MEG実験用液体キセノン検出器におけるデジタル波形処理を用いた パイルアップ事象の研究

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1 MEG実験用液体キセノン検出器におけるデジタル波形処理を用いた パイルアップ事象の研究
日本物理学会２００５年秋季大会　＠大阪市立大　２００５年９月１３日 MEG実験用液体キセノン検出器におけるデジタル波形処理を用いた パイルアップ事象の研究 内山　雄祐 東大素粒子セ,早大理工総研A, 高エネ研B, BINP-NovosibirskC, INFN-PisaD, PSIE 岩本敏幸, 　内山雄祐, 大谷航, 小曽根健嗣,　笠見勝祐B,　菊池順A, 澤田龍, 鈴木聡A, 寺沢和洋A, 名取寛顕, 西口創, 春山富義B, 久松康子,　真木晶弘B, 三原智, 森俊則, 山下了, 山田秀衛, A.A.GrebenukC, D.GrigorievC, Y.YuriC, D.NicoloD, S.RittE, G.SignorelliE

2 Contents Why waveform ? Waveform data Waveform simulation
Pile-up rejection Summary 13/Sep/2005 日本物理学会2005年秋季大会

3 Why use waveform data In the MEG experiment all PMTs are read by
a fast waveform digitizer Photon yield e m g menn + g meg ? signal background 52.8MeV Using Lq.Xe as scintillator large light yield short decay time short radiation length Major background Prompt background Accidental background reject pile-up of g-rays Crucial for the MEG experiment and very difficult without waveform image Unsegmented detector 13/Sep/2005 日本物理学会2005年秋季大会

4 Waveform data Domino Ring Sampler (DRS) ~\10,000/chn
Developed by Stefan Ritt NIM A 518(2004) 470 Domino Circuit Readout Shift Register 10 channels 　　x 1024 bins Analog sampling chip, switching capacitor circuits Max sampling speed 4.5GHz (required 2.5GHz) Sampling cells 8 data ch, 2 calibration ch(voltage and time) / chip Read out speed 40MHz, 12bits Domino wave runs continuously, only stopped by the trigger 2.5GHz sampling Xe waveform data were already taken successfully using prototype detector [mV] Data analysis is going on. I reported at last meeting... ~\10,000/chn [msec] 13/Sep/2005 日本物理学会2005年秋季大会 Xe scintillation pulse

5 Waveform simulation

6 Waveform Pulse shape is a consequence of various effects like,
Scintillation process Light transport in the scintillator PMT response Shaping from circuit Cables Receiver (DRS) a electron t= 45nsec Xe scintillation process for g Decay time 45nsec PMT TTS 0.75nsec (Typ.) TTS : Transit time spread of PMT for individual photoelectrons 13/Sep/2005 日本物理学会2005年秋季大会

7 Waveform simulation Sum up single electron pulses for all photoelectrons Single electron response spread by TTS (Gaussian). Arrival time of each scintillation photon tracked by MC simulation. 8000 p.e. Shaped by low pass filter RC shaping ( integration circuit ) Time constant RC = 5 nsec Data averaged pulse Simulated pulse Simulated waveform is well fitted to real waveform. 13/Sep/2005 日本物理学会2005年秋季大会

8 Simulated waveform We succeed in simulating pulse shape properly
Now we can simulate waveform pulse by pulse. Pulse shapes are not constant especially for small pulses because of statistics. 2000p.e. 500p.e. 100p.e. Data width:height Simulation Distribution of pulse width Fluctuation of pulse shape is well simulated pulse width [nsec] Due to DRS response for small pulses We succeed in simulating pulse shape properly 13/Sep/2005 After this, use these simulated waveform for analysis 日本物理学会2005年秋季大会 pulse height [mV]

9 Pile-up rejection

10 Pile-up event How to reject pile-ups ? g1 distribution of PMT output
Lq. Xe How to reject pile-ups ? distribution of PMT output pulse shape after DT g2 DT = t2 – t1 E1 + E2 = 1 (signal energy) g1 g2 # of p.e. 1 PMT output 2000p.e p.e., DT = 20nsec 13/Sep/2005 日本物理学会2005年秋季大会

11 Set threshold in peak finding with miss-rejection of non-pileup signal
Pile-up rejection How to find pile-ups ? Peak search method simplest way but powerful in case of large DT Take moving average and count peaks DT=75ns, 2000p.e + 400p.e. Differential method powerful in case of DT around rise time Take differentiation and count its peaks Set threshold in peak finding with miss-rejection of non-pileup signal < 0.05% DT=15ns, 600p.e p.e 13/Sep/2005 日本物理学会2005年秋季大会

12 Pile-up rejection Difficult to find pile-up by looking at individual PMT output. # of photons for each PMT is small # of PMTs which can observe event as a pulse is small Noise such as microstructure in pulse shape for small signal Take sum of PMT outputs Larger pulse Microstructure in pulse shape disappear Sum of all PMTs for signal g 13/Sep/2005 日本物理学会2005年秋季大会

13 S/N can be improved considerably
Pile-up rejection Taking all PMTs sum is not good from S/N viewpoint. Sum in order of PMT output How many PMTs to be summed ? 70% 90% 40 130 # of PMTs # of PMTs S/N can be improved considerably 13/Sep/2005 日本物理学会2005年秋季大会

14 Rejection efficiency optimization E1 + E2 = 1 (signal) 13/Sep/2005
日本物理学会2005年秋季大会

15 Rejection efficiency 60% DT 8ns DT 50ns DT 10ns DT 100ns DT 15ns
Weak point DT less than 10nsec Small pulse after large one DT 15ns 13/Sep/2005 日本物理学会2005年秋季大会

16 Summary We Succeed in simulating waveform from LXe detector.
It indicates the detector response is well understood. Algorithm for pile-up rejection is studied and is being optimized. Pile-ups can be separated if , Eg: >5MeV, DT: >10ns 13/Sep/2005 日本物理学会2005年秋季大会

17 Next step Rejection spatially separated pile-up using distribution of PMT outputs Rejection efficiency against m g e g background 13/Sep/2005 日本物理学会2005年秋季大会

18 End of slides

19 DRS principle Inverter “Domino” chain FADC 40MHz
0.2-2 ns Inverter “Domino” chain IN Waveform stored Out FADC 40MHz Clk Shift Register “Time stretcher” GHz  MHz 13/Sep/2005 日本物理学会2005年秋季大会