the saddle points are quite different. Figure 3 gives a contour map of the potential energy of the nucleus 278112. One can see that the map differs very much from that of Fig. 1. Here, two saddle points are obtained with the same energy (-0.9 MeV)

252

Y

Y

X

+x

0.5

.O

Figure 2. Shapes of the nucleus '"Cf at its equilibrium (left side) and saddle (right side) points. Upper plots show cross section of the surface of the nucleus by the ( z , z ) plane, while the lower ones give cross section of it by the (z,y)plane. T h e deformation of the nucleus a t the equilibrium point is: 0; = 0.250, 0: = 0.030, @ = -0.040, yo = Oo, while that a t the saddle point is: 0;= 0.499, 0;= 0.030, 0: = 0.005, ys = 15.7O.

in the case of axial symmetry. Only the second one (at PI = 0.52) is lowered by non-axiality t o the energy -1.9 MeV, i.e. by 1.0 MeV. As B f t is defined, however, by the highest saddle point, it is not decreased by non-axial deformations, because the first saddle point (at pz N 0.32) is not influenced by these deformations. Figure 4 illustrates shapes of the nucleus 278112 at the equilibrium and the saddle points. Similarly as in Fig. 2, the illustration is done by showing cross sections of the surface of the nucleus by ( 2 , ~and ) (z,y) planes. One can see that here only the shapes of cross sections by ( 2 , ~plane ) at the equilibrium and the saddle points are different. Concluding, one can say that non-axial deformations may reduce the barrier height BFt by as much as about 2 MeV. According to [16], such a reduction may lower the cross section for synthesis of the corresponding

253

0.4

0.3

*

E

0.2

B

d 0.1

00 0,O

0,l

0.2

0,3

0.4

0.5

0,7

0.6

P2 UJS Y Figure 3.

Same as in Fig. 1 , but for t h e nucleus 278112.

z

z

0.5 X

Y

Y

.o

1

0.5

0.5

.o

Figure 4. Same as in Fig. 2, but for the nucleus 278112

254 superheavy nucleus by as much as about 2 orders of magnitude, i.e. very essentially. For some nuclei, however, the barrier height is not influenced at all by non-axiality degrees of freedom. Thus, the effect of non-axial shapes on the barrier height Bst is a very individual property of a nucleus and it should be carefully studied for each nucleus separately.

Acknowledgements Support by the Polish State Committee for Scientific Research (KBN), grant no. 2 P03B 039 22, and the Polish-JINR (Dubna) Cooperation Programme is gratefully acknowledged.

References 1. A. Mamdouh, J.M. Pearson, M. Rayet and F. Tondeur, Nucl. Phys. A644, 389 (1998). 2. A. Mamdouh, J.M. Pearson, M. Rayet and F. Tondeur, Nucl. Phys. A679, 337 (2001). 3. I. Muntian, Z. Patyk and A. Sobiczewski, Actu Phys. Pol. B34, 2141 (2003). 4. P. Moller, A.J. Sierk and A. Iwamoto, Phys. Rev. Lett. 92, 072501 (2004). 5. T. Biirvenich, M. Bender, J.A. Maruhn and P.-G. Reinhard, Phys. Rev. C69, 014307 (2004). Larsson, Phys. Scr. 8 , 17 (1973). 6. S.E. 7. S. Cwiok and A. Sobiczewski, 2. Phys. A342, 203 (1992). 8. R.A. Gherghescu, J . Skalski, Z . Patyk and A. Sobiczewski, Nucl. Phys. A651, 237 (1999). 9. I. Muntian, 0. Parkhomenko and A. Sobiczewski, Proc. Intern. Tours Symp. on Nuclear Physics V, Tours (France) 2003 , ed. by. M. Arnould et al. (AIP Conf. Proc., vol. 704, New York, 2004) p. 41. 10. V.I. Zagrebaev, Y . Aritomo, M.G. Itkis, Yu.Ts. Oganessian and M. Ohta, Phys. Rev. C65, 014607 (2001). 11. V.V. Volkov, Actu Phys. Pol. B34, 1881 (2003). 12. A.S. Zubov, G.G. Adamian, N.V. Antonenko, S.P. Ivanova and W. Scheid, Actu Phys. Pol. B34, 2083 (2003). 13. G. Giardina, S. Hofmann, A.I. Muminov and A.K. Nasirov, Eur. Phys. J . A8, 205 (2000). 14. Y . Abe and D. Bouriquet, Actu Phys. Pol. B34, 1927 (2003). 15. W . J . Swi@ecki, K . Siwek-Wilczynska and J . Wilczynski, Actu Phys. Pol. B34, 2049 (2003). 16. M.G. Itkis, Yu.Ts. Oganessian and V.I. Zagrebaev, Phys. Rev. C65, 044602 (2002). 17. I. Muntian, Z. Patyk and A. Sobiczewski, Actu Phys. Pol. B32, 691 (2001). 18. S.G. Rohoziliski and A. Sobiczewski, Actu Phys. Pol. B12, 1001 (1981). 19. S.G. Rohoziliski, Phys. Rev. C56, 165 (1997).

255

DYNAMICS OF THE NECK FORMATION AND ITS EFFECT ON THE FUSION PROBABILITY

T. WADA, A. FUKUSHIMA AND M.OHTA Department of Physics, Konan University, 8-9-1 Okamoto, Kobe 658-8501, Japan E-mail: wadaakonan-u.ac.jp Dynamics of the neck formation in the fusion of two heavy-ions is studied and its effect on the fusion probability is discussed. A multi-dimensional Langevin equation is used to study the fluctuation-dissipation dynamics of the fusion process. It is found that the rapid formation of the neck is due to the smallness of the mass for this motion. Importance of the independent treatment of the neck degree of freedom is stressed. By comparing the results with and without taking account of the neck degree of freedom, it is found that the fusion probability is decreased significantly by the rapid neck formation for the systems that show the phenomenon of the fusion hindrance.

1. Introduction

Synthesis of superheavy elements has been an important subject in nuclear physics l. It will give us the knowledge concerning the nuclear interaction in the extreme condition, namely the extremely strong Coulomb repulsion. The macroscopic fission barrier vanishes at this atomic number and the nucleus is stabilized against fission due to the strong shell correction energy. This special feature makes the synthesis of this element challenging. To synthesize superheavy elements, we first have to fuse two heavy ions to form a compound nucleus which is to cool down with neutron evaporations. Thus the knowledge on the heavy-ion fusion process is essential for the estimation of the cross section to synthesize superheavy elements. In this respect, we should like to study the dynamics of the neck formation in heavy-ion fusion process and to study its effect on the formation probability of the superheavy compound nucleus. The neck formation is supposed to be the first thing to happen when two heavy-ions collide. With the increase of the neck radius, nucleons are exchanged between projectile and target as well as energy, angular momentum and so on. The growth of the neck strengthens the nuclear

256

interaction in the neck region, ie., it enhances the nuclear attraction and also the nuclear friction. For light systems (2< 80), the neck formation enhances fusion. Because the Coulomb repulsion between two nuclei is rather small, the attractive nuclear force makes the system to form a spherical compound nucleus and fusion occurs with large probability. This situation is described successfully by the Bass model z . For heavy systems (2> go), on the other hand, the neck formation diminishes fusion. The Coulomb repulsion becomes very strong and starts to compete with nuclear attraction even after the contact of two nuclei. Therefore, t o go t o the spherical region, the system needs extra energy after the contact and the nuclear friction dissipates this extra energy. This situation is described by the extra push model The hindrance of fusion is observed in the symmetric-like fusion reactions forming 2 > 80 nuclei 4 , where 2 denotes the atomic number of the fused system. Our interpretation is based on the work by Swiatecki and his coworkers the idea of the extra-push is extended to include the fluctuation of the collective motion. In this talk, we would like to discuss the importance of the proper treatment of the neck degree of freedom, in particular, when one considers the fusion probability of two massive nuclei. In Section 2, the nuclear shape parameterization is discussed in connection with the neck degree of freedom. We explain the framework of our fluctuation-dissipation dynamics in Section 3. Results are presented in Section 4. Summary is given in Section 5.

’.

2. Shape parameterization

Fluctuation-dissipation dynamics has been applied for the study of fusion and fission phenomena to understand the nature of the nuclear friction ’. In fission case, this approach clarified the time scale of induced fission ‘. And in fusion case, it was applied to understand the mechanism of fusion hindrance ’. We also applied it for the study of the synthesis of superheavy elements *. In this approach, nuclear shapes are parameterized with several parameters. So far, two or three parameters have been used for the dynamical study of nuclear fusion and fission, though for the static study of energy surface five parameters have been used. Those parameters describe, for example, elongation, mass-asymmetry, deformations of colliding nuclei (fragment de-

257

formation for fission process), and neck. For example, in (c, h, a)-parameterization ’, c and h describe some combination of elongation, fragment deformation and neck while a describes mass asymmetry. Another example is (p, A, A)-parameterization lo. The nuclear shape is expressed with two spheres connected by a quadratic neck. A expresses mass asymmetry and p expresses elongation. X represents a combination of fragment deformation and neck and was called “deck” parameter. In these parameterizations, the neck degree of freedom is not independent from other degrees of freedom, and they are not really suitable to study the role of the neck in the fusion process. In this study, we use a twc-center parameterization, in which neck is an independent parameter ll. The single particle potential for nucleons is expressed with two axially symmetric deformed harmonic oscillators and the nuclear shape is determined as an equipotential surface for this potential. Thus we have four parameters t o describe nuclear shapes: distance between two deformed harmonic oscillator potentials ( z o ) , deformations of the harmonic oscillator potentials (61, 62), and mass asymmetry ( a ) . To avoid a cusped nuclear shape, the single particle potential is smoothed around the point where two harmonic oscillator potentials cross. This smoothing procedure brings one more parameter that is called the neck parameter E . It is the ratio of the smoothed potential height to the original one where two harmonic oscillator potentials cross each other. Thus, a smaller E means a thicker neck. 3. Langevin equation

The multi-dimensional Langevin equation is given in the following form,

where V is the potential energy, mij and yij are the shape-dependent collective inertia and dissipation tensors, respectively. The normalized random force R,(t), is assumed to be a white noise, ie., (R,(t))= 0 and (R,(tl)Rj(t2))= 26ij6(tl - t 2 ) . The strength of the random force gi3 is given by yijT = g i k g j k , where T is the temperature of the compound nucleus calculated from the excitation energy as E, = aT2 with a denoting the level density parameter. The potential is calculated as the sum of a generalized surface energy 12, Coulomb energy, and the centrifugal energy with

258

the moment of inertia of the rigid body. Hydrodynamical inertia tensor is adopted with the Werner-Wheeler approximation for the velocity field, and the wall-and-window one-body dissipation l 3 is adopted for the dissipation tensor. Excitation energy of the composite system Ex is calculated for each trajectory as,

where Eo is given as EO= Ecm- Q with Q denoting the Q-value of this reaction and E,, the incident energy in center-of-mass frame. 4. Results

First, we try to extract the most relevant parameter for the neck formation. To describe (nearly) symmetric fusion reactions, we use three parameters: elongation ZO, fragment deformation 6(= 61 = 62), and neck parameter E . The mass-asymmetry parameter a is fixed to its initial value through the calculation. The initial configuration that corresponds to the contact (zero-neck) configuration has zo=l.GRo, 6=0 and ~ = lwhere , Ro denotes the radius of the spherical compound nucleus. One can change the neck radius by changing these parameters. In the Langevin equation there are three quantities that determine the dynamics: potential V, inertia mass mij , and friction yij. We can increase the neck radius by reducing zo, but it simply increases the Coulomb repulsion. Therefore, we will examine only 6 and E degrees of freedom later on when we talk about the neck formation. For the inertia mass, we examine m 6 6 and me,. It is found that m,, is about ten times smaller than m 6 6 . Thus, E is expected to be a fast degree of freedom. w e also examine the friction tensor. The important quantity in the dynamics is the reduced friction y / m rather than the friction itself. It is found that the reduced friction is in the same order of magnitude in both S and E degrees of freedom. We can expect that the smallness of the inertia mass for c is most responsible for the quick neck formation. Now, we calculate the fusion probability with the multi-dimensional Langevin equation. To see the importance of the neck degree of freedom, we compare the results of the two-dimensional (fixed E ) calculation with that of the three-dimensional (variable E ) one. Figure 1 shows the results for three systems, 1 0 0 M ~ + l o o M looMo+llOPd, ~, and lloPd+llOPd, that are known to show the phenomenon of fusion hindrance. It is seen that the fusion probability is reduced significantly when we include the neck degree

259

of freedom for all cases. The reduction is more significant for lower incident energies and for heavier systems.

1

0.01 190

200

210

220

230

240

250

260

Ecm (MeV) Figure 1. Incident energy dependence of the fusion probability for three systems, 1 0 0 M ~ + l o o M looMo+llOPd, ~, and 'loPd+llOPd. Dashed lines are the results of the two-dimensional calculation with E = 1. Solid ones are those of the three-dimensional calculation with variable E.

An advantageous point of the Langevin approach is that we can follow the Langevin trajectories to really see the time evolution of the process. First) we examine the trajectories in the two-dimensional calculation. The dynamical coordinates are zo and 6. It is found that, starting from the contact configuration, the trajectories move in 6 coordinate first to increase the neck radius. Since the inertia mass for this motion is not very small, the motion is not very fast. Next, we examine the trajectories in the threedimensional calculation including E as the third dynamical coordinate. In this case, the trajectories are found to move in E coordinate first. The development of the neck is much quicker than it was in the two-dimensional case. It is essentially due to the smallness of the inertia mass for E degree of freedom. After the development of the neck, many trajectories find themselves outside of the ridge of the potential energy surface and go down

260

the slope t o re-separation. Thus, the fusion probability is smaller in the three-dimensional calculation. 5 . Summary

The dynamics of the neck formation in fusion process is discussed. Fusion probability is calculated using the multi-dimensional Langevin equa~, and tion for (nearly) symmetric systems, l o o M ~ + ' O O MlooMo+l'OPd, ''oPd+llOPd, that are known to show the phenomenon of fusion hindrance. The nuclear shapes are parameterized with a two-center parameterization which enables us t o treat the neck degree of freedom as an independent one. jFrom t h e examination of the inertia mass and friction tensors, the neck parameter E is expected t o be relevant for the rapid neck formation. The rapid formation of the neck is essentially due to a small inertia mass for this motion. By comparing the results of the two-dimensional calculation without neck degree of freedom with that of the three-dimensional one which includes the neck degree of freedom, it is found that the fusion probability is reduced significantly when we take into account of the rapid neck formation. By examining the Langevin trajectories in two- and three-dimensional calculations, it is confirmed that the neck parameter E is responsible for the quick formation of the neck and for the stronger hindrance of fusion.

References 1. P. Armbruster, Annu. Rev. Nucl. Sci., 35 (1985) 135; Y.T. Oganessian and Y.A. Lazarev, Treatise on Heavy-Ion Science (Plenum, 1985) pp.3-251; G. Munzenberg, Rep. Prog. Phys., 51 (1988) 57. 2. R. Bass, Nuclear Reactions with Heavy Ions (Springer, 1980). 3. W.J. Swiatecki, Nucl. Phys., A376 (1982) 275. 4. K.-H. Schmidt and W. Morawek, Rep. Prog. Phys., 54 (1994) 949. 5. H.A. Kramers, Physica 4 (1940), 284; Y. Abe, S. Ayik, P.-G. Reinhard, and E. Suraud, Phys. Report 275 (1996), 49. 6. T. Wada, Y. Abe, and N. Carjan, Phys. Rev. Lett. 70 (1994), 3538. 7. C.E. Aguiar et al., Nucl. Phys., A517 (1990) 205; T. Tokuda, T. Wada, and M. Ohta, Prog. Theor. Phys., 101 (1999) 607. 8. Y. Aritomo, T. Wada, M. Ohta, and Y. Abe, Phys. Rev., C55 (1997) R1011; Y. Aritomo, T. Wada, M. Ohta, and Y . Abe, Phys. Rev., C59 (1999) 796. 9. M. Brack et al., Rev. Mod. Phys., 44 (1972) 320. 10. J . Blocki and W.J. Swiatecki, Barkeley Report LBL-12811, (1982). 11. J. Maruhn and W. Greiner, Z. Phys., 251 (1972) 431; K. Sato, A. Iwamoto, et al., Z. Phys., A288 (1978) 383. 12. H.J. Krappe, J.R. Nix, and A.J. Sierk, Phys. Rev., C20 (1979) 992. 13. J.R. Nix and A.J. Sierk, Nucl. Phys., A428 (1984) 161c.

26 1

A MICROSCOPIC DESCRIPTION OF a-DECAY CHAINS OF Z=115,118

M. GUPTA*, A. BHAGWAT AND Y. K. GAMBHIR Department of Physics, IIT-Powai, Mumbai-400076, India E-mail: [email protected]

We present theoretical results with experimental comparisons for the two experimentally synthesised 2=115 a-decay chains and predictions for a sample a-decay chain for Z=118. The calculations are based on the self-consistent RMF theory (NL3 parameter set) which yields the ground state properties. RMF densities and Q-values are incorporated within the Double Folding model (with the DDM3Y interaction) and the WKB approximation respectively, to get the theoretical a-decay half-lives. It is seen that a good qualitative agreement exists between theory and experiment with regard to the ground state properties of these nuclei in the case of 2=115. Predictions are made using the same formalism for 293118.

Theoretical Formalism Extensive systematic calculations to describe the ground state properties and alpha-decay half-lives of the super-transactinides with 108 Z 118 have been carried out, the theoretical formalism for which is described in detail elsewhere in these Proceedings A schematic diagram depicting the general flow of calculations is shown below in Figure 1. In a mass region where pairing effects and deformations are expected to be important, the above RMF/RHB prescription with self-consistent pairing, has been applied to the unambiguously measured a-decay chain for 277D~ ', where it was seen that incorporating a deformed potential in the formalism is important for a better agreement with experimental Qvalues '. Similar calculations have been done here. 2. Results Figure 2 shows a comparison of calculated and experimental Q a , and T1/2 ( a ) as well as theoretical binding energies and deformations (pz) for the recently synthesised Element 115 '. There is good qualitative agree1.

<
Tea,(Hg)> Tad,”’(l12), while for adsorption on transition-metal surfaces, Tad8(Hg)> Tad:’(l 12) > T.dP(l12).

1.

Introduction

It was long recognized that relativistic effects scaling as Z2, where Z is atomic number, drastically influence energies and space distribution of the valence electrons of the heaviest elements resulting in major changes in their properties [1,2]. Presently, element 1 12 lies in the focus of the experimental and theoretical investigations: It is expected that due to the very strong relativistic stabilization and contraction of the 7s electrons, as well as the 7s26d” closed shell, it will be as inert and volatile as a noble gas [3,4]. Presently, gas phase termochromatography experiments are under way with the aim to study volatility of element 112 in comparison with that of Hg by measuring their adsorption temperature on gold detectors of a chromatography column [5,6]. At the same time, calculations for element 112 and Hg adsorbed on various surfaces including transition metals are being performed with the use of the fully relativistic density functional theory method. First results for metal-metal dimer and small ad-atom-metal-cluster systems are published meanwhile [7-91. They show that element 112 should form bonding with gold and other transition

310

-

metals of only 0.2 eV weaker than that of Hg. An important aspect of both types of the studies is establishment of the influence of relativistic effects on volatility of element 112. Since nature is relativistic, the only way to "measure" relativistic effects is to compare an experimental outcome with that predicted on the basis of relativistic versus nonrelativistic calculations. Thus, the present theoretical work is devoted to predictions of the relativistic and nonrelativistic adsorption enthalpies and temperatures of element 112 on inert and metal surfaces with respect to those of Hg. For that purpose, relativistic and nonrelativistic atomic and ad-atom-cluster calculations have been performed with the use of the most advanced methods. 2.

Methods and details of the calculations

Calculations for the ad-atom-cluster systems were performed with the use of the fully relativistic (four-component) density functional theory method, 4c-DFT. The relativistic extension of the generalized gradient approximation (RGGA) of Becke and Perdew is used for the exchange-correlation potential. The molecular orbital-linear combination of atomic orbitals (MO-LCAO) approach is used to solve the Kohn-Sham equations. The symmetrization coefficients for double group representations are obtained by the use of the group theoretical projection operators. Recently, the embedded cluster approximation has been incorporated into the code. The full description can be found elsewhere [7-lo]. The calculations were performed for all electrons in the systems. Neutral 1s through the valence ns orbitals were used as the main basis and ionized (+1.5) np and nd virtual orbitals as an additional basis. Nonrelativistic calculations were performed by setting c = 00 in the Dirac equation. For calculations of the adsorption enthalpy of Hg and element 112 on inert surfaces, results of various most accurate atomic calculations were used: the relativistic and nonrelativistic pseudopotentials (PP) [ 1I] for polarizability, a, and Dirac-Fock Couple Cluster Single Double excitations, DF CCSD [12], for ionization potentials, IP. DFT atomic calculations were also performed by us to find a ratio between the relativistic and nonrelativistic values. 3.

Results of the calculations

3.1. Adsorption on inert surfaces

This case can be realized when a quartz chromatography column is used, or if ice is formed in a chromatography column with metal detectors at very low temperatures. The dispersion interaction energy of an atom with a solid surface

311

(slab) is given as E+ - 12 ) [ L E(x)=-- 136 L -

a,,1

+IfL6

) x3’

IPOt

where IP,,,b is roughly the ionization potential of the surface atom, E is the dielectric constant of the surface substance and x is the ad-atom-surface distance. Calculated relativistic and nonrelativistic a,IP and atomic radii, AR, for the adatoms, Hg and element 1 12, are given in Table 1 . For Hg, x can be deduced from the measured AHadsvia eq. (1). For element 112, x can be obtained from its AR using the proportionality between x and AR, which is valid for the case of physisorption. Table 1. Relativistic and nonrelativistic polarizabilities a (a.u.), ionization potentials IP (ev), atomic radii AR (ax.), as well as the adsorption enthalpies AHsda(kJ/mol) and temperatures Tad$(K) for Hg and element 112 on quartz surface Hg

IP a AR

-AH,e Tad”

nr 8.98 57.8 3.91 41.4 124

re1 10.43 33.9 3.43 40.8 123

element 112 nr re1 8.25 11.97 74.7 25.8 4.32 3.20 40.2 44.5 114 129

The relativistic and nonrelativistic atomic properties (Table 1) exhibit opposite trends in group 12. Since a-lIIP, their opposite trends in group 12 will partially cancel in the product aIP according to eq. (l), so that trends in the relativistic and nonrelativistic E(x)=-AHds will finally be determined by trends in the relativistic versus nonrelativistic x-AR (Table 1). By substituting relativistic and non-relativistic values of a and IP into eq. (l), AHd, is obtained as shown in Table 1 . Thus, relativistic effects increase the adsorption enthalpy of element 112 with quartz and it becomes larger than that of Hg. Nonrelativistically, the trend from Hg to element 112 is just opposite. Tds(1 12) relative to Td,(Hg) can be predicted using the following equation for mobile adsorption [ 131

312

T ~ 1 ’ 2 - e - A H ~ IRTB

e-AH~lRTd

Tg112

112

112

*

t$2 rB mB

t$2 rAmA

Here, r M B is AR, rnm is the mass of used isotopes of homologs A and B and tllz is their half-lives. For experimental f112(185Hg) = 50 s and tllz(2831 12) = 3 m, relativistic and nonrelativistic Tds(l12) obtained via eq. (2) are shown in Table 1. Thus, relativistic effects increase Td,(112) by 15 degrees and increase it relative to Td,(Hg) by 6 degrees. They are responsible for a reversal of the trend from Hg to element 112: Tad;e’(I12) > T,dsmP(Hg)> Tdd,”’(l12) (Figure 1). ......

..,.......... . .. . .................

............. .............. . . .

. . . . ..........”.......................................

em.

em. -130

-140

-150

................ .............

-160

-170

-180

-190

T, OC Figure 1, Predicted relativistic (rel) and nonrelativistic (nr) Tadsof element 112 relative to Tadrof Hg and Rn on quartz.

3.2. Adsorption on transition-metal surfaces In the gas-phase chromatography experiment [6], element 112 is supposed to be adsorbed on the surface of gold detectors. To predict AHds on gold, relativistic and nonrelativistic DFT calculations were performed for Hg and element 112 interacting with gold clusters of 14 and 9 atoms. Various adsorption positions on-top, bridge and hollow - on the clusters were considered. Results for the ontop position of M on Auld and the hollow position of M on Aus (M = Hg and element 112) are shown in Table 2. For adsorption in the on-top position, relativistic effects only slightly increase AHadsof element 112, while for adsorption in the hollow position, the difference in AHad,between the relativistic and nonrelativistic values is relatively large. This is connected with the involvement of different valence orbitals in bonding in these two cases: of the relativistically stabilized and inert 7s orbital in the former case, and the destabilized and active 6d orbitals in the latter case.

Table 2. Relativistic and nonrelativistic bond lengths Re (am), adsorption enthalpies AH,,,, (kJ/mol) and temperatures Tada(K) for Hg

313 and element 112 adsorbed on gold clusters of 14 and 9 atoms Hg

Re -A&

Tadr

on-top ( A h ) re1 nr 5.5 5.0 61.7 83.0 428* 338

hollow (Auy) nr re1 4.6 3.8 39.6 82.0 248 428*

element 112 on-top (Auld) hollow (Auy) nr re1 nr re1 5.8 5.2 4.9 4.2 67.5 68.5 50.2 76.2 333 338 198 348

* Experimental value. Relativistic calculations for MAu, with n up to 34 atoms [14] show that the hollow position is preferential. Very recent results for the embedded cluster calculations indicated that the difference in AHds between Hg and element 112 can reach 0.5 eV [14]. Further calculations with improved basis sets are in progress. Taking all the obtained binding energies into account, relativistic and nonrelativistic Tads were predicted using eq. (1) of ref. [9] for localized adsorption, similar to eq. (2) of this work. The results are shown in Fig. 2 by the arrow at 112'". Thus, e.g., for AHads(rel)-AHads(nr)= 26 kJ/mol, Tad;eel-T,,dln' = 150 degrees (Table 2). The relation between AHads(Hg) and AHds(112) for other transition metals, like Pt, Pd, Ag or Cu, was found to be the same [7]. The data of Table 2 show that relativistic effects increase Tad,(112), especially on adsorption in the hollow position. Thus, in contrast to the adsorption on inert (quartz) surfaces, for adsorption on the gold surface the trend is TdFP(Hg) > Td,'el(l 12) >> TadS,,(l12) (Figure 2). ...................................................................................................

112"'

112"' Rn

em.

t----c

200

100

0

-100

-200

T, OC

Figure 2. Predicted relativistic (rel) and nonrelativistic (nr) T,ds of element 1 12 relative to Tad. of Hg and Rn on gold.

4.

Conclusions

Results of the present study have shown that relativistic effects influence adsorption of element 112 in a different way depending on the surface and adsorption position. On adsorption on inert surfaces, relativistic effects lead to

314

the strongest dispersion interaction of element 112 in group 12 due to its relativistic smallest size. Nonrelativistically, element 1 12 would have, on the contrary, had the smallest adsorption energy, so that its volatility would have been the largest in the group. On adsorption on transition metal surfaces, relativistic effects also increase the adsorption energy of element 112, though the increase is much larger for the adsorption in the hollow position than for the on-top position. The reason for that is predominant involvement of the relativistically destabilized 6d orbitals in the former case in contrast to the stabilized 7s orbitals in the latter case. For all the positions, relativistically, element 1 12 should be less volatile than nonrelativistically, i.e., it should be adsorbed at higher temperatures compared to the nonrelativistic case. It should, however, be still more volatile than Hg, but less than Rn due to the metal-metal bond formation. Acknowledgments The calculations were performed on the Linux cluster (Barossa) of the Australian Center for Advanced Computing and Communication (ac3) and the Linux cluster of the GSI, Darmstadt. References 1 . B. Fricke, Struct. Bond. 21, 89 (1975). 2. V. Pershina, In: The Chemistry ofSuperheavy Elements, Ed. M. Sch2de1, Kluwer, 2003,p. 3 1. 3 . K.S. Pitzer,J. Chem. Phys. 63, 1032(1975). 4. B. Eichler, Dubna Report JINR P12-7767 (1974). 5 . A.B. Yakushev et al., Radiochim. Acta, 89,743 (2001). 6. S. Soverna et al., PSI Annual Report 2002, p. 8. 7. V. Pershina et al., Chem. Phys. Lett., 365, 176 (2002). 8. C. Sarpe-Tudoran et al., Eur. Phys. J., D24,65 (2003). 9. V. Pershina et al., Nucl. Phys., A734,200 (2004). 10. S. Varga et al., Phys. Rev., A59,4288 (1999). 1 1 . M. Seth, Doctoral Thesis, University of Auckland, (1998). 12. E. Eliav et al., Phys. Rev. A52,2765 (1995). 13. V. Pershina, Radiochim. Acta, in press. 14. C. Sarpe-Tudoran, Doctoral Thesis, University of Kassel, (2004).

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317

NEUTRON EMISSION IN FISSION AND QUASI-FISSION I. ITKIS, A. A. BOGATCHEV, A. W. CHIZHOV, M. G. ITKIS, J. KLIMAN, G. N. KNYAZHEVA, N. A. KONDRATIEV, E. M. KOZULIN, I. V. KORZWKOV, L. KRUPA, W. TS. OGANESSIAN, I. V. POKROVSKI, E. V. PROKHOROVA, R. N. SAGAIDAK, V. M. VOSKRESSENSKI Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia E-mailgitkis(ii?nr.iinr.ru A. YA. RUSANOV Institute of Nuclear Physics of the National Nuclear Center, 480082, Almaq, Kazakhstan

L. CORRADI, A. M. STEFANINI, M. TROTTA Laboratorio Nazionali di Legnaro, Instituto Nazionale di Fisica Nucleare, Via Romea 4, I-35020, Legnaro (Padova), Italy S. BEGHINI, G. MONTAGNOLI, F. SCARLASSARA Dipartimento di Fisica, Universita di Padova, Via Marzolo 8, I-35100, Padova, Italy G. CHUBARIAN Texas A M University, Cyclotron Institute, College Station, Texas, USA F. HANAPPE, T. MATERNA Universite Libre de Bmelles, I050 Bruxelles, Belgique

0. DORVAUX, N. ROWLEY, L. STUTTGE Institut de Recherches Subatomiques, F-67037 Strasbourg, France G. GIARDINA Dipartimento di Fisica dell'Universita di Messina, 98166 Messina, Itah

The work presents the results of the study of characteristics of the neutron emission in fission and quasi-fission of heavy and super-heavy nuclei, produced in the reactions with heavy ions. These experiments have been performed at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR), tandem accelerator in Legnaro (LNL) and VIVITRON accelerator in Strasbourg (IReS) with the use of the time-of-flight spectrometer of fission fragments CORSET and neutron multidetector DEMON. Massenergy distributions (MED) of the 48Ca+ I6'Er, '"Pb, 238U and "0 + '"Pb reactions products at energies close to and below the Coulomb barrier have been studied. The pre-

318 and post-fission neutron multiplicities as a function of the fragment mass have been obtained. A significant yield of the asymmetric component observed in the fragment mass distributions in the case of "0 + 20sPbreaction denotes the multimodal nature of the fission process. At the same time an increase in the yield of fragment masses M LE 75-85 and M HG 200-210 in the case 48Ca+208Pb, 238Ureactions and MLE 75-85 and M HE 130140 in the case 48Ca+'68Er is rather connected with a quasi-fission process. The obtained neutron multiplicities dependences on fragment masses showed the validity of these assumptions.

1. Introduction

One of the most interesting aspects of modem nuclear physics is the synthesis of heavy and superheavy elements and the study of their properties. The interest in reactions with 48Ca ions is explained by their importance for the presently realized at the FLNR program of the superheavy-ion synthesis in the region of nuclei with Z = 114-1 18 and N = 182-1 84, for which a considerable increase in their stability to spontaneous fission and a-decay has been predicted. A much deeper insight into the mechanism of the fission process and a better knowledge of the fission - quasi-fission competition as a function of the reaction entrance channel and excitation energy is very important. Undoubtedly, all these aspects are of great independent interest to nuclear fission physics. 2.

Experiment

The experiments were carried out in the Flerov Laboratory of Nuclear Reactions and in the Legnaro National Laboratory on the extracted beams of 48Caions of U-400 and XTU Tandem accelerators in the energy range Elab = 180-242 MeV. '68Er,"*Pb and 238Uspectrometric layers 120-200 pg/cm2 in thickness deposited on a 20-50 pg/cm2 carbon backing were used as a target. Reaction products were registered with the use of a double-arm time-offlight spectrometer CORSET [ 1,2], each arm of which consisted of a compact start detector, composed of microchannel plates, and a stop position sensitive (x, y - sensitivity) detector, 6x4 cm in size, also composed of microchannel plates. The estimated mass resolution of the spectrometer is 3-5 a.m.u. The neutron multidetector DEMON [3,4] was used to measure neutrons in coincidence with fission fragments. DEMON consists of NE213 liquid scintillator cells of 20 cm length and 16 cm diameter, optically coupled to XP45 12B photomultiplier tubes. Neutrons are separated from y-rays by pulse shape discrimination. Their energy is determined by time-of-flight measurement over a flight path of about 130 cm. The time resolution of DEMON is about 1.5 ns. The geometry of DEMON was close to spherical one. The discrimination thresholds were set using "Na, I3'Cs, 6oCoand 241Amsources. Efficiency curves

319

for the neutron detection as a function of the energy were obtained both by using 252Cfcalibration run and by Monte Carlo simulations [ 5 ] . 3.

Data Analysis and Results

A standard method presupposing the selection of a two-body process by using the folding correlations was employed in the data processing. The energy losses of the fragments in the target, backings and start detectors foils were taken into account. The decomposition of the total neutron multiplicities into pre- and postscission components was obtained by means of a three moving source fit. Neutrons were supposed to be emitted sequentially with Maxwellian energy distributions fi-om the compound system and two fully accelerated fragments. 48

Ca +z08Pb+ 2 5 6 N ~

240

240

200

200

160

160

120

120

2 240 2 200

’ -240

>

h

w

k

-200 :160

160

‘

120

4

E

5

W

-120

@:=211

=218 MeV E* = 21 M e V

M e V E* = 18

240

200 160 120

50

100

150 M (u)

200

50

100

150

M (4

200

250

Figure 1. Two-dimensional matrices N(M,TKE) for the reactions 48Ca+208Pband 48Ca+”‘u at different excitation energies.

Figure 1 presents the plots of two-dimensional matrices N (M, TKE) for Ca + ’“Pb and 48Ca+ 238Ureactions at several values of the ion energy. One can see that in the case of 2 5 6 N ~N, (M, TKE) at any excitation energy is of distinctive triangle shape, which is typical for the classic symmetric fission of a compound nucleus. On the other hand, in the case of light fission fragments with 65<MLI100 and complementary heavy ones there are “shoulders” in the 48

320

mass distribution which denote an enhanced yield of reaction products as compared with the yield expected in the case of classic fission. In the case of the 286112 decay, the N (M, TKE) matrices have a more complicated structure, determined by domination of quasi-fission process with a characteristic grouping of fragments around the fission fragment mass 208 and complementary masses. Thus extraction of events corresponded to the compound nucleus fission becomes quite a complicated task. Nevertheless, at the excitation energy E* = 26 MeV, between the quasi-fission peaks one can see the region which can be assigned to the fission of the compound nucleus 286112. Obtained neutron total, pre- and post-scission multiplicities are shown in Figure 2. In the case of 48Ca+ 'O'Pb, the total neutron multiplicity is nearly twice higher for the region of masses AcN/2*10 than for the regions A = 6 5 ~ 9 0 and complementary masses. Thus the enhanced yield in this region of fragment masses can be connected with quasi-fission process. For the heavier system the total neutron multiplicity is also essentially higher for AcN/2*20 amu than for the fragment mass region where the mechanism of quasi-fission predominates. Thus it may mean a substantial contribution of the fusion-fission process in the symmetric masses region. 4s

+ 256N0

48

E'= 3 3 MeV1

]*E,.*= 233 MeV

C a +'08Pb

I E,oh= 2 3 0 M e V

Ca

+ 286112

+ 2 3 8 ~

E-= 3 3 M e V

1

280 240 200

160 3000

10 8 > 6

6000 y1

2 4000 8

4

2000

2 50

75

100 125 150 175 200

M (u)

50

7 5 100 125 I50 175 200 225 250

M (u)

Figure 2. Two-dimensional matrices N(M,TKE) (top panels); the mass yields (the solid circles) and neutron total (the stars), pre- (the triangles) and post- (the squares) scission multiplicities as the dependences on the fission fragment mass (bottom panels) for the 48Ca+ "'Pb and 48Ca+ 238U reactions.

Figure 3 shows the mass-energy distributions of fission fragments using two projectile-target combinations, "C + *04Pb and 48Ca+ 168Er,leading to the same compound nucleus 2'6Ra at the excitation energy El- 40 MeV. A nearly triangle shape of the matrix and a practically Gaussian shape of the mass

321

distribution indicate that the influence of shell effects is negligible in the case of 12C+ 204Pbreaction. The TKE(M) dependence slightly deviates from a parabola, describing well the symmetric part of the spectrum starting from the heavy fission fragment mass M~-128. In the dependence c&(M) there is a slight peak in this mass region. The discussed irregularities point to an insignificant contribution of asymmetric fission mode estimated as -1.5%. This is exactly what was predicted for the nucleus under discussion [ 6 , 7 ] . ”C

T EW

+ ’“Pb

i,

’16Ra 4’Ca +l6’Er +’I6Ra

;;; 160 140 120 6000

“ I

c

4500 3000

0

1500

160 v

155

t!7 ;:: h

5 E

150 100

Y

wbg

50 60 80 1 0 0 1 2 0 1 4 0 1 6 0

60 80 1 0 0 1 2 0 1 4 0 1 6 0

M (u)

M (u)

Figure 3. Two-dimensional matrices N(M,TKE), the mass yields, average TKE and the variances G’TKE as a thction of the mass of fission fragments of ‘I6Ra.

In the case of the 48Ca+ ‘68Erreaction the great contribution (-30%) of the asymmetric component manifests itself in the form of wide “shoulders” in mass distribution. The symmetric fission component described by a Gaussian is wider than that for the reaction with ”C. Since an increase in the angular momentum of the fissioning nucleus leads to an increase in 0; of the symmetric fission nearly proportionally to [8], this increase in the variance value is quite expected, The shape of the curve TKE(M) is far from being parabolic. As for the O& (M) curve, the peaks in the region of masses 130-140 are more distinguished. Thus, in the case of the reaction with 48Caions we experimentally observed a sharp increase (by 20 times) of the asymmetric component

-

322 4

48

3 h

Ca +'"Er

E'= 41 MeV

\

\-

0

0

0

g2

+216Ra /

M=AD+lOamu M = 78 *lOamu

0

F 1

0

20 40 60 80 100 120 140 160 I

Ocm

Figure 4. Experimental angular distributions of fission fragments from the reaction 48Ca+'68Erat Elab=195 MeV. The solid circles correspond to the fragments With masses AC@1O, the open circles - to the fragments with masses 78i10.

contribution as compared with the reaction with I2C ions. We would like to emphasize that this effect has been observed at all studied energies of 48Ca projectiles at the vicinity of the fusion barrier. We connect this increase of the asymmetric products yield with the quasi-fission process [9]. Angular distributions for different fragment masses were measured in order to find experimental evidence of the quasifission nature of the "shoulders". Fig. 4 shows the angular distributions for the svmmetrical

range (M=A/2*10, 'lid circles) and for the shoulders (M=78*10, open circles). The solid curve shows the results of calculations made in the framework of the Transition State Model [lo]. As one can see from Fig. 4, the solid line fits well into the data for the symmetrical part of mass distribution, whereas the angular distribution for the "shoulders" has a pronounced forward-backward asymmetry, which is one of the distinctive features of the quasi-fission process. To explain such a difference in the MED a theoretical calculation of the potential energy was made within the three-dimensional version of the "nucleon collectivization" model [ I l l based on the two-center shell model [12, 131. Following this calculation in the case of ''C + "'Pb the process of fusion-fission is the dominating one, whereas in the case of the 48Ca+ 168Erreaction, there are two optimal paths at the potential energy surface. One of these paths leads to the '32Sn+ 84Sr exit channel without the compound nucleus formation. This potential energy minimum is determined by shell effects of the doubly magic '32Sn. The second path corresponds to the classical fusion-fission process and leads to the symmetrical exit channel '08Ru+ '08Ru, the same as in the reaction with 12Cions. That is why mass distribution of the 48Ca+ 168Erreaction products is the superposition of both processes- the symmetrical classical fission and quasi-fission "shoulders".

323

The observed peculiarities of mass-energy distributions can be explained by the influence of the shell structure of the forming fragments on the quasi-fission process. The arrows on the mass scale in Figure 5 (lower panels) show the positions of the closed spherical shells with Z = 28,50, 82 and N = 50,82, 126 and complementary masses, derived from a simple assumption on the proportionality of charge to mass. One can see that in the case of 48Ca+ '68Er reaction the major part of the quasi-fission contribution fits into the region of the above-mentioned shells with Z = 28, 50 and N = 50, 82. Similarly, the shell structure of heavy fission fragments (nuclei in the vicinity of 208Pb)as well as of the light ones (nuclei in the vicinity of 78Ni)strongly influences the quasi-fission process in the case of 48Ca+238Ureaction. It is interesting to note that in the 48Ca+ 208Pbreaction the classical symmetric fission dominates since only the light fragments in the "quasi-fission shoulders" are in the vicinity of magic numbers (Z = 28, N = 50). 48

7

71111 -""

250

17s:

2

150:

200

125-

150 L

.

.

A

100-

I

:

:

: : :

:

:

E*=40.4 MeV I "

"

'

'

*

'

*

'

~

2250

4 2

so

100

150

200

M (u> Figure 5. Two-dimensional matrices N(M,TKE) and the fission fragments mass spectra for the fission of 'I6Ra, 25%10and 286112 compound-nuclei,produced in the reactions with ions

Summing up we can say that in the case of quasi-fission the observed characteristics of mass and energy distributions of the fragments are determined by the shell structure of the formed fragments. It is important to note that in the case of the quasi-fission process the influence of the shell effects on the

324

observed characteristics is much stronger than in the case of classical fission of compound nuclei. 4.

Conclusions

Mass-energy distributions of the 48Ca+ I6'Er, *"Pb, 238Ureaction products at energies close to and below the Coulomb barrier have been studied. The preand post-scission neutron multiplicities as a function of the fragment mass have been obtained for the two heavier systems. A strong manifestation of shell effects has been found in the mass-energy distributions for all the reactions in study. An increase in the yield of fragment masses ML E 65-85 for all three reactions and MHz 200-210 (in the case of 48 Ca + 238U)and MHz 130-150 (in the case of 48Ca+ 16'Er) was observed, which is connected with a quasi-fission process. The analysis of neutron energy and angular distributions has shown that the total neutron multiplicity (M,J in the hsion-fission reactions of compound nuclei formed in the reactions with heavy ions is essentially higher than Mtotin the quasi-fission reactions. It may mean a substantial contribution of the hsionfission process in the case of symmetric masses in the reaction 48Ca+ 238U.

References 1. E. M. Kozulin, N. A. Kondratjev and I. V. Pokrovski, Heavy Ion Physics, Scientijic Report 1995-1996, JINR, FLNR, Dubna, 2 15 (1997). 2. N. A. Kondratiev et al., Fourth Int. Conf. on Dynamical Aspects of Nuclear Fission, (DANF'98) (Casta-Papiernicka, Slovak Republic, October 1998), World Scientific, Singapore, 43 1 (1999). 3. M. Moszytiski et al., Nucl. Instrum. Meth. A 350,226 (1994). 4 . I. Tilquin et al., Nucl. Instrum. Meth. A 365,446 (1995). 5. P.Desesquelles et al., Nucl. Instrum. Meth. A 307, 366 (1991). 6. M.G. Itkis, V.N. Okolovich, A.Ya. Rusanov, and G.N. Smirenkin, Sov. J. Part. Nucl. 19,301 (1988); Nucl. Phys. A 502,243 (1989). 7. I.V. Pokrovsky et al., Phys. Rev. C 60,041304 (1999). 8. M.G. Itkis and A.Ya. Rusanov, Phys. Part. Nucl. 29, 160 (1998). 9. A. Yu. Chizhov et. al., Phys. Rev. C 67,011603 (2003). 10. B. B. Back et al., Phys. Rev. C 32, 195 (1985). 11. V. I. Zagrebaev, Phys.Rev. C 64,034606 (2001); J.Nucl.Radiochem.Sci.,3, No. 1, 13 (2002). 12. U. Mosel, J. Maruhn, and W. Greiner, PhysLett. B 34, 587 (1971). 13. J. Maruhn and W. Greiner, 2.Physik 25 1,43 1 (1972).

325

SHELL EFFECT MANIFESTATION IN MASS-ENERGY DISTRIBUTIONS OF FISSION AND QUASI-FISSION FRAGMENTS OF NUCLEI WITH 2-102-122* E. V. PROKHOROVA, A. A. BOGACHEV, I. M. ITKIS, M. G. ITKIS, M. JANDEL, J. KLIMAN, G. N. KNYAZHEVA, N. A. KONDRATIEV, E. M. KOZULIN, L. KRUPA, YU. TS. OGANESSIAN, I. V. POKROVSKY, A. YA. RUSANOV, V. M. VOSKRESENSKI Flerov Laboratory of Nuclear Reactions. JINR, 141980 Dubna, Russia V. BOUCHAT, F. HANAPPE, T. MATERNA Universite Libre de Bruxelles, I050 Bruxelles, Belgium

0. DORVAUX, N. ROWLEY, C. SCHMITT, L. STUTTGE lnstitut de Recherches Subatomiques, F-6703 7 Strasbourg Cedex, France G. GIARDINA Dipartimento di Fisica dell' Universita di Messina 981 66 Messina, Italy

The mass-energy distributions of the fragments, fusion-fission (OFF) and capture (oCap) cross-sections in the reactions 26Mg+248Cm;48Ca+2"RPb,232Th, 238U , 2 4 4 ~ u2, 4 8 ~ m ; % ~ ~ 2 0 8 p 232~h, b, 244Pu, 248Cm were measured at the U400 accelerator of Flerov Laboratory of Nuclear Reactions (JINR, Russia) with use of double-ann time-of-flight spectrometer CORSET. The influence of reaction entrance channel on the competition between fusion-fission and quasi-fission processes was studied. Strong manifestation of the shell effects in mass distributions of fusion-fission and quasi-fission fragments was observed. The multimodal fission phenomena were found for 2 7 4 H ~and 256N0 nuclei at low excitation energies.

1.

Experiment

In the last few years in view of great interest to the problem of synthesis of new superheavy elements the series of experiments on investigation of the fusionfission dynamics were carried out in FLNR JmR. The influence of the reaction entrance channel, the shell structures of the colliding nuclei and nascent compound nucleus (CN) on the competition of fusion-quasifission processes were studied. Mass-energy distributions of the fragments (MED), fusion-fission (OFF) and ((s,,~) capture cross-sections were measured with use double arm time*

The work has been supported by the Russian Foundation for Basic Research under Grant No 0302-1 6779 and by INTAS 03-5 1-6417

326

of-flight spectrometer CORSET [l]. Each arm of the spectrometer consists of microchannel plate (MCP) Start detector with electrostatic mirror and the set of 4 position-sensitive (XY-coordinates) MCP Stop detectors. The kinematics coincidence method was applied to obtain mass-energy distributions of the fragments. The energy losses of the fragments in Start detector foils, targets and backings were taken into account. The binary events were selected on the analysis of angular correlations of the reaction products. 2.

Reactions with 48Caions Ca + mPb -+ %o

44

*Ca

+ "U

+ mf12

"CS

+

-+ "114

%a + "Cm -+ "116

mass, u

Figure 1. Two-dimensional matrixes TKE-Mass (top panels) and Mass yields of the fragments 244Pu,24XCmat excitation energy E*==33MeV. (bottom panels) for the reactions 4xCa+'("Pb, 23xU,

Figure 1 presents the mass-energy distributions of the fragments of the elements with Z= 102-1 16, produced in the 48Ca induced reaction on the targets "'Pb, 2 3 8 u , 244pu and 248 Cm at the excitation energy E*= 33 MeV. On the top of the Fig. 1 two-dimensional matrixes of Total Kinetic Energy (TKE)/Mass are presented, the mass yields of the reaction products are shown in the bottom. The main feature of the data for trans-fermium nuclei is the sharp change of the MED triangular shape for the reaction 48Ca+20sPb, where fusion-fission process dominates, to the quasifission (QF) [2] shape of MED for the 2s61 12-2961 16 nuclei. The distinctive feature of the quasifission process for these superheavy nuclei is the wide two-humped mass distribution with high peak of heavy fragment near double magic lead (Mp208). In spite of the dominating role of the quasi-fission process for these reactions we assume that in the symmetric region of the fragment masses (A/2+20) fusion-fission process coexists with QF. In the framing on the bottom of Fig. 1 the mass yield of the fusion-fission process obtained as difference between experimental spectra and quasi-fission peak description are shown. One can see on the framing that the mass

327

distribution of the fusion-fission is asymmetric in shape with nearly constant mass of the light fragment ML=132-134 amu. In case of superheavy elements the light spherical fragment plays a stabilizing role whereas the heavy 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 fragment with MH=140 is playing Mass of Composite Nucleus, u the same role for actinide nuclei Figure 2. The ratio of ( T Q F / ( T ~ as ~ ~ function of [3]. For superheavy nuclei 286112the composite nucleus mass number for the 296116 QF the magic shells as in reaction with 48Ca-ions and different targets. light (Z=28, N=50) as heavy fragments (Z=82, N=126) produce two-humped structure of the mass distribution [4]. In the Fig.2 we show the ratio of the quasi-fission to the capture cross-sections oqF/ocap as the function of the mass of composite nucleus for the reaction with 48Ca-projectiles at the excitation energies E*=33-40 MeV. Solid circles are measured reactions [4,5,6]; the question signs are the reactions to be investigated. One can see that in the case of heaviest targets 238U,244Puand 248Cmthe ratio oQF/o,,changes weakly, and that tendency was observed in the CJER excitation functions for the superheavy elements [7]. Conspicuous decrease of QF contribution is observed for the reaction where both target and projectile are spherical magic nuclei (208Pb,144Sm-targets). In contrast to the reaction on 144 Sm QF component make up till 30 % in case of deformed target 154Sm[ 6 ] .In this case the system of amalgamating nuclei has only compact configuration in the contact point in comparison with deformed target 154Smwhere the prolate orientation of the target nucleus leads to QF process. To investigate this effect one should chose the reactions with 48Ca-projectiles and targets 142-15%d,136140 130-138 Ba, where both spherical and deformed targets are available. Ce, 3.

Reactions with "Fe and 64Niprojectiles

Figure 3 shows the data for the reactions of 58Fe and 64Ni projectiles on 232Th, Cm targets, leading to the formation of the compound system from 290116 up to 302120and 306122(where N = 182-184), i.e. to the formation of the spherical compound nucleus, predicted by theory [8]. As seen from Fig. 3, we observe here even stronger manifestation of the asymmetric mass distributions of 306122and 302120fission fragments with the light fragment mass MLs132 amu. The corresponding structures are seen well in the (TKE)(M) dependence. Only for the reaction "Fe + 232Th+ 290116 (E*= 53 MeV) the

2 4 2 , 2 4 4 ~and ~ 248

328

valley in the region of M=A/2 disappears - this is seen from the mass distribution as well as from the (TKE)(M) and O * ~ ~ ~ dependences. (M) This fact is connected with a damping of the shell effects with increase of the excitation energy. On the right-hand side of Fig. 3 the characteristics of 'O6I22 fission fragments, formed in the reactions 64Ni+242P~ and 58Fe+248Cmare demonstrated. Despite the fact that the compound nucleus 306122 undergoes fission at approximately the same excitation energy E*=3 1.5 MeV and the coefficients of the entrance channel asymmetry differ slightly nevertheless the form of the energy distributions changes to more flat (TKE)(M) dependence in case of 64Niinduced reaction. "Fe +='Th +*90116

"Fe +lUPu +3021Z0 E'= 44 MeV

E'= 53 MeV

50

1W

IS0

2W

1%

50

IW

150

2W

250

MNi+"'Pu -+'06122 E'= 31 5 MeV

Fe+'"Cm +'O6lZZ E'= 3 1 5 MeV

50

IW

150

2M

250

50

100

150

2W

:

Figure 3 . Two-dimensional TKE-Mass matrixes, the mass yields, (TKE)(M) and GTKE(M)for 29"1 16, 3"2120and 3"h122nuclei.

4.

The influence of the reaction entrance channel to the superheavy nuclei formation

The dynamics of the fusion reaction and the probability to form compact compound nucleus strongly depends on the reaction entrance channel [5,9]. Figure 4 shows the formation of Hs-isotopes in the "cold fusion" (58Fe+208Pb-+ 266 Hs) and "hot fusion" (26Mg+248Cm+274Hs)reactions. Though in the former reaction compound nucleus 266Hsis produced at lower excitation energies, that should increase the CN survivability in the de-excitation process, nevertheless

329

the quasifission process is the main reaction mechanism at the asymmetry of the entrance channel (q=(AI-A2)/(A~+A2)=0.56). 3 0 0 4

E'= 31.7 M e V h

1 300

150

300 0

150

100

mass, u

mass, u

Figure 4. Two-dimensional TKE/Mass matrixes for the reaction sXFe+2"RPb+Zh6Hsat E*=1439.5MeV (left panels) and for the reaction 26Mg+2"Cm+274Hs at E*=31.7-63.4 MeV (central panels). The excitation functions for both reactions are shown at the right panel.

For the 26Mg-induced reaction more n-rich isotope 274Hsis formed with the asymmetry coefficient q=0.8 1. As one can see from the central part of the Fig.4 TKE/mass matrixes have the triangular shapes typical for fission of heated nuclei, described by Liquid Drop Model (LDM) [ 101. Only at lower excitation energies E*=31.7-35.3 MeV some difference in matrix shape from the LDM predictions appears. Right panel of Fig.4 shows the excitation functions for both reactions. 5.

Bimodal fission of 274H~ and 256N0 nuclei

Figure 5 presents the mass yields, (TKE)(M) and variances oZrKE(M)and oZM(TKE)for the reaction 26Mg+248Cmat energies Elab=129, 143 and 160 MeV (E*=35.3, 48 and 64.3 MeV). Fig 5 a) shows the shoulders in mass yield for Mp193-220 amu and additional light fragment masses for the Elab=129, 143 MeV. The fit of the mass yield by the Gaussian according to the LDM

330

model is shown by solid line. The experimental dependence (TKE)(M) was fitted by parabola (dash line in Fig.5 b):

TKE =TKE(A/2)( 1-p2)(1+pp2), where p=(M-A/2)/(A/2),

600

-02

400

200 0 2500 2000

-.=

1500

1000 500 0

160 180 200 220 240 260 280 300 320

TKE. MeV

Figure 5. Mass yield, (TKE) GTKE (M) and LTM(TKE) for the reaction 26Mg+248Cmat energies El,b=129, 143 . .- - .-.

p=0.17 is the empirical parameter, defming the parabola width [3]. One can see from FigSa) for all three excitation energies (TKE)(M) is higher for the masses MH-193-220 amu than the LDM parabolic dependence. The similar increase of (TKE)(M) was already observed for QF component of '"No [4]. The variance dTKE(M)increases for the mass region Mp200-220 amu, revealing indirectly the presence of QF and FF processes. QF component on the edges of the mass distribution is caused by the closed shells with 2=28 in light fragment and 2=82, N=126 in the heavy fragment

(FigSa). Fig. 5d) shows the mass variance oM as function of kinetic energy. For TKE=l60-220 MeV ( T * ~ increases with decreasing of excitation energies of compound nucleus. Only for TKE>220 MeV 0 2 ~ for the lowest energy (E*=35.3 MeV) is reduced, and mass distribution becomes narrowest. Figure 6 shows the mass yields of the fragments at E*=35.3 MeV for different TKE ranges. For TKE>220 and TKE>208 MeV one can observe that mass yield consists of two fission modes - symmetric narrow and wider one, which manifests itself as a pediment in " mass yield. In case of symmetric fission Figure 6. Mass yields of the 2 7 4 H ~ 274Hs (NcN=166) both fragments are of (E*=35,3MeV) fission fragments for I

26Mg +24RCrn+274Hs (E*=35.3 MeV)

rn185S.

different TKE ranges.

331

close to the magic shell N=82, that results to narrow mass distribution. The bottom panels of Fig.6 exhibits the wide mass distribution for TKE201 MeV, Fig.8~) the narrow two-humped structure with M p 1 3 1135 amu, typical for the actinide fission [13] is observed. For TKE201 MeV

Mi132

400

2000

300

1500 1000 500

200 100

0 300

0 60

200

40

0 100

20

b)

5 40v)

0

"

3

20-

. .

0.. 60-

TKE c 201 MeV

.

.

.

. c) .

40~

20-

0

0 160

40

120

30

80

20

40

O- 60

so

160 120 I40 180 160 200

0

100

10

150

200

250

100

150

200

250

3000

Figure 8. Mass yields of the fission fragments

Figure 9. TKE distributions for the total mass

for the reaction

range (left panels) and for the masses M=124total mass yield;

132 amu (right panels) at 242 MeV.

Elab = 21 I ,

217 and

332

of superheavy nuclei [14]. Fig. 9 shows the energy distributions for all masses and for the mass range M=124-132 amu. For Elab=211 MeV (E*=17.6 MeV) two components with (TKE),,,+200 and (TKE)hi,p233 MeV are observed. These TKE and mass values are characteristic for the Standard-I and Super Short modes of superheavy nuclei [ 15 3 6.

Conclusion

Mass and energy distributions of the fragments and the excitation functions have been studied for the superheavy nuclei with Z = 102-122 produced in the reactions with 26Mg,48Ca, "Fe and 64Ni-proJectilesat energies close and below the Coulomb barrier. Strong manifestation of the shell effects were found for quasifission fragments of nuclei with Z=112-122 caused by the shells Z=28, N=50 in light fragment and Z=82, N=126 in heavy fragment. Analysis of the fusion-fission components showed that the mass distribution of the fusion-fission fragments is asymmetric in shape with the mass of the light fragment ML=132134 amu. The competition between the hsion-fission and quasifission processes was investigated in dependence of the reaction entrance channel, the shell structures of the reaction partners and compound nucleus. The multimodal fission phenomena were observed for superheavy nuclei 2 7 4 Hand ~ 256N~.

References I

N.A. Kondratiev et al., in Dynamical Aspects ofNuclear Fission, Proc. of 4th Intern. Conf., Casta-PapierniEka 1998, p.43 1 (WS, Singapore, 1999). J.Toke et al., Nucl. Phys. A440, 327 (1985) 3 M.G.Itkis et al., Sov.J.Part.Nucl., 19,301 (1988). M.G. Itkis et al., Nucl. Phys. A 734, 136 (2004). A.Yu. Chizhov et al., Phys.Rev. C67,011603(R) (2003) G.N. Knyazheva et al., In this Proceedings. 7 V.I. Zagrebaev, Phys. Rev. C64 034606 (2001) 8 Z. Patyk, A. Sobiszevski, Nucl. Phys. A 533, 132 ( 1 99 I). 9 M. Dasgupta, D.J. Hinde, Nucl.Phys. A734, 148 (2004) l o J. R. Nix and W. J. Swiatecki, Nucl. Phys. 71, 1 (1965) ' I D.C. Hoffman et al., Radiochim.Acta 70/71, 135 (1995) 12 M.G.Itkis et al., Phys.Rev. C59, 3172 (1999) 13 T.J. Hamilton et al., Phys.Rev. C46, 1837 (1992) l 4 J.F. Wild et al., Phys.Rev. C41, 640 (1990) 15 U.Brosa, S.Grossman, A. Muller, Phys.Reports,l97, 167( 1990)

333

THE INFLUENCE OF ENTRANCE CHANNEL PROPERTIES ON QUASIFISSION* G.N. KNYAZHEVA, A.YU. CHIZHOV, M.G. ITKIS, N.A. KONDRATIEV, E.M. KOZULIN, R.N. SAGAIDAK, V.M. VOSKRESSENSKY Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia B.R. BEHERA, L.CORRADI, E. FIORETTO, A. GADEA, A. LATINA, P.M. STEFANINI, S. SZILNER INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova) Italy

S. BEGHINI, G. MONTAGNOLI, F. SCARLASSARA INFN and Universita di Padova, Padova, Italy

M. TROTTA Dipartimento di Fisica and INFN, Napoli, Italy

V.A. RUBCHENYA Khlopin Radium Institute, St.-Petersburg, Russia V.G. LYAPIN, W.H. TRZASKA Department of Physics, University of Jyvaskyla, Finland and Helsinki Institute of Physics, Helsinki, Finland

Mass-energy and angular distributions of ~~-192.2(12pb and 44ca+206pb+25Y\lo,

fragments for the No reactions were measured. Fusion suppression and the presence of quasifission at energy near and below the Coulomb barrier were observed for the reaction with the deformed target Ij4Sm. In the case of the spherical 144Smtarget no quasi-fission manifestation was found. A quasifission component is observed for the more symmetric combination h4Ni+lX6W compared with the 44Ca+2"6Pbsystem leading to the same compound nucleus 25"N0. 48ca+144.1S4

1.

fission

w

6 4 ~ i + l R 6 ,25(

Introduction

Better understanding the formation of a compact superheavy compound system requires clear separation of the fusion-fission events from the quasi-fission component in experiments. The competition between compound nucleus (CN) fission and quasifission is, probably, determined by the properties of di-nuclear * The work has been supported by the RFBR under Grant No 03-02-16779.

334

configuration at contact, where entrance-channel effects are expected to play the major role in the reaction dynamics [I]. In recent years great efforts were concentrated on the investigation of entrance channel effects on heavy-ion reaction dynamics. The experiments show that the complete-fusion cross section for two very massive nuclei is strongly reduced at incident energies around the expected fusion barrier [2], due to the quasifission effect. This is clearly manifested in the comparison of evaporation residue cross sections for reaction leading to the same CN, but having a different mass-asymmetry in the entrance channel [3, 41. It was found by Hinde et al. that a deformation as well as shape orientation play a major role in the dynamics of nuclear collisions with deformed heavy nuclei [5, 61.

2.

Experimental set-up

Experiments were carried out at the XTU Tandem + ALP1 accelerator complex of the Laboratori Nazionali di Legnaro (LNL) and at the K-130 cyclotron of the JYFL Accelerator Laboratory of the University of Jyvaskyla. The targets were metal evaporations of highly enriched isotopes of 206Pb, '44,'54 Sm and '86W03 (50-200pg/cm2) onto thin carbon backings (1 5-20 pg/cm2). The beam intensity was monitored continuously using four silicon surface-barrier detectors to measure Rutherford scattering from the target. Precise mass-energy distributions of binary reaction events were measured using the ToF-ToF spectrometer CORSET [7] consisted of compact start detectors and position-sensitive stop detectors (designated as St4, Sp4-St5, Sp5 in Figure 1). The angular acceptance

335

for both arms was 25’ in-plane and + l o o out-of-plane, the mass resolution is about 2-3 amu. To measure mass-angular distributions of fission fragments we also installed three ToF-E telescopes at the angles of 15’, 25’ and 35’ to the beam line (designated as Stl-Spl-Sil, St2-Sp2-Si2, St3-Sp3-Si3 in Figure I). The angular acceptance of each telescope was * I S o and mass resolution corresponded to 4-5 amu. Position-sensitive microchannel plate detectors Sp5, Sp6, Sp7 were used for measuring complementary fission fragments to those detected by the three ToF-E telescopes.

The 48Ca+144’ 154Sm reactions

3.

The effects of the deformations of the colliding nuclei on the reaction dynamic were studied with the reactions 48Ca + ‘443154Sm, going from the closed-shell spherical 144Smnucleus (p2 = 0, p 4 = 0 [ S ] ) to the well-deformed ‘54Smone (p2 = 0.27, p 4 = 0.133 [S]). “cat’”SlIl+?Jb ElalF202MeV Hab=I9.%2MV Elab=183.2MV

“ca+l“*l% ElalF2022MV

Rab=I88MV

Figure 2. Fission fragments mass-energy distributions at different energies of %a for the 4xC2 144. ISJ Sm reactions.

+

Mass-energy distributions of fission fragments, fission cross sections have reactions from well below to well above been measured in the 48Ca+ 144,154Sm the Coulomb barrier. For the 48Ca+ lS4Smreaction we observed an asymmetric component in the fission-fragment mass distributions. A contribution of this component into the total mass distribution increases with respect to the symmetric CN-fission as the projectile energy decreases. At the Coulomb barrier energy, such mass-asymmetric contribution corresponds to about 30%. The symmetric fission conponents are described by a Gaussian shape with mass variances 0; =174 amu’ at Elab = 202 MeV, 151 amu’ at Elab = 195.2 MeV,

336

123 amu2 at Elab=183.2 MeV. These values of the varience are consistent with the results of Ref. [9]. In the case of the 48Ca+ 144Sm-+ I9*Pb*reaction no asymmetric “shoulders” are observed. The same behavior one can see in average kinetic energy distribution as a function of the fragment mass (Figure 2). The arrows in Figure 2 show the position of the spherical closed shells with Z = 28, 50 and N = 50, 82 derived from the simple assumption of charge/mass equilibration. One can see that in the case of 48Ca + ‘54Sm the shell structure of the fragments strongly favors the “asymmetric shoulders”. We connect this yield of asymmetric products with the quasifission process, for which manifestation of shell effects become evident. Angular distributions for different fragment masses were derived in attempts to find experimental evidences for the quasifission nature of the massasymmetric “shoulders”. In Figure 3 the angular distributions for the selected mass bins of fission-like fragments are shown. The solid curves are fits to the experimental data given by

where W ( 0 )is the angular distribution of the CN-fission [lo], j3 is a slope parameter in an exponential decay function reproducing the evident forward backward asymmetry. a and b are normalization parameters corresponding to symmetrical and asymmetrical parts of angular distributions. E,,=l82MeV

EIab=202MeV

ZlO.

-r e E

v

0

9 310’ I]

o

30

60

90 120 150 180

o

30

Ocm(deg)

60

90 120 150 180

Figure 3. Angular distributions obtained in the 48Ca+ IS4Smreaction for fission-like fragments corresponding to the selected fragment mass bins. The solid lines are the best fits to the data using Eq (1).

01

:”,

,

,

,

,

,

,

,

1 170

180

IYO

200

,

1

210

E,av MeV

Figure 4. Fusion-fission, quasifission and capture cross sections for 4 8 ~+a1 5 4 ~ m .

As we see, for masses around 70-80 at Elab = 182 MeV the angular distribution is very asymmetric. The contributions of quasifission extracted from

337

angular distributions are -32% at Elab= 182 MeV and -2% at Elab = 202 MeV. Figure 4 presents fusion-fission (circles), quasifission (open triangles) and capture (stars) cross sections for the 48Ca + lS4Smreaction as a hnction of the projectile energy. Fusion-evaporation cross sections have been measured for the 48Ca + lS4Sm reaction in a parallel experiment at LNL. The comparison of fusion-evaporation cross section for this reaction with I6O + Ig6W, leading to the same "'Pb* CN, shows an inhibition of fusion for the reaction with Ca ions [ I I]. 4.

The 44Ca+ '''Pb and 64Ni+ "'W reactions

The effect of the mass-asymmetry in the entrance channel a = (M,,, - Mion)/(Mt, + Mion)was studied in the 44Ca+ 'O'Pb (a= 0.648) and 64Ni+ Is6W (a = 0.488) reactions leading to the same 254\10*CN. Notice that Is6W is a strong deformed nucleus, whereas 206Pbis a spherical one. In the study of the spontaneous fission properties of heavy actinide nuclei (Z > 98) it was found that the transition from asymmetric to symmetric fission in the No isotopes takes place somewhere at N = 154 [12]. So, the 25%0 (N = 148) spontaneous fission should has an asymmetric mass division. 44

Ca(218 MeV)+ZU6Pb+250No

210

I 4, . ,

,

,

. , . ,

.

,

,

,

. ,

~.

L9G 40

60

80

LOO

120

140

mass, u

160

180 20QA3

60

80

100

120

140

mass u

160

180 200

40

60

80

100 120 140 160 180 200 220

mass, u

Figure 5. Mass distributions for the reactions 4'4Ca+206Pb (a) and 64Ni+'86W(b) and average kinetic energies as a function of fragment mass (c) for these reactions at excitation energy about 30 MeV.

Figure 5 (a) and (b) show the mass distributions, (c) shows distributions as a function of the fragment mass for 44Ca+ '06Pb and 64Ni+ Ig6W at the energy close to the Bass barrier (the CN excitation energy is about 30 MeV). One can see that mass-energy distributions for these systems are very different. In the case of 44Camass distribution has a complicated structure: i) the asymmetric fission connected with the formation of the deformed shell near the heavy fission fragment mass 140; ii) the symmetric fission component determined by the effect of the Z = 50 proton shell; and iii) the quasifission component, visible around Z = 20, 28 and N = 28, 50. In contrast to this reaction, the contribution of the quasifission component into the total mass distribution in the case of 64Ni + Is6W greatly increases. This observation is confirmed by the different behavior of the distributions for fission

338

fragments in the systems (see Figure 5c). In the mass region AcN/2i20 the distributions are similar for both reactions, while for the asymmetric mass region for 64Ni+ Is6W is higher than the one for 44Ca+ 'O'Pb. Our analysis shows that only a small part (-25%) of the fission cross section can be associated with complete hsion for the 64Ni+ Is6W system, the remainder should be attributed to quasifission. In the case of 44Ca+ *06Pb,the contribution of CNfission component into the total mass distribution is -70%.

5. Conclusion Fusion suppression and the presence of quasifission at energies near and below the Coulomb barrier were observed in the 48Ca+ '54Sm reaction on the deformed target nucleus. In the 48Ca+ 144Smreaction (on the spherical target nucleus), no evidence of quasifission was found at the same CN excitation energy and angular momentum as in the case with 154Sm.A pronounced quasifission component was observed for the more symmetric combination 64Ni + Is6W (a= 0.488), when compared with the 44Ca + 206Pbsystem (a= 0.648), leading to the same 25%0* CN. We point out that shell effects in the exit channel determine the main characteristics of fission-like fragments, as in the case of projectile-target combinations leading to superheavy CN systems [13]. References

W.Q. Shen et al., Phys. Rev. C 36, 115 (1987). R. Bass, Lect. Notes in Phys. 117, 2 18 (1 980). R.N. Sagaidak et al., Phys. Rev. C 68,014603 (2003). A.C. Berriman et al., Nature 143, 144 (2001). D.J. Hinde et al., Phys. Rev. C53, 1290 (1996). D. J. Hinde et al., Eur. Phys. J. A 13, 149 (2002). N.A. Kondratiev et al., in Dynamical Aspects of Nuclear Fission, Proc. of 4Ih Intern. Con$, casta-Papiernitka 1998, p.43 1 (WS, Singapore, 1999). 8 P. Moller et al., At. Data Nucl. Data Tabl. 59, 185 (1 995). 9 M.G. Itkis and A.Ya. Rusanov, Fiz. Elem. Chastits At. Yadra 29, 389 (1998). 10 I. Halpern and V.M. Strutinski, Proc. of the Second UN Intern. ConJ on the Peaceful Uses ofAtomic Energy, Geneva, 1957, p.408 (UN, Geneva, 1958). 1 1 M. Trotta et al., Nucl. Phys. A 734,245 (2004). 12 E.K. Hulet, Yad. Fiz. 57, 1 165 (1994). 13 M.G. Itkis et al., Nucl. Phys. A 734, 136 (2004).

1 2 3 4 5 6 7

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YIELDS OF CORRELATED FRAGMENT PAIRS IN THE REACTION 208Pb('80,F)OBTAINED IN y-y-y COINCIDENCE METHOD A. BOGACHEV', 0. DORVAUX2, E. KOZULIN', L. KRUPA', A. ASTIER', M. G.PORQUET', LDELONCLE', D.CURIEN2, G.DUCHENE2, B.J.P.GALL2, F.HANAPPE6, M.G.ITKIS', F.KHELFALLAH2,R.LUCAS', LPIQUERAS', M.ROUSSEAU*, M.MEYER', N.REDON', 0.STEiZOWSKI' AND L.STUTTGEZ JINR, Joliot-Curie 6, 141980, Dubna, Moscow region, Russia

' IReS, IN2P3-CNRS and Universite Louis Pasteur, 6703 7 Strasbourg, France ' CSNSM IN2P3-CNRS and Universite Paris-Sud, 91405 Orsay, France IPNL, INZP3-CNRS and Universite Claude Bernard, 69622 Villeurbanne Cedex, France

' CEA/Saclay, DSM/DAPNlA/SPhN, 91 191 Gif-sur-YvetteCedex, France 'Universite LIbre de Bruxelles, CP 229, B-1050 Bruxelles, Belgium Via spectroscopic method we aimed: - to investigate the symmetric and different asymmetric fission modes in the fission fragment mass distributions. - to obtain experimental information on the total y-ray energy and y-ray multiplicity event by event, which could give the access to the description of the prescission shapes of the fissioning nucleus at the saddle point.

1.

Introduction

We studied the fission process of light neutron deficient Th nuclei produced in the reaction 1 8 0 + 208Pb + 226Th. The beam energy was Elab = 85 MeV (the excitation energy of the compound nucleus is 32 MeV). The measurements were carried out using the y-ray spectrometer EUROBALL IV (Ge-detectors) coupled with the Inner Ball (BGO-detectors). The EUROBALL IV set-up [I], consisting of 36 Tapered detectors (single crystals), 26 Clover detectors (4 crystals in common cryostat), 15 Cluster detectors (7 crystals in common cryostat). In the center of the set-up, the Inner Ball detectors are placed. It consisted of about 210 units. 2.

Experiment

Thick ( 1 00 mg/cm2) enriched 208Pb target was used. A Doppler shift correction was not needed because fission fragments were completely stopped in the target.

340

To suppress the Compton scattering events, anti-Compton shield was used. The pairs of even-even isotopes from symmetric part (charge splits 44/46 and 42/48) were taken to analyze. In previous fusion-fission experiments it was demonstrated that by setting two conditions ("gates") on the y-ray energies, it is possible to identify fission fragments (both the mass and the charge) [3,5]. In the Figure 1 the "double gated" spectrum is shown.

I

I

I

I

I

I

The double gated spectrum for 112 Pd, gates on transitions (2'4') and (4+-2')

,,zF 61

mRu(2*-0'), ""Ru(2*-0') 6n 4n 41,6 keV '"Ru(4'-2*), 110Ru(4+-2') "Pd(10'-8')

731,7 keV

300

3

500

E,

1

Figure 1. The double gated spectrum for "2Pd. The gates are set on transitions 4'-2+ and 2+-O+

Two gates were set which correspond to transitions (2+-0+) and (4+-2+) to obtain this spectrum. The peaks corresponding to the complementary fragments (Ru isotopes) and to the higher transitions of 112Pd (up to 12+-lo+) are shown. Below (Fig. 2) the example of double gated spectrum is given for the same nucleus 1 12Pd, but the gates were set on the y-ray energies corresponding to the transitions (4+-2+) and (6+-4+). From this spectrum the independent yield of the isotope 112Pd is possible to obtain (if to sum the events in the peak corresponding to the transition (2+-0+). But it's worth noting that we didn't take into account the side-feeding contribution to the yield. In other words, we take only isotopes which suffered the y-ray transitions from the level at least 6+ of the rotational band. It is expected that for these even-even nuclei the side-feeding does not give very significant contribution to the yield (approximately 10%). Partly, for this reason we did not put the error bars in the yield distributions for isotopes that we took for analysis (Fig. 3).

341

The double gated spectrum for Pd, gates on transitions (6'-4') and ( 4 - p ) 1,5XIO'

+ y1

a

Ru(2*-0*). 'lo Ru(2 '-0')

lrn

0

l,(h10'

0,o

IOD

200

300

4*)

500

700

800

Elt keV 1 Figure 2. The double gated spectrum for "ZPd. The gates are set on transitions 6+-4+and 4'-2'.

I

-I-

RU

Figure 3. The relative independent yield of different isotopes of Pd, Ru, Mo and Cd.

As it was mentioned above, side-feeding is not included in these yields. Error

bars are under estimation. It should include the uncertainty of peak efficiency, energy calibration and side-feeding contribution.

342

M Ca.ma) Figure 4. The mass distribution and y-ray multiplicity for the reaction 1 8 0 + 2 0 8 ~ b+ 226Th.

Mass distribution and y-ray multiplicity distribution are well-known for this reaction (see Fig. 4). Those are the results of the previous experiment [4] for the investigation of the same reaction with the incident energy of 1 8 0 ions Etab=78 MeV. We suppose that it will be possible to restore partially the mass distribution for symmetric and asymmetric regions using y-y-y coincidence method and to restore the mean y-ray multiplicity and mean y-ray energy associated with the restored regions of mass distribution. It will give us the possibility to make any conclusions concerning the shape of the fissioning compound nuclei in the saddle point leading to a better understanding of the dissipation process. References 1 . J.Simpson, Z.Phys. A358, 139-143 (1997). 2. Z.Hu et al., NIM A419, 121-131 (1998). 3. D.C.Biswas et al., Eur.Phys.J. A7, 189-195 (2000). 4. G. G. Chubarian et al., Physical Review Letters 87,052701 (2001). 5. M. Houry et al.,Eur. Phys. J. A6,43-48 (1999).

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NEUTRON AND PROMPT GAMMA RAY EMISSION IN THE PROTON INDUCED FISSION OF 239NpAND 243AmAND SPONTANEOUS FISSION OF 252Cf L. KRUPA''7, G.N. KNIAJEVA', J. KLIMAN'97,A.A. BOGATCHEV', G.M. CHUBARIAN6,0. DORVAUX3,I.M. ITKIS', M.G. ITKIS', S. KKLEBNIKOV', N.A. KONDRATIEV', E.M. KOZULM],V. LYAPIN4>',T. MATERNA', W. RUBCHENIA4>',I.V. POKROVSKY', W. TRZASKA4,D. VAKHTIN', V.M. VOSKRESSENSKY' 'Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia 'Universite Libre de Bruxelles, PNTPM, CP229, BI 050, Bruxelles, Belgique 'Institut de Recherches Subatomiques, CNRS-INZP3, Strasbourg, France 4Deparment of Physics, University of Jyvaskyla, FIN-40351, Jpiiskyla, Finland 'V. G. Khlopin Radium Institute, St.-Petersburg 194021, Russia 6TexasA M Universiy, Cyclotron Institute, College Station, Texas, 77843-3366, USA 'Institute of Physics SASc. Dubravska cesta 9, 84228 Bratislava, Slovak Republic

Average prescission < MI" > and postscission < MI""' > neutron multiplicities as well as average y-ray multiplicity <MS, average energy < E S emitted by y-rays and average energy per one gamma quantum <E> as a function of mass and total kinetic energy (TKE) of fission fragments were measured in proton induced reactions p+'42Pu+243Am, p+'38U+'3%Ip (at proton energy E,=13, 20 and 5 5 MeV) and spontaneous fission of ''Cf. The solid angle aberration and the Doppler shift in the laboratory angular distribution of y-ray emission were utilized to obtain the number and energy of y-rays as functions of single fragment mass. The results in the case of "'Cf, for both average number and average energy as functions of single fragment mass, are characterized by a sawtooth behavior similar to that which is well known for neutron emission. The similar behavior is seen for proton induced fission of 23%Ipand '43Am. The fragment mass dependence < MY' > (m) and <MS(m) show a clear sawtooth structure that is gradually washed out with increasing proton energy E,. Using the response matrix techniques we were able to distinguish between the statistical dipole (E 1) and collective quadrupole (E2) yray emission of single fission fragments. 1.

Introduction

The investigation of light-particles-induced fission of heavy nuclei at intermediate energy is connected to large extent with the problem of understanding the fission process with increasing excitation energy of the compound nucleus. The measurement of prescission < M:x > and postscission

344

< M r ' > neutron and y-ray <My> multiplicities in coincidence with primary fission fragment mass and total kinetic energy (TKE) provides access to fission dynamic. The multiplicity < MY' > is a measure of the excitation energy distributed to both fragments, whereas the <My' is more closely related to the spin and deformation of fission fragments. At fission of nuclei the main part of the energy set free in the fission process is released in the form of kinetic energy of the fragments. Relatively large amount of energy remains just after scission as excitation of the fragments. The excitation being stored in the forms of fragment deformation, rotation and vibration and also as internal heat due to dissipation process from saddle to scission is released mainly by evaporation of neutrons and emission of gamma rays. The study of integral characteristics of y-ray emission in fission is necessary in order to obtain information on final steps of the deexcitation process of fission fragments. A spontaneous or excited compound nucleus, after neutron evaporation, decays towards the yrast line mostly by statistical dipole (E 1) and collective quadrupole (E2) y-ray emission. The first one is responsible for the major part of the cooling of the nucleus, while the second one slows down the rotation. The behaviour of the y-flow is governed to a large extent by the interplay of these two decay modes. By analyzing the multiplicity and energy of gamma rays one can derive information about this interplay and knowing the neutron multiplicity evaluate the absolute values of fragment spins. The dependence of gamma rays on the mechanism of formation and decay of compound heavy systems has been studied in this work in proton induced reactions p+242Pu+243Am,(with proton energy E,=13, 20 and 55 MeV), P+*~~U+*~%P (&=20 and 55 MeV) and spontaneous fission of 2 s 2 ~ f . 2.

Experimental techniques

The measurements were carried out at the Accelerator Laboratory, University of Jyvaskyla, (Finland) using a setup that included: the two-armed time-of-flight reaction products spectrometer CORSET built with the use of microchannel plates (MCP) [ 11; 0 a 8 detector time-of-flight neutron spectrometer DEMON; 0 a High Efficiency Neutron DEtection System (HENDES) facility; 0 six 76.2 x 76.2 mm NaI(T1) detectors for y-ray detection. The solid angle aberration and the Doppler shift in the laboratory angular distribution of y-ray emission were utilized to obtain the number and energy of y-rays as functions of single fragment mass. In the case of 252Cfthe techniques

345

from paper [2] was used. In proton induced fission we used the method presented in paper [3]. We generalized this method for the case when the angle between fission fragments in c.m. and lab systems are different. The response matrix techniques [4,5] was used to carry out the data processing of y-rays. This method makes it possible to acquire both the average y-ray multiplicity <MY> and the average total energy <E,> emitted by y-rays. The total efficiency as a function of energy and response matrix of the NaI(T1) detectors were taken from experimental calibration and simulation using EGSnrc code [6]. Separation of < MZm> from < MY' > was achieved under the assumption of isotropic emission in the respective source frames [7]. The error bars shown on subsequent figures of the data represent only the statistical counting uncertainty, which is the relevant quantity, when assessing the systematic trends seen in the data.

3.

Experimental results

We calculated average y-ray multiplicity <My>, average energy emitted by y-rays, average energy per one gamma quantum ,average statistical y-ray multiplicity < M;ut > , average statistical energy < E:@' > and average prescission < MP," > and postscission < MP,"' > neutron multiplicities. Table 1. Experimental results from this experiment. The second through tenth columns give the following quantities: second - the proton energy Ep; third - average prescission neutron multiplicities < MPm> ; fourth - average postscission neutron multiplicities < M Y > ; fifth -

<w;

-

average y-ray multiplicities <Mp;sixth average energy emitted by y-rays seventh - average energy per one gamma quantum