The CMS Silicon Strip Tracker: Concept, Production and Commissioning

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Oliver Pooth The CMS Silicon Strip Tracker

VIEWEG+TEUBNER RESEARCH

Oliver Pooth

The CMS Silicon Strip Tracker Concept, Production, and Commissioning

VIEWEG+TEUBNER RESEARCH

Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de.

Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der Rheinisch-Westfälischen Technischen Hochschule genehmigte Habilitationsschrift.

1st Edition 2010 All rights reserved © Vieweg+Teubner | GWV Fachverlage GmbH, Wiesbaden 2010 Editorial Office: Dorothee Koch |Anita Wilke Vieweg+Teubner is part of the specialist publishing group Springer Science+Business Media. www.viewegteubner.de No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the copyright holder. Registered and/or industrial names, trade names, trade descriptions etc. cited in this publication are part of the law for trade-mark protection and may not be used free in any form or by any means even if this is not specifically marked. Cover design: KünkelLopka Medienentwicklung, Heidelberg Printing company: STRAUSS GMBH, Mörlenbach Printed on acid-free paper Printed in Germany ISBN 978-3-8348-1003-8

To Matti, Merle and Carolin

Preface When the experiments at the Large Hadron Collider (LHC) at CERN begin data taking the biggest high energy physics experiments ever will be underway. One of these experiments is the Compact Muon Solenoid (CMS) with more than 3,000 collaborators working at the energy frontier of particle physics. The silicon strip tracker of the CMS experiment is the largest silicon based tracking detector system worldwide. A sensitive silicon surface of about 200 m2 is realised on more than 15,000 individual detector modules. The silicon strip tracker is one of the central subdetector components inside the CMS experiment at the LHC. The CMS experiment is conceived to study proton-proton collisions at a centreof-mass energy of 14 TeV at luminosities up to 1034 cm−2 s−1 . To deliver best possible particle track identification in the very harsh radiation environment inside the CMS detector a tracking device with high granularity, high readout speed and radiation hardness is needed. The construction of the CMS silicon tracker required production methods and quality control mechanisms that are new to the field of particle physics: An easy to use detector module test system that was used by the entire CMS community and partners in industry, and specialised tests for larger subdetector structures where detector modules were tested together with final optical readout components in cold environment. This report gives an overview of the silicon strip tracker and the production and commissioning phase. After an introduction to the CMS detector project and the LHC physics motivation in chapter 1 the basic concepts of silicon based particle detectors are explained in chapter 2. Chapter 3 describes the concept, layout and realisation of the CMS silicon strip tracker in detail. In chapter 4 the production phase and tracker commissioning is described together with first experiences running the tracker, while chapter 5 draws a final conclusion. The results in this report represent the status as of autumn 2008. Building a huge device like the CMS silicon strip tracker is team work with many enthusiastic colleagues worldwide. I wish to thank the entire CMS tracker collaboration for the fantastic working atmosphere over the past years. This atmosphere made the work described here possible. I also wish to thank all colleagues

VIII

Preface

at the participating institutes at the RWTH Aachen, I. Physikalisches Institut b (Prof. Dr. Stefan Schael, Prof. Dr. Lutz Feld) and III. Physikalisches Institut b (Prof. Dr. Günter Flügge, Prof. Dr. Achim Stahl). Furthermore I would like to thank Prof. Dr. Achim Stahl, Prof. Dr. Joachim Mnich and Prof. Dr. Gian Mario Bilei for proof-reading and refereeing this report.

Oliver Pooth

Contents 1

Introduction 1.1 The LHC project . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The LHC physics programme . . . . . . . . . . . . . . . . . . . . 1.3 The CMS experiment . . . . . . . . . . . . . . . . . . . . . . . .

1 1 6 8

2

Semiconductor Detectors 2.1 The p-n junction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Signal creation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Radiation effects . . . . . . . . . . . . . . . . . . . . . . . . . .

21 23 27 36

3

The CMS Silicon Strip Tracker 3.1 Tracker concept . . . . . . . . . 3.2 Silicon strip detector modules . 3.3 Readout, triggering and services 3.4 Radiation hardness . . . . . . . 3.5 Tracker substructures . . . . . . 3.6 Laser Alignment System . . . . 3.7 Cooling system . . . . . . . . . 3.8 Material budget . . . . . . . . . 3.9 Expected performance . . . . .

39 39 44 53 66 67 80 81 82 83

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4

Detector Production and Commissioning 87 4.1 Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 4.2 Commissioning experiences . . . . . . . . . . . . . . . . . . . . 116

5

Conclusion

Bibliography

129 131

List of Figures 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13

Schematic view of the LHC. . . . . . . . . . . . . . . . . . . . . Cross section of the LHC dipole and quadrupole magnets. . . . . . Schematic layout of the LHC machine. . . . . . . . . . . . . . . . CERN accelerator complex. . . . . . . . . . . . . . . . . . . . . QCD predictions for hard-scattering cross sections at the LHC. . . Higgs boson production cross section and branching ratios. . . . . CMS exploded view. . . . . . . . . . . . . . . . . . . . . . . . . Longitudinal r − z view of the CMS layout. . . . . . . . . . . . . Layout of the CMS muon detector system. . . . . . . . . . . . . . Location of the electromagnetic and hadronic subdetectors inside the CMS magnet coil. . . . . . . . . . . . . . . . . . . . . . . . . One quarter of the hadronic calorimeter. . . . . . . . . . . . . . . The electromagnetic calorimeter. . . . . . . . . . . . . . . . . . . CMS pixel detector system. . . . . . . . . . . . . . . . . . . . . . Overview of the CMS tracking system and its substructures. . . .

2 3 4 6 8 9 11 12 13

Bond representation of n-type and p-type silicon. . . . . . . . . . Schematic view of a p-n junction. . . . . . . . . . . . . . . . . . Model assumption to describe a p-n junction. . . . . . . . . . . . Mean energy loss in different materials according to the BetheBloch equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . Rate of energy loss due to ionisation versus the kinetic energy of a traversing pion in silicon. . . . . . . . . . . . . . . . . . . . . . . Parameterisation of Landau distributions for pions. . . . . . . . . Experimental energy loss distributions for 2 GeV/c positrons, pions and protons. . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross section of a partially depleted silicon pixel sensor. . . . . . Two different pixel layouts. . . . . . . . . . . . . . . . . . . . . . Photograph of one silicon pixel module. . . . . . . . . . . . . . . A typical layout of a silicon based strip detector. . . . . . . . . . . Response function of the detector with digital readout. . . . . . . Particle detection with reversely biased diodes. . . . . . . . . . .

24 25 26

14 15 16 17 18

29 30 30 31 32 32 33 33 34 35

XII

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 3.31 3.32 3.33 3.34 3.35

List of Figures

Effective doping concentration as a function of high energy proton flux. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layout of the CMS silicon tracker. . . . . . . . . . . . . . . . . . Number of measured hit positions as a function of the pseudorapidity. Silicon strip detector modules. . . . . . . . . . . . . . . . . . . . All tracker end cap module types. . . . . . . . . . . . . . . . . . Layout of the CMS silicon sensors. . . . . . . . . . . . . . . . . . Technical drawing of one corner of a sensor and a photograph of the same area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . The different silicon sensor geometries. See table 3.3 for the exact dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photograph of a frame assembly plate and all parts necessary for a TEC ring 6 frame. . . . . . . . . . . . . . . . . . . . . . . . . . . Gluing scheme of a tracker end cap module with two sensors. . . . Pitch adapter used on a tracker outer barrel stereo module. . . . . Microscopic views of the pitch adapter. . . . . . . . . . . . . . . Front-end hybrid design. . . . . . . . . . . . . . . . . . . . . . . Hybrid cross section. . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram of one channel in the APV25-S1. . . . . . . . The APV25-S1 deconvolution mode. . . . . . . . . . . . . . . . . The APV25-S1 data frame. . . . . . . . . . . . . . . . . . . . . . Encoding and decoding of the trigger and clock signal. . . . . . . Block diagram of the Detector Control Unit. . . . . . . . . . . . . Photograph of a front-end hybrid used for a TEC module. . . . . . Analogue and digital opto hybrid. . . . . . . . . . . . . . . . . . Photograph of a front-end driver. . . . . . . . . . . . . . . . . . . The CMS tracker read out scheme. . . . . . . . . . . . . . . . . . Depletion voltage after irradiation. . . . . . . . . . . . . . . . . . Behaviour of irradiated TOB modules. . . . . . . . . . . . . . . . Integrated TIB modules on half-shells. . . . . . . . . . . . . . . . Inner barrel modules placed on a cooling loop. . . . . . . . . . . . Sketch of a TOB rod. . . . . . . . . . . . . . . . . . . . . . . . . Photograph of a TOB rod. . . . . . . . . . . . . . . . . . . . . . Fully equipped TOB rod. . . . . . . . . . . . . . . . . . . . . . . Photograph of one tracker end cap. . . . . . . . . . . . . . . . . . Photograph of one end cap turned by 90 degrees. . . . . . . . . . Integrated long front petal. . . . . . . . . . . . . . . . . . . . . . Integrated long back petal. . . . . . . . . . . . . . . . . . . . . . Cooling pipes inside a long front petal. . . . . . . . . . . . . . . .

41 43 44 45 45 47 47 49 51 52 53 53 54 55 56 57 58 59 60 61 62 63 65 67 68 69 69 71 72 74 75 76 77 78 79

X III

List of Figures

3.36 3.37 3.38 3.39 3.40

Layout of the laser alignment system. . . . . Silicon tracker material budget. . . . . . . . . Expected performance. . . . . . . . . . . . . Transverse momentum resolution for muons. Global CMS track reconstruction efficiency. .

. . . . .

81 82 85 86 86

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Production flow of detector modules for the tracker end caps. . . . Basic ARC system. . . . . . . . . . . . . . . . . . . . . . . . . . Single module test box. . . . . . . . . . . . . . . . . . . . . . . . Mounting precision for all TEC modules. . . . . . . . . . . . . . Mechanical precision of all TEC modules. . . . . . . . . . . . . . Production and quality control flow. . . . . . . . . . . . . . . . . Mechanical defects of the sensor surface. . . . . . . . . . . . . . Pinholes and shorts. . . . . . . . . . . . . . . . . . . . . . . . . . Leakage current versus bias voltage in a single module test. . . . . Behaviour of open bonds and pinholes in the common mode subtracted noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Response of the APV channels to injected charge measured in peak mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Behaviour of defects. . . . . . . . . . . . . . . . . . . . . . . . . Detection of pinholes. . . . . . . . . . . . . . . . . . . . . . . . . Weekly assembly and bonding rates of all TEC detector modules. . Average pull force necessary to break a bond wire. . . . . . . . . Leakage current measured at a depletion voltage of 450 V. . . . . Number of faulty channels on all TEC detector modules. . . . . . Leakage current at a depletion voltage of 450 V. . . . . . . . . . . Number of faulty channels. . . . . . . . . . . . . . . . . . . . . . Photograph of the petal long term test set-up. . . . . . . . . . . . Temperatures during a cold cycle measured on the modules. . . . Petal long term test results. . . . . . . . . . . . . . . . . . . . . . Common mode subtracted noise. . . . . . . . . . . . . . . . . . . Petal long term test results. . . . . . . . . . . . . . . . . . . . . . Leakage current measured at a bias voltage of 450 V. . . . . . . . The petal production period. . . . . . . . . . . . . . . . . . . . . Noise distribution for all channels in the TOB. . . . . . . . . . . . Noise distribution for both TECs. . . . . . . . . . . . . . . . . . . Equivalent noise charge values for all seven end cap rings. . . . . The silicon strip tracker part in the MTCC set-up. . . . . . . . . . Cluster charge distributions. . . . . . . . . . . . . . . . . . . . .

88 89 90 91 92 94 95 95 97

4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29 4.30 4.31

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98 99 100 102 104 105 105 106 106 107 108 111 111 112 112 113 113 114 115 116 117 118

XIV

4.32 4.33 4.34 4.35 4.36 4.37 4.38 4.39 4.40 4.41 4.42

List of Figures

Cluster noise distributions. . . . . . . . . . . . . . . . . . . . . . Example distributions for a ring 4 module on a front petal. . . . . Signal-to-noise ratio for various modules without magnetic field. . MTCC cosmic trigger scintillator positions. . . . . . . . . . . . . Test results from the tracker cosmic challenge. . . . . . . . . . . . Signal-to-noise ratio measured in the tracker slice test. . . . . . . Mean common mode subtracted noise corrected for the tick mark height. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mean common mode subtracted and tick height corrected noise as a function of the strip length . . . . . . . . . . . . . . . . . . . . Hit reconstruction efficiency. . . . . . . . . . . . . . . . . . . . . An event of the third phase of the Cosmic Run at Zero Tesla. . . . An event of the Cosmic Run at Four Tesla. . . . . . . . . . . . . .

118 119 120 122 123 124 125 126 126 128 128

List of Tables 1.1

LHC parameters for proton-proton collisions. . . . . . . . . . . .

5

2.1

Basic properties of silicon. . . . . . . . . . . . . . . . . . . . . .

22

3.1 3.2 3.3 3.4 3.5

Radiation levels for different radii in CMS. . . . . . . . . . . . . Design parameters of thin and thick silicon sensors. . . . . . . . . Specifications of all sensors of the CMS tracker. . . . . . . . . . . Distribution of silicon strip modules across the subdetector systems. Low voltage groups on front and back petals. . . . . . . . . . . .

40 48 49 66 80

4.1

Produced TEC detector modules according to the individual ring geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Petal grading after petal integration. . . . . . . . . . . . . . . . . 110

4.2

1 Introduction To pinpoint the smallest fractions of matter in the universe the largest machine in high energy physics ever is being commissioned at CERN near Geneva in Switzerland. The main motivation of the Large Hadron Collider project LHC is to study the nature of electroweak symmetry breaking for which the Higgs mechanism is presumed to be responsible. The experimental study of the Higgs mechanism will explore the consistency of the Standard Model in particle physics at energy scales above about 1 TeV. Many other compelling reasons motivate the investigation of the TeV energy scale provided by the LHC. Possible alternatives or extensions to the Standard Model invoke new symmetries or new fundamental forces and constituents. Important discoveries pointing towards a grand unified theory could be in reach. The expected discoveries at the LHC experiments could take the form of supersymmetric extensions of the Standard Model or extra space dimensions, the latter requiring the gravitational force at the TeV scale to be modified. Past experiments proved that hadron colliders are best suited to explore new energy domains. The region of 1 TeV constituent centre-of-mass energies can only be explored at very high luminosities and proton energies – both provided by the LHC which is specially designed to study physics at this energy scale. A wide range of physics will be possible with the increase in energy by a factor of seven and the increase in integrated luminosity by a factor of 200 over the current hadron collider experiments. The experimental challenge to build and run a reliable high energy physics experiment is taken by four giant detector projects. Each of these detectors consists of state-of-the-art subdetector components. One of these subdetectors – the silicon strip based inner tracking system of the CMS experiment – its concept, the mass production phase and the commissioning is described in this report.

1.1 The LHC project The Large Hadron Collider is a proton proton particle collider with a circumference of 26.7 km, 50 to 175 m underground. Two counter rotating particle beams with bunches of protons provide collisions at an energy of 7 TeV per proton beam. In a later phase of the LHC project heavy ion collisions are foreseen up to the range

2

1 Introduction

of PeV – more than 30 times the energy used by present day accelerators to study a new state of matter called the quark-gluon plasma. Inside a continuos vacuum the particle beams are guided by a 8.33 T magnetic dipole field. This field is produced by superconducting magnets operating at a temperature of 1.9 K. Two apertures per magnet allow for the counter-rotating proton beams in each of the 1,232 dipole magnets with radio frequency cavities providing an increase in the proton energy of 485 keV/turn. A single iron yoke and the cryostat are shared by the two beam pipes. Several hundreds of quadrupoles and higher order magnets keep the particle beams focused and the motion stable for hours. In total the length of more than 23 km of the LHC circumference is equipped with superconducting magnets (18 km with dipole magnets, 4.5 km with quadrupole magnets). Eight straight segments along the collider (LHC points 1 to 8) are potentially foreseen for beam collisions. Figure 1.1 shows a schematic view of the LHC and the experimental areas. Four experiments where particle collisions will be studied are located along the circumference. Figure 1.2 shows the cross sections of LHC dipole and quadrupole magnets. To provide the highest possible luminosity for the four experiments 11.5 × 1010

Figure 1.1: Schematic view of the LHC and the experimental areas [1].

1.1 The LHC project

3

Alignment target Main quadrupole bars Heat exchanger pipe Superinsulation Superconducting coils Beam pipe Vacuum vessel Beam screen Auxilliary bars Shrinking cylinder / He I vessel Thermal shield (55 to 75 K) Non-magnetic collars Iron yoke (cold mass, 1.9 K) Dipole bus-bar Support post

Alignment fixture Beam screen

Heat exchanger pipe

Cold bore

Cold mass assembly

Superconducting coils

Radiation screen

Stainless steel collars

Superinsulation

Iron yoke laminations

Thermal shield

He II vessel

Vacuum vessel

Bus bars

Support post

Figure 1.2: Cross section of the LHC dipole (top) and quadrupole magnets (bottom) [2].

protons per particle bunch are squeezed into a tiny space region at the interaction points. This leads to approximately 20 proton proton collisions per bunch crossing when the LHC is operating with nominal beam currents. The particle bunches

4

1 Introduction

will collide with a frequency of 40.08 MHz. At the design luminosity, each beam consists of k = 2808 bunches with N = 11.5 · 1010 protons per bunch. With a revolution frequency of f = 11.25 kHz and beam sizes at the collision points in the directions perpendicular to the beam axis of σx = σy = 16 μm, the peak luminosity of the collider is kN 2 f = 1034 cm−2 s−1 Ldesign = F 4πσx σy with F = 0.83, a factor introduced to take into account the beam crossing angle of 283 μrad at the collision points. During the first years of LHC operation, the so-called low-luminosity phase, the luminosity will be considerably lower (up to about Linitial = 1033 cm−2 s−1 ), leading to an integrated luminosity of about 20 fb−1 per year. In the so-called high-luminosity phase, integrated luminosities of the order of 100 fb−1 per year are expected. A schematic view and the parameters of the LHC are given in figure 1.3 and table 1.1.

Figure 1.3: Schematic layout of the LHC machine [3]. The location of the experiments, the sections for beam injection, acceleration (RF), cleaning and beam dumping are shown.

1.1 The LHC project

5

particles beam energy circumference dipole field bending radius beam crossing points L frequency bunch spacing pp collisions per b.c. crossing angle bunch length beam radius L lifetime fill time acceleration period fill-in energy radio frequency particles per bunch bunches per ring beam current stored energy

7 TeV,

protons √ s = 14 TeV 26.659 km 8.33 T 2,804 m 4

1034 cm−2 s−1 40.08 MHz 25 ns (7.48 m) 20 (inelastic) 283 μrad 7.5 cm 16 μm 10 - 20 h 360 s 1200 s 450 GeV (SPS) 400.8 MHz 11.5 × 1010 2808 536 mA 334 MJ

Table 1.1: LHC parameters for proton-proton collisions.

At a centre-of-mass energy of 14 TeV the total proton proton cross section is 110 mbarn with a contribution of approximately 60 mbarn for inelastic scattering. Elastic scattering and diffractive events do not give rise to particles with sufficient large transverse momentum pt with respect to the LHC beam axis so that these events are difficult to detect in the large detectors. The LHC beam structure is determined by the injection scheme and properties of the beam dump system. Figure 1.4 shows the entire CERN accelerator complex needed to operate the LHC. Protons stemming from a duoplasmotron source are accelerated to 750 keV by a radio frequency quadrupole over a length of 1.75 m. LINAC 2 then provides protons with an energy of 50 MeV after 30 m of acceleration. In 1.2 seconds the PS Booster accelerates all protons by means of four

6

1 Introduction

staggered beam pipes to 1.4 GeV and defines the LHC beam emittance. The Proton Synchrotron (PS) needs 3.6 seconds to accelerate all protons to 25 GeV and forms 72 bunches with a length of 4 ns and a time spacing of 25 ns. The Super Proton Synchrotron (SPS) with a circumference of 6,911 metres employs 744 dipole magnets (2 Tesla field) and 216 quadrupole magnets and accelerates the proton bunches to 450 GeV before they are injected to the LHC. This operation is repeated 12 times for each counter-rotating beam. At each transfer enough space is reserved to accommodate the rise time of the injection kicker magnets. Finally a longer time gap is reserved for the rise time of the dump kicker magnets by eliminating one PS batch.

Figure 1.4: CERN accelerator complex [4]. Protons for the LHC are accelerated in the sequence LINAC, BOOSTER, PS, SPS, LHC.

1.2 The LHC physics programme In the LHC starting phase with the relatively low luminosity of L = 1032 − 1033 cm−2 s−1 Standard Model processes like W+/− and Z0 pair production and top quark decays will be investigated with high precision. The large production

1.2 The LHC physics programme

7

cross section of bb quark pairs allows studies of CP violation effects in B meson systems with very high accuracy. Of particular interest are the parameters of the Cabbibo-Kobayashi-Maskawa matrix elements violating the CP symmetry in the weak interaction. The LHC will produce 1012 to 1013 bb events per year thus allowing a detailed study in both the B sector and rare B decays, like B→ μ μ to test physics beyond the Standard Model. The production and decay of top quarks can be studied in detail. The mass of the top quark and its branching ratios can be measured in various decay channels and it may also be possible to detect rare top decays like t→bH+ or t→Zc. Approximately 10,000 tt quark pairs will be produced per day in the low luminosity phase. In the later phase of LHC operating at design luminosity of L = 1034 cm−2 s−1 new phenomena can be studied in a wide range of cross sections. Figure 1.5 shows the cross section from QCD predictions and expected event rates as a function of the centre-of-mass energy for proton proton collisions. The quest for the Higgs boson is one of the major goals of the LHC experiment. The expected production cross section for the Standard Model Higgs boson is as small as several femtobarn. This challenge requires an accelerator with high luminosity and experiments able to observe up to 1017 particle collisions in total. The two omnipurpose LHC experiments CMS (Compact Muon Solenoid) and ATLAS (A Toroidal LHC Apparatus) are designed to find the Higgs boson in a mass range between 90 GeV and 1 TeV by the detection of its decay products. The Higgs boson production cross sections and branching ratios are shown in figure 1.6. In so-called supersymmetric extensions of the Standard Model superpartners of the known particles are introduced. The strongly interacting squarks and gluinos should have large cross sections at hadron colliders. The LHC allows to search for these supersymmetric particles in a mass range of up to 2 TeV. Especially the identification of the lightest supersymmetric particle (LSP) which is predicted to be stable in some supersymmetric models would provide an interesting candidate to explain the existence of dark matter in the universe. In total four large experiments are located along the Large Hadron Collider ring: ALICE, ATLAS, CMS and LHC-b. ATLAS and CMS are general purpose experiments, while ALICE (A Large Ion Collider Experiment) and LHC-b are designed to study specific aspects of the physics possible at the LHC. The ALICE collaboration proposed a detector experiment for heavy nucleon collision to study the physics of strongly interacting particles at extremely high densities. The formation of a new phase of matter is expected, the quark-gluon plasma. The LHC-b experiment is specially designed to study CP violation in B meson decays with the help of a one arm spectrometer. The entire LHC physics programme is described in the physics design reports of the four experiments ([7], [8], [9], [10]).

8

1 Introduction 109 mtot 7

10

Tevatron

LHC

10

8

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105

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34

> 3s/20)

101

102

mW mZ jet

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jet

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10

3s (TeV) Figure 1.5: QCD predictions for hard-scattering cross sections at the LHC [5]. Production cross section and event rates for some characteristic processes in proton proton collisions as a function of the centre-of-mass energy for the LHC design luminosity of L = 1034 cm−2 s−1 . The cross section for the Tevatron experiments (a proton anti-proton √ collider project at Fermilab in the United States) is given for comparison up to s = 4 TeV.

1.3 The CMS experiment CMS will be installed about 100 metres underground close to the French village of Cessy, between Lake Geneva and the Jura mountains. More than 2,800 scientists from over 180 institutes worldwide work on the CMS project.

1.3 The CMS experiment

9

(ppH+X) [pb]

10 2

s = 14 TeV Mt = 174 GeV

ggH

10

CTEQ6M

1 _

10 10 10 10

qq’HW

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qqHqq

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gg,qqHtt -3 _

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Figure 1.6: Higgs boson production cross section (top) and branching ratios for various possible decay modes (bottom) [6].

The main challenge of the general purpose experiment CMS is the registration of reaction products coming from the hadron collisions by measuring their energy, mass and electric charge with highest possible precision. A reliable detection requires a precise track reconstruction of charged particles, determination of coordinates and energy depositions. At the design luminosity of LHC about 1,000 particles from approximately 20 overlapping proton proton interactions traverse the experiment starting in the inner tracking volume for each proton bunch crossing every 25 ns. Searches for the Standard Model Higgs boson, supersymmetric

10

1 Introduction

particles and all events containing neutrinos, escaping the detector without leaving a signal, depend crucially on the measurement of missing momentum. Therefore all detector subsystems and the entire experiment have to be hermetic. The requirements for the CMS experiment to meet the LHC physics programme are: - Excellent muon identification and momentum resolution over a wide range of momenta and angles. Excellent di-muon mass resolution with ≈ 1% at 100 GeV/c, and the determination of the charge of muons with p < 1 TeV/c. - Excellent momentum resolution for charged particles and reconstruction efficiency in the inner tracking system. Most efficient triggering and offline tagging of τ-leptons and b-jets. - Excellent electromagnetic energy resolution, di-photon and di-electron mass resolution with ≈ 1% at 100 GeV/c, providing wide geometric coverage, measurement of the direction of photons and/or correct space points of the primary interaction vertex, π 0 rejection and efficient photon and lepton isolation. - Excellent missing transverse energy and dijet mass resolution, requiring hadron calorimeters with a large hermetic geometric coverage and with fine lateral segmentation. An overview of the CMS detector is given in figure 1.7 showing the cylindrical shape and the symmetry with respect to the azimuthal angle φ . CMS is 21 m long with a diameter of 15 m and a total weight of 12,500 t. The experiment is built around a superconducting solenoid creating a 3.8 T magnetic field parallel to the beam axis inside the coil of 13 m length and a diameter of 3 m. Here the inner tracking devices (a silicon pixel and a silicon strip detector system) and calorimeter systems are located. The inner part of the inner tracking system is equipped with silicon pixel detectors whereas in the outer part silicon strip detectors are used. The tracking section is then followed by an electromagnetic calorimeter and a hadronic calorimeter inside the superconducting solenoid. Outside the coil the muon spectrometer is embedded in the iron return yoke. The detection of the two-photon decay of the Standard Model Higgs boson for example requires an excellent electromagnetic calorimetry. The calorimeter must not be disturbed by material causing the photons to convert before reaching the calorimeter system. The aperture of the magnet coil is large enough to accommodate the inner tracking system and the calorimetry inside. The iron yoke equipped with the muon spectrometer of CMS returns the magnetic flux making the entire experiment compact as indicated by its name. The


11

Figure 1.7: CMS exploded view indicating all subdetector systems [8]. The inner tracking system is surrounded by the calorimeter system both embedded inside the coil of the superconducting magnet. The magnetic flux is returned in the joke with embedded muon spectrometer.

magnetic return field is large enough to saturate 1.5 m of iron, allowing four muon stations to be integrated to ensure full geometric coverage. The integral bending power of the magnetic field is 17 Tm in a pseudorapidity range of |η| < 1.5, with η = − ln(tan(θ /2)), decreasing to 6 Tm in the end cap region up to |η| = 2.5.1 In the following subsections all subsystems are briefly described starting from the outer to the inner parts. A longitudinal view of CMS is given in figure 1.8. 1 The

right handed CMS coordinate system: origin centred at the nominal collision point inside the experiment, the y-axis pointing vertically upwards, the x-axis pointing inwards radially to the centre of the LHC ring, the z-axis directs parallel to the beam direction toward the Jura mountains from LHC Point 5. The azimuthal angle φ is measured from the x-axis in the x-y plane. The polar angle θ is measured from the z-axis. The distance from the beam pipe centre is denoted as radius r. Therefore the momentum and energy measured transverse to the beam direction, denoted as pt and Et respectively, are derived from the x and y components. The imbalance of energy measured in the transverse plane is denoted by Etmiss .

12

1 Introduction η=1

η= 0.5

4.02 m 2.95 m

Barrel ECAL Tracker

1.79 m 1.29 m 0.44 m 0.00 m

0.00 m

Endcap HCAL

Endcap ECAL

Barrel HCAL

3.88 m

6.45 m

7.24 m

8.49 m

9.75 m

14.96 m

10.86 m

Forward HCAL

η= 5.31

4.91 m

Superconducting COIL

ME1

η= 3.0

5.98 m

MB 1 Iron Yoke

η= 2.4

ME3 Endcap ME2

η= 1.479

Endcap Muon Station 4 (ME4)

-z

7.38 m 7.00 m

Barrel Muon Station 4 (MB 4) Iron Yoke MB 3 Barrel MB 2

5.68 m

η= 1.1

r

Figure 1.8: Longitudinal r − z view of the CMS layout (based on [8]).

1.3.1 The muon spectrometer Muon identification, momentum measurement and triggering are the main tasks for the muon spectrometer. Muons that do not interact much with matter give rise to signals in the muon spectrometer after traversing sixteen radiation lengths of the subdetector components inside. Muons are very important decay particles in the Higgs boson identification strategy. For this reason CMS houses a muon system that takes part in the event trigger decision. Detectors capable of delivering extremely fast signals allow bunch crossing association and application of decisive cuts on the transverse momentum (pt ) already at trigger level. For a reliable muon detection three different detector technologies are used: drift tubes, cathode strip chambers and resistive plate chambers. Each muon station consists of several layers of aluminium drift tubes in the barrel region and cathode strip chambers in the end cap region, complemented by resistive plate chambers. Drift tubes are relatively common drift chambers filled with a gas mixture of Ar/CO2 and used in the CMS barrel region (|η| < 1.3) where the expected particle rate is below 10 Hz/cm2 . Inside the drift tubes the electron and ion drift lines are nearly undistorted since the magnetic field is almost zero between the barrel return yoke plates. Four stations each housing twelve planes of drift tubes are organised in three subunits, two measuring the coordinates in the bending plane (r, φ ) and one measuring the coordinate along the beam axis (z). For each layer the spatial resolution is better than 250 μm allowing a determination of

R (c m)


13

800

DT

MB 4

eta = 0.8

1.04

RPC

1.2

700 MB 3

600 MB 2

500

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400

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400

600

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ˇ

300

2.4

ME 4

CSC 800

1000

1200 Z (c m)

Figure 1.9: Layout of the CMS muon detector system in the r − z projection [11]. DT: drift tubes, CSC: Cathode strip chambers, RPC: resistive plate chambers.

the muon direction with an accuracy of about 1 mrad. Cathode strip chambers, in the CMS case multiwire proportional chambers filled with an Ar/CO2 /CF4 gas mixture, are used in the forward muon spectrometer part (0.9 < |η| < 2.4), in the high magnetic field region between the iron return yoke plates and the forward part. Trapezoidally shaped detectors are arranged in rings around the beam pipe and each end cap houses two stations separated by an iron plate from the return yoke. One station provides six measurements of φ by the radial cathode strips and six measurements of r by the anode wires arranged perpendicular to the middle cathode strip. In the r − φ plane a spatial resolution of approximately 75 μm is expected. Because of their excellent time resolution resistive plate chambers are used to trigger within one bunch crossing on muon tracks. They are added to both the barrel and the end cap part (|η| < 2.1). Six layers in the barrel region and four layers in the end cap region trigger on high and low pt muons. The momentum resolution of the entire system is between 8% and 15% for 10 GeV muons and between 20% and 40% for 1 TeV muons. It depends on the pseudorapidity. Figure 1.9 shows the layout of one quarter of the muon detector system.

14

1 Introduction

1.3.2 The magnet The magnetic field inside CMS is produced by a 12.5 m long superconducting magnet with an inner diameter of 5.9 m. In total 2,168 aluminium reels are able to conduct a current of 19,500 A producing a solenoidal magnetic field of up to 4 Tesla. This results in a stored field energy of about 2.7 GJ. Its return yoke weighs 11,000 tons and consists of two end caps, each of which has three disks and a barrel yoke that is made of five rings. The return yoke is equipped with the detectors of the muon spectrometer mentioned above (see the outer white areas in figure 1.9).

1.3.3 The calorimeter system Strongly interacting particles (hadron jets) are measured in the hadronic and electromagnetic part of the calorimeter while electron and photon showers are detected and measured by the electromagnetic calorimeter part. A good missing transverse energy resolution, important for several physics processes, is ensured by the calorimeter system. An overview of the calorimeter system is given in figure 1.10.

Figure 1.10: Location of the electromagnetic and hadronic subdetectors inside the CMS magnet coil.

Hadronic calorimeter: The hadronic calorimeter is a very hermetic detector with a coverage of up to |η| = 5.0. It consists of a barrel part, an end cap part and a very forward part at a distance of 6 m from the interaction point very close


15

to the beam axis. It contains 9,072 readout channels organized into four subsystems: barrel (HB, 2,592 channels), end cap (HE, 2,592 channels), outer (HO, 2,160 channels) and forward part (HF, 1,728 channels). It is segmented with Δη × Δφ = 0.087 × 0.087, necessary for the separation of nearby jets, the determination of their direction and an adequate mass resolution. Plastic scintillators are sandwiched between 5 cm thick copper plates or steel absorbers in the end caps and read out by wave length shifting fibres. The light is detected by novel photodetectors (hybrid photodiodes, or HPDs) that can provide high gain and operate in high axial magnetic fields. All material sums up to an absorber thickness of seven nuclear interaction lengths. The expected hadronic energy resolution is: σE /E = 70%/ E/eV ⊕ 4.5% Figure 1.11 shows the layout of one quarter of the hadronic calorimeter. 2 1 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

3

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Figure 1.11: One quarter of the hadronic calorimeter with hadron barrel (HB) |η| < 1.4 and 1.8 m < r < 2.9 m, hadron outer (HO) and hadron end cap (HE) part 1.3 < |η| < 3.0 [12].

Electromagnetic calorimeter: High resolution lead-tungstenate (PbWO4 ) crystals are used in the electromagnetic calorimeter with a coverage in pseudorapidity up to |η| < 3.0. One of the most important search strategies for a light Standard Model Higgs boson or the lightest MSSM Higgs boson is the decay into two photons (H→ γγ). In this mass range the expected width of the Higgs boson is very small and therefore the observed signal width is mainly driven by the energy resolution of the γγ system. To achieve a very compact and fast calorimeter lead-tungstenate crystals (density ρ = 8.28 g/cm3 ) were chosen for their short scintillation decay time matching the 25 ns bunch spacing (80% of the light is emitted in the LHC clock period of 25 ns), their short radiation length of 9 mm and a small

16

1 Introduction

Moliere radius of 2 cm. Each of the 83,000 crystals has a length of 23 cm and a front area of 2×2 cm2 (Δη × Δφ = 0.014 × 0.014) guaranteeing a high granularity. A very good shower containment is guaranteed by providing 25.8 radiation lengths. A block of 5 × 5 crystals matches one hadronic calorimeter segment. The scintillation light is detected by silicon avalanche photodiodes (APDs) in the barrel region and vacuum phototriodes (VPTs) in the end cap region to compensate the relative low light yield of PbWO4 (about 4.5 photoelectrons per MeV at 18◦ C). The energy resolution is measured to be σE /E = 0.4% for electrons and photons of 120 GeV [8]. A pre-shower detector in front of the end caps is used to improve the π 0 /γ separation in the region 1.65 < |η| < 2.61. Figure 1.12 shows a photograph of one PbWO4 crystal and the layout of one quarter of the electromagnetic calorimeter.

Barrel ECAL (EB)

9

.47

y

=1

= 1.

653

Preshower (ES) = 2.6

z

= 3.0

Endcap ECAL (EE)

Figure 1.12: Left: Photograph of a PbWO4 crystal used in the electromagnetic calorimeter. Scintillation light is collected and amplified by avalanche photodiodes connected to the front side of the crystal. Right: Layout of the CMS electromagnetic calorimeter in the y − z projection [13].

1.3.4 The inner tracking system The trajectories of charged particles originating from primary interactions and secondary vertices are measured with high precision and efficiency by the inner tracking system. Inside the homogeneous magnetic field of 3.8 T a full silicon based tracking device with a length of 5.8 m and a diameter of 2.5 m is installed. To cope with the extremely high particle flux and to ensure safe and reliable trajectory identification, a detector with high granularity and readout performance is required. Vertex recognition inside the strong magnetic field is a crucial factor to reach the physics goals of the experiment. The tracking system must reconstruct isolated high pt muon tracks with an efficiency of more than 98%, tracks from charged particles inside a jet with an efficiency of more than 85% for pt ≈ 1 GeV/c and 95% for


17

pt > 10 GeV/c and provide a momentum resolution of δ pt /pt = (15 × pt ⊕ 0.5)% (pt in TeV) for particles in the rapidity range |η| < 1.6. CMS employs three layers of silicon pixel detectors and ten layers of silicon strip detectors covering a sensitive area of approximately 200 m2 . In total 1,440 silicon pixel detector modules and 15,148 silicon strip detector modules are used. Interesting events are likely to contain b-jets and τ-jets originating from the decay of heavy particles e.g. the top quark or the Higgs boson. To allow an efficient tagging of these jets the inner tracking system needs to be as close as possible to the interaction point. The innermost detector layers are composed of hybrid silicon pixel devices delivering high resolution space points with rectangular pixels of 100 μm in r − ϕ and 150 μm in z. The pixelated sensors are bump bonded via indium solder bump bonds to the amplifier and readout chip CMOS. At least two hits per charged particle track will be measured by the pixel system that is divided into a barrel and an end cap region. With respect to the beam axis the pixel detectors are contained in a cylindrical volume defined by -50 cm < z < 50 cm and r < 110 mm with three barrel layers at r = 4.3 cm, r = 7.2 cm and r = 11.0 cm. The silicon pixel system and a sketch of one silicon pixel module are shown in figure 1.13. To guarantee a precise impact parameter resolution in r − ϕ and z, the single hit spatial resolution is σ (z) ≈ σ (rϕ) ≈ 15 μm. Due to the harsh radiation environment the innermost layer has an expected lifetime of just two years.

Figure 1.13: CMS pixel detector system. Overview of the pixel detector system in the high luminosity configuration. Drawing of one silicon pixel module with approximately 65,000 pixels (size: 65 mm × 22 mm).

1 Introduction

2.2 m

18

5.5

m

Pixel Barrel (TPB) Pixel End Cap (TPE) Inner Barrel (TIB) Outer Barrel (TOB) Inner Disks (TID) End Cap (TEC)

Figure 1.14: Overview of the CMS tracking system and its substructures. Silicon pixel detectors set up the innermost part (TPB, TPB) while the outer part is equipped with silicon strip detectors in the four different regions TIB, TOB, TID, and TEC.

The following chapters of this thesis describe in detail the CMS silicon strip tracking detector and the expected performance. The layout of the entire silicon based inner tracking system is shown in figure 1.14. Detailed information of all CMS subsystems can be found in the CMS technical design reports ([11], [12], [13], [14], [15], [16], [17], [18]).

1.3.5 The trigger system At the design luminosity of L = 1034 cm−2 s−1 a huge amount of data is generated inside the CMS detector that is impossible to read out and write to mass storage. A two step trigger system is employed to reduce the amount of data by several orders of magnitude. The level-1 trigger system (L1) is based on custom made electronics and uses data from the calorimeter and muon systems. The data from all subdetectors are stored in pipeline memories and wait for about 3.2 μs for a level-1 trigger decision before the data is discarded or accepted for further processing. The level1 trigger decision is based on the identification of muons, electrons, photons, jets and missing transverse energy. The initial event rate of 40 MHz is reduced to about


19

100 kHz. Data from events accepted by the level-1 trigger are read out and assembled by an event builder system. The so-called high level trigger (HLT) employs a set of sophisticated software algorithms running on a PC farm close to the experiment to analyse the entire event information. The accepted event rate is further reduce for permanent storage and analysis. Finally an event rate of about 100 Hz is accepted by the trigger system and a corresponding data flow of approximately 100 MB/s is stored.

2 Semiconductor Detectors Particle detectors based on semiconducting materials are used in a wide range of applications in various physics fields. The two main applications are tracking of charged particles and the precise energy spectroscopy of photons. Since the 1950s p-n junctions are used to detect signals from charged particles and photons traversing the depletion zone between an n-doped and p-doped material. For 25 years in coincidence with the detection of short lived mesons containing charm and bottom quarks and the study of decaying tau leptons, the particle physics community developed great interest in very fast particle detectors with high resolution. First applications of semiconducting particle detectors in high energy physics experiments date back to the 1970s. Today nearly every large scale high energy physics experiment makes use of silicon strip and/or silicon pixel detectors to precisely determine the trajectories of traversing charged particles. The tracking device of the CMS experiment with a sensitive silicon area of approximately 200 m2 is the largest project of this type today. The principle of operation of semiconductor detectors is similar to an ionisation chamber but is based on solid state material. Compared to the low density of the counting gas in gaseous detectors, semiconductor detectors are able to measure particles with higher material densities. In tracking applications the segmentation of electrodes allows a finer separation of the detection cells and therefore higher spatial resolution compared to gaseous detectors. Charged particles or photons create electron hole pairs in the semiconductor material. Inside an electric field the produced charge carriers are collected and converted to an electric signal that can be amplified and shaped into the appropriate needs. Compared to the counting gas in gaseous detectors the average energy necessary to produce an electron hole pair in a semiconductor is one order of magnitude smaller (2.8 eV for germanium, 3.6 eV for silicon). Because of the small energy gap between valence band and conduction band (0.67 eV for germanium, 1.14 eV for silizium) the detectors are often operated below room temperature to reduce the effect of thermal noise. Basic properties of silicon are summarised in table 2.1. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the determination of the energy of incident photons.

22

2 Semiconductor Detectors

Z (A) atoms density

14 (28.08 u) 4.99 · 1022 /cm3 2.3 g/cm3

energy gap effective state density: conduction band valence band mobility: electrons holes diffusion constant: electrons holes intrinsic charge carrier density resistivity melting point thermal expansion coefficient critical electric field

Eg = 1.14 eV nc = 2.8 · 1019 /cm3 nv = 1.04 · 1019 /cm3 1350 cm2 /(Vs) 480 cm2 /(Vs) 34.6 cm2 /s 12.3 cm2 /s 1235 kΩcm 480 cm2 /(Vs) 1415◦ C 2.5 · 10−6 /◦ C 30 V/μm

Table 2.1: Basic properties of silicon.

Energy spectroscopy is therefore possible using semiconductor detectors with excellent energy resolution compared to gaseous devices. Furthermore diamond based detectors [19] are an alternative to silicon detectors and are expected to offer better radiation hardness compared to silicon detectors. But today they are much more expensive and more difficult to produce even on a small scale. Silicon is the material of choice for the tracking detectors built for the experiments at the LHC. Silicon offers reliable, radiation hard and fast detectors. Using microelectronic planar technology, segmented detectors are produced and used to precisely measure the tracks of charged particles. Radiation hardness is essential for all detectors at the LHC. When irradiated with high fluxes of neutrons or high-energy hadrons (> 5 · 1014 particles/cm2 ) the performance of the semiconductor detector is compromised. Radiation-induced defects completely transform the electrical properties in the crystal lattice. As a result the charge released by

2.1 The p-n junction

23

a traversing particle may be changed drastically. The detectors used at the LHC need to be resistant to the extreme radiation environment.

2.1 The p-n junction A semiconductor is defined as a material with a specific resistance in the range of 10−4 Ωcm < ρ < 1012 Ωcm falling between isolators and conductors. A current in a semiconductor is introduced by the motion of free electrons and holes. At very low temperatures close to 0 K the valence band is completely filled with electrons while the conduction band is free of electrons. The number of free intrinsic charge carriers is close to zero (ni = pi = 0 at 0 K). At ambient temperature approximately 10−12 of all electrons are excited to the conduction band. Ec − EF EF − Ev ni = nc · exp − pi = nv · exp − kT kT with: Ec and Ev : energies at the edge of the conduction band and the valence band nc and nv : volume densities of possible states in the conduction band and in the valence band EF : Fermi energy k: Boltzmann constant T : temperature For pure silicon with ni = pi this leads to the number of free intrinsic charge carriers: Eg √ √ = 1.5 · 1010 /cm2 ni = pi = np = nv nc · exp − 2kT with: Eg = Ec − Ev : the band gap This density of free charge carriers is many orders of magnitude higher compared to the number of charge carriers produced by the ionisation of a traversing charged particle. Therefore a zone free from charge carriers must be created for particle detection. Usually two kinds of doped semiconductor materials are used to achieve a zone depleted from free charge carriers. In n-type silicon atoms having five valence

24


electrons (e.g. phosphorus, arsenic) are implanted with a concentration of approximately 1 : 106 . In p-type silicon the same is done with atoms having only three valence electrons (e.g. aluminium, boron, indium) as indicated in figure 2.1.

Figure 2.1: Bond representation of n-type and p-type silicon [20]. Left: n-doped silicon. The phosphorus atom provides an extra electron (energy level close to the conduction band). Right: p-doped silicon. The aluminium atom provides an extra hole (energy level close to the valence band).

Additional donor and acceptor energy levels are introduced close to the conduction band (n-type) and close to the valence band (p-type) with an energy separation of the order of kT ≈ 0.05 eV. The concentration of free charge carriers is therefore increased drastically compared to the intrinsic free charge carriers in pure silicon. A p-n junction (diode) is then produced by mechanical contact between n-type and p-type material as shown in figure 2.2. Free electrons and holes recombine and a depletion zone is established free from charge carriers. To form a particle detector this diode is used in reverse-biasing to provide an electrical field inside the depletion zone, to remove charge carriers produced by a traversing particle, and to enlarge the depletion zone and therefore the sensitive detector volume. A simple model is used here to discuss the p-n junction parameters. Assuming a charge density distribution as shown in figure 2.3 simple calculations allow to determine the parameters of the junction. The model allows different charge carrier concentration on both sides (xn · nD = x p · nA and d = xn · xd ). The electric field and potential is derived from the Maxwell equation ∇D = ρ with D = εε0 E. ρ d 2V dEx = 2 = dx dx εε0

2.1 The p-n junction n

25 p

n

p

d Ec EF

Ec

Ec

EF

EF Ev

Ev

VC

Ev

Figure 2.2: Schematic view of a p-n junction. The edges of the conduction band and valence band are given for the individual materials (left) and the p-n junction (right). In the equilibrium state after charge annihilation in the contact zone and levelling of the Fermi energies EF a contact potential VC is established. When applying an external potential difference the p-n junction acts like a diode in reverse-biasing.

with: Ex : electric field in direction of the p-n junction V : potential difference With the charge density ρ(x) (see figure 2.3) ⎧ 0 < x < xn ⎨ e · nD ρ(x) = −e · nA −x p < x < 0 ⎩ 0 else the electric field is given by ⎧ ⎨ e · nD (x − xn ) εε0 Ex = −e · nA (x + x p ) ⎩ 0

0 < x < xn −x p < x < 0 else

with the boundary conditions Ex (x ≤ −x p ) = Ex (x ≥ −xn ) = 0. The potential is derived by integration of the electric field. It is shown in figure 2.3 (right). −e·nD 2 0 < x < xn 2 (x − xn ) + εε0V0 εε0V = −e·nA 2 (x + x ) −x p p<x 0 holes undepleted E 0

ionising particle track p+ implant (-300 V)

Figure 2.8: Cross section of a partially depleted silicon pixel sensor. The charged particle leaves a trace of electrons and holes that are collected by the electrodes after drifting in the electric field. The E × B effect deflects the charge carriers perpendicular to the electric field. The signal is spread over several pixels. The effect can be compensated by tilting the sensor according to a given Lorentz angle when the field configuration is known.

Figure 2.9: Two different pixel layouts [26]. Left: p-spray with bias grid. Right: open p-stop rings.

of a pixel barrel module is shown in figure 2.10. Futher details on the CMS silicon pixel detector can be found in [27]. To achieve very fast detectors in regions with a reduced number of charged particles, strip based detectors can be used. The segmented electrode of a silicon strip detector acts like several neighbouring strip-like diodes that give rise to the information of the position where a particle traversed the detector perpendicular to the strip orientation. The basic concept of a strip detector is explained in figure 2.11. Details on the silicon sensors used in the CMS strip tracker are explained in the

2.2 Signal creation

33 readout side

sensor side

Figure 2.10: Photograph of one silicon pixel module with approximately 65,000 pixels (size: 65 mm × 22 mm). See also figure 1.13.

next chapter. The precision of the measurement of the space coordinate perpendicular to the strip orientation depends mainly on the distance between neighbouring strips and the readout method. If only digital information is used, e.g. by taking the strip hit as the measured coordinate, and if effects coming from charge diffusion during the signal collection time and inclination of incoming particles are neglected, the spatial resolution of the strip detector can be derived from the prob-

p SiO 2

Al p+

n bulk

0,3 cm

n+ Al backplane bias

Figure 2.11: A typical layout of a silicon based strip detector (dimensions are not to scale). The p+ strips are implanted in the n-bulk of the sensor. The induced signal is coupled capacitively to the readout electronics. The capacitor is realised by a thin isolating layer between the p+ strips and the readout strips above. The reverse bias voltage is usually contacted to the sensor backplane, realised by a thin aluminium layer on the thin n+ layer on the n-bulk material. On the right side a traversing charged particle is indicated producing a cloud of electrons and holes along the trajectory.

34


ability distribution shown in figure 2.12. F(x) 1

ïp/2

x

p/2

Figure 2.12: The traversing particle left a signal in the vicinity of the central strip. F(x) shows the response function of the detector with digital readout.

With F(x) being

F(x) =

1 0

−p/2 < x < p/2 else

the expectation value and variance is the result of the measurement. The expectation value is given by:

p/2

x =

−p/2 x · 1dx

p/2 −p/2 dx

=

x2 p/2 2 |−p/2 p/2 x|−p/2

=0

The variance is given by:

p/2

2 −p/2 (x − 0) · 1dx

p/2

p2 1 x3 p/2 1 p3 p3 |−p/2 = ( + )= p p 3 3p 8 8 12 −p/2 √ The spatial resolution is therefore given by σx = p/ 12. Strip pitches usually lie in the range between 20 and several hundred micro-metres. The measurement precision can be improved substantially with analogue readout of the induced signals that spread over more than one strip due to diffusion and Lorentz angle effects. The strip pitch must be chosen such that the signal can spread over many strips. Figure 2.13 explains the advantage of analogue readout, where the measured signal heights on the strips are used to improve the spatial resolution with the centre-of-gravity method. Detectors based on germanium are often used for energy spectroscopy applications. While silicon detectors cannot be thicker than a few millimetres germanium σx2

=

=

1 p

x2 dx =

2.2 Signal creation

35

measured signal

Amplifier Shaper Coupling Capacitor p-plane p-strip drifting holes

particle trajectory high energetic primary electron

position

(a)

Signal Distribution measured position of passage

position

(b)

measured position of passage

Amplitude

Amplitude

Amplitude

n-bulk

position

(c)

Figure 2.13: Particle detection with reversely biased diodes [28]. (a) A traversing particle is measured by the induced signal on the p-plane. (b) The p-plane is segmented in strips connected to individual amplification stages. The induced charge is shared between neighbouring strips and measured individually. Weighting the signal heights improves the spatial resolution of such a device compared to the highest possible spatial resolution of strip √ pitch/ 12 for devices with digital readout (only hit or no hit information available). (c) Along the particle track the creation of high energetic primary electrons is possible leading to a deterioration of the highest possible spatial resolution. These electrons are the reason for high entries in the Landau distribution.

can be depleted up to centimetres and therefore can absorb photons up to energies of a few MeV. Present-day high purity germanium detectors use lithium diffusion to make an ohmic n+ contact and boron implantation to make a p+ contact. In medical imaging (mainly in tomography, mammography, and dental x-ray) pixelated detectors are often used based on the charge coupled device (CCD) technology. The analogue shift register is too slow for application in high energy physics experiments, but has many advantages in the other fields. Due to their high quantum efficiencies, linearity of their outputs, e.g. one count for one photon, ease of use compared to photographic plates CCDs are used by astronomers for nearly all UV-to-infrared applications.

36


2.3 Radiation effects Semiconductor detectors in harsh radiation environments suffer from different defects. Two general types of radiation damages to the sensitive detector material can be distinguished: surface damages due to ionising energy loss and bulk (crystal) damages due to non-ionising energy loss (so-called NIEL). As a consequence of the ionising energy loss insulating layers on the surface of detectors can charge up when charged particles traverse. This affects the interstrip capacitance (and therefore the noise of the detector) and the breakdown behaviour of the sensor. Bulk damages appear in the semiconductor crystal lattice as displaced atoms because of the interaction with the traversing particles. These defects, lattice vacancies and atoms at interstitial sites (so-called Frenkel pairs), have the effect of temporarily trapping electrons and holes. Since electrons and holes form the signal, the detector response can be strongly reduced leading to an unusable detector when large amounts of these defects are produced. Damage to the bulk material by charged and neutral particles is more difficult to control. Defects in the lattice induced by knocked-off silicon atoms can be electrically active leading to increased space charge, leakage current or charge trapping. Increased space charge can prevent the electric field from full penetration of the material unless a very high bias voltage is used. Space charge induced by radiation can increase after the irradiation stopped, a phenomenon called reverse annealing. To reduce the effects of high leakage currents and reverse annealing the detectors usually need to be cooled to at least −10◦ C. It has also be shown that adding oxygen into the wafer material reduces the space charge build-up and reduce the reverse annealing for damages caused by charged particles [29]. The breakdown behaviour of sensors can be optimised by so-called multiguard structures allowing a safe detector operation at high depletion voltages. Biased guard rings surround the active area of the sensor. With guard structures long term stability and reduced leakage currents can be achieved and avalanche breakdown can be avoided when high depletion voltages are required. Guard structures have the disadvantage of a larger insensitive area at the sensor borders. A further reduction of the working temperature has shown to reduce the space charge induced defects (Lazarus effect [30]). Electrons and holes trapped by local defects remain trapped for a long period due to the very low thermal energy of the lattice. Trapping defects get filled and therefore inactive. Details are described in [31]. A large electric field minimises the risk of trapping charge carriers. The effective drift length depends on the electric field via the drift velocity. Electrons with a three times longer drift length in silicon compared to holes make a larger

2.3 Radiation effects

37

contribution to the detector signal. The drift length decreases linearly with the radiation dose making detectors with large charge collection distances very inefficient in harsh radiation environments. A detailed description of the stability of detector devices with respect to radiation hardness can be found in [21]. The radiation hardness and properties of the silicon sensors used in the CMS strip tracker is discussed in the following chapter.

3 The CMS Silicon Strip Tracker The inner tracking system is designed to measure as precisely as possible the trajectories of charged particles with a full silicon based system. Originally CMS was foreseen to use two different detector technologies in the central strip tracking part: silicon based strip detectors and micro strip gas chambers (MSGC, [32]) in the outer regions. Finally in the year 1999 the CMS collaboration decided to use a full silicon solution for the entire tracking device, because of superior radiation hardness and dropping prices for silicon sensors. In total more than 9.3 million silicon strips on 24,244 sensors on 15,148 detector modules are read out by about 73,000 front-end chips. More than 1,000 power supply modules deliver the required power to the detectors and more than 150 km of cables and optical fibres provide electrical connections and optical links. Over the expected period of more than ten years of CMS operation the high particle flux will cause severe radiation damages to the entire system including the sensitive silicon volumes, read out electronics and all additional material. One major challenge in the silicon tracker project is the power dissipation and the efficient heat removal by cooling strategies that must introduce as little material as possible. The supply of power to the detector modules and electrical infrastructure plus efficient cooling is in direct contrast to the need for a detector with a small material budget avoiding multiple scattering, bremsstrahlung, photon conversion and nuclear interactions. Finally CMS makes use of three layers of silicon pixel detectors and ten layers of silicon strip detectors covering a sensitive silicon area of approximately 200 m2 . In total 1,440 silicon pixel detector modules and 15,148 silicon strip detector modules were built, tested and commissioned on different detector substructures inside the CMS tracker volume.

3.1 Tracker concept Taking into account the decrease of density of traversing particles from 1 MHz/mm2 at a radius of 4 cm to 60 kHz/mm2 at 22 cm and to 3 kHz/mm2 at 115 cm, detectors with different cell sizes are used at different radial positions to keep the occupancy below a certain level that can be handled by the detectors and readout electronics.

40

3 The CMS Silicon Strip Tracker

Below a radius of 10 cm pixel detectors with a pixel size of 100 × 150 μm2 in φ × z are used to achieve a cell occupancy below 1%. In the radial range between 20 cm and 50 cm silicon strip detectors with a typical cell size of 80 μm × 10 cm are used, leading to a cell occupancy in the order of 2-3% per bunch crossing. In the outer tracking region up to a radius of 110 cm the cell size is further increased by making the silicon strips longer and allowing for a wider strip separation (pitch). Here typical cell sizes are 200 μm × 25 cm. With increasing strip length the noise performance is degrading since the capacitance at the preamplifier input and the leakage current increases. To compensate for this effect silicon sensors slightly thicker compared to the inner region are used in the outer tracker region (500 μm compared to 320 μm thickness). Higher depletion voltages can be tolerated for the thicker sensors since the radiation damages are smaller in the outer regions. An overview of the expected radiation levels after ten years of operation at LHC at different radii in CMS are given in table 3.1. A large fraction of the hadron flux radius [cm] 4 11 22 75 115

fast hadron density [1014 /cm−2 ]

dose [kGy]

flux of charged particles [cm−2 s−1 ]

32.0 4.6 1.6 0.3 0.2

840 190 70 7 1.8

1 · 108 6 · 106 3 · 105

Table 3.1: Radiation levels for different radii in CMS in terms of fast hadron density, the flux of charged particles and accumulated dose after 500 fb−1 [8].

originates from neutrons that are produced by hadronic interactions in the electromagnetic calorimeter. In the outer layer of the silicon tracker these neutrons dominate the hadron flux. The design of the silicon tracker components was driven by the constraints coming from this harsh radiation environment. All components, i.e. silicon sensors, front-end electronics, and mechanics must survive ten years of operation. The following points are taken into account: - The front-end electronics mainly suffer from additional space charge changing the MOS structure. Space charges are created by traversing ionising particles producing holes in the surface. These damages scale linearly with the accumulated dose.

3.1 Tracker concept

41

- Non-ionising energy losses, so-called NIEL are responsible for bulk defects in the sensitive silicon volumes. The silicon lattice structure is modified and introduces additional energy levels in the band gap that can completely change the sensor characteristics in terms of effective doping. The doping changes from n- to p-type silicon after a certain accumulated dose. Therefore the behaviour with respect to depletion of the sensor changes (see figure 3.1). The depletion voltage will change by several hundreds of volts over the lifetime of the experiment. The increased depletion voltage leads to a higher current and therefore higher silicon temperatures. The produced heat must be removed by an efficient cooling system where the heat producing silicon is to be coupled to the cooling system as efficient as possible. The case of so-called thermal runaway must be avoided, or else it would not be possible to operate the silicon tracker at a stable low temperature. - The read out electronics in general is affected in a non-predictable way which leads to so-called single event upsets. The handling of these must be foreseen in the read out scheme.

Effective Doping Concentration [cm-3 ]

Reverse annealing, an effect produced by the radiation induced defects in the silicon sensors, can lead to an even worse degradation of depletion voltage as a func10

14

Nd0 = 1013 cm-3

1013 10

12

10

11

10

10

10

9

12

Nd0 = 10 cm

-3

108 10

11

12

13

14

10 10 10 Fluence [1-MeV-neutron-equivalent cm-2]

10

15

Figure 3.1: Effective doping concentration as a function of high energy proton flux given for two different initial donor concentrations [33].

42


tion of the particle flux. A permanent temperature below 0◦ C has shown to minimise the effects caused by reverse annealing. Except for short maintenance periods the CMS tracker volume will be held at below 0◦ C for the entire lifetime. The radiation hardness of the silicon detector modules was verified prior to the large scale production and is described later in this section. The entire tracker volume will be operated in a cold environment with the silicon sensors at −10◦ C. A radiation hard cooling fluid C6 F14 at approximately −27◦ C circulates the tracker volume and removes the heat at the places where it is produced. Thermal stress on all components is one of the consequences of the cooling scheme. All components in the tracker volume were tested for thermal stress effects in temperature cycles between ambient temperature of 20◦ C and −30◦ C. To reduce the effects of multiple scattering for particles traversing the tracker and nuclear interaction of pions and other hadrons the amount of material used was minimised. To conserve the best possible energy resolution of the electromagnetic calorimeter it is mandatory to limit photon conversion and bremsstrahlung by keeping the material budget as low as possible. This results in an extremely challenging task of holding the detector modules precisely in place by high precision stiff mechanical structures, bringing the required 60 kW of power to all electrical components and removing the produced heat to avoid thermal runaway and overheating of the front-end electronics with as little material as possible.

3.1.1 Layout of the silicon tracker Two or three high precision space points (in three dimensions) are delivered by the tracker pixel system. Starting from the interaction point particles traverse three layers of hybrid pixel detectors at the radii 4.4 cm, 7.3 cm and 10.2 cm. In the end cap regions two disks complete the pixel based detector part. A total of 66 million pixel channels cover an area of approximately 1 m2 . After traversing the pixel layers particles enter the silicon strip system. Four different subsystems (Tracker Inner Barrel (TIB), Tracker Inner Disks (TID), Tracker Outer Barrel (TOB), two Tracker End Caps (TEC)) compose the strip based system. At a radius below 55 cm the inner barrel and the inner disks provide four φ measurements using 320 μm thick silicon strip sensors. The strips are oriented parallel to the beam axis in the barrel and radially to the beam axis in the end caps. Strip pitches vary between 80 μm and 120 μm in the inner barrel, while the inner disk sensors have a pitch between 100 μm and 141 μm. The single point resolution of the strip detectors in the inner part is in the range between 23 μm and 35 μm. The outer barrel covers a radial range up to 116 cm and extends in z between ±118 cm. Six outer barrel layers make use of 500 μm thick silicon strip sensors with strip pitches between

3.1 Tracker concept

43

122 μm and 183 μm, six φ measurements with a single point resolution between 35 μm and 53 μm are provided. Both sides are closed by two tracker end caps covering the regions 124 cm < |z| < 282 cm and 22.5 cm < r < 113.5 cm. Each end cap is made of nine disks carrying up to seven rings of detector modules with radial strips having pitches between 97 μm and 184 μm. Rings 1 to 4 carries 320 μm thick silicon sensors, rings 5 to 9 500 μm respectively. Each end cap delivers up to nine φ measurements per traversing particle depending on the pseudorapidity range. A schematic cross section of the silicon tracker is shown in figure 3.2. To provide the measurement of a second coordinate so-called stereo modules are mounted back-to-back with a stereo angle of 100 mrad to regular modules in some cases. The position of the stereo modules is indicated in figure 3.2. The additional measurement of z is done in the barrel region with the first two layers and rings of TIB/TID and TOB. In the end cap region rings 1, 2 and 5 are equipped with stereo modules providing an additional measurement of r. The expected number of measured hit positions for traversing particles as a function of η is shown in figure 3.3. At least nine points are measured per trajectory in the η range up to 2.3, with at least four out of the nine being stereo measurements. η -1.5

-1.3

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

-1.7 -1.9 -2.1 -2.3 -2.5

1.9

1000

TOB

800

2.1 2.3 2.5

600 400

r (mm)

1.5 1.7

1200

TID

TIB

TID

200

TEC-

0

PIXEL

TEC+

-200

TID

-400

TIB

TID

-600 -800

TOB

-1000 -1200 -2600

-2200

-1800

-1400

-1000

-600

-200

200

600

1000

1400

1800

2200 z (mm)

2600

Figure 3.2: Layout of the CMS silicon tracker [34]. Each line represents one detector module. Stereo modules are indicated by double lines.

3 The CMS Silicon Strip Tracker N points

44 16 14

total hits

12 10 8 6 4

stereo hits

2 0

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

Figure 3.3: The number of measured hit positions as a function of the pseudorapidity η [34]. The closed circles give the number of total hits (stereo modules counted as one), the open squares give the number of measured hit positions in stereo modules. The error bars reflect the RMS of the distribution for many tracks with smeared primary vertices in the given η range.

3.2 Silicon strip detector modules A silicon strip detector module in CMS is a hybrid object consisting of many components. Each module carries either one thin (320 μm) or two thick (500 μm) silicon sensors. Depending on the position inside the tracker the module is supported by a frame made of carbon fibre and/or graphite. To insulate the silicon sensor from the frame a Kapton layer is used, also providing the electrical connection to the silicon sensor back plane (i.e. bias voltage supply and temperature probe readout). The module frame carries the front-end hybrid and the pitch adapter necessary to adapt the front-end chip readout pitch to the strip pitch on the silicon sensor. An exploded view and a photograph of a ring 6 tracker end cap module is shown in figure 3.4. All different modules used in the tracker end cap are shown in figure 3.5. Stereo modules consist of two of these modules, e.g. modules R1 and R1-S form a stereo module on TEC ring 1.

3.2.1 CMS Silicon Sensor CMS uses p-in-n single sided silicon microstrip sensors. In a standard planar process they are manufactured on 6 inch wafers and each silicon sensor is produced

3.2 Silicon strip detector modules

45 Kapton foil

silicon sensors

aluminium carrier plate

siliconsensors sensors silicon far far sensor sensor Kapton Kaptonfoil foil

pitch adapter near nearsensor sensor

far sensor pitchadapter adapter pitch front end front-end hybrid hybrid

frame frame (carbon fibre) fibre) (carbon

front end hybrid near sensor

frame (carbon fibre) hybrid supply and readout

ceramic

ceramic ceramic

cross piece (graphite)

cross piece piece cross (graphite) (graphite)

HV connection

Figure 3.4: Silicon strip detector modules. Left: Exploded view of a module housing two sensors. Right: Photograph of a TEC ring 6 module. The near sensor is connected to the front-end hybrid. The far sensor is located on the opposite side of the module.

R2

R1-S

R2-S

R3

R1

R4

R6

R5-S

R7

R5

10 cm

Figure 3.5: All tracker end cap modules types. On modules labelled with S the 100 mrad rotation of the silicon sensor with respect to the module frame is visible.

from one wafer within a maximum fiducial circle of 13.9 cm in diameter. The silicon bulk material is n-doped float zone silicon with a crystal orientation 100. This orientation has shown to be less prone to the accumulation of surface charges than the more common 111 crystal orientation. Therefore less inter strip capacitance is expected after irradiation which was confirmed in test measurements [35].

46


The substrate resistivity is in the range of 1.55 kΩcm < ρ < 3.25 kΩcm for the thin 320 μm sensors. For the thick 500 μm sensors the resistivity is in the range 4.0 kΩcm < ρ < 8.0 kΩcm. The achieved sensor thickness accuracy is ± 20 μm for both the thin and thick sensors, while the curvature from one end to the other is less than 100 μm for all sensors. At the back side of all sensors the ohmic contact for the bias voltage is realised by a thin n+ implantation layer covered with aluminium. To achieve full sensor depletion bias voltages up to approximately 500 V can be applied to the sensor back plane. The n+ layer also acts as a barrier for minority charge carriers coming from the depleted n bulk and for majority charge carriers coming from the metal contact leading to an overall low leakage current. To form the read out strips p+ implants create diodes in the n bulk. Each implant strip is covered by an aluminium strip that is isolated electrically from the p+ strip by a thin multilayer of silicon dioxide (SiO2 ) and silicon nitride (Si3 N4 ). By means of this integrated capacitor the signal is AC coupled to the read out electronics which is thus protected from high leakage currents (i.e. possible after strong irradiation). Two bonding pads on each end of all metal strips can be used to either connect the front-end electronics or to daisy chain sensors in modules housing two sensors. On each end a so-called DC pad is connected to the p+ strips for testing purposes. The p+ bias ring enclosing all strips and defining the sensitive detector region is connected to each strip via a polysilicon bias resistor of (1.5 ± 0.5) MΩ. The polysilicon bias resistor has a wiggled shape to provide the required resistance in a small area. This polysilicon biasing technique has proven to be radiation hard. A uniform total strip capacitance per unit length is achieved by a constant ratio of strip width over strip pitch of 0.25 for all strips. On each side of the p+ implant the aluminium extends between 4 μm and 8 μm over the edge of the implant (15% wider than the p+ strip underneath) giving a metal overhang that pushes the electric field lines into the silicon oxide layer where the break down voltage is much higher compared to the n bulk. The thickness of the metal layer is at least 1.2 μm to limit the noise contribution of the electrode resistivity. To further increase the high voltage stability of the sensors the bias ring is surrounded by a floating p+ guard ring that degrades gradually the electric field between the n+ at the edge of the sensor and the bias ring. The mask alignment tolerance in the process and the precision of the implant dimensions are of the order of 1 μm. A schematic cross section of the sensitive silicon region and the sensor edge are shown in figures 3.6 and 3.7. A passivation layer on the surface protects the active area, improves the sensor stability and allows safer handling during the sensor testing and module production phase and later during the substructure integration. The basic properties of the thin and thick silicon sensors are summarised in table 3.2. Further design details for rectangular and wedge shaped sensors can be

3.2 Silicon strip detector modules bias ring

guard ring

47

wire bond

bias resistor DC pad AC pad

aluminium strip

lk

oxide (thin layers of SiO2 and Si3N4) p -strips

+

w

p -implants below bias and guard ring

+

p -implants below bias and guard ring + n -layer aluminium backplane

bu

+

n-

+

n -ring

p

Figure 3.6: Layout of the CMS silicon sensors, based on [28]. Guard ring Bias ring

guard ring bias ring Bias bias resistors resistors DC pads

DC pads

AC pads AC

pads

Figure 3.7: Technical drawing of one corner of a sensor [36] and a photograph of the same area.

retrieved from table 3.3. In total 15 different geometries are used to achieve full spatial coverage. Two rectangular sensors for the TIB (IB1, IB2), two for the TOB (OB1, OB2) and eleven wedge shaped sensors for the TID and TEC.

3.2.2 Mechanics Modules for the inner barrel, the inner disks and ring 1 to 4 in the end caps are equipped with one silicon sensor, while modules in the outer barrel and rings 5 to 7 in the end caps have two sensors. In the case of two sensors the corresponding strips are connected electrically via wire bonds. Depending on the geometry

48


thin sensors

thick sensors

(320±20) μm

(500±20) μm

100 μm ±20 μm ±1 μm 1.55 - 3.25 kΩcm

100 μm ±20 μm ±1 μm 4 - 8 kΩcm

0.25 80 μm - 158 μm 20 μm - 39 μm 28 μm - 51 μm

0.25 122 μm - 205 μm 30 μm - 51 μm 40 μm - 67 μm

breakthrough voltage

> 500 V

> 500 V

interstrip resistance interstrip capacitance strip leakage current

> 1 GΩ < 1.2 pF/cm < 100 nA at 400 V

> 1 GΩ < 1.2 pF/cm < 100 nA at 400 V

1.5 ± 0.5 MΩ

1.5 ± 0.5 MΩ

thickness flatness cutting accuracy implant precision resistivity width/pitch pitch width of p+ implant width of aluminium

bias resistor

Table 3.2: Design parameters of thin and thick silicon sensors.

and the number of sensors the active area of a module varies between 6241.1 mm2 (TEC modules, ring 1) and 17202.4 mm2 (TOB modules). In total 29 different module designs, 15 different sensor designs and twelve different front-end hybrid designs are used in the entire strip tracker subsystem. As explained later special modules are prepared for alignment purposes with etched holes in the sensor aluminium back plane to allow a laser ray traversing up to five modules. The different sensor geometries are displayed in figure 3.8. The module frames provide the stability and safety necessary for the sensor support and module handling and carry the readout electronics. In the inner barrel a 500 μm thick carbon fibre frame surrounds the silicon sensor on all sides. Outer barrel module frames are made of carbon fibre. The U-shaped frame is obtained by gluing two 625 μm thick carbon fibre legs (K800 carbon fibre composite, 5 × 125 μm fabric) on a carbon fibre cross piece made of the same material. In the end caps the frames for the one-sensor modules are U-shaped and made of 500 μm thick graphite (FE779 carbon) in one piece. For the two-sensor-modules a similar U-shaped support frame is obtained by gluing two 625 μm thick carbon fibre legs (K800 carbon fibre composite, 5 × 125 μm fabric) on a 800 μm thick graphite cross piece (FE779 carbon) which holds the front-end electronics. Both graphite and carbon fibre fulfil the requirement of high stiffness, low mass and efficient heat removal from the silicon sensors. They are radiation hard and have a thermal expansion coefficient similar to silicon (2.6 · 10−6 /K). Since the module frames are


sensor

L1 [mm]

L2 [mm]

49

height [mm]

volume [cm3 ]

pitch [μm]

thickness [μm]

number of strips

number of sensors

IB1 IB2

61.5 61.5

116.9 116.9

2.30 2.30

80 120

320 320

768 512

1536 1188

OB1 OB2

93.9 93.9

91.6 91.6

4.30 4.30

122 183

500 500

768 512

3360 7056

85.2 110.9 88.2 110.7 115.2 81.2 63.2 96.1 84.9 106.9 94.9

2.03 2.73 2.78 2.55 2.39 4.19 3.64 4.28 4.23 4.05 4.00

81-112 80.5-119 113-143 123-158 113-139 126-142 143-156 163-185 185-205 140-156 156-172

320 320 320 320 320 500 500 500 500 500 500

768 768 768 512 512 768 768 512 512 512 512

288 288 864 880 1008 1440 1440 1008 1008 1440 1440

W1 TEC W1 TID W2 W3 W4 W5A W5B W6A W6B W7A W7B

63.1 62.1 86.6 63.3 58.1 96.5 110.0 83.6 94.5 71.5 80.4

85.8 91.7 110.1 81.1 71.3 109.5 120.1 94.6 104.7 80.1 88.0

Table 3.3: Specifications of all sensors of the CMS tracker. The values are given for the active areas defined as the area inside the bias ring. Two inner barrel (IB) thin sensors and two outer barrel (OB) thick sensors, and thin (W1-W4) and thick (W5a-W7b) wedge shaped sensors for the inner disks and the end caps are displayed. The number of sensors do not take into account spare sensors.

Inner innerBarrel barrel 15

End Cap rings(1-4) (1-4) end cap inner inner rings

10 IB 1/2

0

thin sensors

thick sensors

20 OB 1/2

W1 ring 1

5

Ring ring 4 4

TID

Thick sensors

15

Ring W5A 5 ring5 near near W5B 5 Ring ring 5 far near

5 0

outer Barrel barrel Outer

Ring ring 22

Thin sensors

10 OB 1/2

W4

W3 Ring 3 ring 3

W2

Ring TEC/ 1

W6A 6 Ring ring6 near near

Ring W6B 6 ringfar 6 far

Ring W7B 7 ring 7 far near

55 cm cm

W7A7 Ring ring7 near near

cm endCap cap outer outer rings End rings(5-7) (5-7)

Figure 3.8: The different silicon sensor geometries. See table 3.3 for the exact dimensions.

50


used to efficiently remove the generated heat, carbon fibre with high thermal conductivity of 800 W/(mK) is used. Glue joints between the frames and the silicon compensate remaining differences in the expansion coefficients. The chosen glue complies with requirements for radiation hardness, good thermal conductivity and long term stability. Three types of glue were used. Epoxy AW 106 [37], silicon glue Dow Corning RTV 3140 [38] to compensate for different thermal expansion coefficients and the electrically conductive glue Epotek EE 129-4 [39] to connect the silicon sensor back plane and the high voltage line on the Kapton bias circuit. The high voltage supply to the back plane is provided by 131 μm thin metallized Kapton bias circuits running along the legs of the modules between the silicon sensor and the carbon fibre support frame. The connection of the bias voltage to the back plane is done via wire bonds. In addition, this connection is supported by electrically conductive glue (Polytec EE 129-4). Temperature probes are placed on the Kapton foil to measure the temperature of the silicon. The glue joint between the temperature sensor and the back plane is done with the silicon glue RTV 3140. Figure 3.9 shows all parts of a TEC ring 6 frame before mounting on the frame assembly plate. Figure 3.10 shows the gluing scheme of a tracker end cap modules with two sensors. The pitch adapter between the front-end hybrid and the silicon sensor adjusts the strip pitch of the sensor (between 80 μm and 205 μm depending on the sensor type) to the pitch of the front-end readout chip of 44 μm. It also places the heat producing front-end electronics further away from the sensors. The bias return line is a wide line on the pitch adapter connecting the bias ring on the sensor side and the hybrid ground on the opposite side. The pitch adapter is realised on a 550 μm thick glass substrate, patterned with low resistivity aluminium strips and cut to the correct dimensions depending on the detector module geometry. The 30 μm narrow lines are etched on a 1.5 μm thick aluminium layer deposited on a Cr base resulting in less than 25 mΩ/2. Specially designed and cut pitch adapters are used on the stereo modules where the silicon sensors are tilted by 100 mrad with respect to the sensors on normal modules. A photograph of one pitch adapter for an outer barrel stereo module is shown in figure 3.11. Figure 3.12 shows microscopic views of the lines on the pitch adapter with the bonding pads and the wire bonds between the pitch adapter and the front-end electronics. Fixation: Different types of aluminium inserts and precision bushings in the module frames are used to position and restrain the modules to the larger support structures with high precision. TIB/TID and TEC modules are mounted using four points, two being high precision bushings that allow a mounting precision bet-


51 Kapton circuit

carbon fibre leg graphite cross piece

assembly plate

ceramic reinforcement strip

carbon fibre leg

Kapton circuit

Figure 3.9: Photograph of a frame assembly plate and all parts necessary for a TEC ring 6 frame.

ter than 20 μm and provide thermal contact between the detector module and the cooling pipes. For TOB modules two Cu-Be springs give the precision positioning and four screws ensure good thermal contact.

3.2.3 Readout Hybrids The front-end readout hybrid carries the various ASIC chips necessary for the readout and control of the detector modules. It distributes and filters the voltages of 1.25 V and 2.5 V and routes the clock, control and data lines between the ASICs. The heat produced by all chips needs to be removed efficiently from the front-end hybrid. The hybrid is build up of a four layer PCB with a flex cable soldered to a 50 pin NAIS1 connector. Three different designs are used in TIB/TID, TOB and TEC, where space constraints are less strict in the last. The only difference between TOB and TEC hybrids is the length of the flexible cables. In figures 3.13 and 1 Matsushita

Electric Works, Ltd. 1048, Kadoma, Kadoma-shi Osaka 571-8686, Japan.

52


Figure 3.10: Gluing scheme of a tracker end cap module with two sensors [40]. On the tracker end cap modules two thin ceramic reinforcement strips are glued underneath the bonding wires between the pitch adapter and the near sensor and between the two sensors.

3.14 both hybrid layouts and the hybrid layer structure are shown. The most critical issues in the hybrid design are the extremely small dimensions, especially the inter-layer electrical connections, called vias, with a diameter of just 100 μm, and the integration of the flexible cable. One hybrid carries four or six APV25-S1 chips and three auxiliary chips, one multiplexer (MUX), one chip to decode the trigger signal (Tracker Phase Locked Loop, TPLL) and one chip surveying the environmental parameters (Detector Control Unit, DCU). All chips are glued to their positions shown in figure 3.13. The wedge bonding technique is used to connect all ASICs to the electrical lines on the hybrid. The ASICs are described in the

3.3 Readout, triggering and services

53

Figure 3.11: A pitch adapter used on a tracker outer barrel stereo module. Patterned low resistivity metal connectors are formed on 0.55 mm thick glass substrates that are precision cut to the correct dimensions. The broader bias return line is located on the right side next to the outermost regular strip.

Figure 3.12: Microscopic views of the pitch adapter. Left: Detailed picture of the lines on the pitch adapter and the bonding pads. Right: Wire bonds between the pitch adapter (top) and the front-end electronics (bottom).

following sections.

3.3 Readout, triggering and services The signals from the silicon strips are amplified, shaped and stored by the custom made APV25-S1 chips [43] before they are converted to optical signals by lasers on the analogue-opto-hybrids close to the detector modules. After being converted back to electrical signals they are digitised by analogue-to-digital converters (ACD) on the front-end driver (FED) modules [44] in the service cavern of the CMS experiment. Each silicon microstrip is read out by a charge sensitive amplifier with a time constant of 50 ns. The output voltage is sampled with 40 MHz corresponding to the

54

3 The CMS Silicon Strip Tracker 60 mm APV25

28 mm

28 mm

47 mm APV25

MUX TPLL DCU MUX

TPLL

DCU

Figure 3.13: Front-end hybrid design [41]. Left: High density hybrid layout for TIB/TID modules. Right: Slightly larger circuit for TOB/TEC modules.

LHC beam crossing frequency. An analogue pipeline is capable of storing samples up to the first level trigger latency. Over a short distance to a laser driver data are multiplexed from pairs of 128 channels (corresponding to two front-end chips) on the front-end chips on the hybrid. After electrical to optical signal conversion the data stream is transmitted over approximately 100 m via optical fibre cables to the counting room adjacent to the CMS experiment cavern. Edge emitting laser transmitters operating at a wave length of 1,300 nm transmit data through single mode fibres. A VME bus system in the underground area outside the central cavern is the central part of the data acquisition. Non zero suppressed pulse height data are converted back to electrical signals matching the range of a 10 bit ADC. Baseline variations of the systems are absorbed by approximately 2 bits of the range while the remaining 8 bits provide enough resolution for the signal range. After signal digitisation the FED performs signal processing as explained later in this section. The front-end controller (FEC), also realised as a VME bus module, controls and monitors the electronic system by distributing the LHC machine timing signals and first level triggers via the Timing Trigger and Command (TTC) system. All trigger, clock and control data are distributed through digital optical links using photodiodes and amplifiers and are send electrically by Communication and Control Units (CCUs) to the individual detector modules. The following paragraphs explain all system components in detail. A summary is given in figure 3.23 at the end of this section.


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Figure 3.14: Hybrid cross section [42]. The final version of the hybrid consisting of layers made from copper, polyimide, and acrylic glue.

3.3.1 On-detector module readout electronics 3.3.1.1 The APV25-S1 readout chip The APV25-S1 (Analogue Pipeline Voltage Mode, revision S1 with 0.25 μm feature size) is a 128 channel pre-amplifier chip and the main component of the readout chain. It is manufactured in a 0.25 μm CMOS process taking advantage of radiation hardness, low noise amplification, low power consumption and high circuit density. The APV25-S1 requires a 40 MHz clock input and two operation voltages of 1.25 V and 2.5 V respectively. The power consumption per channel is approximately 2.81 mW. The aluminium strips on the silicon sensors are connected to one of the 128 input bonding pads. The input charge is amplified by a low noise amplifier and transformed into a voltage signal. An inverter stage allows signal inversion on demand. All signals pass a CR-RC shaper with a time constant of 50 ns before being sampled continuously every 25 ns. The samples are stored in a pipeline of

56


192 switched capacitors per channel. A write pointer determines the pipeline cell that stores the actual signal. Depending on the operation mode of the APV25-S1 up to 32 or 10 triggers can be processed on the pipeline that stores the signal for more than 4 μs. If no trigger signal is received within 4.8 μs the pipeline cell is overwritten and the stored signal is lost. After reception of a trigger signal a read pointer marks the specific pipeline cell for readout. The read and write pointers are separated by an adjustable latency respecting the time necessary for trigger decision and run times on cables. All charges stored in the 128 pipeline cell capacitors belonging to the appropriate time slice are routed to a 128:1 multiplexer before leaving the APV25-S1 at a rate of 20 MS/s as a differential bi-directional current signal together with additional information in a digital header. The channel output is non-consecutive so that re-ordering is necessary prior to actual data processing. Figure 3.15 shows a schematic view of one channel of the APV25-S1.

MUX gain

low noise charge pre-amplifier

50 ns CR-RC shaper inverter

128:1 MUX differential current output amplifier

192 analogue pipeline cells

signal input -1

APSP

Figure 3.15: Schematic diagram of one channel in the APV25-S1 (based on [43]).

Three different operation modes are possible with the Analogue Pulse Shape Processor (APSP) on the APV25-S1 leading to different signal peak times. In the so-called peak mode the stored charges of one pipeline row is routed to the signal processing chain. Due to the 50 ns shaping time of the pre-amplification stage two particle hits within 25 ns cannot be disentangled and this operation mode is not suitable for the high luminosity running of the LHC. In the so-called deconvolution mode a weighted sum of three subsequent pipeline cells is routed to the subsequent signal processing. In case of a CR-RC shaper (shaping time τ = 50 ns) and sampling intervals Δt = 25 ns the signal can be restored and the weights are given by:


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Δt Δt = exp −1 / = 1.213 τ τ −1 Δt ·e = −2 · = −1.472 τ −1 Δt Δt · exp +1 = = 0.446 τ τ

w1 w2 w3

The method of weighted summation is shown in figure 3.16.

Signal [ADC counts]

Method of Deconvolution f1 = fpeak(t) x w1

100 measured

fpeak

(t)

50 f3 = fpeak(t-50ns) x w3 0 measured

fdec

(t)

fdec = f1 + f2 + f3 -50 f2 = fpeak(t-25 ns) x w2 -100

0

50

100

150

200

250

300 350 Time [ns]

Figure 3.16: The APV25-S1 deconvolution mode shown for measured data in peak mode and deconvolution mode [28]. A convoluted pulse (blue) calculated from the measured ) in deconvolution mode pulse in peak mode is compared to a reconstructed pulse (fmeasured dec (red).

On the chip the weights are realised by three capacitors of C1 = 0.61 pF, C2 = 2.04 pF and C3 = 1.69 pF. Four clock cycles are needed to pass on all data after receiving a trigger signal. In three clock cycles the corresponding capacitors get loaded while the fourth cycle is needed to feed the multiplexer. In the so-called three sample mode of the APV25-S1 all three pipeline cells are returned without applying the deconvolution weights. The analogue data of the silicon strip sensor coming from the APV25-S1 is accompanied by additional information on pipeline and chip related properties.

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A full data frame is shown in figure 3.17. The frame starts with a 12 bit digital header (the first three bits indicate the beginning of the header, the following eight bits indicate the pipeline address, the last bit is an error bit) followed by 128 data points corresponding to the 128 input strips on the silicon sensor that are connected to one APV. A synchronisation pulse (so-called tick mark) terminates the APV data frame. An error bit of 0 indicates a FIFO error (APV25-S1 receiving more triggers than allowed) or a latency error (read and write pointer separated by a number not corresponding to the latency value set). APV Data Frame

12 bit digital header

ADC counts

250

level of digital 1

8 bit pipeline address

tick mark

200 analogue data tick mark

150

100

50

level of digital 0 0

-40

-20

0

20

40

60

80

100

120

140

160

180

Figure 3.17: The APV25-S1 data frame [28]. Channels 1 to 12 set up the digital header. Channels 13 to 140 show the digitised analogue data from the 128 input strips. Tick marks are visible at channels -34, 141 and 176.

Via a two wire serial interface conform to the Philips I2 C standard [45] 17 registers can be adjusted in the APV25-S1 to allow chip tuning under various operation conditions, i.e. temperature, strip capacitance, radiation damage. The operation mode (peak mode, deconvolution mode, three sample mode), latency, pedestal, currents and voltages for the pre-amplifiers, shapers, APSPs and multiplexers can be adjusted. The electronics noise of the analogue read out chain is dominated by the MOSFET transistor in the APV25-S1. The equivalent noise charge ENC of the APV25S1 at room temperature is measured to be ENC peak

= 270e− + (38e− /pF) ·Cdet

ENCdeconv

= 430e− + (61e− /pF) ·Cdet


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in peak mode and deconvolution mode respectively, depending linearly on the contemperature the ENC will nected detector capacitance Cdet . At the CMS operating √ be lower by approximately 10% due to ENC ∼ T . 3.3.1.2 The multiplexer chip, APVMUX To decrease the number of readout channels the analogue data of two APV25-S1 chips are multiplexed to a single differential line by interleaving the two 20 MS/s streams to one 40 MS/s stream. Via eight resistors the current output is converted to voltages. Each of the eight resistors with a resistance of 400 Ω can be connected in parallel to a given number of others leading to a total resistance between 50 Ω and 400 Ω. It is therefore possible to adjust the signal height to the dynamic range of the analogue-opto-hybrid that converts the electrical signals to optical signals. Via the I2 C interface on the chip an 8 bit register sets the number of resistors used. 3.3.1.3 The Tracker Phase Locked Loop chip, TPLL To minimise the bandwidth and power consumption the level one trigger signal in CMS is encoded in the clock line. Figure 3.18 shows the encoding and decoding of the trigger and clock signal. ~ 25 ns

CLK

Encoder

CLK_T1

T1

Decoder CLK_T1

CLK

PLL T1

Figure 3.18: Encoding and decoding of the trigger and clock signal [28]. Left: Clock (CLK) and trigger signal (T1) are encoded to a single signal. Right: The TPLL chip decodes the incoming signal and distributes the clock and trigger signal on two separate lines to the ASICs on the front-end hybrid.

The TPLL chip (Tracker Phase Locked Loop) on the front-end hybrid serves both the mulitplexer and the APV25-S1 with decoded trigger and clock signals. The TPLL employs a voltage controlled oscillator (VCO) with adjustable oscillation frequency. The voltage driving the VCO is determined in a feedback loop by measuring the phase difference between the incoming and outgoing clock signal.

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The TPLL also compensate for different time-of-flight of particles coming from the interaction point to different positions in the tracker.

3.3.1.4 The Detector Control Unit chip, DCU Three functions are provided by the DCU chip (Detector Control Unit): monitoring the sensor leakage current, monitoring the APV25-S1 supply voltages V125 (nominal 1.25 V) and V250 (nominal 2.5 V) and monitoring the temperatures of the hybrid and the silicon sensor. A unique 24 bit identifier on each DCU allows to identify each individual detector module in the tracker. A schematic view of the DCU is given in figure 3.19. The sensing resistor Rs is used to monitor the sensor leakage current. A 10 μA constant current source is used to drive the thermistor near the APV25-S1 measuring the hybrid temperature while a 20 μA constant current source drives two thermistors connected in parallel under the silicon sensor back plane. The DCU chip itself is able to measure its own temperature by the voltage from a self-biased current source proportional to the absolute temperature. Two external resistor dividers maintain the APV25-S1 supply voltages V125 and V250. All analogue data measured on the DCU is converted by a 12 bit ADC and can be read via the standard I2 C protocol. Figure 3.20 shows a fully equipped front-end hybrid equipped with four APV25-S1s. In the case of modules with 768 strips two more APV25-S1s can be mounted on the same front-end hybrid.

bus

Figure 3.19: Block diagram of the Detector Control Unit and connections on the front-end hybrid, based on [46].


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APV25S1: amplifier, shaper, buffer, multiplexer

Alignment Hole Four-Layer Circuit

MUX: multiplexer

Flexible Cable

NAIS Connector

PLL: clock distribution, trigger reconstruction and distribution

DCU: monitor of operation parameters

Ceramic Support Piece

70mm

Figure 3.20: Photograph of a front-end hybrid used for a TEC module with 512 strips (corresponding to the four APV25-S1) [42]. The two positions in the middle are left free and provide space for two more APV25-S1 in the case of modules with 768 strips.

3.3.2 Off-detector module readout electronics 3.3.2.1 Optical links To reduce the material budget and to prevent the analogue data stream and the digital control data from being disturbed by electrical interference all links over longer distances are realised as optical connections. Data streams are transmitted over a distance of about 100 m between the tracking system and the CMS service cavern by means of analogue optical links at 40 MS/s. Digital timing and control signals are also transmitted by digital optical links. Commercially available Multi-Quantum-Well InGaAsP edge emitting transmitters are chosen for good linearity and low threshold currents, while InGaAs photodiodes serve as receivers. Standard single-mode, non dispersion-shifted telecommunication fibres are used between the lasers and the photodiodes. Ribbons of twelve fibres are grouped in bunches of eight forming a 96-fold ribbon with a diameter below 10 mm and a minimum bending radius of 8 cm. For the analogue data three transmitters are connected to a laser driver ASIC on an analogue opto hybrid (AOH) (see figure 3.21, left) in the case of modules with 768 strips (six APV25-S1) while only two transmitters are needed for modules with 512 strips

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Figure 3.21: Analogue and digital opto hybrid. Left: Photograph of an analogue opto hybrid. Laser diodes are covered by white ceramic protection shields. Right: Photograph of a digital opto hybrid.

(four APV25-S1). The electric analogue signal from the APVMUX is transmitted differentially over a distance of only a few centimetres between the detector hybrid and the AOH. The laser diode current is modulated by the signal. To transmit the control signals two receivers and two transmitters are located on digital opto hybrids (DOH) (see figure 3.21, right). The optical signals are converted to low voltage differential signals (LVDS) and vice versa. All digital opto hybrids are mounted on separate printed circuit boards delivering electrical power to the DOHs, forming the digital opto hybrid module (DOHM). 3.3.2.2 The front-end driver In total 96 optical fibres are received by one front-end driver (FED), a VME based 9U format module processing all 96 channels in parallel. Opto-receivers convert the optical to electrical signals and digitise them with a 10 bit ADC at 40 MHz. The APV data is re-ordered to the geometrical order of the strips on the silicon sensor. Pedestal correction, common mode subtraction and cluster finding is performed in the FED. A look up table contains all pedestals and thresholds that are applied for the cluster finding algorithm. For each trigger decision and APV25S1 the common mode subtraction is done separately. In the regular data taking mode (so-called zero suppressed mode) the FED delivers a list of clusters that pass the cluster thresholds together with address and signal amplitude information for each channel in the cluster. Only information relevant for global particle track reconstruction is therefore transferred to the central CMS data acquisition. In the


63

final system about 450 FEDs are used. A photograph of one FED VME module is shown in figure 3.22.

Figure 3.22: Photograph of a front-end driver VME card (FED). It is double-sided, with around 6,000 components, and high component density. The 14-layer boards have almost 25,000 tracks, serving 96 input channels (corresponding to 192 APV25-S1). Signal processing is performed with FPGAs. The FED is equipped with unique custom analogue optical receiver modules (on the left).

3.3.2.3 Services All modules are grouped in power supply groups to limit the number of power supply channels and cables. In cooperation with the company CAEN2 a new custom made power supply system was developed and commissioned at INFN Firenze for the CMS silicon tracker. Three different power supply boards were developed for the system featuring combined low voltage and high voltage macro-channels delivering floating output voltages to the detector modules and the corresponding front-end electronics. Remote sensing lines guarantee the correct voltages in place. 2 CAEN

S.p.A., Via Vetraia, 11, 55049 - Viareggio (LU) - Italy

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Low and high voltage supply: One power supply module (CAEN, A4601H/F) houses two power supply units (PSU). One unit provides two floating low voltage and two floating high voltage sources. The two low voltage sources deliver sensed 1.25 V (6 A, programmable in the range 1.15 V - 1.35 V with an accuracy of ±50 mV) and 2.5 V (13 A, programmable in the range 2.3 V-2.75 V with an accuracy of ±50 mV) to the power groups. A voltage drop of up to 4 V can be compensated along the approximately 50 m long cables between the service cavern and the detector modules. The two high voltage power regulators can be set independently in the range between 0 V and 600 V with an accuracy of ±0.5 V. They are able to deliver a maximum current of 12 mA. Ramp-up and down speeds can be adjusted between 1 V/s and 100 V/s. The two independent reference voltages for high voltage and low voltage are DC isolated and can be connected outside the power supply module. The low voltage power supplies are protected against shorts.

3.3.2.4 Slow control and triggering The Front-End Controller: Front-end controller cards (FEC) realised in VME standard and located in the CMS service cavern distribute clock, trigger and service signals to the individual detector modules via the digital optical link. All global CMS clock and trigger signals are received from the Timing Trigger and Command (TTC) system. The digital opto hybrids convert the optical signals to LVDS token ring signals. Inside the tracker volume several Communication and Control Units (CCU) set up the token rings and interface it to the ASICs on the front-end hybrids. The combined clock and trigger signal is delivered to the TPLL chips and processed as explained in the previous section. All DCUs are read out via the control ring and the optical link. Slow control information, i.e. temperatures etc. are only available if the control ring is operational and all front-end hybrids are powered. All CCUs are powered by a dedicated power supply unit (CAEN A4602). It was developed by CAEN and is able to operate in a magnet field and radioactive environment. One module houses four power supply units with each channel providing 2.5 V (7 A) with an accuracy of ±50 mV to the CCUs. A maximum of 6 V can be compensated along the distance between the power supply and the tracker via sense wires. One CCU chip mounted on a PCB called CCUM proving services to the CCU is dedicated to a specific set of detector modules.


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3.3.2.5 Summary

The read out scheme is summarised in figure 3.23. It combines the possibility of optimum spatial resolution using charge sharing information between adjacent strips, and monitoring due to the full analogue signal availability.

Optical transmitter APV APV MUX 256:1

analogue optical link PLL

Detector CLK

Front End Module

T1 I2C

DCU

CCU

PLL Tx/Rx

Control module

ADC

DSP

digital optical link

TTCrx

Tx/Rx

TTCrx

P

RAM

Front End Driver

Front End Controller

Figure 3.23: The CMS tracker read out scheme [34]. The upper part shows the silicon strips on the left and the APV front-end chips plus multiplexer and PLL chip on the detector hybrid. On the right side the signal path of the analogue data is shown including conversion from electrical to optical signals by the optical transmitter and conversion back to electrical signals inside the front-end driver (bottom right). The left side shows the slow control part. The CCU close to the detector module transmitting and receiving signals form the front-end controller via the digital optical link.

The distribution of silicon strip modules across the four subsystems with the number of optical channels and APV readout chips are given in table 3.4. The

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TIB

TID

TOB

TEC

tracker

single sided modules stereo modules individual modules

1,188 768 2,724

240 288 816

3,048 1,080 5,208

4,096 1,152 6,400

8,572 3,288 15,148

one sensor modules (thin) two sensor modules (thick) individual sensors

2,724 0 2,724

816 0 816

0 5,208 10,416

2,512 3,888 10,288

6,052 9,096 24,244

512-strip modules (4 APVs) 768-strip modules (6 APVs)

1,188 1,536

240 576

3,528 1,680

4,096 2,304

9,096 6,052

6,984 13,968 1,787,904

2,208 4,416 565,248

12,096 24,192 3,096,576

15,104 30,208 3,866,624

36,392 72,784 9,316,352

number of optical channels number of APV chips number of strips

Table 3.4: Distribution of silicon strip modules across the subdetector systems.

total active area on all modules is 198.34 m2 with 19.58 m2 in the inner barrel, 89, 59 m2 in the outer barrel, 7.45 m2 on the inner disks, and 81.72 m2 in the end caps. The total silicon area on all modules is 209.1 m2 , approximately 10 m2 of the silicon area lies outside the bias rings.

3.4 Radiation hardness During the lifetime of ten years and after an integrated luminosity of approximately 500 fb−1 the silicon strip tracker will suffer from an enormous particle flux. Detector modules in TIB/TID and TEC will experience a flux of up to 1.8 × 1014 1-MeV-neutron-equivalent per cm2 . With 0.5 × 1014 1-MeV-neutron-equivalent per cm2 for TOB modules there will be less radiation damages in this detector region. Depending on the distance from the beam axis two regions can be distinguished with respect to the main particle types that cause the radiation damages. In the inner region up to a radius of about 0.5 m the dominant fraction will come from fast hadrons, while for the outer tracker region the main part will come from neutrons backscattered off the electromagnetic calorimeter. Before the CMS collaboration endorsed the final detector module concept several irradiation tests with both neutrons3 and protons4 were performed. - proton irradiation of several TOB modules: 0.1 − 0.7 × 1014 1-MeV-neutron-equivalent per cm2 3 At 4 At

the cyclotron of the Centre de Recherches du Cyclotron, Louvain-la-Neuve, Belgium. the compact cyclotron of the Forschungszentrum Karlsruhe, Germany.

3.5 Tracker substructures

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- neutron irradiation of one TOB module: 1.2 × 1014 1-MeV-neutron-equivalent per cm2 - proton irradiation of two TEC modules: 0.1 − 0.7 × 1014 1-MeV-neutron-equivalent per cm2 - proton irradiation of three TIB modules: â 0.5 − 2.1 × 1014 1-MeV-neutron-equivalent per cm2

Depletion voltage [V]

After irradiation annealing was simulated by heating the detector modules at 60◦ C for about 80 minutes. Reverse annealing was avoided by storing the detector modules in a freezer at approximately -20◦ C. As expected the full depletion voltage increased with the flux but stayed well below 500 V as shown in figure 3.24. 600 500 400 300 200 100 0 0

0.5

1

1.5

2 14

2.5

Fluence [10 neq /cm2]

Figure 3.24: Depletion voltage after irradiation for TOB and TEC detector modules (triangles and dots in upper curve) and TIB detector modules (dots in lower curve) [34]. All modules annealed for 80 minutes at 60◦ C after each radiation. The two curves stem from simulations for silicon sensor thicknesses of 320 μm (lower curve) and 500 μm (upper curve).

Figure 3.25 shows the behaviour of various irradiated TOB modules for different irradiation levels. These test beam measurements showed that after the end of the CMS lifetime a sufficient signal-to-noise ratio is guaranteed. With the bias voltage set to 400 V a signal-to-noise above 16 was achieved.

3.5 Tracker substructures As mentioned above four different subsystems (Tracker Inner Barrel (TIB), Tracker Inner Disks (TID), Tracker Outer Barrel (TOB) and two Tracker End Caps (TEC))

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Figure 3.25: Behaviour of irradiated TOB modules. Left: Signal-to-noise ratio of TOB modules after irradiation versus the electron beam energy (measured at a bias voltage of 450 V) [47]. Right: Signal-to-noise ratio of four TOB modules after irraditon versus the bias voltage [47]. All radiation levels are given in 1-MeV-neutron-equivalent per cm−2 .

compose the entire system. According to their positions with respect to the centre of the CMS detector (+z-direction or −z-direction) the subsystems are labelled TIB+ or TIB- etc. The following sections describe the individual subsystems in detail.

3.5.1 Tracker Inner Barrel and Tracker Inner Disks Four concentric rings made of carbon fibre at radii of 255.0 mm, 339.0 mm, 418.5 mm and 498.0 mm from the beam axis with a length of 1,400 mm parallel to the beam set up the inner barrel part. Stereo modules with a strip pitch of 80 μm are mounted on the two inner layers while the outer layers house single sided modules with sensors having a strip pitch of 120 μm. Each individual concentric cylinder of this subdetector is subdivided into four parts, so-called half-shells. Each half-shell is a self-contained system with respect to electrical connections and cooling which has the advantage that one half shell can be fully assembled and tested before integration in the final system. Both ends of the TIB are connected to service cylinders providing all service connections to a service distribution disk, the socalled margherita (see figure 3.26). The TID± subsystem consists of three identical disks in the z-range 800 mm < |z| < 900 mm carrying three rings of modules forming a sensitive area between 200 mm and 500 mm in radius. On the two inner rings stereo modules are mounted while the outer ring consists of single sided modules. In the TIB/TID all detector modules are mounted directly on a carbon fibre structures. A picture of integrated modules on half-shells can be seen in figure 3.26.


detector Silicon detectors modules

service Service cylinder (disks are cylinder hidden inside) (disks hidden)

4 shells TIBLayer layer 4 shells

69

electrical power Electrical power patch panels patch panels

Optical fibres optical fibres patch panel patch panel

layer 1

TIB/TID + with margherita. TIB/TID+ withthe margherita

Figure 3.26: Integrated TIB modules on half-shells. Left: Sketch of one half of the tracker inner barrel (TIB+). The margherita provides electrical and optical connections as well as the connection to all cooling pipes. Right: Photograph of one half (+z side) of the first TIB layer surrounded by layers 2 and 4.

Cooling: Aluminium pipes with a diameter of 6 mm and a wall thickness of 300 μm are bent to cooling loops and interconnected by input/output manifolds so that several loops can be used in parallel. Aluminium ledges are glued to the cooling pipes to provide the thermal contact to the silicon modules. Three modules are mounted per cooling loop as shown in figure 3.27. The mechanical positioning precision of the modules is guaranteed by the position of the ledges on the cooling pipes.

i

Figure 3.27: Three inner barrel modules placed on one cooling loop. At the right end side a CCUM provides the interface to the slow control ring. One analogue opto hybrid per module is placed very close to the front-end electronics and converts the electrical signals to optical signals.

Before integration of the electrical parts all cooling loops are tested at −30◦ C and a pressure of 20 bar. The number of modules that need to be cooled by one cooling circuit varies between twelve stereo modules on four daisy chained cooling loops on the inner shell and 45 single sided modules on 15 cooling loops in the outer shell. In total 70 cooling circuits are used to remove the dissipated power from the TIB/TID.

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Electronics: Three TIB modules mounted on one cooling loop is the basic electronic group, a so-called string. A mother cable (a Kapton based circuit) connects the three modules and provides all services e.g. low voltages, high voltages and control signals for the detector modules. Mother cables are grouped in control rings that are self contained systems with respect to trigger distribution, clock distribution and slow control via the CCUM. After the DOHM the electrical LVDS signals are distributed to up to 45 detector modules on 15 mother cables. The power distribution via the power supply units is done in groups according to the control rings. The cooling pipes made from aluminium are used to provide the electrical ground. All mother cables and DOHMs are connected to the cooling manifolds. Cooling in- and outlet pipes are running along the service cylinders to the margherita and are connected to the common CMS detector ground outside the inner tracker volume.

3.5.2 Tracker Outer Barrel The detector modules of the tracker outer barrel are integrated on 688 so-called rods, a similar concept to the tracker inner barrel strings. Rods are inserted in a single mechanical TOB structure called TOB wheel. Three inner and outer carbon fibre composite cylinders support four identical disks also made from carbon fibre with a core of aramid-fibre honeycomb. On the disks 344 holes allow for rod insertion with each rod being supported by two disks. The full TOB length is covered by two consecutive rods. The TOB wheel is 218 cm long and has a coverage in r between 555 mm and 1,160 mm. Rods in the wheel form six detector layers around the beam axis at distances of 608 mm, 692 mm, 780 mm, 868 mm, 960 mm, and 1,080 mm. To achieve hermetic coverage rods are shifted by ±16 mm from layer to layer without a gap at z = 0 by an overall shift of 1.5 mm along z. To guarantee an overlap in r − φ adjacent rods overlap by 1.5 mm (corresponding to approximately 12 silicon strips). Precision elements glued to the carbon fibre support structures and assembly in a temperature surveyed environment guarantee a mechanical precision of approximately 100 μm. Mechanics and cooling: A single rod with either six or twelve individual detector modules is the basic TOB detector substructure. Rods provide mechanical support and all electrical connections. Two 1,130 mm long carbon fibre profiles (C-shaped) are connected by carbon fibre pieces running perpendicular. As a central part of a rod the cooling pipe is realised by a U-shaped CuNi 70/30 alloy pipe with a wall thickness of only 100 μm. A sketch of the rod mechanics and module positions is shown in figure 3.28.


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modules

rod

Figure 3.28: Sketch of a TOB rod [15] . Detector modules are mounted on both sides of the mechnical support structure.

In total 24 precision inserts are glued to the carbon fibre frame and the cooling pipe. Each detector module is mounted on four of these inserts (two close to the hybrid and two close to the sensor-to-sensor bonds). Close to the detector hybrid the inserts employ pins used to clamp Be-Cu springs on the module frame to guarantee mechanical positioning precision. Efficient cooling contact between the detector module and the cooling pipe is realised by special (cup shaped) washers and screws fixed with a certain torque in the threaded holes inside the inserts. Size and material of the inserts are optimised with respect to efficiency of the cooling performance and material budget (minimal cross section of the cooling pipe). A picture of a rod frame is shown in figure 3.29. Two different kinds of rods are used in the different TOB layers. Layers 1 and 2 are equipped with rods carrying stereo modules, while layers 3 to 6 house rods equipped with single sided modules. Six modules are mounted on rods with only single sided modules with each module surface towards the central rod plane. On rods with stereo modules two modules are mounted on each of the six positions. In addition to the inner modules that are mounted, as for the rods in layers 1 and 2 a second module is mounted on top of each module with the two sensor back planes

72


facing each other. The distance between the silicon sensor and the central plane of a rod is ±3.3 mm for the inner modules and ±7.6 mm for the outer modules. Due to the different heat load of the different rod types the outer diameter of the cooling pipe is 2.2 mm for rods with single sided modules and 2.5 mm for rods with stereo modules. All cooling pipes are combined in 44 independent cooling manifolds serving on average 15 rods or 118 detector modules with 550 front-end chips. Electronics: One power supply unit serves one rod forming a power group. The main part of the rod electronics is the TOB Inter-Connect-Bus (TOB-ICB), a printed circuit board integrated in the central plane of each rod (see figure 3.29). It carries one CCUM that distributes all slow control information across the TOBICB to the front-end hybrids and the AOHs. Four Inter-Connect-Cards (ICC) provide all connections between the TOB-ICB and the detector front-end hybrid on a rod. Two specific ICCs serve one module position and two other ICCs serve two module positions. Since the number of detector modules per module position is different for rods with single sided modules and rods with stereo modules four different ICCs are necessary in total. The TOB-ICB is integrated on thin carbon fibre plates inside a rod and carries the connectors for all ICCs and the CCUM. All ICCs are screwed to the opposite side of the same inserts that carry also the detector modules to remove the dissipated power from the ICCs efficiently. On all ICCs the AOHs are mounted very close to the front-end hybrids. AOHs are cooled by their electrical connector over the ICC. Low voltages and slow control signals are connected to the detector modules via the ICCs. Temperature information from the detector modules is routed to the TOB-ICB via the ICCs. The optical fibres are routed inside the rod frame profiles together with the high voltage cables (the return current line is integrated in the TOB-ICB). On rods with single sided modules each module is served by one high voltage line, while on rods with stereo modules four high voltage lines serve four individual detector

Figure 3.29: Photograph of a TOB rod equipped with all electronic boards before integration of the detector modules.


73

modules and four lines serve two modules in parallel. Each rod employs a panel at the end where all high voltage cables, low voltage cables and temperature sense wires come together in separate connectors. Via a multi-service cable all signals and services are guided to the TOB back end. The six (or eight for rods with stereo modules) high voltage lines are connected to individual high voltage supply lines such that each line powers one side of a rod. Slow control signals and the low voltage line powering the CCUM are transmitted from rod to rod via short cables forming one control ring. The first and the last rod is connected to one of the 92 TOB DOHMs housing the DOHs. Control rings do not contain more than ten CCMs per ring. For the TOB two or three control rings are implemented per cooling segment, thus avoiding control rings going across cooling segment borders. This results in 7.5 rods (CCUMs) per control ring on average. Inside a control ring rods are clustered to match FEDs such that readout groups do not belong to different control loops under the constraint of minimising the number of unused FED channels. In total 134 FEDs convert the TOB detector signals in the service cavern. The central ground connection of a rod is its cooling manifold. Inside the CCUM the return line of the DOHM and CCUM low voltage and the return lines for the detector module low voltages and high voltages are connected. Via a short multi-wire cable they are connected to the cooling manifold. Additional grounding connections are implemented by metallisation around each ICC mounting hole. Figure 3.30 shows a fully equipped rod with stereo modules.

3.5.3 Tracker End Caps Two almost identical end cap detectors (called TEC+ and TEC- according to their position along the beam axis) with 3,200 detector modules each close the tracker on both sides. The tracker end caps are located in the region 22 cm < r < 113.5 cm and 124 cm < |z| < 280 cm. Substructures called petals are mounted on nine disks per end cap and carry the individual detector modules that are arranged in rings. Again the strategy of combining detector modules on larger substructures, like rods and strings in TOB/TIB, is applied in the end caps. Two additional disks providing the tracker side termination are added. Along their outer radius disks are joined together by eight U-shaped carbon fibre profiles per end cap that also serve as holders for all services. The outer six disks have a larger inner aperture (r < 30.9 cm) compared to the inner three disks (r < 22.9 cm). All disks are made from a carbon fibre/honeycomb structure covered on both sides by 0.4 mm thin carbon fibre skins and are linked to the inner support tube at four points. The inner honeycomb structure is 16 mm thick and

74


Figure 3.30: Both sides of a fully equipped TOB rod with stereo modules before integration into the TOB wheel. At the side the optical ribbon cables transmitting the analogue optical signals are shown.

reinforced at the borders by epoxy potting. Carbon fibre panels with a thickness of 0.4 mm close the end caps on the outside and provide a cylindrical envelope for the dry nitrogen atmosphere. The front plates (5 mm honeycomb NOMEX plus 0.2 mm carbon fibre skins on both sides) close the end caps on the inside, while the outside is closed by the TEC back plate (45 mm honeycomb NOMEX plus 1.5 mm carbon fibre skins on both sides). The back plate function is threefold. It serves as thermal shield, provides rigidity along z and carries the bulkhead, another carbon fibre disk that serves as patch panel for all TEC services. The bulkhead is held by the tracker support tube and covered with heating foils to close the thermal screen of the tracker support tube. Petals: As explained above all detector modules in the end caps are arranged in rings around the beam axis. Disks 1 to 3 carry all seven rings of modules, on disks 4 to 6 the ring 1 modules are missing, on disks 7 and 8 the inner two rings are missing, while on disk 9 rings 1 to 3 modules are absent. Stereo modules are used on rings 1, 2 and 5, all other rings are equipped with single modules. To provide easy access to the detector modules and to ease the production of the end caps all modules are grouped on petals. Sixteen petals are mounted on each disk, eight socalled front petals (facing the proton proton interaction point) and eight so-called back petals (mounted on the opposite side of a disk). Four different types of petals


75

exist, long and short front petals and long and short back petals. Long petals are used on disks 1 to 3 while the short petals are used on all outer disks where less rings of modules are used. Modules belonging to rings 1, 3, 5, 7 are mounted on the side facing the interaction point (so-called A side for front petals and C side for back petals). Even numbered rings are mounted on the side facing the outside of CMS (so-called B side for front petals and D side for back petals). To guarantee overlap of the sensitive regions detector modules belonging to the same ring overlap azimuthally and front petals overlap with back petals on the opposite side of the disk. Radial overlap is achieved by the mounting positions for detector modules of adjacent rings on opposite sides on a petal. Figures 3.31 and 3.32 show two photographs of a TEC.

Figure 3.31: Photograph of one tracker end cap. The detector modules on petals arranged in rings around the central aperture are visible. Panels made from carbon fibre form the outer protection cylinder of the end cap.

Figure 3.33 shows sides A and C of a fully integrated long front petal. Figure 3.34 shows sides B and D of a fully integrated long back petal5 . 5 Photographs

by courtesy of Dr. R. Bremer.

76


Figure 3.32: Photograph of one end cap turned by 90 degrees in the position for petal integration without outer protection cylinder.

The mechanical structure of petals is similar to disks. A honeycomb structure of 10 mm thickness is covered by 0.4 mm thin carbon fibre skins on both sides. A three point fixation is used to hold the petals on inserts in the disks. The modules are fixed on petals by means of four precision inserts machined to an accuracy of 20 μm required for module positioning [48]. In addition the inserts serve as heat sinks. Each insert is connected to one of the two cooling loops that are embedded inside the honeycomb core of a petal. Cooling pipes are made of titanium with an outer diameter of 3.9 mm and a wall thickness of 250 μm. The routing of the two interconnected cooling pipes inside a petal body is shown in figure 3.35. They are used to remove the generated heat of all components that are mounted on both sides of a petal. After ten years of operation the heat load from one petal equipped with up to 28 detector modules will be about 87 W. A mass flow of 2.3 kg/min of the cooling fluid C6 F14 creates a temperature difference of 2 K between petal inlet and petal outlet. To deliver the coolant fluid to the petals 64 stainless steel pipes (11 mm inner diameter) run on the outside along the service channels on each endcap. Five or six petals are served by one pair of pipes.


77

side A, front petal

bridge

ring 1 ring 3 ring 5 ring 7

side B, front petal

CCU CCU

cooling manifold

bridge

ring 2 ring 4

ring 6

Figure 3.33: Both sides (A and C) of a long front petal equipped with 28 detector modules. Azimuthal overlap is guaranteed by detector modules mounted on top of each other by means of special bridges. Close to the front-end hybrids all AOHs with the corresponding fibres can be seen. All fibres are routed to the petal side and run in special grooves to the petal edge. The cooling manifold interconnects the two cooling loops inside the petal body and provides the connectors to the outside cooling service.

78


side C, back petal

bridge

cooling manifold

ring 1 ring 3

ring 5 ring 7

side D, back petal

bridge

CCU

CCU ring 2 ring 4 ring 6 DOHM

Figure 3.34: Both sides (B and D) of a long back petal equipped with 23 detector modules. Structure as described in figure 3.33. The digital opto hybrid module (DOHM) is mounted on the D side.


79

Figure 3.35: The two cooling pipes inside a long front petal [40]. Both are connected inside the cooling manifold on the left so that the cooling fluid runs serially.

Electronics: Similar to the TOB-ICB all end cap modules, AOHs and CCUMs are mounted on five individual printed circuit boards called TEC interconnect boards (TEC-ICB). The main board ICB46 mounted on the B and D sides of the petals carries the detector modules of rings 4 and 6, all connectors to the petal periphery, two CCUMs and the connectors to the four smaller boards ICB2 (also mounted on sides B and D), ICB1 ,ICB3 , ICB57 (all mounted on sides A and C). The ICB indices indicate corresponding module rings. All slow control signals, power for all electrical devices and data to the AOHs are transmitted electrically via the ICBs. Detector modules are organised in groups to limit the number of low voltage supplies and cables. Table 3.5 shows the module grouping on front and back petals. Each group is served by an individual power supply unit. A maximum sensed current of 12 A is transfered by the eleven lines on ICB46 . To reduce ripples and overvoltages when switching the power, dedicated capacitors are placed near the front- end hybrid and the power input connector. Two high voltage groups are arranged per low voltage group. Up to four high voltage lines per group serve the modules such, that one or two detector modules are biased by one line. Slow control rings are realised by a pair of one front and one back petal, the front petal being the first in line. The two CCUs on CCUMs serve the modules of rings 1 to 4 and rings 5 to 7, respectively. Two DOHs on the DOHMs mounted on each back petal distribute slow control signals to the CCUs. In case of problems each CCU

80


front petals

rings

# modules

# APVs

group 1 group 2 group 3

1,2 3,4,6 5,7

8 11 9

48 44 44

1,2 3,4,6 5,7

4 8 11

24 32 56

back petals group 1 group 2 group 3

Table 3.5: Low voltage groups on front and back petals.

can be bypassed to maintain the functionality of the control ring. A fifth CCU is mounted on the DOHM to allow the last CCU of a ring to be bypassed. If two consecutive CCUs fail the control ring is broken.

3.6 Laser Alignment System To align the individual tracker sub-components with respect to each other and to provide a link to the muon system without the information from traversing particles, a Laser Alignment System (LAS) is implemented in CMS. Some silicon modules in the end caps are transparent for infra red laser light. The laser beam position can be measured by the silicon sensors to an accuracy of 10 μm. Special so-called alignment modules are mounted on rings 4 and 6 on back petals. A laser beam penetrating ring 4 in both TECs connects the end caps with TIB and TOB. Light is reflected to modules of the inner TOB layer and the outer TIB layer. A link to the muon system is provided by an extra laser beam. The schematic layout of the LAS is shown in figure 3.36. The LAS is designed to provide alignment information on a continuous basis at the level of 100 μm for all tracker substructures. Special alignment runs and operation during physics data taking is foreseen and data can be identified by special identification headers. Detailed information on the LAS is given in [49].

3.7 Cooling system η 0.1 0.2 0.3 0.4

81 0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

r/mm 1200 TOB TEC 1100 1000 900 ray 2 BS 800 700 ray 3 BS 600 AT 500 ray BS 4 AR 400 ray 1 300 200 TIB TID Pixel 100 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 z/mm

1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5

Figure 3.36: Layout of the laser alignment system in the r − z projection [49]. Four laser light rays induce charge signals in special alignment modules in the end caps and normal modules in barrel region. Ray 1 connects the silicon strip tracker to the muon system.

3.7 Cooling system The surface of the tracker support tube faces the electromagnetic calorimeter, which is operated at ambient temperature and requires temperature stability for excellent performance. The outside of the electromagnetic calorimeter is kept at 18 ± 4 ◦ C while the tracker volume needs to be cooled to below −10 ◦ C. This thermal gradient over a very limited radial thickness is realised by an active thermal screen. It guaranties a temperature below −10 ◦ C inside the silicon tracker volume even when the sub-detectors and their cooling are switched off, and a temperature above +12 ◦ C on the outer surface of the support tube in order to avoid condensation. The thermal screen consists of 32 individual panels. Cooling fluid is circulated on the inside in a thin aluminum plate whilst, separated by 8 mm of Rohacell foam, several polyimide-insulated resistive circuits are powered to heat the outer surface to the required temperature. The system is feed-back controlled, based on 64 temperature sensors. The total power dissipation inside the silicon tracker volume is expected to be approximately 60 kW. Mainly for robustness in operation, the CMS tracker is equipped with a mono-phase liquid cooling system. The liquid used for refrigeration of the silicon strip detector, the pixel detector and the thermal screen is C6 F14 . It has a sufficiently low viscosity even at the lowest required temperature, excellent behavior under irradiation and C6 F14 is extremely volatile thus avoiding eventual

82


damages from accidental leaks. The cooling system provides up to 77 m3 /hour of C6 F14 to the tracker, at a temperature down to -35 ◦ C and with a pressure drop of up to 8 bar. This corresponds to a cooling capacity of 128 kW. The entire tracker volume (about 25 m3 ) is flushed with chilled dry nitrogen gas at a maximum rate of one volume exchange per hour.

3.8 Material budget

x/X0

x/X0

As mentioned above the material budget plays an important role in the performance of the overall detector. Figure 3.37 shows the material budget in units of radiation length as a function of pseudorapidity. In the region around η ≈ 0 the material budget is as low as 0.3 X0 . It increases to 1.7 X0 at |η| ≈ 1.4 and decreases again to 1.0 X0 at |η| ≈ 2.5 (the acceptance limit of the tracker). The sensitive silicon surface makes up only a small fraction of the total material budget that is mainly driven by cables, cooling infrastructure, electronics and support structures.

Outside TEC TOB TIB+TID Pixel Beam Pipe

2

1.8 1.6

1.8 1.6

1.4

1.4

1.2

1.2

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0 -4

-3

-2

-1

0

1

2

3

Outside Other Support Cooling Cables Electronics Sensitive Beam Pipe

2

4 η

0 -4

-3

-2

-1

0

1

2

3

4 η

Figure 3.37: The silicon tracker material budget in units of radiation length [34]. Left: Material budget broken down to the individual subdetector systems. Right: Material budget broken down to the individual components.

3.9 Expected performance

83

3.9 Expected performance Simulations of the entire tracker (pixel part plus strip tracker) show the expected performance for single muons. Figure 3.38 shows five track parameters for single muons with transverse momenta in the range between 1 GeV and 100 GeV. Up to η < 1.6 the transverse momentum resolution is 1 − 2% for high momentum muons. In the pseudorapidity range above 1.6 the resolution degrades because of the reduced lever arm. The degradation in the region around η = 1.0 is due to the gap between the barrel and the end cap disks. The degradation beyond η = 1.2 is due to the lower hit resolution of the last hits of the tracks measured in the TEC ring 7 modules with respect to the hit resolution in the TOB layer 5 and 6 modules. For high momentum muons the material of the tracker accounts for approximately 2030% of the transverse momentum resolution. For low momentum muons multiple scattering dominates the resolution. For high pt muon tracks the transverse impact parameter resolution reaches 10 μm. For lower transverse momenta again multiple scattering of the muons degrades the expected performance. At high momenta, the d0 resolution is constant and dominated by the hit resolution of the first hit in the pixel detector system. At lower momenta the d0 resolution is degraded by multiple scattering until it becomes dominant. The longitudinal impact parameter resolution reaches 20 μm for high pt muon tracks. The z0 resolution of high momentum tracks is also dominated by the hit resolution of the first hit in the pixel system, again multiple scattering dominates at low momenta. The improvement of the z0 resolution up to a η = 0.5 is explained by the fact that in the barrel region, when the incident angle of tracks crossing the pixel layers increases, the clusters get wider and the pixel hit resolution is improved. Combining the information from the inner tracking and muon systems improves the transverse momentum resolution. Muons produced in the interaction are measured three times, in the inner tracking system, after the superconducting coil and in the return yoke. When using the muon system alone, the momentum measurement is determined by the bending angle at the exit of the 3.8 T coil, with the origin of the muon being the interaction point which is known to approximately 20 μm. Up to pt ≈ 200 GeV/c the resolution is dominated by multiple scattering, before the muon chamber resolution starts to dominate. For low momentum muons the resolution is determined by the measurement of the silicon tracker. For muons with a pt 1 TeV/c it is in the order of Δpt /pt ≈ 10−1 − 3 · 10−1 . Figure 3.39 shows the transverse momentum resolution for muons for the muon system alone, the silicon tracking system alone and for a combination of both.

84


The global track reconstruction efficiency is shown in figure 3.40 for simulated single muons and pions. About 99% of all muons are reconstructed in almost the entire acceptance range. The slight decrease in efficiency at η = 0 is due to a small gap between the silicon pixel layers at z = 0. In the high η range the efficiency decreases due to the reduced coverage with silicon pixel modules on the forward disks. The lower reconstruction efficiencies for hadrons, e.g. pions, is due to interactions with the material of the tracker.

σ(δ φ) [mrad]

85

μ, pt=1GeV

t

10

μ, pt=10GeV

t

σ(δ p /p ) [%]

3.9 Expected performance

μ, pt=100GeV

10

μ, pt=1GeV μ, pt=10GeV μ, pt=100GeV

1

1 10-1 0

0.5

1

1.5

2

0

η

0.5

1.5

2

1.5

2

1.5

2

η

σ(δ z ) [μm]

b)

μ, pt=1GeV μ, pt=10GeV

0

μ, pt=100GeV

0

σ(δ d ) [μm]

a)

1

2

10


103

μ, pt=100GeV

102

10

0

0.5

1

1.5

2

10 0

η

0.5

η

d) 2


χ2 /ndf

σ(δ cot(θ)) [10-3]

c)

1

μ, pt=100GeV

10

1.5

1 1

0.5 μ, pt=1GeV

μ, pt=10GeV

μ, pt=100GeV

-1

10 0

0.5

1

e)

1.5

2

η

0 0

0.5

1

η

f)

Figure 3.38: Resolution of the five track parameters for single muons with transverse momenta of 1, 10 and 100 GeV/c [8]: a) transverse momentum, b) φ , c) transverse (d0 ) and d) longitudinal impact parameter (z0 ), e) cot(θ ), f) reduced χ 2 .

86


Δ p/p

Δp/p

1

0.0

-0 ,2 5% 0, 25 % -0 , 5% 0, 5% -0 ,7 5% 0, 75 % -1 % 1% -1 ,2 5% 1, 25 % -1 , 5% 1, 5% -1 ,7 5% 1, 75 % -2 %

2% >

0%

-0 ,2 5% 0, 25 % -0 ,5 0, % 5% -0 ,7 5% 0, 75 % -1 % 1% -1 ,2 5% 1, 25 % -1 ,5 1, % 5% -1 ,7 5% 1, 75 % -2 %

0

faulty channels [%]

faulty channels [%]

Figure 4.19: Number of faulty channels. Left: Detector modules with HPK sensors. Right: Detector modules with STM sensors. Only detector modules with two sensors are taken into account.

needed produced %

R1N

R2N

R5N

R1S

R2S

R5S

R3

R4

R4A

R6

R6A

R7

144 161 112

288 305 106

720 761 106

144 159 110

288 304 106

720 771 107

640 667 104

576 641 111

432 437 101

864 900 104

144 161 112

1440 1489 103

Table 4.1: All produced TEC detector modules according to the individual ring geometry. The goal of the production with a minimum of 3% spare detector modules for all TEC rings was achieved for all geometries but for the special alignment modules for ring 4 (R4A).

ised to match the sector by sector integration of the TECs. In the first production step the TEC-ICBs were mounted on 314 petal bodies5 between December 2004 and January 2006 and delivered for integration of analogue opto hybrids and detector modules. Integration of analogue opto hybrids and optical fibre routing on the petal body was organised centrally6 and done in the period between March 2005 and August 2006. Petal integration and petal quality: Six institutes (so-called petal integration centres, PICs) were responsible for the integration of modules and all services on petal bodies, already equipped with the interconnect boards and analogue opto hybrids as explained above. Two set-ups are necessary to integrate modules with high precision in a clean room environment and to test the completed petal at ambient temperature and at the CMS working temperature of -10◦ C. 5 Manufactured

at the I. Physikalisches Institut b, RWTH Aachen. at Physikalisches Institut, Universität Hamburg. 31 petal were equipped with analogue opto hybrids at CERN.

6 Done

108

4 Detector Production and Commissioning

fridge cooling plant

DAQ rack

slow control

DAQ PC

Figure 4.20: Photograph of the petal long term test set-up in the Aachen PIC. The fully equipped petal is installed inside the fridge. The petal cooling pipe is connected to the cooling plant providing the chilled cooling liquid. A rack with DAQ components and two PCs are necessary to read out the petal under test.

In the petal assembly set-up all modules are mounted on the petal bodies and basic connectivity and communication tests are performed. The identity of all components and their location on the petal are stored in special xml-files. After optical inspection the petal long term test is the first place where all petal components are read out simultaneously and tested in a cold environment. A typical PIC long term test set-up is shown in figure 4.20. In three temperature cycles the petal is cooled down to -20◦ C and qualified. All silicon sensors are tested at a temperature lower than -10◦ C (see figure 4.21, left). In figure 4.21 (right) the measured front-end hybrid temperature indicates that the hybrids carrying six APVs are warmer compared to hybrids carrying only four APVs that produce less heat. To control the petal quality and compare the test results of the different petal integration centres a new data format was created. It provides both the most important information of the long term test for all production petals (petal properties like ID and test centre, module properties like position and leakage current and channel properties like noise and failure flag) and results of the single module test (ARC test) which was performed for each module before it was mounted on a petal. Figure 4.22 (left) shows the summarised result of these

4.1 Production

109

two tests. Out of roughly 3.8 × 106 channels that belong to the two tracker end caps 99.8% are of good quality. Only 0.1% show a conspicuous behaviour in the ARC test and 0.1% in the petal long term test with 3,584 strips being flagged in both tests. The small difference shown in figure 4.22 (left) is caused by different test procedures and analysis algorithms that are performed on the data taken in the two set-ups. In the long term test the results of several pedestal runs are combined to get the final noise behaviour of each strip which reduces the impact of random noise that is induced for example by the set-up itself. Thus the number of faulty channels is decreasing although the petal test is a much more complicated environment where different failures are harder to detect than in the single module test. A more reliable estimation of real defects independent from the performed test is given by the number of channels that are tagged as faulty both in the long term test and the ARC test (0.1%, see figure 4.22, right). Strips receiving a flag in only one of the two tests are mostly noisy channels that probably picked up noise from the test set-up. The petals are graded by using relative cuts (± 10%) on the common mode subtracted noise (normalised and averaged) and on histograms taken with the APV internal calibration pulse logic (the normalised pulse height distribution and the rise time of the calibration pulses). In addition the leakage current must stay below 10 μA for one-sensor modules and 20 μA for two-sensor modules. Figures 4.23 and 4.24 show example distributions for one out of four APV readout modes and one out of three temperature cycles. In total 0.16% of all strips are flagged as bad reflecting the overall channel quality for all modules on all production petals7 . The outermost channels of each APV (these are the channels 1, 2, 127 and 128) are shown separately as it is a known behaviour of the APV chip that these edge strips have higher noise. Special cuts are applied and the channels are qualified as good if they are only conspicuous in the noise test. A very good agreement between the ARC- and the petal long term test appears in the leakage current measurement at a depletion voltage of 450 V (see figure 4.25, left). In figure 4.25 (right) this leakage current is shown with respect to the number of sensors belonging to the module. It is evident that all detector modules mounted on the production petals fulfill the criterium of < 10 μA per sensor. The long term test set-up does not provide a separate high voltage line for each single module but several detectors share one high voltage line. Therefore it is not possible to disentangle the current contributions from the modules belonging to the same line. The total leakage current of a group of detectors is divided by the number of corresponding modules and the average current is assigned to 7 for

the common mode subtracted and normalized noise in peak mode with inverter on in the last cycle of the long term test

110

4 Detector Production and Commissioning

all modules in the group. Taking this into account no sensor exceeds the limit of 10 μA per sensor. Petals are classified according to the criteria given in table 4.2. grade A B C D

number of bad channels

number of modules with worst grade

Ileak (450 V)

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