Astroparticle, Particle And Space Physics, Detectors And Medical Physics Applications (Proceedings of the 9th Italian Conference)

Editors

Michele Barone Emilio Borchi Andrea Gaddi Claude Leroy Larry Price Pier-Giorgio Rancoita Randal Ruchti

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Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Pmi eedings ol the 9th ( onference

Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Proceedings of the 9th Conference

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Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications Proceedings of the 9th Conference

Villa Olmo, Como, Italy

1 7 - 2 1 October 2005

Editors

Michele Barone Demokritos Athens, Greece

Emilio Borchi IN FN & University of Florence, Italy

Andrea Gaddi CERN, Switzerland

Claude Leroy Universite de Montreal, Canada

Larry Price ANL, USA

Pier-Giorgio Rancoita INFN-Milano, Italy

Randal Ruchti University of Notre Dame & US National Science Foundation, USA

\fc World Scientific N E W JERSEY • L O N D O N

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ASTROPARTICLE, PARTICLE AND SPACE PHYSICS, DETECTORS AND MEDICAL PHYSICS APPLICATIONS Proceedings of the 9th Conference Copyright © 2006 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher.

ISBN 981-256-798-4

Printed in Singapore by Mainland Press

Organizing Committee: E. Borchi C. Leroy L. Price P.G. Rancoita R. Ruchti

INFN and University of Florence University of Montreal ANL INFN-Milano (chairman) University of Notre Dame/US National Science Foundation

Industrial Organizing Committee: M. Barone A. Gaddi

Demokritos Athens CERN Geneva

International Advisory Committee: D. Abbaneo S. Baccaro P. Binko A. Breskin A. Capone C. Fabjan K. Freudenreich S. Giani V. Hagopian E. Longo E. Nappi S. Pospisil P.L. Riboni T.J. Ruth D. Saltzberg J. Seguinot V. Sossi S. Volonte C. Waltham

CERN Geneva ENEA-Rome Observatoire de Geneve & SynSpace SA Weizmann Institute INFN and University of Romel CERN Geneva ETH-Zurich CERN Geneva Florida State University INFN and University of Romel INFN-Bari CTU Prague ETHZ-Zurich TRIUMF UCLA College de France, Paris University of British Columbia, Vancouver ESA Paris University of British Columbia, Vancouver

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Plenary Session Organizers: A. Capone (INFN & Univ. of Romel) S. Giani (CERN Geneva) E. Nappi (INFN-Bari) L. Price (ANL) R. Rutchi (Univ. of Notre Dame) D. Saltzberg (UCLA) J. Seguinot (College de France) V. Sossi (Univ. of British Columbia) S. Volonte (ESA)

Astroparticle Experiments Symposium on GEANT4 Applications Advanced Detectors and Particle Identification High Energy Physics experiments Accelerator and Data/Networking Developments Astroparticle Experiments Advanced Detectors and Particle Identification Radiotherapy and Medical Imaging Space experiment

Parallel Session Organizers: D. Abbaneo (CERN Geneva) S. Baccaro (ENEA, Rome) S. Giani (CERN Geneva) P. Binko (Observatoire de Geneve & SynSpace SA) D. Saltzberg (UCLA) A. Capone (INFN & Univ. of Romel) C. Fabjan (CERN Geneva) V. Hagopian (Florida State Univ.) C. Leroy (Univ. of Montreal) E. Nappi (INFN-Bari) J. Seguinot (College de France) S. Pospisil (CTU, Prague) L. Price (ANL) V. Sossi (Univ. of British Columbia) S. Volonte (ESA)

Secretariat: A. Cazzaniga N. Tansini

Centro Volta, Como Centro Volta, Como

Tracking Devices Radiation Damage Software Applications Software Applications Astroparticle Experiments Astroparticle Experiments Miniaturization of Particle Detectors Poster Session Calorimetry (both sessions) Advanced Detectors and Particle Identification Advanced Detectors and Particle Identification Medical Application Instrumentation High Energy Physics Experiments Radiotherapy and Medical Imaging Space experiment

PREFACE The exploration of the subnuclear world is done through increasingly complex experiments covering a wide range of energy and performed in a large variety of environments going from particle accelerators, underground detectors up to satellites and space laboratory. The achievement of these research programs calls for novel techniques, new materials and new instrumentation to be used in detectors, often of large scale. Therefore, particle physics is at the forefront of technological advance and also leads to many applications. Among these, medical applications have a particular importance due to health and social benefits they bring to the public. The International Conference on Advanced Technology and Particle Physics is held every two years. The conference returned to the "Centro di Cultura Scientifica A. Volta" for the 2005 Edition. The Conference welcomed about 300 participants. These participants were representing more than 140 institutions from about 30 countries. The conference allows a regular review of the advances made in all technological aspects of the experiments at various stages: preparation, data taking or upgrade. The open and flexible format of the Conference is conducive to fruitful exchanges amongst participants, shows the progresses made and gives research directions. The medical sessions gave an interesting example of merging advanced technologies: particle physics and numerical techniques. Industries specialized in advanced technologies were present at the Conference through a presentation of their products (organized by M. Barone and A. Gaddi) Plenary and parallel sessions covered Advanced Detectors and Particle Identification (organized by E. Nappi and J. Seguinot), Miniaturizing of Particle Detectors (organized by C. Fabjan), Astroparticle Experiments (organized by D. Saltzberg and A. Capone), Calorimetry (organized by C. Leroy), Software Applications (organized by S. Giani and P. Binko), High Energy Physics Experiments (organized by L. Price), Accelerator and Data/Networking Developments (organized by R. Ruchti), Medical Application Instrumentation (organized by S.Pospisil), Radiotherapy and Medical Imaging (organized by V. Sossi), Radiation Damage (organized by S. Baccaro), Space Experiments (organized by S. Volonte), and Tracking Devices (organized by D. Abbaneo). This edition of the conference was combined with a symposium on the applications of the GEANT4 simulation software. The symposium was organized by S. Giani. The poster session has always received special attention by the organizers of the conference. This session was organized this year by V. Hagopian. The posters were a great success. The number of posters has more than doubled to about 45 since the 8th IC ATPP Conference in 2003. Having their own rooms was conducive of numerous discussions by various interested parties. This year, there were several innovations in the poster presentations. First, one afternoon was designated as the Poster Parallel Session and all the presenters requested to be near their poster. Second, it was requested attaching a small picture of the presenter to every poster so that interested

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parties could recognize the person for possible discussions. Many of the presenters also left copies of their paper on the poster board, so interested parties could pick up a copy. Most of the posters were very elaborate with excellent graphics, printed on photographic type glossy paper. The posters were divided into three general categories as follows: 1. Astroparticle, Underground Experiments, Space Physics and Cosmic Rays; 2. High-Energy Physics Experiments, Trackers, Calorimetry, Software and Data Systems; 3. Advanced Detectors and Medical Physics Applications. Each poster submission was reviewed for relevance to the conference and accepted or rejected. The final poster papers accepted for publications have been included in the appropriate section of the Proceedings, as function of their topic. From the quality of the material of the posters, as well as the poster presentation, it is clear that a lot of effort went into the preparation. We would like to thank the poster presenters for their considerable effort that made the posters a truly important part of this meeting. We would like to thank the staff of the Centro A. Volta for the excellent support provided to the Conference organization at Villa Olmo. In particular, we would like to extend our appreciation and thanks to the Secretariat of Centro di Cultura Scientifica A. Volta for their help and efficiency with the organization of the Conference and its running. We are particularly indebted to Dr G. Castiglioni (President of Univercomo), who, by providing a sponsorship, allowed the Conference Organizers to largely reduce or waive the registration fees for the Ph.D. students. The organizers would like to thank the strong support of INFN and ESA which made the conference possible. Finally, we would like to thank the speakers for the high quality of their contributions and the participants for their enthusiasm in attending the Conference and contribution to the discussions.

Michele Barone Emilio Borchi Andrea Gaddi Claude Leroy Larry Price Pier-Giorgio Rancoita Randal Ruchti December 2005

CONTENTS

Preface

vii

Advanced, Miniaturized Detectors and Particle Identification A Novel Micromegas Detector for In-core Nuclear Reactor Neutron Flux Measurements The ALICE TPC The ATLAS RPC Test Stands Performances in High Magnetic Fields of Fine-mesh Photomultipliers for Fast Time-of-flight Detectors Recent Results on GridPix Detectors: An Integrated Micromegas Grid and a Micromegas Ageing Test RICH Detector at Jefferson Lab, Design, Performance and Physics Results Hybrid-Photon-Detectors in the LHCb RICH System Development of a Fast Transition Radiation and Tracking Detector for CBM at FAIR R&D on a Detector for Very High Momentum Charged Hadron Identification in ALICE The Design and Test of the ATLAS Diamond Beam Conditions Monitor The AMS02 Transition Radiation Detector (TRD) - A Gasfilled Detector for the International Space Station A New Automatic Microscope for High Speed Analysis of Nuclear Emulsions A Novel Type of Proximity Focusing RICH Counter with Multiple Refractive Index Aerogel Radiator A Subminiature Scintillation Detector for Catheter Operation . . . Analysis of Test-beam Data from a Prototype LHCb RICH Detector Single Crystal CVD Diamond Detectors for Hadron Physics Improvement of Particle Identification by Energy Loss in a Stack of Silicon Detectors Studies for a Fast RICH Properties of Scintillator Strip with Wavelength Shifting Fiber and Silicon Photomultiplier IX

1 3 10 21 26 31 36 42 47 54 60 66 71 77 82 87 92 98 103 108

X

Astroparticle Experiments The Underground Baradello Laboratory: Characterization of the Site and Early Results on Gamma-Ray and Neutron Spectrometry Measurements The Art of Detecting Neutrinos, 2 km Underground - The Three Phases of the Sudbury Neutrino Observatory Neutrino Physics at the South Pole - Recent results from the AMANDA Experiment First Results from GLAST-LAT Integrated Towers Cosmic Ray Data Taking and Montecarlo Comparison Background Analysis of Cuoricino in View of the Future Experiment CUORE Application of Nuclear Track Detectors in Astroparticle and Nuclear Physics Recent DAMA Results Double Beta Decay Measurement with COBRA Ultraviolet Diamond Photodetector Status of the KATRIN Experiment The RPC System for the OPERA Spectrometers Electromagnetic Shower Reconstruction in Emulsion Cloud Chamber A Low Background Micromegas Detector for the CAST Experiment Imaging Optical Adapters for Multi-Anode Photo Multipliers Detectors A 2D Stochatsic Montecarlo for the Solar Modulation of GCR: A Procedure to Fit Interplanetary Parameters Comparing to the Experimental Data Spatial Distribution of Energetic Heavy Ions near the Earth . . . . A Photon Tag Calibration Beam for the AGILE Satellite Observation Program of Isotope Composition in the Ultra Heavy Cosmic Rays High-Energy Cosmic Rays Investigated by Air-shower Measurements with KASCADE, KASCADE-Grande, and LOPES Scintillating Optical Fibers for Astroparticle Physics The T2K Experiment: Status and Instrumentation of the 280m Near Detector Results and Status of the PICASSO Experiment Data Acquisition System and Trigger Electronics for CACTUS . . . The Central Pixel of the MAGIC Telescope for Optical Observations Status report of ARDM Project: a New Direct Detection Experiment, Based on Liquid Argon, for the Search of Dark Matter

113

115 122 132 139 144 149 158 168 173 178 185 190 196 201

206 212 217 223 229 239 245 253 262 267 272

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Development of Multiple Photon Counting Coincidence (MPCC) Technique for Characterisation of Scintillators for Cryogenic Applications Results from Cuoricino Experiment and Prospects for CUORE . . . Timing Calibration of the NEMO Optical Sensors Design, Production, and First Results from the IceCube Digital Optica] Module

298

Calorimetry

305

Study of Electromagnetic Calorimeter Trigger-Primitive Performances for CMS Data Selection The CMS Electromagnetic Calorimeter The CMS ECAL Laser Monitoring System Pointing Calorimeter for Measuring KQL —> ifivv Decay and Development of Extruded Scintillator CMS Preshower In-situ Absolute Calibration Dedicated Front-End Electronics for an ILC Prototype Hadronic Calorimeter with SIPM readout Using Single Photoelectron Spectra in the Calibration of the CMS-HF Calorimeter Jet Energy Scale Determination for the Run II D0 Calorimeter . . . The Hadron Calorimeter of the CMS Experiment at the Large Hadron Collider Properties of a 256-channel Prototype of the ALICE/PHOS Calorimeter Source Calibration of the CMS Quartz Fiber Forward Calorimeter . The Calibration Strategy of the CMS Electromagnetic Calorimeter . The Very Front-End Cards for the CMS Electromagnetic Calorimeter: Description, Calibration and Performance Uniformity of Response of the ATLAS Electromagnetic Calorimeter Series Modules Plans for the Very Forward Region of ATLAS - the LUCID Luminosity Monitor Fast, Long-Wavelength Scintillators and Waveshifters Test Beam Results of the CMS Electromagnetic Calorimeter . . . . Electron and Pion Results from the ATLAS Foward Calorimeter 2003 Test Beam Linearity, Energy and Position Resolution of the ATLAS Electromagnetic Calorimeter Series Modules Preshower Silicon Strip Detectors for the CMS Experiment at LHC Test Beam Results of the CMS Forward Quartz Fibre Calorimeter .

277 282 292

307 312 318 323 328 333 338 343 348 353 358 363 369 374 379 389 395 401 406 411 416

XII

Inter-Calibration and Offline Compensation Studies of the ATLAS Calorimeter Using Beam Tests and Monte Carlo

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G E A N T 4 and Software Applications

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Symposium on the Applications of the GEANT4 Simulation Software GEANT4 Applications for NASA Space Missions GEANT4 Application to Hadrontherapy in Japan AMS02 Italian Data Transfer System: Real Life Experience Application of GEANT4 in the Development of New Radiation Therapy Treatment Methods G4 Accelerator Applications A Full Monte Carlo Simulation of Silicon Strip Detectors Simulation of Radiation Monitors for Future Space Missions . . . . Electromagnetic Physics Modeling in GEANT4 Package Perspectives in Medical Applications of Monte Carlo Simulation Software for Clinical Practice in Radiotherapy Treatments GEANT4 Monte Carlo Simulation to estimate Gamma-Ray Emissions from Lunar Surface and SELENE Spacecraft CORAL, a Software System for Vendor-Neutral Access to Relational Databases GEANT4 Simulation of the ATLAS Muon Spectrometer Track Extrapolation with Intrinsic Navigation in the New ATLAS Tracking Scheme Applications of GEANT4 for the ESA Space Programme Detector Simulation in High Energy Physics

427 429 437 444

High Energy Physics

531

Fast Shower Parametrisation for Electromagnetic Cascades Technical Aspects of the Manufacture of 4 Grease Pad Assemblies for the CMS Experiment Implementation and Performance of a Tau Lepton Selection within the ATLAS Trigger System at the LHC ATLAS Detector Simulation: Status and Outlook The ATLAS Experiment at CERN: Two Years Before the Start of the Data Taking Progress in Grid Computing for Particle Physics Design, Production and Quality of the Sensors for the Silicon Strip Tracker of CMS Running MINOS

533

451 462 470 475 480 485 490 495 500 506 511 519

539 546 551 556 565 576 590

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ATLAS Inner Detector Results from the 2004 Combined Test Beam Data The CMS Muon System Performances of the ATLAS Level-1 Muon Trigger Processor in the Barrel Certification, Installation and Commissioning of Muon Drift Tube Chambers for the CMS Central Muon Detector The LHC Collider: Status and Outlook to Operation PANDA: A Detector for Research with Antiprotons Test of QED with the Reaction e+e" -> 77(7) Muon Reconstruction and Identification for the Event Filter of the ATLAS Experiment

Medical Applications a n d I n s t r u m e n t a t i o n s Spectral and Spatial Distribution of the Scattered Radiation of an Electron Radiotherapy Beam A Liquid Ionization Chamber as Monitor in Radiotherapy The Mammographic Head Demonstrator Developed in the Framework of the "IMF Project: First Imaging Tests Results Optimized Infrared Detectors for Infrared Synchrotron Radiation Microspectroscopy and Biomedical Imaging MATRIX: An Innovative Pixel Ionization Chamber for On-line Beam Monitoring in Hadrontherapy Development of a Dual Tracer PET Method for Imaging Dopaminergic Neuromodulation Rethinking Positron Emission Technology for Early Cancer Detection Positron Emission Tomography - Tracer Kinetic Modelling in Drug Development 3D-Reconstruction of Absorbed Dose Obtained from Gel-DosimeterLayers Accurate Determination of Radionuclidic Purity and Half-Life of Reactor Produced Lu-177g for Metabolic Radioimmunotherapy . . . Spatial Linearity Improvement for Discrete Scintillation Imagers . . High Resolution, High Sensitivity Detectors for Molecular Imaging of Small Animals and Tumor Detection Strip Ionization Chamber as Beam Monitor in the Proton Therapy Eye Treatment Low Dose, Low Energy 3D Image Guidance During Radiotherapy . Alpha Cyclotron Production Studies of the Alpha Emitter 2 n At/ 2 1 l 5 , Po for High-LET Metabolic Radiotherapy

600 605 616 621 626 637 643 648

653 655 661 666 671 677 682 692 697 705 710 715 720 725 732 742

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Treatment Planning with IVIS Imaging and Monte Carlo Simulation Monte Carlo Simulations of a Human Phantom Radio-Pharmacokinetic Response on a Small Field of View Scintigraphic Device Applications of the Monte Carlo Code GEANT to Particle Beam Therapy Charge Sharing in Pixel Detectors for Spectroscopic Imaging . . . . Direct Thickness Calibration: Way to Radiographic Study of Soft Tissues A portable Pixel Detector Operating as an Active Nuclear Emulsion and Its Application for X-Ray and Neutron Tomography

747 752 758 768 773 779

Radiation Damage

785

Statistical Study of Radiation Hardness of CMS Silicon Sensors . . SIC PbWC-4 Crystals for the Electromagnetic Calorimeter of CMS Experiment MDT Chamber Ageing Test at ENEA Casaccia Neutron and Gamma Facilities Behavior of Thin Film Materials Under 7 Irradiation for Astronomical Optics Full Characterization of Non-Uniformly Irradiated Silicon Micro-Strip Sensors Beam Energy Monitor for 4-10 MeV Electron Accelerators Optical Link of the ATLAS Pixel Detector Ion Electron Emission Microscopy for SEE Studies An Analysis of the Expected Degradation of Silicon Detectors in the Future Ultra High Energy Facilities Investigation of VLSI Bipolar Transistors Irradiated with Electrons, Ions and Neutrons for Space Application Radiation-Hardness Studies of High OH~ Content Quartz Fibres Irradiated with 24 GeV Protons Development of Radiation Hard Silicon Detectors: The SMART Project Scintillator and Phosphor Materials: Latest Developments and Applications Crystals for High-Energy Physics Calorimeters in Extreme Environments Radiation Testing of GLAST LAT Tracker ASICS Magnetic Field and Radiation Tests of a Programmable Delay Line LCFI Charge Transfer Inefficiency Studies for CCD Vertex Detectors

787 792 797 802 807 812 817 822 827 832 838 843 850 857 864 871 876

XV

Photoluminescence and 7-Ray Irradiation of SrO-B203-P20s:Eu 2+ and SrMo0 4 :Eu 3 + Phosphors

883

Space Experiments

889

The Extreme Universe PAMELA Data Acuisition System AGILE MCAL, the Mini-Calorimeter Primary Helium CR Inside the Magnetosphere: A Transmission Function Study Lanthanum Halide Scintillators and Optical Fiber Readout for XRay/Gamma-Ray Astronomy and National Security Applications . The AGILE Silicon Tracker: Construction and Calibration Results . Ions Abundance Close to the Earth Surface: The Role of the Magnetosphere Design of a Silicon Transition Radiation Detector (SiTRD) for Accelerators and Space Applications The Origin of Helium-3 Isotope Enhancement in the Magnetosphere Observed by TSUBASA Satellite The Microscope Mission and Pre-Flight Performance Verification . . Performance of the Integrated Tracker Towers of the GLAST Large Area Telescope Performance of Neutron Detector and Bottom Trigger Scintillator of the Space Instrument PAMELA High Granularity Silicon Beam Monitors for Wide Range Multiplicity Beams Environmental Test Activity on the Flight Modules of the GLAST LAT Tracker The Time of Flight Detector and Trigger for the PAMELA Experiment in Space Fundamental Physics in ESA's Cosmic Vision Plan The AMS-02 Electronics System Launch in Orbit of the Space Telescope PAMELA and Ground Data Results CZT Detector Development for New Generation Hard-X/GammaRay Astronomical Instruments

891 898 904

Tracking Devices

1005

A Barrel IFR Instrumented with Limited Streamer Tubes for BaBar Experiment

909 917 922 928 935 940 945 952 957 962 968 973 979 989 994 999

XVI

A New Inner Layer Silicon Strip Detector for DO Calibration System for the Silicon Drift Detector of the ALICE Experiment Experience with the Resistive Plate Chamber in the BaBar Experiment Muon Identification and Reconstruction in the ATLAS Detector at theLHC Micrometric Position Monitoring Using Fiber Bragg Grating Sensors in Silicon Detectors Commissioning of the CMS Tracker Outer Barrel New Effects Observed in the BaBar Silicon Vertex Tracker: Interpretation and Estimate of Their Impact on the Future Performance of the Detector Tests of Substructures of the CMS Silicon Strip Tracker Test, Qualification and Electronics Integration of the ALICE Silicon Pixel Detector Modules Layout and Status of the CMS Silicon Tracker The CMS-Tracker Detector Controls System Optimization of the Readout Pad Geometry for a GEM-based Time Projection Chamber Tracking Strategy and Performance for the ATLAS High Level Triggers The LHCb Silicon Tracker The HI Silicon Tracker The LHCb Muon System Two- and Three-Dimensional Reconstruction and Analysis of the Straw Tubes Tomography in the BTeV Experiment Cryogenic Operation of Edge-Sensitive Silicon Microstrip Detectors The DEPFET Active Pixel Sensor as Vertex Detector for the ILC . Final Assembly and Intergration of the ATLAS Semiconductor Tracker and Transition Radiation Tracker 3-Dimensional Position Control for the AMS-02 Tracker with Infrared Laser Beams Development of a GEM-based High Resolution TPC for the International Linear Collider STAR Inner and Forward Tracking Upgrade

1012

1126 1131

List of participants

1137

1017 1022 1027 1032 1038

1044 1049 1054 1060 1067 1072 1077 1082 1087 1092 1098 1104 1109 1114 1120

Advanced, Miniaturized Detectors and Particle Identification Organizers: E. Nappi and J. Seguinot (Advanced Detectors and Particle Identification)

C. Fabjan (Miniaturizing of Particle Detectors)

S. Andriamonje J. Baechler M. Bianco M. Bonesini M. Chefdeville E. Cisbani J.M. Dickens C. Garabatos A. Gallas A. Gorisek J. Hattenbach M. leva P. Krizan L. Montani M. Patel M. Pomorski M. Regler

A Novel Micromegas Detector for In-core Nuclear Reactor Neutron Flux Measurements The ALICE TPC The ATLAS RPC Test Stands Performances in High Magnetic Fields of Fine-mesh Photomultipliers for Fast Time-of-fiight Detectors Recent Results on GridPix Detectors: an Integrated Micromegas Grid and a Micromegas Ageing Test RICH Detector at Jefferson Lab, Design, Performance and Physics Results Hybrid-Photon-Detectors in the LHCb RICH System Development of a Fast Transition Radiation and Tracking Detector for CBM at FAIR R&D on a Detector for Very High Momentum Charged Hadron Identification in ALICE The Design and Test of the ATLAS Diamond Beam Conditions Monitor The AMS02 Transition Radiation Detector (TRD) - A Gasfilled Detector for the International Space Station A New Automatic Microscope for High Speed Analysis of Nuclear Emulsions A Novel Type of Proximity Focusing RICH Counter with Multiple Refractive Index Aerogel Radiator A Subminiature Scintillation Detector for Catheter Operation Analysis of Test-beam Data from a Prototype LHCb RICH Detector Single Crystal CVD Diamond for Hadron Physics Improvement of Particle Identification by Energy Loss in a Stack of Silicon Detectors 1

2

F. Sozzi E. Tarkovsky

Studies for a Fast RICH Properties of Scintillators Strip with Wavelength Shifting Fiber and Silicon Photomultiplier

A NOVEL MICROMEGAS DETECTOR FOR IN-CORE NUCLEAR REACTOR NEUTRON FLUX MEASUREMENTS S. ANDRIAMONJE*, S. AUNE, A. GIGANON, I. GIOMATARIS, J. PANCIN, M. RIALLOT CEA Saclay DSM/DAPNIA

91191 Gif-sur-Yvette,

France

C. BLANDIN, S. BREAUD, B. GESLOT, C. JAMMES CEA/DEN/Cadarache,

13108 Saint-Paul Lez

Durance-France

Y. KADI, L. SARCHIAPONE CERNCH1211

Geneva-Switzerland

G. BAN, P. LABORIE, J.F. LECOLLEY, J.C. STECKMEYER, J. TILLIER IPC - CNRS/1N2P3 - ENS1CAEN - UCBN, 6 bd MarechalJuin,

14050 Caen, France

R. ROSA ENEA-Casaccia,

Via Anguillarese,

00060 Roma, Italy

G. ANDRIAMONJE IXL - Universite Bordeaux 1 F-33405 Talence Cedex, France

Future fast nuclear reactors designed for energy production and transmutation of nuclear wastes need new neutrons detectors able to measure the neutron flux over a large energy range from thermal energies to several MeV. A novel compact and very small detector, named Piccolo-Micromegas has been developed for this purpose. Description of the detector configuration especially dedicated to neutron detection inside nuclear reactor is given. The advantage of this detector over conventional neutron flux detectors and the results obtained with the first prototype are presented.

1. Introduction Micromegas is a gaseous detector [1] that has been developed initially for tracking in high rate high-energy experiments. At present, Micromegas detector is used in many experiments [2,3] and due to its high performances is being " Corresponding author: sandriamoniefecea.fr

3

4

employed for searching rare events such as the Solar axion CAST [4] and also for neutron detection [5,6]. The detection principle is simple: the gas volume is separated in two regions by a thin micromesh, the first one where the conversion and drift of the ionization electrons occur and the second one, 50-150 micron thick, where the amplification takes place. In the amplification region, a high field (40 to 70 kV/cm) is created by applying a voltage of a few hundred volts between the micromesh and the anode plane, which collects the charge produced by the avalanche process. The anode can be segmented into strips or pads. One of the main advantages of Micromegas is its robustness and its high resistance to radiations. These qualities have been exploited to develop a new detector to be used in nuclear reactor environment: operation with very high neutron flux, high gamma ray background and possibly at high temperature. In order to be placed inside an empty rod of a reactor (see Figure la) the detector needs to be compact, sealed and very small (3.5 cm x 3.5 cm x 3.5 cm), contrary to the usual Micromegas detector used in particle physics experiments hence the origin of the Piccolo-Micromegas name. After a short description of motivations, the full description of PiccoloMicromegas is given. The advantage of this detector over conventional neutron flux detectors and the results obtained with the first prototype at the CELINA 14 MeV neutron source facility at CEA-Cadarache are presented.

2. Motivations Fast nuclear reactors and Accelerator Driven Systems (ADS) are considered as an alternative for energy production and transmutation of nuclear wastes. One important step needed for approval of a demonstrator is the experimental validation of simulations. Of particular interest is the determination of the neutron spectrum (i.e neutron flux as a function of the neutron energy) for different configurations of the subcritical device. As well known, the neutron flux in ADS consists of neutrons produced via spallation reactions in the target and fissions from the multiplying blanket. Unfortunately the neutron spectra cannot be measured using only one type of detector. To cover the complete energy range of the produced neutrons, a new neutron detector concept based on Micromegas technology has been developed. For illustration the possible subcritical configuration based in TRIGA-ADS project which consists of coupling a 1 MW TRIGA reactor with a 140 MeV proton accelerator has been chosen [7]. The core has a cylindrical shape, the fuel

5

rods being arranged in seven concentric rings labeled A to G. The central rod and the rods named ring B are reserved for the spallation target and its cooling system. The measurement can be only achieved starting from ring C. The distribution of neutrons in three-dimensional space as a function of energy and time is simulated using innovative simulation codes (FLUKA and EA-MC [8,9] at CERN, and MCNP-4C [10] at ENEA/Casaccia). b)

a)

35 mm 1 0 ' 3 10" 2 10" 1 10°

101

102

103

104

105

106

10r

Neutron energy (eV) b)

Figure 1. a) View of the detector Piccolo-Micromegas placed inside the empty rod of TRIGA Neutron flux spectra at selected locations of the TRIGA-ADS reference configuration

An example of the neutron energy spectrum for different regions of the core for k src = 0.97 is given in Figure lb. The measurement of neutron flux spectra is a necessary step in order to characterize the neutronics of the system.

3. Description of Piccolo-Micromegas It has been demonstrated, from the results of CERN n_TOF experiments [11], that a Micromegas detector equipped with an appropriate solid neutron converter is well adapted for neutronic studies providing a fast response, a large dynamic energy range and a low sensitivity to gamma-ray background. This method has been extended to the measurement of neutron flux in a nuclear reactor core. Fissile elements such as 235U, 232Th are used simultaneously as neutron/charged particle converter in addition to B and recoil ions of the gas (Ar + iC4Hi0 quencher) filling the detector. The Piccolo-Micromegas detector shown in Figure 2a is designed to cope with severe constraints encountered in a nuclear reactor environment (high radiation, high temperature, neutron activation, sealed detector). For this reason

6

the detector structure, frames and sensitive elements have been carefully chosen and most of them are made out of stainless steel and ceramics. The detector consists of a drift electrode, a thin stainless steel cathode grid and four anode pads connected to fast amplifiers. The drift gap is 1 mm wide and the amplification gap 160 jim. In a second prototype, different amplification gaps were used in each compartment, in order to adapt the dynamic range of the collected charge (see paragraph 4.1). Because of the high reactor temperature and the high radiation yield, the electronics were placed outside and connected to the anode pads through special radiation hardened and low capacitance coaxial cables 10 meter long.

Figure 2. a) Schematic view of the Piccolo-Micromegas detector for neutron flux measurements inside a nuclear reactor, b) Variation of the amplitude of the signal of one PAD of PiccoloMicromegas detector versus High Voltage (HV) of the PAD for fixed Mesh Voltage at +110 V, the drift cathode in the ground and 1 bar Ar+iC4Hio.

The drift electrode is composed of four neutron/charged particle converters: • l0B for thermal and epithermal neutrons, • 235U for monitoring as the fission cross section of 235U from thermal energies up to several MeV is well known, • 232Th with a fission threshold of about 1 MeV is dedicated to high energy neutrons. The elastic scattering of neutrons with light atoms from the gas filling the detector (hydrogen) will produce nuclear recoils than could be detected with a typical threshold of several keV. The use of the Micromegas read-out was motivated by the following advantages:

7 •

a low sensitivity to gamma background has been demonstrated by many experiments [11,14] • a high radiation resistance [ 12] • a large dynamic energy range [11] allows for measuring simultaneously neutrons, alphas, fissions and recoil ions. • a very good robustness Using four converters with a unique detector will permit extracting practically on line a large range of the neutron flux spectrum in a specific position in the reactor. The large dynamic range of Piccolo-Micromegas will permit precise measurements and a detailed scanning of the flux into the whole reactor volume. The use of the pre-mixed gas of 4He + iC4Hi0 or other quencher like CH4 combined with the four neutron/charged particle converters before mentioned allows to obtain a large part of the neutron spectra of fast nuclear reactor and is well adapted to the requirements of the future ADS. At very high counting rate (>100 MHz) measurement will be performed on a current mode basis. At low counting rate, the fast response of the detector will allow counting one by one the incident particles using a low noise fast-preamplifier. It opens the way to measure the neutron flux at the peripheral part of the reactor and also in some cases when the full reactor power is not used. 4. Experiments and First results 4.1. Characteristics of Piccolo-Micromegas detector 55

Fe X-ray source and two types of gas mixtures (Ar-10%CO2 and Ar2%iC4Hio) have been used to determine the main characteristics of PiccoloMicromegas. The pressure of the sealed detector has been set to 1 bar. In order to adapt the dynamic range of the collected charges, two methods have been used: 1. Different amplification gaps in each compartment : this is ensured by the use of different thickness of anode pad. 2. To have more flexibility with the amplification gap, an identical amplification gap for the four pads (160 um) has been chosen. The drift electrode is grounded. The mesh and the four reading pads have been individually positively polarized. The signal from each pad is collected and shaped by a homemade fast amplifier. An oscilloscope-PC has been used for the data acquisition. The energy resolution of the detector at 5.9 keV 55Fe X-ray is about 20%. The gain of the detector as a function of the voltage applied on the different pads has been

8

measured (Figure 2b). The results show that a very low polarization of the pad is largely sufficient for the observation of the signal from fission fragments (several MeV compared to 5.9 keV energy deposited by the X rays of the 55Fe radioactive source).

4.2. Experiments and results with neutron source A first test with sealed prototype of the detector has been performed with 14 MeV neutrons given by the Cadarache CELINA facility. An example of the pulse obtained from the fission fragment emitted in the interaction of neutron s with 235U is shown in figure 3a). This figure shows clearly the contribution of the detected electrons (fast part of the pulse) and ions (slow part) with a total width of 150 nsec. A special routine has been developed in Matlab Code [13] to analyze and clean the data for possible spark and saturation. 1 a),:. 5" 0 £ -5

Pi

n c

1j

3

_ii

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k r

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|

•200

200

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400

600

40

60

80

Amplitude (mV)

Figure 3. a) Example of the fission fragment pulse b) Amplitude spectrum of 235U fission fragments

An example of the spectrum of 235U fission fragment as a function of the amplitude of the pulse is reported in figure 3b). A full simulation of the PiccoloMicromegas detector placed inside the CELINA-Cadarache neutron source facility has been performed using FLUKA code [8]. The study has been centered on the particular case of 235U, because of the well-known fission cross section. For 50 |j,g of U the measured fission rate given in figure 3b) is in good agreement with the prediction (4 fissions/s). 5. Conclusion A new miniaturized Micromegas has been developed for the measurement of the neutron flux spectra in nuclear reactor. The first results presented here are obtained using a neutron source facility. The next step will be the measurement inside a nuclear reactor. Further improvements are necessary one of them is the

9 use of special connectors and cables, in order to operate the detector at high temperatures, up to 300°C. References 1. 2. 3. 4. 5. 6.

7. 8. 9.

10.

11. 12. 13. 14.

I. Giomataris et al, Nucl. Instr. and Meth. A 376 (1996) 29. I. Giomataris, Nucl. Instr. and Meth. A 419 (1998) 239. G. Charpak et al, Nucl. Instr. and Meth. A 478 (2002) 26. K.Zioutas et al, Phys. Rev. Letters, 94,121301, (2005) S. Andriamonje et al, Nucl. Instr. Meth. A 481 (2002) 36 S. Andriamonje et al, Advances in Neutron Scattering Instrumentation. Edited by Anderson, Ian S.; Guerard, Bruno. Proceedings of the SPIE, Volume 4785, pp. 214-225 (2002). The TRADE Working group, "TRiga Accelerator Driven Experiment (TRADE)" Feasibility Report. ENEA Report, March 2002. A. Fasso, et al., "Intermediate Energy Nuclear Data: Models and C", Proceedings of a Specialist's Meeting, Issy les Moulineaux (France) 1994 C. Rubbia, et al., "Conceptual Design of a Fast Neutron Operated High Power Energy Amplifier", CERN/AT/95-44 (EET) Geneva, September 29, 1995. Y. Kadi, et al., "The EA-MC Monte Carlo Code Package", in Proc. of the 5th Int. Meeting on Simulating Accelerator Radiation Environment — SARE-5: Models and Codes for Spallation Neutron Sources, Paris, France, July 17-18, 2000. J. F. Briesmeister, Editor ^"MCNP™, General Monte Carlo N-Particle Transport Code, Version 4C", Los Alamos National Laboratory report LA13709-M, March 2000). J. Pancin, et al., Nucl. Instr. and Meth. A 524 (2004) 102 G. Puill, et al., IEEE Trans. Nucl. Sci. NS-46 (6) (1999) 1894 http://www.mathtools.net/MATLAB/ S. Boyer, et al, Proceeding of GLOBAL 2005, Tsukuba, Japan, Oct 9-13, 2005

THE ALICE TPC JOACHIM BAECHLER European Organization for Nuclear Research, CERN, Department of Physics 1211 Geneva 23, Switzerland for the ALICE collaboration The TPC is the main tracking detector in the central region of the ALICE experiment. This paper describes the main components of the ALICE TPC and addresses the specific technological challenges. Furthermore, the results of comprehensive system tests are summarized.

1. Introduction ALICE [1] is one of the four experiments presently under construction for the Large Hadron Collider (LHC) at CERN in Geneva. ALICE is a general-purpose heavy-ion experiment designed to study the physics of strongly interacting nuclear matter and the quark-gluon plasma in nucleus-nucleus collisions. At a maximum cms energy of 5.5 TeV per nucleon in Pb-Pb collisions, up to 8000 charged particles per unit rapidity are produced within a central collision. Owing to the enormous particle multiplicity per event, very specific requirements are made on the performance of the detectors, the electronics and the data acquisition. The ALICE TPC, operated in a solenoid of 0.5 T, is the main detector of the ALICE experiment for tracking, momentum measurement and particle identification of charged particles. In conjunction with the Transition Radiation Detector (TRD), the Inner Tracking System (ITS), Cherenkov counters (HMPID) and Multi-gap Resistive Plate Chambers for time of flight (TOF), the TPC will provide identification of leptonic and hadronic particles in the momentum range from 0.5 to 10 GeV/c. 10

11 The specific requirements on the TPC for hadronic physics are: • Good two track separation • dE/dx resolution of better than 10% • Track matching with 85-95% efficiency, using inner and outer detectors. In combination with the ITS and the TRD, the TPC needs to satisfy the following criteria for electron physics: • • •

Track finding efficiency for tracks with p, > lGeV/c of > 90% . Momentum resolution for electrons of 2 % with a momentum of~4GeV/c. Rate capability for central collisions to be at least 200Hz.

1.1. TPC - Main Components The ALICE TPC is composed of a cylindrical gas volume (barrel), divided into two half volumes of equal size, separated by a 30um thick HV electrode to generate the drift field. It has an inner radius of about 80 cm and an outer radius of about 280 cm, with an overall length of 500 cm in the beam direction (Figures 1 and 2). At both ends of the barrel, conventional multi-wire proportional chambers with pad readout are mounted into end plates with 18 trapezoidal sectors each.

HVBtoctrode(i'''KV] field cage *^^

readout chamber

jtiSHBl

M&BSBt

Figure 1. Layout of the ALICE TPC showing the central electrode, the field cage and the end plates supporting the readout chambers.

12 The front-end (FE) electronic cards with an optical interface to the DAQ are connected to the backplane of the readout chambers via ~8cm long flat Kapton cables.

Figure 2. Photography of the ALICE TPC positioned in the clean room with readout chambers mounted, in preparation for electronics installation.

1.2. Field Cage To keep the structural material (mass) at a minimum, the field cage is composed of two outer cylinders (outer containment vessel, outer field cage vessel) and two inner cylinders (inner field cage vessel, inner containment vessel). The drift gas is confined in the outer and inner field cage vessel; the volume created between the containment- and field cage vessels is filled with C0 2 and serves as an electrical insulator. At the center of the of the field cage the central electrode of 30um thickness is strung and mechanically supported by the outer and inner field cage cylinders, to generate the drift field in the two half volumes. Two opposite axial potential degraders with potential strips create a uniform drift field on either side of the central electrode. A unique feature of this field cage is the method to remove the potential strips from the field cage surface and wind them instead around support rods held away from the surface. This avoids instabilities and discharges over the huge surface of the field cage cylinder walls.

13

In total, 18 such axial rods, in line with the 18 sectors of the endplate, support the potential strips over a length of 250 cm in either direction. This is shown in Figures 3 and 4.

Figures 3 and 4. TPC field cage prototype with the six support rods and potential strips (left), and the ALICE TPC field cage (right) with 18 support rods plus strips; the TPC is mounted vertically on one endplate with an opening for a readout chamber.

To ensure minimal multiple scattering and low secondary particle production, the material budget of the TPC field cage components is kept as low as possible. Furthermore, the oxygen content of the drift gas has to be maintained below 5ppm in order to assure good detector efficiency. Consequently, we use, wherever possible, only low-mass composite material providing sufficient mechanical rigidity and the required leak tightness. After assembly, the field cage was leak-tested with dummy chambers in the end plates. An oxygen level of < 5ppm was measured with one volume exchange of gas per day. It is a major challenge to guarantee the mechanical stability and mounting precision of 250um for the central electrode and the readout planes within the entire assembly. With detailed photogrammetry, before and after chamber installation, it was shown that the entire detector stayed within the required mechanical tolerances. 1.3. Readout Chambers The ALICE TPC readout [1] is based on conventional multi-wire proportional counters with cathode pad readout. The segmentation of the readout pad plane was chosen to optimize, in the high multiplicity environment of central Pb-Pb collisions, the momentum and dE/dx resolution. The radial dependence of the track density led to two different types of readout chambers and a radial segmentation of the readout plane. The active area varies radially from 84.1 to 132.1 cm and from 134.6 to 246.6 cm for the inner and outer chamber, respectively. The total active area is about 32.5 m2. The readout

14 chambers are made of standard wire planes, i.e. a grid of anode wires above the pad plane, followed by a cathode plane and gating wire plane facing the drift volume. To account for the different pad sizes chosen, the wire geometry is different for the inner and outer readout chambers. The general wire layout was tuned with Garfield (electrostatic simulation program [2]) to obtain the maximum charge coupling to the pads. As a consequence, the field wires between the anode wires were suppressed. The wire pad geometry was optimized [2] based on the specific event topology of the particle spectra and densities as they are expected to occur in central Pb-Pb collision. This optimization procedure led to three different pad sizes of 4 x 7.5 mm2 in the inner readout chambers and 6 x 10 mm2 and 6 x 15mm2 in the outer readout chambers. 1.4. Gas Mixture The requirements on momentum and position resolution in the high multiplicity environment, as well as on minimum interaction processes inside the detector, determined the choice of gas for the ALICE TPC. c

•5

100000

Ne-CO -N2 [90-10-5]

Ne-CO, [90-10] 10000

1200

1300

1400

1500 1600 A n o d e voltage (V)

Figure 4. Comparison of gas gain as a function of the anode wire voltage for a Ne-C0 2 and a NeC0 2 -N 2 gas mixture. The N2 admixture allows a stable operation with a higher gain.

The Ne-C0 2 (90%-10%) mixture provides low diffusion, low Z and large ion mobility. It was therefore chosen as the optimum gas mixture for our application. From further optimization work [3] it was found that a small addition of nitrogen (to improve quenching) to the Ne-C02 gas, results in higher operational stability of the readout chamber at the expense of a tolerable reduction in drift speed for constant E/p (Figure 4).

15

The physics results presented here were obtained with a Ne-C0 2 -N 2 (85.7-9.5-4.8) mixture. Unfortunately, the drift velocities in both the Ne-C0 2 and Ne-C02-N2 gas mixtures are not saturated at E-fields of 400 V/cm. Therefore, small variations in temperature and gas mixture have a significant bearing on the drift velocity and herewith produce distortions in the space coordinates. To limit these distortions to the intrinsic space resolution of the TPC of < lOOOum (limited by diffusion) the temperature gradient [4] needs to be smaller than 0.1K. These constraints require that the TPC be operated at high drift fields of > 400V/cm to achieve acceptable drift times of < 88us. Therefore, the HV on the central electrode is > 100 kV.

1.5. Front-End Electronics The front-end electronics [5] has to read out the charge detected by the 570132 pads from the cathode plane of the readout chambers. Each single pad is red out by a charge sensitive preamp/shaper, a 10-bit 10 MHz low power ADC, and an ASIC that contains a digital filter for tail cancellation and baseline subtraction as well as zero-suppression circuits and a multi event buffer. The charge induced on a single pad is amplified and integrated via a low input impedance amplifier. The continuously sensitive charge amplifier is followed by a semi-gaussian pulse shaper of second order (Figure 5). The output of the amplifier/shaper chip is fed into the ALICE TPC Read Out (ALTRO) chip containing 16 channels. Upon arrival of the first level trigger the data stream is stored in a memory. Once the second level trigger ("accept" or "reject") is received, the latest data stream will either be frozen in the data memory, until its complete readout has taken place, or discarded.

Figure 5. Schematic of the ALICE TPC readout electronics

16 The ALTRO chip contains 16 times the TSA 1001 ADC from ST Microelectronics in the form of 8 ADC IP blocks. This ADC has differential inputs and can work up to a clock frequency of 20 MHz with a resolution of 10 bits, while consuming only 12 mW. After digitization a Baseline Correction Unit removes systematic perturbations of the baseline by subtracting a pattern stored in memory. Between triggers, a self-calibration feature can correct any baseline shifts due to temperature shifts or low frequency perturbations. The tail cancellation filter is a three-stage IIR digital filter that removes the long complex tail of the detector signal, thus narrowing the charge clusters to improve identification. It also removes the undershoot that typically distorts the signal amplitude when pile-up occurs. A second correction of the baseline is performed based on the moving average of the samples that fall within a certain acceptance window. The algorithm removes non-systematic perturbations of the baseline. Zero suppression finally removes all the data that is below a certain threshold, except for a specified number of pre- and post-samples around each peak.

1.6. TPC Test Facility We have set up a comprehensive test facility in which every individual component of the ALICE TPC can be tested, either separately or jointly with other parts of the detector system. A cosmic ray scintillator array allowed to trigger on single particles or on cosmic showers. Figure 6 shows the signals generated on a single pad by a cosmic shower event and the effect of the ALTRO filter on the baseline. Since the local occupancy is similar to that in global central Pb-Pb collisions a detailed study and optimization of the baseline filters could be performed. In spring 2004, a final "integral test" was performed in the PS-T10 test beam facility at CERN. An inner readout chamber (IROC) with the final electronics - DAQ chain was operated using the newly proposed gas mixture. The purpose of this test was to check the performance of the detector in combination with the entire readout chain, including the readout software. Thanks to the smooth and stable operation of the complete system, we were able to extract from the data the Bethe-B loch-curve in a momentum range from 1.0 up to 7GeV/c.

17 f —— MwtXaiw

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^ W-Wil^J lyyutyft,Vv* Figure 6: High-multiplicity cosmic ray event in the prototype TPC. Signal on a single pad, before ALTRO action (upper), and after ALTRO action (lower).

Because of the high performance DAQ system with a data throughput of 100 MB/s it was possible to take events even without zero suppression allowing the study of the original signal characteristics with sufficient statistics. In total we have accumulated about 6'000 non-zero suppressed events for each momentum setting.

30 95%

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20% in the wavelength range 250 - 450 nm. 2.1.2. Pixel response Pixel operation was checked using a pulsed LED. The majority of tubes had >99% working pixels, well above the 95% specification. One tube was found to have a noisy pixel and another a bad internal wire bond, resulting in a lost connection to one column of 256 pixels. This tube was still found to have a working pixel fraction of 94.8%. The leakage current of the silicon sensor for the majority of tubes was found to be 1.115-

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Figure 5 Simulated count rate per total activity unit vs. LDD at 30keV (left) and 140keV (right). Data corresponding to different uptaking ratios are plotted together.

simulates a small lesion even in presence of background, suggesting that the method could be used for lesion localization. However, first simulations of clinical conditions have demonstrated a very poor lesion visibility even at high

86

lesion-to-background ratios and at very low energy (30keV). Nevertheless, we believe that the detector assembly can be significantly improved introducing a directional shield to reduce the background radiation reaching the detector. Moreover a number of specific clinical conditions have to be investigated, less severe than the one considered in this work. As an example the case of lung lesions detection could be studied which presents the advantages of a reduced background contribution, due to the low organ density and to its proximity to the body surface. Further studies will be performed to foresee the detector response in such conditions, also investigating different detector geometries. Acknowledgment This work has been supported by Italian National Council for New Technologies, Energy and the Environment (ENEA) under Project P466. References 1. J.R.Davies, J.H.Rudd, P.L.Weissberg: "Molecular and Metabolic Imaging of Atherosclerosis"; J Nucl Med, Vol.45-11 pp. 1898-1907 (2004) 2. T.Mukai, R.Nohara, M.Ogawa, S.Ishino, N.Kambara, K.Kataoka, T.Kanoi,K.Saito, H.Motomura, J.Konishi, H.Saji: "A catheter-based radiation detector for endovascular detection of atheromatous plaques"; Eur J Nucl Med Mol Imaging, Vol.31 pp.1299-1303 (2004) 3. J.Narula, H.W.Strauss: "Imaging of unstable atherosclerotic lesions"; Eur J Nucl Med Mol Imaging, Vol.32 pp. 1-5 (2005) 4. R.Scafe, D.Della Sala, T.Fasolino, G.Alonge: "Rivelatore a Scintillazione Sub-miniaturizzato", Italian Patent Application, RM2005A000204, (2005) 5. Hamamatsu Photonics K.K.: "Si APD S8664 series data sheet"; KAPD1012E02, Sep.2003 DN, Japan 6. R.Scafe, G.Iurlaro, L.Montani, T.Fasolino, G.Alonge, D.Della Sala: "Silicon avalanche photodiodes evaluation for low-energy single-photon scintillation readout", Eurosensors XVIII, Digest of technical papers, ISBN 88-7621-282-5, Palombi Editore, Rome, Italy, (2004) 7. J.F.Briesmeister (Editor): "MCNP 4C - A general Monte Carlo N Particles transport code", Los Alamos National Laboratory Report, LA-13709-M (2000)

ANALYSIS OF T E S T - B E A M DATA F R O M A P R O T O T Y P E LHCB RICH D E T E C T O R

M.PATEL * CERN, Geneva, Switzerland E-mail: [email protected]

A prototype of the LHCb Ring Imaging Cherenkov detector has been constructed. The prototype module contained six pre-production pixel Hybrid Photon Detectors which were read out at the full LHC speed of 40 MHz using a prototype on-detector electronics readout system. The photon detectors were mounted in a gas vessel and Cherenkov rings were observed from an N2 gas radiator using electron and pion beams. The photoelectron yields and Cherenkov angle resolutions observed are in line with expectations.

1. Introduction The LHCb experiment 1 will make high precision studies of CP violation and other rare phenomena in B meson decays. Particle identification is essential for this physics programme and will be provided by two Ring Imaging CHerenkov (RICH) detectors 2 . Three different radiator materials: silica aerogel, and the fluorocarbon gases C4F10 and CF4, will be used to produce Cherenkov light from charged particles traversing the detectors. The resulting single photons must be detected for wavelengths in the range 200 to 600 nm, over a total active area of 2.6 m 2 . The photon detectors are required to have a granularity of 2.5mmx2.5mm, a time resolution better than the LHC bunch crossing rate of 25 ns and an active-to-total area ratio of ~70%. Photon detector prototypes that satisfy these constraints have been developed and subjected to extensive laboratory and beam tests. Before launching the production of ~550 of these detectors, a beam test was performed with the first six pre-production photon detectors and prototypes of the on-detector electronics system. In the following sections, the photon detectors, the readout system and prototype RICH detector used are briefly described, before presentation of the test-beam data. *On behalf of the LHCb RICH Collaboration.

87

88

2. The Hybrid Photon Detector The photon detector adopted for the LHCb RICH detectors is the novel pixel Hybrid Photon Detector (HPD). The HPD consists of a pixelated silicon detector anode assembly which is encapsulated in a vacuum envelope (Fig 1). The silicon sensor chip of the anode assembly is divided into 1024 effective pixels of 500x500^m, with each pixel bump-bonded to one channel of a LHCb PIX1 binary readout chip 4 . The HPD tube has a 7mm-thick quartz entrance window with an S20 multialkali photocathode deposited on the inside. The typical quantum efficiency is ~25% at 270nm. Photoelectrons (p.e.) emitted from the photocathode are accelerated onto the anode assembly by a 20 kV cross-focussing electron optics system. This results in ~ 5000e - in the silicon. A number of early HPD prototypes have been subject to a programme of laboratory and beam tests and all devices have fulfilled the requirements of LHCb 3 . SI pixel array (1024 dements) Ceramic carrier V L I C

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Figure 2. The RICH readout electronics.

3. The Readout Electronics The test-beam readout scheme is shown in Fig 2. The first stage is the binary front-end pixel chip, LHCbPIXl, which is encapsulated inside the HPD vacuum envelope. This chip discriminates single p.e. hits, resulting in binary data which are then read out on receipt of a triggered event. A pair of HPDs are connected via a kapton cable to the Level-0 readout board, mounted on-detector. The Level-0 board interfaces the HPD pixel chips to the trigger, timing and control systems, and receives and distributes the local low-voltage power to the chip. On a receipt of a Level-0 trigger accept, the data from a pair of pixel chips are multiplexed out at a 40 MHz rate. The Gigabit-Ethernet protocol is used to drive the data through optical fibres to the off-detector (Level-1) optical receiver. The receiver card was mounted onto an S-link to PCI bridge (FLIC) 5 . Triggered events

89

were buffered on the FLIC until being read out to a PC through the PCI interface. Test-beam data were driven through the readout system at full LHCb speed with the pixel chip running at 40 MHz and the optical links channelling the data to the receiver at the full 1.6 GHz rate. 4. The Prototype RICH Detector Beam tests were performed at the CERN-PS in the T9 facility which provided 10 GeV/c negative pions and electrons. The Cherenkov detector used is shown schematically in Fig 3. A light-tight vessel contained the N2 gas used as a Cherenkov radiator. Beam particles entered the vessel through a thin aluminum foil and impinged on the centre of a parabolic mirror which was inclined at an angle of 13.4° to the mirror axis. The mirror had a focal length f= 1016 mm, a diameter of 200 mm and a reflectivity of more than 90% in the range 225-450 nm. The detector plane was fixed at a distance of 1047 mm from the mirror centre with dimensions chosen to ensure that saturated (P—l) electron rings could be fully contained within a single HPD. The HPDs were isolated from the Cherenkov gas by a 5mm thick quartz plate through which the Cherenkov photons entered the detector housing.

Figure 3. Schematic of the prototype RICH detector.

Figure 4. Ring image on an from a pion beam and N 2 ra di a tor.

H PD

5. Photoelectron Yields and Cherenkov Angle Resolution A ring image obtained on the pixel chip from a 10 GeV/c pion beam with the N2 radiator is shown in Fig 4. The image is integrated over ~ 30k events. Very few hits are seen outside of the ring indicating the low level of noise.

90

The Poisson distribution of the number of measured p.e.'s is modified by three main effects. Firstly, due to charge sharing effects at the pixel boundaries, a single p.e. may give rise to two pixel hits. Conversely, two p.e.'s can give a hit in the same pixel. Each of these effects was determined to be at the ~ 2.5% level. Secondly, in the test-beam, the 25 ns clock and beam trigger were asynchronous. If a track arrived late in time with respect to the clock, the hit signal may be assigned to a clock tick that is not subsequently strobed and the data may be lost. In contrast to the situation in the final RICH detector, the detection efficiency is therefore reduced by a timing efficiency factor of ~0.9. Thirdly, the efficiency of the HPD, defined as the probability for the detection of a p.e. after it has been converted at the photocathode, is typically 85% from laboratory measurement. The number of measured p.e.'s per event was fitted with a Poisson distribution, modified for the above effects.The data and the fit are shown for a typical HPD in Fig 5. A summary for all HPDs is given in Table 1. The uncertainty in the expectation is dominated by measurements of the quantum efficiency of the HPDs and is at the 10% level. The data are generally in good agreement with the expectation. The discrepancy for HPD 1 originates from a known timing problem. The low number of p.e.'s observed in HPD 6 is currently under investigation.

10

12

14

16

18

Figure 5. The number of p.e.'s observed on a single HPD from pion data (points). The fit to these data is also shown (solid line).

Cherenkov angle (mrad)

Figure 6. The Cherenkov angle distribution from data (black) and Monte Carlo (red) from a pion run.

The Cherenkov angle distribution observed on a single HPD from a pion run is shown in Fig 6. The distribution from a GEANT-4 based Monte Carlo (MC) simulation that includes the contributions from chromatic, pixelisation, emission point, beam divergence and charge sharing effects is also shown. A Gaussian fit to the data gives a width, a, of 1.1 mrad compared to 1.0 mrad from the MC. The data and MC agree within the systematic

91 errors of ~ 10%, which are currently under study. The MC allows the width to be broken down into the contributions from the effects listed in Table 2. Here the pixelisation error dominates, in contrast to the situation in the final RICH detectors which have different geometrical configurations.

HPD

Mfit)

^(theory)

Ratio

1

7.8

6.0

1.3

2

11.5

11.2

1.0

3

8.8

8.9

1.0

4

9.7

10.7

0.9

5

10.1

10.0

1.0

6

8.5

11.4

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a{8c)l Chromatic

0.4

Pixel

0.8

Emission point

0.3

Beam divergence

0.2

Charge sharing

0.3

MC Total

1.0

DATA

1.1

mrad

Table 2.

6. Conclusions A test of a prototype LHCb RICH detector has been performed, employing six pre-production pixel Hybrid Photon Detectors with prototypes of their readout electronics. The data were read out at the full LHC speed of 40 MHz with a prototype on-detector readout system, using optical fibre transmission at 1.6 GHz. Using lOGeV/c electron and pion beams from the CERN-PS T9 facility and an N2 gas radiator, Cherenkov rings were observed with low background. Resolutions and photoelectron yields were in line with expectations. The HPD performance meets the requirements of LHCb, and full production of ~550 HPDs is now ongoing. References 1. S.Amato et al, LHCb Technical Proposal, CERN-LHCC-98-4, CERN, 1998 2. The LHCb Collaboration, LHCb Technical Design Report: Reoptimized detector design and performance, CERN-LHCC-2003-030, 2003 3. M.Moritz et al, Performance Study of New Pixel Hybrid Photon Detector Prototypes for the LHCb RICH Counters, Nucl.Instrum.Meth.A442,164-170,2000 4. R. Dinapoli et al, An analog front-end in standard 0.25/im CMOS for silicon pixel detectors in ALICE and LHCb, Proceedings of the 6th Workshop on Electronics for LHC Experiments, CERN/LHCC/2000-041, 2000. 5. A.Guirao et al.,Readout of high speed S-LINK data via a buffered PCI card, Conference Proceedings, Frascati, PCaPAC 2002

SINGLE CRYSTAL CVD DIAMOND DETECTORS FOR HADRON PHYSICS MICHAL POMORSKI*, E. BERDERMANN, M. CIOBANU, B. E. FISHER, A. MARTYMYIANOV, P. MORITZ, M. REBISZ, B. VOSS Gesellschaft fur Schwerionenforschung (GSI), Planckstr. 1 Darmstadt, 64291 .Germany J. MORSE, M. SALOME European Synchrotron Facility (ESRF) Grenoble, 38043 CEDEX, 6 rue Jules Horowitz, France We report on characterization of several single crystals CVD diamond detectors tested for the use in heavy-ion spectrometry and minimum ionizing particles time of flight measurements. An energy resolution of 16 keV (FWHM) is achieved using a 24l Am asource, which is comparable to the energy resolution of silicon detectors. The collected charge maps, measured using 12C micro beams confirm the excellent homogeneity of the novel material. Using low impedance broadband electronics and a transient-current technique the mobility and saturation drift velocity of electrons and holes are estimated.

1. Introduction Due to its remarkable physical properties [1-3] diamond is one of the most promising wide band-gap materials for particle detection in present and future hadron physics experiments, where radiation hardness and speed are the crucial requirements. Recent progress in the growth of ultra pure Single-Crystal CVD diamond [4] has opened perspectives to fast and high-resolution radiation sensors based on this novel type of artificial diamond 2.

Detector Preparation and Experimental setup

The samples characterized in this work are single crystal diamond films made by Chemical Vapor Deposition (CVD) at Element Six, Ascot, UK. Cleaning and oxidation in boiling mixture of H2SO4/KNO3 is performed before electrodes in "sandwich" configuration are applied by sputtering. Different metallic layers have been tested consisting of: Ti(20nm):Pt(30nm):Au(100nm),

92

93

Cr(50nm):Au(100nm) and Al(lOOnm). No differences in IV characteristics or other detector properties are found so far for various metallization. Vacuum 10° mbar

.

^^^_n

Air

SC-DD

•

: '"'Am •'erhMI

»D> I

IT QND j

.•/- HV E [V/|im]

GND

Fig. 1. (left) IE characteristics for several SC diamond samples. Unexpected huge spread between samples and low break down field is found for most of the tested samples; (right) Mounting of the samples and geometry of irradiation.

Before detector irradiation the I-V characteristics is probed in order to determine the safe range of detector HV bias. The current-voltage curves of various samples, normalized to the contact surface are shown in Fig. 1 (left). For most of the samples, unexpected huge spread and low break-down fields have been observed. Optical microscopy and AFM studies (not presented here) of the samples surfaces show damaged regions probably due to polishing processes after diamond growth. Such undefined state of the interlayer diamond-metal can be the reason for the observed behavior. Further work is ongoing indenting to improve post-growth surface preparation and thus the electrode metallization. The diamond detector is clamped between two printed circuit boards (Fig. 1, right). This mounting provides electrical contact and a particles collimation as well. A 24lAm a-source is placed above the collecting electrode. Impinging a particles of 5.468 MeV have a short range of-12 urn in diamond, thus, for the presented irradiation geometry, the choice of HV polarity enables mainly holes drift at positive bias, and electrons drift at negative potential [5]. 3. Charge Collection Efficiency (CCE) and Energy resolution Using a classical nuclear electronics chain, cross-calibrated by silicon detector and spectroscopic pulser [6], the collected-charge characteristics is measured. Results for 4 different samples are displayed in Fig. 2 (left). Blue curves correspond to the electrons collection efficiency and red data to the

94

efficiency of holes. In all cases we observe saturation of collected charge to ~68.7 fC at relatively low bias. BDS14 E-0.8 V/um 241Am a 1.6x10*

holes

1.4x10*

electrons

1.2x10*

5.486 MeV a particle injection

w c g

•

1.0x10'

:

8.0x10*

|

6.0x10

• 0.00

0.25

0.50

0.7S

1.00

— holes electrons 1.25

1.50

4.0x10" 2.0x10'

'

.J

66

E|V/pm]

y

& collected charge [fC| 69

I

•

i ,

;

69

Fig. 2 (left) Collected charge characteristics of electrons (blue), and holes (red) obtained from four SC diamond detectors, (right) The a-spectra recorded for holes and electrons drift, respectively. All satellite lines are distinguished. The energy resolution (FWHM) for holes drifts is AE/E ~ 0.003.

Good energy resolution of monoenergetic a-particles impinging randomly an area of a few square millimeters, is an indicator of the diamond bulk's homogeneity. The spectrum presented in Fig. 2 (left) is an evidence for excellent homogeneity of this diamond crystal, as well as for long term stability (time of measurements >12h). An energy resolution of AED (FWHM) - 17keV is extracted from a Gaussian fit of the 24lAm main peak, which is comparable to the value of AESiD= 14keV obtained with a silicon detector under same experimental conditions. In addition having such excellent energy resolution and detection stability we can assume charge collection efficiency close to 100% at plateau of curves presented in Fig.2 (left). Thus estimation of average energy needed for eh pair creation is possible, which amounts to -12.8 eV/e-h. 4. Timing Properties - Time of Flight Technique We are using the intrinsic ToF technique [6-8], in order to probe the timing properties of SC diamond detectors. The electric field is applied along the crystal direction. The average (over 500 single shots) current signals, recorded with a DBA-II broad band voltage amplifier of 50Q impedance [7] and a digital scope of 3GHz bandwidth, are presented in Fig. 3 for holes- (red) and electron drift (blue). The FWHM of these signals marks the transition time t,r of the charge carriers cloud between the moment of its generation near one contact and its arrival to the opposite electrode. The flat top of the signals indicate a uniform internal electric field (no space charge and negligible trapping). Thus the drift velocity v^ of carriers is constant at fixed bias and can be easily calculated from the simple relation Vi,=Aliu, where d is the sample thickness. The

95 dependence of the drift velocity from the applied external electric field E for six tested samples is displayed in Fig. 3 (right). - 2.0x10''

E=1.23W|im

E=1.23V/|im

< 1.0x10*

«

E=0.12 V/|im 2.0x10* 3.0x10*0

1.0x10* 2.0x10

3.0x10

104

time [s]

10*

E[v/xrl Fig.3 (left) An example of broad-band current signals (average from 500 single shots) for holes(rcd) and electrons (blue) drift at increasing electric field. The oscillation on top is due to a weak 50il mismatching, (right) Charge carriers drift velocity as a function of electric field for 6 tested SC diamond samples.

Since the transport process at very high electric field is difficult to be theoretically described, the data are fitted by the empirical equation [6-10]: 'dr

(E) =

l+-*-£

(1) Mo Where u 0 - low field mobility, vsal-saturation velocity. Excelent reproducibility of the drift velocity of holes is found for various samples, in contrast to the electrons, where spread of experimental points affects significantly the precision of the fit. Thus, the extrapolated parameters are given for an interval. Table 1 shows the timing parameters, extracted for several tested samples. Table 1. Measured cfective mobilities (u*rr) and drift velocities (Vdr) at typical detector operation range 0.6 < E < 2 [V/u,m], as well as low filed mobilities (u«) and saturation velocities (v s „) extrapolated from fits of the experimental data.

electrons holes

Measured Hen [cm2/Vs] vdr Inm/ns] 1000-450 50-90 1400-550 70-110

Extrapolated vsat [cm/s] Ho [cmVVsl -1300-3000 1.3-1.9*107 ~1.4*107 -2400

96

5. Mapping with heavy-ion micro beams In order to study spatial homogeneity, a SC CVD diamond detector has been irradiated with l2C2* ions of a kinetic energy E = 9.5 A MeV at the micro beam facility of GSI. The irradiated area was 60x93 [urn2] and the sample was scanned in a lum raster. Classical charge sensitive nuclear electronics was used for the signal readout. The obtained 3D distribution of the detector's pulse heights is presented in Fig. 4 (left); no regions of lower (±2%) CCE in scanned area are observed. In Fig. 4 (right) the calibrated ID spectrum is shown, where the left peak is the pulser line used to control the stability of the electronics, the intermediate one the l2C2+ ion distribution and the right one due to an unavoidable beam contamination (~ 0.6%) with 18 0 +3 ions having the same A/Q. The unexpected oxygen peak has been used for energy calibration of the spectrum. The obtained energy resolution of the 12C peak is AE/E = 0.01.

200

Energy [MeV] Fig.4 (left) Pulse height map of a SC CVD diamond detector measured with

12

C ions of E = 9.5

AMeV using nuclear spectroscopy read-out. The irradiated area was 60 x 93 [urn2], (right) The Corresponding calibrated ID spectrum.

The obtained map and the good energy resolution of the 12C peak confirm the excellent homogeneity of single crystal diamond, which was already indicated during the irradiation tests with ct-particles. 6. Summary and Outlook The study conducted here gives a clear evidence that SC CVD diamond is a competitive material for silicon pin diodes, frequently used in hadron physics experiments. Most of the tested samples show stable operation, negligible charge

97 carriers trapping and material homogeneity. The energy resolution found for alpha particles of E a = 5.5 MeV and 12C ions of E = 9.5 AMeV is close to the values obtained from silicon detectors. The timing properties of diamond are superior to silicon. Fast and comparable drift velocity for holes and electrons is measured, which results in the absence of slow signal components. However, the low e-h pair creation compared to silicon, is a drawback for MIP- and weaklyionizing particles detection. The important radiation hardness tests are planned for the next future. Nevertheless, considering the extreme physical properties of diamond and the experimental data obtained from polycrystalline CVD diamond films [12], a sufficient high radiation dose tolerance is expected. Acknowledgments This work is supported by the EC Integrated Infrastructure Initiative Physics Project under contract number: RII 3-CT-2004-506078.The express their gratitude to Element Six, Ascot, UK for the SC diamond provision and to the Target Laboratory crew of GSI for the metallization.

Hadron authors samples samples

References 1. E. Berdermann, et al, ICATPP,2001, p246-251. 2. M. Marinelli.et al, Diamond Related Mater. 7 (1998) 519. 3. J. Isberg et al., Science 297 (2002). 4. Element Six, Ascot, UK 5. S. Ramo, Proc. IRE 27, 584 (1939). 6. M. Pomorski, E. Berdermann phys. stat. sol. (a) 202, No. 11, 2199-2205 (2005) 7. P. Moritz, E. Berdermann, K. Blasche, H. Stelzer, and B. Voss, Diam. Relat. Mater. 10, 1765 (2001).

8. H. Pernegger, et al, Journal of Applied Physics 97, 073704 (2005) 9. C. Jacoboni, C. Canali, G. Ottaviani, and A. Alberigi Quaranta, Solid-State Electron. 20, 77 (1977). 10. T. Lari, A. Oh, N. Wermes, H. Kagan, M. Keil, and W. Trischuk, Nucl. Instrum. Methods A 537, 581 (2005). 11. E. Berdermann, K. Blasche, P. Moritz, H. Stelzerl, and B. Voss, Diam. Relat. Mater. 10,1770(2001). 12. W. Adam et al. MM A: , Vol. 476, Issue 3, 11 January 2002, Pages 686-693

I M P R O V E M E N T OF PARTICLE IDENTIFICATION B Y E N E R G Y LOSS IN A STACK OF SILICON D E T E C T O R S

M. R E G L E R , R. F R U H W I R T H A N D M. F R I E D L Institute

of High Energy

Physics of the Austrian Academy Nikolsdorfer Gasse 18, 1050 Wien, Austria E-mail: [email protected]

of

Sciences,

We have studied the optimal use of information from silicon track detectors for particle identification, using a recent measurement with 4 GeV/c pions with low electronic noise. The raw data were modeled by the convolution of a density obtained by detailed simulation with a Gaussian that accounts for the measurement noise only. We observe excellent agreement with the data. Then a stack of fifteen detectors has been simulated from the experimental distribution. Using these data, the performance and robustness of some location estimators has been compared, including traditional methods such as truncated means and the Maximum Likelihood estimator, as well as a novel method, the optimal L-estimator.

1. Introduction The study of energy loss in silicon has a long history. Due to the complex structure of the cross section (thresholds and multiple modes in the low energy domain, and the possibility of very large energy transfer in a single collision leading to a large skewness), the number of collisions is too small for the Central Limit Theorem. Many attempts for practicable approximations have been made with some well-known milestones like the work of Landau 1 , Vavilov2, and Shulek3, along with the earlier work of Blunck and Leisegang4 who used a Gaussian sum approximation. The model used in this contribution is a probability density from a more refined simulation (without experimental noise), followed up by a convolution procedure, performed for us by Hans Bichsel 5,6 .

98

99

2. The experimental data 2.1. Detector

and experimental

set-up

A prototype silicon detector module with double sided readout was built for an upgrade of the BELLE experiment 7 at KEK (Tsukuba, Japan) and studied in a beam test with 4 GeV/c pions in April 2005. The sensor is 300 /xm thick and has AC-coupled strips with a readout pitch of 51 fxm on either side. The strip signals are amplified by the low-noise APV25 front-end chip 8 , which was originally developed for the CMS experiment at CERN (Geneva, Switzerland). 2.2. Raw data

analysis

Out of the various measurements performed in that beam test, we will only show data of the p-side measured at "standard" conditions like perpendicular incidence and moderate over-depletion. The raw strip data was processed with the usual pedestal subtraction and common mode correction algorithms, followed by a cluster finding procedure with two different thresholds: Once a seed strip with a signal above 5 times the RMS of the noise is found, neighbouring strips are added to the cluster as long as their signals exceed 3 times the RMS of the noise. Statistical tests were carried out to ensure that the noise of individual strips is largely uncorrelated as expected. Consequently, the noise of a cluster signal depends on the number c of strips involved. Assuming identical RMS noise ns on each strip (which is a good approximation of the real system), we obtain the RMS cluster noise nc by nc = y/cns. In the data presented here, we found an average RMS strip noise of 522 e-h pairs and a mean cluster size of 2.49. Hence, the cluster noise is expected to be 824 e-h pairs in average. By modifying the cluster finding procedure such that the same number of strips are summed up but displaced from the actual cluster strips above threshold, we found an RMS cluster noise of 739 e-h pairs. Note that the data has been taken in a clean test-beam environment. 3. The electron-hole pair distribution 3.1. The Bichsel

model of the energy

loss

We thank H. Bichsel for providing us with a (pointwise) probability density function of the energy loss corresponding to our experimental setup (4 GeV/c pions, detector thickness of 300 //m). The mode (most probable value) of the Bichsel density ist at Emode = 82.64 keV, corresponding to

100 22637 e-h pairs. We have "standardized" the Bichsel PDF by an afnne transformation such that its location (mode) b is at zero and its scale (one quarter of the full width at half maximum) a is equal to 1. The standardized Bichsel density can be approximated quite well by the convolution of a Landau density (with mode b = -0.26 and scale a = 0.791) with a Gaussian density with \i = 0 and a — §.711\.. This explains the apparent success of the Landau-Gaussian-convolution in modeling energy loss data. 3.2. Modeling model

the data by the Bichsel

and by the

Landau

The observed e-h pair counts are corrupted by the electronic noise. We have modeled the noise by Gaussian fluctuations of the counts with zero mean and a width to be determined from the data. The convolution of the Bichsel density with a Gaussian gives an excellent fit (Figure 1). The Bichsel density has mode b = 19785 and scale a = 2058. The standard deviation of the Gaussian is equal to that has the smallest variance. Under the constraints E(Copt) = t and ^Wi = 1 the optimal weights are given by wopt = A - 1 B(te — ft), where jl = E(x), B = G(/2e T - ep,T)G, G = [cov(x)] -1 , A = /uT-Be*and e is a vector of ones. The optimal weights can be estimated from the data. 4.2. Results from experimental

data

The separation of different parent distributions (particle types) is optimal if the ratio p = mean/std of the location estimator is maximal. We have investigated the robustness of various truncated mean estimators and of the optimal L-estimator by contaminating the samples with various amounts a

A location estimator l(x) is equivariant iff l(ax + b) — a l(x) + 6.

102 of noise, uniformly distributed between 0 and 100,000. The ratio p drops by only about 5 percent with a 2%-contamination of noise. Figure 2 shows the results for 2% contamination. The separation is optimal when the expectation of the L-estimator is equal to about 19200, somewhat below the mode of the parent distribution. The optimal Lestimator performs only slightly better than the best truncated mean estimators (£(2,5) and £(2,7))- Besides being not equivariant, the maximum likelihood estimator is slightly worse than the best L-estimators and much slower to compute.

8

3

9

7

6

1

11

4

Noise probability, xlO"4

5

7

18

5

2

9

12

Number of random trigger, xlO

multiplying of number of detected photons after simulation of cross talk value. Then the width of single photoelectron peak from LED spectrum was .added to have the final SiPM response. It turned out that one might have more than 98%

112 efficiency if sum response was larger than 8 pixels. This was checked with data. As the data were taken with inclined tracks we had to increase the threshold by factor 1.1. Numbers of cosmic triggers and number of events with Asum < 9 are given in the Table 1. One may see that for all data the detector non-efficiency is less than 1%. In order to estimate the noise rate at the threshold > 8 pixels we used the spectra obtained with random trigger. Number of random trigger and number of events with amplitude higher than 8 pixels are also given in the Table 1. -4 All runs data gave 1.7*10 noise probability with 120 ns gate width. The noise rate may be reduced if one uses for SiPM pulses coincidence faster electronics. 4. Conclusions The detector consisting of 200 x 2.5 x 1 cm3 plastic scintillator, the WLS fiber and two novel photodetectors (SiPM) has been tested. Such kind of detectors may be used in muon systems. It has high efficiency and low noise rate as compare with resistive plate chambers often proposed as muon detector. The SiPM has similar gain and efficiency as the traditional photomultiplier. Despite Contrary to vacuum photomultipliers the SiPM can operate in magnetic field, its small size allows incorporate it into scintillator body, SiPM operational voltage does not exceed several tens volt. The light collection efficiency may be increased by gluing of a fiber into the groove [3], References 1. 2.

3.

P Adamson et al., The MINOS scintillator calorimeter system, IEEE Trans. Nucl. Sci. 49, 861-863 (2002). G.Bondarenko et al., Limited Geiger-mode silicon photomultiplier with very high gain, Nucl. Phys. Proc. Suppl. 61B, 347-352 (1998) G.Bondarenko et al., Limited Geiger-mode microcell silicon photodiode: new results, Nucl. Instr. Meth. A442, 187-192 (2000) P.Buzhan et al., An advanced study of silicon photomultiplier, ICFA Instr. Bull. 23, 28-41 (2001) P.Buzhan et al., Silicon photomultiplier and its possible applications, Nucl. Instr. Meth. A504, 48-52 (2003) V.Andreev et al. A high-granularity scintillator calorimeter readout with silicon photomultipliers, Nucl. Instr. Meth. A540 368-380 (2005) R.Wojcik Embedded Waveshifting Fiber Readout of Long Scintillators, Contribution to the III International Conference in High Energy Physics, Sep.29-Oct. 2, 1992, Corpus Christi, Texas

Astroparticle Experiments Organizers: D. Saltzberg and A. Capone

E. Andreotti

S.N. Ahmed

J.K. Becker M. Brigida S. Capelli S. Cecchini R. Cerulli J.V. Dawson A. De Sio O. Dragoun S. Dusini L.S. Esposito E. Ferrer Ribas L. Gambicorti D. Grandi

M. Hareyama S. Hasan

The Underground Baradello Laboratory: Characterization of the Site and Early Results on Gamma-Ray and Neutron Spectrometry Measurements The Art of Detecting Neutrinos, 2 km Underground The Three Phases of the Sudbury Neutrino Observatory Neutrino Physics at the South Pole - Recent results from the AMANDA Experiment First Results from GLAST-LAT Integrated Towers Cosmic Ray Data Taking and Montecarlo Comparison Background Analysis of Cuoricino in View of the Future Experiment CUORE Application of Nuclear Track Detectors in Astroparticle and Nuclear Physics Recent DAMA Results Double Beta Decay Measurement with COBRA Ultraviolet Diamond Photodetector Status of the KATRIN Experiment The RPC System for the OPERA Spectrometers Electromagnetic Shower Reconstruction in Emulsion Cloud Chamber A Low Background Micromegas Detector for the CAST Experiment Imaging Optical Adapters for Multi-Anode Photo Multipliers Detectors A 2D Stochatsic Montecarlo for the Solar Modulation of GCR: A Procedure to Fit Interplanetary Parameters Comparing to the Experimental Data Spatial Distribution of Energetic Heavy Ions near the Earth A Photon Tag Calibration Beam for the AGILE Satellite 113

114 N. Hasebe A. Haungs

M.H. Israel Yu.G. Kudenko C. Leroy J. Lizarazo F. Lucarelli M. Messina

V. Mikhailik

M. Pedretti M. Ruppi O. Tarasova

Observation Program of Isotope Composition in Ultra Heavy Cosmic Rays High-Energy Cosmic Rays Investigated by Air-shower Measurements with KASCADE, KASCADE-Grande, and LOPES Scintillating Optical Fibers for Astroparticle Physics The T2K Experiment: Status and Instrumentation of the 280m Near Detector Results and Status of the PICASSO Experiment Data Acquisition System and Trigger Electronics for CACTUS The Central Pixel of the MAGIC Telescope for Optical Observations Status report of ARDM Project: a New Direct Detection Experiment, Based on Liquid Argon, for the Search of Dark Matter Development of Multiple Photon Counting Coincidence (MPCC) Technique for Characterisation of Scintillators for Cryogenic Applications Results from Cuoricino Experiment and prospects for CUORE Timing Calibration of the NEMO Optical Sensors Design, Production, and First Results from the IceCube Digital Optical Module

T H E U N D E R G R O U N D BARADELLO LABORATORY: CHARACTERIZATION OF T H E SITE A N D EARLY RESULTS O N G A M M A - R A Y A N D N E U T R O N SPECTROMETRY MEASUREMENTS

E. A N D R E O T T I Universita

dell'Insubria, Dipartimento di Fisica e Maternatica, INFN- Laboratori Nazionali del Gran Sasso

Como

A. G I U L I A N I , M. P E L L I C C I A R I , C. R U S C O N I Universita

dell'Insubria,

Dipartimento

di Fisica e Maternatica,

Como

A. C E S A N A , L. G A R L A T I , M. T E R R A N I Politecnico

di Milano,

Dipartimento

di Ingegneria

Nucleare,

Milano

G.L. R A S E L L I INFN-

Sezione

di Pavia,

Pavia

An underground station for the measurement of low level radioactivity is in operation in the town of Como under the Baradello hill. The laboratory is covered by 100 m of rock (~ 300 m w.e.). The main activities carried out at the laboratory concern the measurement of the radioactive content in different kind of matrices by means of an HPGe detector shielded with radio-pure copper. Another research subject of the laboratory is the analysis of fast neutron flux produced by natural radioactivity of the rocks using liquid scintillator cells. This paper reports about the preliminary measurements carried out to characterize the site. Moreover gamma and neutron spectrometry set-up are described and some measurement results performed on low level radioactivity samples and on fast neutron flux are also reported.

1. Introduction In the frame of an interdisciplinary project funded by the University of Insubria, a station for the measurement of low level radioactivity was developed starting from the year 2002. The aim of the project was the characterization of the environment surrounding the town of Como, with particular attention for the distribution of natural radioactive nuclides. The 115

116 results obtained from preliminary measurements allowed to perform some other activities, in connection with fundamental research experiments. The chosen site for the installation of this station is an artificial cave under the Baradello hill housing the water purification plant of Como. A small area of about 10 m 2 in the deepest point of the main underground hall is dedicated to the low background laboratory. In that point the rock cover reaches about 100 m, corresponding to about 300 m water equivalent. The air conditioning system operates continuously and in standard condition the temperature is 13 °C and the humidity level about 80%.

2. C h a r a c t e r i z a t i o n of t h e site Some preliminary measurements were carried out in order to evaluate the potentiality of the site as described in Ref. 1. The measurements concerned the following items: radon ( 222 Rn) concentration in air, gamma ray background, muon background, neutron background, natural radioactivity of soil, rocks and concrete. Radon concentration was measured inside the underground hall at several different points, using detectors based on different operations principles. The Radon concentration resulted to be about 34 Bq/m 3 , i.e. a rather low value in agreement with the high forced ventilation rate (31.000 m 3 /h). Gamma-ray background was measured approximately at the centre of the area housing the Laboratory with an unshielded HPGe detector. A simple model able to account for the contributions from rock and atmosphere to the main gamma peaks enabled to extract useful informations on the activities of the rocks forming the cave walls. Activity values resulted to be about 20-30 Bq/Kg both for 238 U and 2 3 2 Th, in agreement with concentrations found in some rock samples by means of high resolution gamma spectroscopy. Measurements performed with Nal detector showed that the muon flux is reduced by about two orders of magnitude with respect to the outside sea level value (result consistent with the 300 m w.e. rock cover). Thermal neutron flux was measured at the centre of the Laboratory by a BF3 proportional counter showing a value reduced by a factor of ~ 103 with respect to sea-level external laboratories. Fast neutron flux, measured using the set-up described further on in this article, resulted to be a factor ~ 102 suppressed with respect to sea level values. These results confirmed the suitability of the underground site located under the Baradello hill for scientific activities for which a low level radioactivity is required. In fact, in spite of an observed higher radioactivity content in the constituent rocks with respect to other underground sites (that can anyway

117

be made harmless by a proper shielding), the 300 m water equivalent cover ensures a sizeable reduction of various components of the cosmic radiation. 3. Gamma spectrometry measurements 3.1. The HPGe

detector

A high purity Ge detector (sensitive volume ~ 148 cm 3 and 30% relative efficiency at 1.33 MeV) was then installed at the Baradello Laboratory under a in shield of ultra-pure OFHC copper with a minimum thickness of 20 cm: thanks to this configuration the background was reduced by about four orders of magnitude with respect to that obtained without shield. A metallic box flushed continuously with nitrogen surrounds the detector, in order to keep it in a Radon free atmosphere. 3.2. Comparison

with other

sites

Comparisons between the Baradello background spectrum and those of some similar above ground and underground installations were performed. In Table 1 we report the count rate (Counts/d/Kg) obtained with p-type HPGe detectors in the characteristic energy range of gamma spectrometry, (between 40 and 2700 KeV) for some Low Background European Laboratories. 2 We point out the lower background counting rate in the case of Baradello Laboratory, obtained thanks to the ~ 300 m w.e., with respect to the above ground ACR Austrian Laboratory: in that case the developement of a high sophisticated passive and active shielding was needed in order to reduce the HPGe background. On the contrary Baradello background is worse with respect to those of the other underground laboratories, mainly due to the much more thicker rock cover and to the more absorbing shielding used, in most cases provided by a 15 cm thickness external lead shield plus an electrolitic copper core. Table 1. Count rate (Counts/d/Kg) in the energy range 40-2700 KeV for some Low Background European Laboratories. ARC

Baradello

IRMM

PTB

LNGS

LSCE

Seiberdorf

Laboratory

HADES

UDO

(Italy)

Modane

(Belgium)

(Germany)

(Austria)

(France)

Counts/d/Kg

8200±200

7000±17

260±4

277±4

87±1

186±2

Depth m w.e.

ca. 1

300

500

2100

3800

4800

118

3.3. MDA calculations and upper bounds on content for CUORE

radioactive

Finally, to give a quantitative estimation of the potentiality of the Baradello site, Minimum Detectable Amount (MDA) were calculated for different typical environmental matrices and different isotopes, using the procedure given in Ref. 3. As an example we report MDA calculations in the case of soil, obtained assuming SHO2, p= 2.5 g/cm 3 , a cylindrical geometry and considering a hypothetical measurement time of ~ 1 day. Sensibility resulted to be about 1 mBq/g for 226 Ra, 0.7 mBq/g for 40 K and 0.1 mBq/g for other radionuclides ( 228 Ac, 137 Cs, 60 Co, 26A1). MDA calculations are also useful in order to derive upper bounds on radioactive content in the case of very high purity materials, allowing to select those suitable for fundamental research experiments. The CUORE experiment aims at the study of neutrinoless Double Beta Decay of 130 Te using TeC-2 crystals as bolometric detectors: the reduction of background induced by bulk radioactivity in the detectors themselves and in the other components which constitute the set-up is of paramount importance for the experimental results. So at the Baradello Laboratory we performed some measurements on a Te02 crystal (mass ~ 200 g), costantan wires (mass ~ 7 g) and connectors (total mass ~ 60 g). Results for TeC-2 crystal are given in Table 2 together with reference activities, calculated by means of Monte Carlo code in order to produce a background counting rate not exceding the one expected for the fulfilment of CUORE radiopurity goal.4 In this case the upper limits attainable at the Baradello Laboratory are greater than the required values (and the same happens in the case of costantan wires), so a further analysis is needed, for example by means of measurements performed at LNGS; however a first selection can be provided, as it is evident from the results on connectors, also given in Table 2: in that case a large amount of radioactive contaminants was found, thus precluding their usage in the experimental set-up. 4. Fast neutron flux measurements 4.1. Set-up

description

The tecnique used for the estimation of fast neutron flux inside the laboratory is based on proton recoil in an organic liquid scintillator. The equipment consists of three commercial cells, each one viewed by two photomultipliers, filled with liquid scintillator (Bicron B501A) manifactured by Bicron with total sensitive volume of about 1 liter. This type of scin-

119 Table 2. Radioactive upper bounds in g/g for the TeC>2 crystal (measured upper bounds are derived from MDA calculated considering a measurement time of 5 days) and radionuclides concentrations found in connectors (Bq/Kg). Nuclide

23S

Measured upper bound

Required upper bound

Activity in connectors

for T e 0 2 g/g

for T e 0 2 g/g

Bq/Kg

s

13

U

232 T h 60

Co

40

K

< 1.2 x 1 0 "
BJ

171) 101)

80

~ — -

Data Fit result Neutrons CC ES External neutrons

•

'-

60 40

i 1i

20

'-

52 Figure 3.

Separation of CC, NC, and ES events based on variable /3i4.

phase 4 . The equivalent 8B neutrino fluxes in units of 106 cm~2s"1 as obtained by energy-unconstrained maximum likelihood fitting of integral number of events from each event type are 4>cc = l-68i0;°66(stat.)i0:o89(syst.)

4,ES = 2.35tg;i(stat.)lS:ls(syst.) ^c=4.94iO;2J(stat.)+o:i!( s y s t -)Another important analysis performed in salt phase was investigation of matter enhancement effects in the earth through day-night asymmetry. The results were found to be consistent with no asymmetry. After including results from other experiments the best fit values for two-neutrino mixing parameters were found to be Am 2 = (8.0±o.o 4 ) x 10" 5 eV 2 and 0 = 33.9_tl;|. 5. N C D P h a s e In the third and final phase of the SNO experiment, 40 helium filled long proportional counters were deployed in the heavy water. The deployment activity was carried out between November 2003 and February 2004 with the help of a specially designed remotely operated vehicle. 36 of the counters or strings are filled with 3He while the remaining 4 are filled with 4He. The

130 helium-3 counters are used for direct neutron detection while the helium-4 ones are used to assess the background. A SNO neutral current detector is shown in Fig.4. The counter body is made of ultrapure nickel tube. The end caps are made by chemical vapor deposition on stainless steel mandrels and are welded into the tubes by Nd-YAG laser welder. Due to the restriction on the mine elevator size, approximately 3m long tube segments were welded together underground in the SNO lab. The final welds were done right over the opening of the acrylic vessel since the height of the deck did not allow the full length counter to be directly deployed. The anode is made of 50/xm diameter low radioactivity copper wire. This enables a gas gain of approximately 100 at 1650V. A long delay line at the end of each string allows position measurement of the neutron event along the counter axis by measurement of relative time of arrival of direct and reflected pulses.

Figure 4.

Sketch of a neutral current detector of SNO experiment.

The filling gas in the 36 neutron counters is a mixture of 85% 3He and 15% CF4 at a pressure of 2.5atm. A neutron entering the active volume of the counter reacts with helium according to 3

He + n —> p +1 + 764keV,

where t stands for tritium. The energy released then produces electron ion pairs, which cause avalanche resulting in a voltage pulse traveling on the anode wire in both directions. The cross section of the above reaction for the NC neutrons is about 53306, which is more than two orders of magnitude higher than for chlorine that was used in the salt phase. However even

131

with such a high cross section the number of neutrons detected are fairly small due to relatively small volume occupied by NCD strings. The real advantage is the possibility of directly counting neutrons, eliminating the need to isotropic separation of events. 6. Summary The two phases of SNO have contributed towards our understanding of neutrino physics specially neutrino flavor transformations. The salt phase results have confirmed and improved the results obtained in the first phase of the experiment. The results strongly favor solar neutrino flavor transformations and are consistent with no day-night asymmetry. The integral fluxes obtained are consistent with 8B neutrino standard solar model predictions. It is expected that these results will get significantly improved in the third phase where the neutral current signals will be directly measured through NCD strings without involvement of PMTs. The NCD phase of the experiment has started and data is being collected. This final phase will end at the end of 2006. After decommissioning the detector and retrieving the heavy water, there is a proposal under consideration to fill the cavity with liquid scintillator. This proposal has very rich physics motivation including tweaking of low energy regime of 8B neutrino flux, pep neutrinos, geo-neutrinos, and double beta decay. Acknowledgments This work is supported in Canada by NSERC, Industry Canada, NRC, Northern Ontario Heritage Fund, Inco Ltd, AECL, Ontario Power Generation, HPCVL, and CFI; in USA by DOE; and in UK by PPARC. We thank the SNO technical staff for their strong contributions. References 1. The SNO Collaboration, Nucl. Instr. Meth. 87, 071301 (2001). 2. Q.R. Ahmad et. al. (The SNO Collaboration), Phys. Rev. Lett, 87, 071301 (2001). 3. Q.R. Ahmad et. al. (The SNO Collaboration), Phys. Rev. Lett. 89, No.l, 011301 (2002). 4. S.N. Ahmed et. al. (The SNO Collaboration), Sumitted to Phys. Rev. Lett, 92, arXiV:nucl-ex/0502021 (2005). 5. J.N. Bahcall, M. H. Pinsonneault, and S. Basu, Astrophys. J.. 555, 990 (2001).

N E U T R I N O PHYSICS AT T H E SOUTH POLE - R E C E N T RESULTS FROM T H E A M A N D A E X P E R I M E N T

J. K. BECKER FOR THE ICECUBE COLLABORATION* Universitat Dortmund, Institut fur Physik, D-^221 Dortmund, Germany E-mail: julia@physik. uni-dortmund. de

In this contribution, recent results from the Antarctic Muon And Neutrino Detector Array (AMANDA-II) on searches for high-energy neutrinos of extraterrestrial origin are presented. In particular, different methods to investigate the diffuse neutrino flux, the single and stacked search for neutrino point sources, the search for transient sources (GRBs and AGN time dependencies) and the search for neutrinos from WIMP annihilation are discussed.

1. Very high energy n e u t r i n o s in a s t r o p a r t i c l e physics One of the primary goals of the Antarctic Muon And Neutrino Detection Array (AMANDA-II) is the search for very high energy (VHE) neutrinos from extraterrestrial sources. Due to their low interaction probability with matter, neutrinos are one of the most promising particles to provide information about core processes of sources of VHE emission. The low cross section is also the reason that very large volumes have to be instrumented to achieve a significant rate of events. AMANDA is located in the antarctic ice, between 1500 m and 2000 m below the surface and covers a geometric volume of ~0.016 km 3 . The detector consists of 19 strings with a total of 677 photomultipliers (PMTs). When a muon-neutrino interacts with a nucleon, a muon is produced, emitting Cherenkov light since traveling faster than the speed of light in ice. The Cherenkov light can be detected by the PMTs and both the incidental direction and the neutrino energy are reconstructible. To guarantee that the observed muons are neutrino-induced, the Earth is used as a filter: While muons produced in the atmosphere are absorbed by the Earth, neutrinos traverse the Earth and the signature 'http://icecube.wisc.edu

132

133 is unique. After the filtering of atmospheric muons, the remaining signal mainly consists of neutrinos produced in hadronic showers in the atmosphere. Different analysis methods have been developed to separate this background from a potential signal from extraterrestrial sources. In addition to the neutrino-induced lepton, a hadronic cascade produced at the interaction vertex contributes to the signal. This contribution will focus on four different types of analyses: Limits on an additional diffuse flux contribution over the expected atmospheric signal will be discussed as well as limits on signals from steady and transient point sources. Additionally, an approach for dark matter searches will be discussed. The presented results are mainly based on a four-year data sample taken between 2000 and 2003 in which 3329 neutrinos could be identified1.

2. Diffuse searchFigure 1 shows a summary of the energy spectra of diffuse predictions compared to AMANDA limits. The dashed lines represent the atmospheric neutrino flux2'3, the lower line giving the vertical flux (nadir angle of 6^ = 180°), the upper line showing the horizontal contribution (0/v = 90°). Predictions 1 to 4 give possible extragalactic contributions: Model 1 (BBR 4 ) predicts a neutrino flux from steep and flat spectrum Active Galactic Nuclei (AGN). Model 2 (WB 5 ) shows the diffuse neutrino spectrum which is expected from Gamma Ray Bursts (GRBs). Model 3 (MPR) gives the maximum signal contribution from blazars, while 4 (MPR bound) represents theoretical upper limits on the same source class. The upper bound of the shaded region represents the limit using a high neutron-photon opacity, rny » 1, the lower, energy dependent, bound is calculated using r „ 7 < 1, see Ref. 6 . All extragalactic contributions turn out to be flatter than the atmospheric flux, so that the total spectrum is expected to flatten at a certain energy where extragalactic neutrino sources start to dominate isotropically. The data circles show the unfolded energy spectrum of AMANDA7. The spectrum is complementary to Frejus measurements at lower energies (squares) 8 . The measured spectrum can be determined up to energies of 100 TeV and follows the atmospheric prediction. An upper limit on an extragalactic E~2 muonneutrino signal is given by E2dN/dE = 2.6 • 10 _7 GeV cm~ 2 s _ 1 s r _ 1 for data from the year 2000. Using cuts sensitive to very high neutrino energies to look for an excess over the atmospheric signal, a sensitivity of

134 E2dN/dE = 9.0 • 10 _8 GeV c m - 2 s _ 1 s r _ 1 is reached for four years of data (2000-2003)9. At ultra high energies, limits to all three neutrino flavors are achieved by considering events from near the horizon (labeled UHE in Fig. 1) or cascades.

Atmospheric ^ f c1 b -

10

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V sensitivity (4yr)

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Figure 1. Summary of AMANDA results for a diffuse search in the context of extragalactic neutrino flux predictions.

3. Looking for steady point sources The search for point sources yields an opportunity to reduce the background of atmospheric neutrinos by selecting positions in the sky where potential objects of neutrino emission are located. The average sensitivity to single point sources for four years of data (2000-2003) is E2dN/dE

1 = 6 • 1 0 - 8 GeV cm -2 s"

Here, an E~2 spectrum has been assumed as the expected signal. The complete northern hemisphere has been scanned for a clustering of events. In addition, a catalog of 33 sources has been preselected. No excess over the atmospheric background could be observed. Preliminary limits to the flux from the 33 single sources and a significance map of the northern hemisphere are given in .

135 Active Galactic Nuclei (AGN) are one of the most prominent source candidates for the extremely high energy flux of Cosmic Rays. This charged component is likely to be accompanied by a neutrino flux.Therefore, there is good potential to detect high energy neutrinos from AGN. In a stacking approach, different AGN samples have been defined according to their geometry and photon luminosity at different wavelengths. Method and results are discussed in detail in 10 . It is assumed, that the neutrino luminosity is proportional to the observed photon luminosity. Each sample contains about 10 sources. Figure 2 shows the stacking limits per source for one year of data taking (year 2000) that are given for the 11 AGN samples. The dashed line represents the single source sensitivity for four years of data taking. The one year stacking limits are already at the sensitivity level of four years of data, which shows that the stacking method provides a very effective way of improving the sensitivity.

Figure 2. Upper limits to a signal from various AGN samples given per source. The classes on the x-axis are GeV blazars (GeV), unidentified GeV blazars (unid), infrared sources (IR), keV blazars from HAEO-A (keV(H)) and from ROSAT (keV(R)), TeV blazars (TeV), Compact Steep Sources and GHz-Peaked Sources (CSS/GPS), FR-I galaxies including M-87 (FR-I(M)) and excluding it (FR-I) as well as FR-II galaxies (FR-II) and Quasi Stellar Objects (QSO). The stacking limits for one year of data are comparable to the single point source sensitivity of four years (dashed line).

136

4. Transient sources 4 . 1 . Flaring

states

Many permanent objects in the sky reveal a variability in the observed photon luminosity. Based on the assumption that the observed photons are accompanied by neutrinos, a variability in the neutrino luminosity is examined by using a sliding window of a fixed duration. For this purpose, the catalog of the 33 sources presented in Sec. 3 has been used. The sliding window has been optimized to a duration of 40 days for extragalactic and 20 days for galactic sources. Additionally, a multi-wavelength approach has been pursued for three objects (Mkn 421, 1ES 1959+650 and Cygnus X-3) where it has been searched for an excess in the neutrino data at times of flaring states. No significant excess could be observed for any of the methods, the results are summarized in Ret 1 1 . 4.2. Gamma

Ray Bursts

(GRBs)

-~10 rolling, i asc (, ?0 11. prelin ' c a s c a d e s ( 2 0 0 0 )

7

8 9 10 log(Ev /GeV)

Figure 3. Summary of the limits on the average GRB spectrum.

GRBs are the most luminous objects in the sky and they release their energy within a few seconds: The duration ranges from milli-seconds (short

137

bursts) up to several hundred seconds (long bursts). To increase the sensitivity, all GRBs that occurred during the time of the neutrino data sample are stacked. The limits on the average GRB spectrum given in Ref.5 are summarized in Fig. 3. It can be seen that the sensitivity of the predicted spectrum (WB) is not reached with AMANDA. Using neutrino induced muons (solid lines) gives limits about an order of magnitude higher than the predicted spectrum 12 . One year of data has been analyzed in the cascade channel with limits as indicated in Fig. 3 (dashed lines, see 13 ).

^

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12 13 14 15

12 13 14 16

8

9

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9

10 11

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9

10 11

4

5

6

7

4

5

6

4

5

6

7

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6

7

0

1

2

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2

3

0

2

3

I Towers

2

7 3

4 Towers

12 13 14 15

8

6 Towers

9

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10 11

8 Towers

Figure 2. Towers in the grid during hardware integration.

4.

Data/MonteCarlo comparison

In our analysis we used runs of cosmic ray data taking in eight towers configuration. We selected a data sample consisting of single muons tracks fully contained in a single TKR tower. The event selection has been performed by imposing following cuts: • • • •

events triggered by the TKR; only a single muon tracks; events fully contained in a tower; minimum ionizing particles;

The data sample that we used in our analysis come from cosmic ray data taking at sea level, in standard hardware configuration, for a total of more than 2-106 events. Less than 10% of events survived to the cuts. We studied the dependence of the hit strip multiplicity by the zenith angle 0. As shown Fig.3, the hit multiplicity increases linearly with l/cos6, that is proportional to the track length in the SSDs: the plot shows a good agreement between data and MC predictions.

142 In order to validate the MC digital output simulation we examined the ToTs distributions. Fig.4 shows the distribution of ToTs in each track layers for real muon data and the MC expectations for all integrated towers. As shown by the plot, the real data are well reproduced by the MC and the mean ToT value is consistent with expected charge deposition in 400jim thick silicon layers [4,5]. Finally, the dependence of the ToT in the track layers on the geometrical parameters (

4 MeV), a continuum (E>3 MeV) and 7 and /3 region. The Geant4 package has been used to write a C + + based code able to simulate the entire structure of CUORICINO and CUORE. Decay processes due to natural ( 232 Th , 238 U , 2 1 0 Pb , 4 0 K ) and cosmogenic isotopes in the bulk and on the surfaces of the various experimental parts have been simulated, as far as LNGS environmental radioactivity. 4.1. CUORICINO

background

analysis

From the analysis performed on the CUORICINO data the main sources responsible of the background in the P/3(0v) region have been identified, a decays occurring on the crystals surface have been clearly identified from their characteristic asimmetric peak at the alpha transition energy and the coincidence pattern, while a decays occurring in the crystals bulk show a gaussian peak at the Q value of the decay. The flat continuum measured between 3 and 4 MeV can be ascribed to deep (1-5 /xm depth) surface contaminations of materials facing the crystals (the biggest surface is the one of the copper structure) as well as to surface contamination of the detector themselves. The contribution to the /30(Ov) region from these sources has been evaluated to be of ~10±10% and ~50±20% respectively for the Te02 and Cu surfaces. The remaining background in the /?/?(0z/) region can be ascribed to 2 3 2 Th contaminations of experimental part distant from

147 the detector ( ~30±10%). A limit of ~10 _ 8 Bq/cm 2 has been evaluated for the surface contamination of copper and crystals in 2 3 2 Th , 238 U or 2 1 0 Pb . For the bulk contamination a limit of 1 pg/g and 0.1 pg/g for copper and TeC>2 has been set. 4.2. Extrapolation

to

CHORE

In order to reach the sensitivity required to investigate (mee) in the inverted hierarchy, it is necessary to reach in CUORE a background rate in the /3/?(0f) region of the order of 1 0 _ 2 - 1 0 ~ 3 c/keV/kg/y. The construction of the CUORE detector will require about five years; a dedicated R&D is on the way, aiming to decrease the background sources that could limit the sensitivity of the experiment. The background reachable with CUORE was evaluated on the basis of the background model developed for CUORICINO and on the basis of the radioactive contaminations measured so far for the various materials commercially at our disposal [5]. Negligible contributions are expected from thermal neutrons and environmental gammas, thanks to the use of specially designed neutron and lead shields. The evaluated contribution from high energy neutrons (both produced from rock radioactivity and from muon interactions in the rock and in the lead shields) is of ~ 4 x l 0 - 4 c/keV/kg/y. Copper and crystals bulk contamination expected rate in the flfi{Qv) region is less than 3xl0~ 3 c/keV/kg/y. The most worrisome background source is the surface contamination: for contamination values of ~10~ 8 Bq/cm 2 a rate of 2xlO~ 2 c/keV/kg/y and 5xlO~ 2 c/keV/kg/y could arise for crystals and copper surface contaminations respectively. 5. R & D for background reduction In order to fulfill the CUORE goal with respect to background reduction an intense R&D had begun with respect to surface cleaning and passivation, aiming at reaching at least reduction by factors 10 and 4 in copper and crystals surface contamination respectively. Dedicated measurements have been performed in a second and smaller cryostat, installed in the hall C of LNGS, provided with 5 cm of Copper shield and 15 cm of commercial lead. No neutron shield is present. In the first measurement (RAD1) 8 crystals of 760 g subject to surface etching and polishing with clean and measured powders have been mounted in CUORICINO like tower. The copper used for the tower structure was

148

also subject to a specially studied cleaning procedure: etching, electropolishing and passivation. The measured background, with a statistics of ~6750 h x detector, gave very encouraging results with respect to crystal surface 2 3 2 Th and 238 U contamination reduction. The rate of the a peaks due to these sources were in fact reduced by a factor of ~ 4 with respect to CUORICINO, fulfilling the first CUORE milestone. The continuum background between 3 and 4 MeV was compatible with the one measured with CUORICINO. Further testswere therefore necessaries in order to find out the source responsible for this contribution. In a second test (RAD2) with the same detector array the crystals were faced to a higher mass and surface of teflon (a 10x10x0.5 cm 3 sheet facing 4 crystals), gold wires (12.7 m facing 2 crystals) and silicon heaters (~30 cm 2 facing 2 crystals), thus increasing the sensitivity for these components contaminations with respect to CUORICINO. The measured background have proved that this source can't be responsible of the rate measured in CUORICINO and in RAD1 in the 3-4 MeV energy region. Another test (RAD3) is presently running in hall C in order to have a clear answer with respect to copper surface contamination. All the copper parts facing the detectors have been covered with 5 layers of ~12/im low contaminated polyethylene sheet, in order to stop a particles generated by decays occurring in the Cu surface. In a second run thermal neutrons will be tested by changing the shields: the 5 cm of copper will be sostituted by lead and 1 cm of CB4 and 6 cm of paraffin will be added. Preliminary results show a background reduction in the 3-4 MeV region of a factor ~ 2 . 6. Conclusion The analysis performed on CUORICINO data was fundamental in order to disentangle the main sources responsible of the background measured in the (3/3(Qv) energy region. And intense and promizing R&D has begun in order to reduce the identified contaminations in view of the next generation CUORE experiment. References 1. 2. 3. 4. 5.

A.Strumia and F.Vissani Nucl.Phys. B726, 294-316 2005 S.Pascoli and S.T.Petkov Nucl. Phys. B580, 280 2004 C.Amaboldi et al. (hep-ex/0501034) D. Twerenbold, Rep. Prog. Phys. 59 349 1996 A.Arnaboldi et al, CUORE proposal

APPLICATION OF NUCLEAR TRACK DETECTORS IN ASTROPARTICLE AND NUCLEAR PHYSICS S. CECCHINI IASF/INAF, 40129 Bologna, Italy INFN, Sez. Bologna, 40127Bologna, Italy S. BALESTRA, M. COZZI, G. GIACOMELLI, M. GIORGINI, S. MANZOOR, E. MEDINACELI, L. PATRIZII, V. POPA, G. SIRRI, M. SPURIO, V. TOGO Dipartimento di Fisica, Universita di Bologna, 40127Bologna, Italy INFN, Sez. Bologna, 40127 Bologna, Italy T. CHIARUSI

Dipartimento di Fisica, Universita la Sapienza, 00185 Roma, Italy After a brief presentation of the technical methods employed and the calibrations performed on different Nuclear Track Detectors (NTDs), the results obtained by their use in different fields of astroparticle and nuclear physics are presented. In particular the results of searches for magnetic monopoles, nuclearites and other exotica in the cosmic radiation carried out by the Bologna Group. Also an application of these detectors in an experiment for the determination of the charge spectrum of primary cosmic ray particles is described. Example of the use of the same detectors for the measurement of the total and partial charge changing fragmentation cross sections of relativistic ions in several targets are also reported.

1. Introduction The technique of Solid-state Nuclear Track Detectors (SSNTDs) was born almost 50 years ago. Since then the field has grown enormously; numerous Conferences have been dedicated to the topic and several books cover different aspects of this technology [1]. Starting from 1986 the Bologna group has devoted part of its activity to the use of plastic materials (CR39, Makrofol, Lexan) in various research fields and to the improvement of industrial production of CR39 expressly for science applications [2]. Here we briefly summarize the results obtained during the decennial experience acquired in Astroparticle and Nuclear physics experiments using this detection technique. 149

150

2. The Nuclear Track Detectors (NTDs) The working principle of plastic NTDs is qualitatively presented in Fig.l. When a particle crosses such a detector the polymeric chains along its path are affected giving origin to the so-called "latent track". In the case of a particle with an electric charge Z and velocity (3 (= v/c), the produced damage can be related to Z/p. The latent tracks become visible under an optical microscope after chemical etching, typically in aqueous solution of NaOH or KOH - for the materials considered here - when the etching velocity along the latent track (vx) is larger than the one for the bulk of the material (vB). The temperature and concentration of the etching bath are chosen according to the type of application. For a through going particle, in the initial stage of the etching two cones are formed on both sides of the detector. The size and shape of such cones depend on the energy loss of the incident particle; the measurement of the cone base area or heights allows, through an appropriate calibration, to determine the characteristics of the particle.

Figure 1. (left) The breaking of polymeric chains of plastic NTD by a crossing particle; (right) crossview of the formation of the track after etching for two different times [1].

The relation between the parameter p =vT /vB, the observable quantity, and the REL is obtained through calibrations. A typical set-up at an accelerator ion beam would consist in few sheets of detectors upstream of some target material (Cu, Pb, C, Al and so on) used for the ion fragmentation, and several NTD sheets placed downstream of the target in order to record the tracks of the nonfragmented ions as well as of the fragments generated in the target. Fig 2 shows the distribution of the etch cone base areas produced by 158A GeV In + ions and their fragments at the CERN SPS in CR39. By computing the REL values corresponding to the different relativistic ions, the NTD calibration curves (p vs. REL) can be obtained [3].

151

Different calibrations for different materials and etching conditions appropriate for the specific experiment carried out have been obtained [4]. 200

pyr-

180

7

1 1 1 1 1 1 1 1 1 1

SOFT ETCHING

160 '7

Z=49