STUDY OF MUONS PRODUCED IN EXTENSIVE AIR SHOWERS DETECTED IN HANOI USING A WATER CHERENKOV DETECTOR

 STUDY OF MUONS PRODUCED IN EXTENSIVE AIR SHOWERS DETECTED IN HANOI USING A WATER CHERENKOV DETECTOR

Ph.D: Nguyen Phuong Thao - Pierre Darriulat

(Luận án trình bày nghiên cứu chi tiết về hoạt động của detector Cherenkov VatLy- bản sao của một trong 1660 detector mặt đất tại đài thiên văn Pierre Auger. Đề tài tập trung vào nghiên cứu tín hiệu nhỏ được quan sát bởi Detector (nhỏ bằng 1/10 tín hiệu hạt muốn xuyên qua detector theo phương thẳng đứng - ta thường gọi là VEM) và mở rộng vùng nghiên cứu lên đến 10⁴..)

 A detailed study of the performance of the VATLY Cherenkov detector, a  replica of one of the 1660 detectors of the ground array of the Pierre Auger  Observatory, is presented. The emphasis is on the response to low signals down  to a tenth of the signal produced by a vertical feed - through muon (VEM),  implying a dynamical range in excess of 10⁴. The method is to look for decays of  muons stopping in the water volume of the detector, of which only a few produce  sufficient Cherenkov light to be detected before stopping. The subsequent muon  decay produces an electron (or positron) that carries an average energy of only  ~35 MeV. The experimental set - up detects the signals produced by both the  stopping muon and the decay electron. Such pairs have been detected under  various experimental conditions and the amplitude of the electron signal has been  recorded together with the time separating the two signals. A scintillator  hodoscope that brackets the Cherenkov detector from above and below provides

a precise calibration. A large sample of data has been collected that give very  clear evidence for muon decays with the expected time dependence. The  amplitude of the electron signal is observed at the level of a fraction of a VEM,  and only the upper part of its distribution can be detected. The muon distribution  requires the additional contribution of a soft electron/photon component, which  appears particularly important in the present experimental set - up due to the large  sensitive volume of the Cherenkov detector. A model of the physics mechanism  at play and of the detection process has been constructed, giving good  descriptions of the measured charge and time distributions. This allows for  obtaining useful evaluations of the number of photoelectrons per VEM, 13. 0±0. 9,  and of the mean muon energy, 4. 0 ±0. 4 GeV. The detection efficiency of  electrons implies an effective electron shower size, ~36±6 cm, at the scale of the  radiation length in water. The end point of the electron charge distribution,  corresponding to a kinetic energy of 53 MeV, is measured to be  Eend=0. 275±0. 018 VEM in agreement with expectation. The measured event  rates are found in good agreement with predictions and the occurrence of muon  pairs from a same shower is measured with a rate of 7. 0±0. 5 Hz. A simulation of the light collection mechanism suggests the presence of a small zenith angle  dependence of its efficiency, which is found consistent with observation. At the  same time as this study contributes useful information to the detailed  performance of large Cherenkov detectors in general, and particularly of the  ground array of the Pierre Auger Observatory, it contributes to the training of  students of experimental particle and nuclear physics by making available to  them a tool particularly well suited to the task.

Abstract - Page: 5
Key to Abbreviations - Page: 7
Acknowledgements - Page: 8
Table of content - Page: 9
1. Introduction - Page: 11
   1.1 Generalities on cosmic rays - Page: 11
   1.2 The Pierre Auger Observatory - Page: 13
   1.3 Cosmic rays in Hanoi - Page: 19
   1.4 The VATLY Cherenkov detectors - Page: 21
   1.5 Overview of the present work - Page: 24
2. Response of the VATLY Cherenkov Detector to feed-through muons - Page: 26
   2.1 The trigger hodoscope - Page: 26
      2.1.1 Description - Page: 26
      2.1.2 High voltages and delays - Page: 27
      2.1.3 Rate - Page: 29
   2.2 Electronics - Page: 30
 2.3 Analysis of hodoscope data - Page: 32
 2.3.1 Charge distributions  - Page: 32
 2.3.2 Time of flight  - Page: 35
 2.3.3 Event selection - Page: 37
  2.3.4 Stability - Page: 38
 2.4 Analysis of Cherenkov data - Page: 40
 2.4.1 Response of the Cherenkov counter to a hodoscope trigger  - Page: 41
 2.4.2 Selection of good muons  - Page: 42
 2.4.3 Conclusion - Page: 43
3. Muon decays in the VATLY Cherenkov tank - Page: 44
 3.1. Basic processes - Page: 44
 3.2. Simulation of the detector and muon signal - Page: 47
4. Auto-correlations: rates and time distributions - Page: 53
 4.1 The problem - Page:53
 4.2 No correlation - Page:54
 4.3 Cosmic rays - Page:54
 4.4 Muon decays and muon captures - Page:55
 4.5 Decays, capture and multi-muons - Page:57
 4.6 Simulation - Page:58
5. Auto-correlations: electronics and data acquisition - Page:61
 5.1 Auto-correlation measurement - Page:61
 5.1.1 Timing considerations - Page:63
 5.1.2 Calibration - Page:65
 5.1.3 Spikes - Page:67
 5.2 Charge measurement - Page:70
6. Auto-correlations: data analysis - Page:72
 6.1 Time spectra - Page:72
 6.1.1 Introduction - Page:72
 6.1.2 Cherenkov detector - Page:73
 6.1.3 Scintillator detector - Page:78
 6.2 Charge spectra - Page:81
 6.2.1 Introduction - Page:81
 6.2.2 Cherenkov detector - Page:81
 6.2.3 Scintillator detector - Page:90
7. Results and interpretation - Page:93
 7.1 A simple model - Page:93
 7.2 Comparison with the data - Page:94
 7.3 Including a soft component - Page:96
 7.4 Threshold cut-off functions - Page:98
 7.5 Dependence on zenith angle - Page:99
 7.6 Comparison between data and simulation - Page: 102
 7.7 Decoherence and shower size - Page: 109
8. Summary and conclusion - Page: 111
References - Page: 115