High rate and ageing studies for the drift tubes of the Atlas muon spectrometer [Elektronische Ressource] / vorgelegt von Stephanie Zimmermann

Sujets

• Physik

Informations

• Publié par: albert-ludwigs-universitat_freiburg
• Publié le: 01 janvier 2004
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 19 Mo

Extrait

High Rate and Ageing Studies
for the Drift Tubes of the
Atlas Muon Spectrometer
DISSERTATION
zur
Erlangung des Doktorgrades
der
Fakultat? fur? Mathematik und Physik
der
Albert-Ludwigs-Universit?at Freiburg im Breisgau
vorgelegt von
Stephanie Zimmermann
M?arz 2004Dekan: Prof. Dr. R.Schneider
Referent: Prof. Dr. G.Herten
Korreferent: Prof. Dr. A.Bamberger
Tag der mundlichen Prufung: 18. Mai 2004? ?High Rate and Ageing Studies
for the Drift Tubes of the
Atlas Muon Spectrometer
by
Stephanie Zimmermann
March 2004
Universitat Freiburg Fakultat fur Physik
im Breisgau und Mathematik
Abstract
The muon spectrometer of Atlas, one of the 4 experiments currently under construction
at the Large Hadron Collider LHC, relies on Monitored Drift Tubes (MDTs) for track
reconstruction in most of its regions. The MDTs will have to sustain count rates up
to 1500Hz/cm and must be able to survive an accumulated charge of up to 0.6C/cm
during 10 years of operation. This thesis presents results of high rate and ageing studies
conducted at the CERN Gamma Irradiation Facility GIF; a series production Atlas muon
chamber and a prototype of the gas recirculation system planned at LHC were used for
the first time.
TestbeammuonswereutilizedtoevaluatetheMDThighratebehaviour; questionsofres-
olution, efficiency and changes in the drift properties of the operating gas were addressed.
ThemeasurementswerecomplementedbysimulationswiththeprogramGARFIELD.For
the ageing study the MDTs were irradiated over a period of several months, in which
their performance was monitored weekly with cosmic muons. A loss in pulse height was
observedforMDTsundergasrecirculationafteranaccumulatedchargeequivalenttoonly
1 year of LHC operation, while tubes operated without gas recirculation did not show any
evidence of ageing. Extensive material analyses were performed in order to determine the
origin of the performance degradation.Contents
Preface 1
1 Introduction 3
1.1 The Standard Model of particle physics . . . . . . . . . . . . . . . . . . . . 3
1.2 The Large Hadron Collider LHC . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 The Atlas detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4 Atlas physics potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 (Atlas Monitored) Drift Tubes 17
2.1 Atlas MDTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Electron drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Ion drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.4 Gas amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.5 Signal formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 Background environment at LHC . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.1 MDT rate capability requirements . . . . . . . . . . . . . . . . . . . 28
3 Experimental setup 30
3.1 Data taking periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Gamma Irradiation Facility GIF . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 The MDT chamber BIS ’Beatrice’ . . . . . . . . . . . . . . . . . . . . . . . 31
3.4 On-chamber gas system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5 Scintillator trigger hodoscope . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.6 MDT electronics and readout; data acquisition . . . . . . . . . . . . . . . . 34
3.6.1 Passive front end electronics . . . . . . . . . . . . . . . . . . . . . . . 34
3.6.2 Active front end . . . . . . . . . . . . . . . . . . . . . . . 35
3.6.3 Event building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.6.4 Pulse height readout . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.6.5 Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.7 Mechanical setup during the different run periods . . . . . . . . . . . . . . . 40
3.8 Coordinate systems and naming conventions . . . . . . . . . . . . . . . . . . 42
3.9 Off-chamber gas system and gas recirculation . . . . . . . . . . . . . . . . . 43
3.9.1 The gas circulator ATC . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.9.2 Gas supply and regulation system . . . . . . . . . . . . . . . . . . . 45
3.9.3 Slowcontrol and safety interlocks . . . . . . . . . . . . . . . . . . . . 49
3.9.4 Experiences with operating the recirculation system . . . . . . . . . 49
III CONTENTS
4 Test beam studies – MDT performance under high rates 53
4.1 Radiation environment and muon beam . . . . . . . . . . . . . . . . . . . . 53
4.2 Drift time spectrum analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.1 Spectrum unfolding . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.2 Maximum drift time changes with background rate . . . . . . . . . . 58
4.2.3 Temperature dependence . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3 Track reconstruction and autocalibration. . . . . . . . . . . . . . . . . . . . 60
4.3.1 Finding an initial rt-relation. . . . . . . . . . . . . . . . . . . . . . . 60
4.3.2 Autocalibration method . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.4 Single tube resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.4.1 Determining tube resolutions without external reference . . . . . . . 62
24.4.2 Imposing ´ -cuts on the track quality . . . . . . . . . . . . . . . . . 63
4.4.3 Results in the absence of background radiation . . . . . . . . . . . . 64
4.4.4 for operation under high background rates . . . . . . . . . . 66
4.5 Single tube efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.5.1 Hit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.5.2 3?-efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5 Simulation of MDT high rate behaviour 70
5.1 Space charge and drift field modifications – steady-state models . . . . . . . 70
5.2 Changes in the maximum drift time . . . . . . . . . . . . . . . . . . . . . . 71
5.3 Gain drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.4 Resolution degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.4.1 Resolution when no radiation background is present . . . . . . . . . 73
5.4.2 change under high rates . . . . . . . . . . . . . . . . . . . 74
5.5 Efficiency loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.5.1 3?-efficiency when no radiation background is present . . . . . . . . 76
5.5.2 3?-efficiencies under high rates . . . . . . . . . . . . . . . . . . . . . 78
6 Ageing studies 81
6.1 Mechanisms of wire chamber ageing . . . . . . . . . . . . . . . . . . . . . . 81
6.2 Integrated charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.3 Methods of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.4 Accuracy and systematic errors . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.5 Ageing results for BIS multilayer 2 — vented gas system . . . . . . . . . . . 96
6.6 for BIS multilayer 1 — gas recirculation . . . . . . . . . . . . 96
6.7 Chemical and surface analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7 Summary 105
A MDT response and transfer functions 107
A.1 Abstract description of electrical systems . . . . . . . . . . . . . . . . . . . 107
A.2 Transfer function for a drift tube . . . . . . . . . . . . . . . . . . . . . . . . 108
A.3 Front end electronics: Amplification and pulse shaping . . . . . . . . . . . . 109
A.4 Front end Tail cancellation . . . . . . . . . . . . . . . . . . . . . 110
A.5 Response function for the MDT ASD-lite . . . . . . . . . . . . . . . . . . . 110
B Hodoscope calibration and track reconstruction 112
B.1 Track reconstruction in each hodoscope double layer . . . . . . . . . . . . . 112CONTENTS III
B.2 Reconstruction of the track angle . . . . . . . . . . . . . . . . . . . . . . . . 113
B.3 of the track position in MDT wire direction . . . . . . . . . 113
C Gas system components 115
D DAQ data format 117
D.1 Event block structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
D.2 Event data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
List of Figures 120
References 123Preface
”Dass ich erkenne, was die Welt im Innersten zusammenhalt” (’so that I understand?
what keeps the world together’) – these words, said by the character Dr.Faustus in J.W.
Goethe’s drama ’Faust’, are a good description of the goal of fundamental physics re-
search, namelytodescribenaturebyasmallnumberofuniversallawsandequations. The
question as to what makes up matter in particular attracted the interest of philosophers
and scientists from ancient Greece over the middle ages up to the present times.
It is remarkable that already the earliest models assume the existence of fundamental
entities from which all matter is built. This idea is present both in the discussions of
Aristotle, who postulated all things to be based on the elements water, air, fire and earth,
and the work of Demokrit, to whom we ow