MEG II 実験液体キセノン検出器実機 MPPC のコミッショニング. Commissioning of all MPPCs for MEG II LXe detector 小川真治 他 MEG II 日本物理学会 2017 年秋季大会

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1 1 MEG II 実験液体キセノン検出器実機 MPPC のコミッショニング Commissioning of all MPPCs for MEG II LXe detector 小川真治 他 MEG II 日本物理学会 217 年秋季大会

2 Table of contents 2 1. Introduction 2. MPPC commissioning 3. Detector commissioning by γ-ray

3 3 LXe detector upgrade top We have upgraded LXe detector for MEG II to significantly improve the performance. γ 2 inch PMT outer MEG II MEG γ mm2 MPPC lateral (DS) We have replaced inch PMTs on the γ-entrance face with mm2 MPPCs. Better granularity Better position resolution Better uniformity of scintillation readout Better energy resolution Less material of the γ-entrance face Better detection efficiency inner bottom lateral (US)

4 Expected performance Significant improvement of all resolutions and efficiency are expected. Detector performance for signal γ-ray MEG (measured) Efficiency 65% 7% MEG II (simulated) Position ~5 mm ~2.5 mm Energy ~2%.7-1.5% Timing 67 ps 5-7 ps MEG 4 MC Log scale Imaging power improves MEG II Position resolution [mm] Position resolution (horizontal) 1 9 MEG I 8 MEG II Conversion depth [cm] Reconstructed Energy MEG I MEG II Energy (MeV) MC Log scale

5 Table of contents 5 1. Introduction 2. MPPC commissioning 3. Detector commissioning by γ-ray

6 Photo sensor commissioning 6 Many photo sensors are used in LXe detector. 492 MPPCs PMTs. Performance of all photo sensors is being evaluated in LXe. Confirm reasonable MPPC PDE for LXe scintillation light in VUV. PDE over 1% is needed for detector operation. PDE ~2% has been measured for small number of sample MPPC. Timing resolution (ps) Timing resolution vs. MPPC PDE MC PDE PDE vs Over Voltage MPPC PDE (%).1.5 measurement in test setup Over Voltage (V)

7 Signal check for all photo-sensors 7 Signal check has been done for all sensors in LXe. With scintillation light from cosmic ray. Several dead and bad channels have been found. Dead MPPC (9 ch) : Short circuit or bad connection of cable. Dead PMT (9 ch) : Short circuit of HV cable. Bad PMT (6 ch) : HV cable connection is not stable. Effect of dead MPPC is negligible. Dead PMT may affect energy resolution. Under investigation. Dead & bad channel T D U dead channel bad channel B

8 Setup for photo-sensor calibration 8 Two kinds of data have been taken for MPPC calibration. LED run Observe 1 p.e. peak to estimate gain. Alpha run Observe scintillation light from alpha source. Compare measured # of p.e. with MC to estimate PDE. WaveDREAM (electronics developed for MEG II) has been used. WaveDREAM has the functionality of waveform digitization, amplifier, trigger, HV supply for MPPC. All MPPCs are grouped into 12 DAQ sets due to limited number of readout channels. Dark noise rate(@ LXe temp.) is too small to use it for calibration. (~5 Hz/mm 2 )

9 Noise in LED run 9 Large noise level are being observed in LED run. Seems to be due to GND treatment and switching power supply of electronics. Needs hardware modification to solve it. MPPC 1p.e. peak cannot be seen due to this noise. Apply following treatment as a temporally solution. Subtract coherent noise by using adjacent channel waveform. Operate MPPCs at relatively large voltage (over voltage 7V) Use short integration range (7 ns) to improve S/N. Charge distribution from LED light 1 p.e. can be seen except for noisy channel of electronics (6% of total). before coherent noise subtraction after coherent noise subtraction

10 MPPC Gain 1 MPPC gain has been estimated from 1p.e. peak. Reasonable gain has been confirmed. 8 x 1 over voltage ~7V Gain spread is 5% at same over voltage. Gain 54V 8 7 mean : 8.2 x

11 Alpha run 11 Alpha event trigger by lateral PMT. Event selection Separate alpha and others by pulse shape discrimination Select events from each alpha source by position reconstruction charge QAratio / height α cosmic ray etc... h Entries 211 Mean Std Dev PMT Reconstructed alphaz[] {QAratio position < 6 && QAratio (α event) >.5} α MPPC (not to scale) scintilation photons h2 Entries 88 Mean.739 Std Dev cm

12 MPPC PDE MPPC PDE has been estimated. PDE = (measured # of p.e.) / (expected # of photon in MC) Mean of PDE : 18% Sufficiently large PDE (> 1%) has been confirmed. Roughly consistent with previous measurement result (~2%). 12 Several effects which affect the measured PDE. crosstalk & afterpulse short charge integration range should be LXe light yield still improving ~1-3% 2 MPPCs one bin = one MPPC 66 Detected # of hmppccharge p.e. from alpha source MPPCs QEnearest*.22 MPPC {QEnearest> PDE&& ipm<492} htemp Entries 3765 Mean.1814 Std Dev QEnearest*.22

13 Summary of MPPC commissioning 13 We have upgraded LXe detector for MEG II to significantly improve the performance. Commissioning of the detector is ongoing. Performance of all VUV-MPPCs is being evaluated. Reasonable gain (~ 8 x 1 5 at over voltage 7V) and sufficiently large PDE (~18%) have been confirmed for most MPPCs.

14 Table of contents Introduction 2. MPPC commissioning 3. Detector commissioning by γ-ray

15 Detector commissioning by γ-ray 15 We have started to see γ signal. 4.4MeV γ from AmBe source has been successfully observed. From excited $% # C from ( ) Be(α,n) # C Hit the detector from inner face. $%. Detector performance is under investigation. Detector response will be checked by several γ-ray sources. ~9MeV from thermal neutron capture. 4 & 12 MeV from $$ / B(p,γ) $% # C. 18 MeV from 4 5 Li(p,γ) 6 ( Be. Pilot run of the detector will be performed with μ beam Typical event display (4.4 MeV γ) Energy spectrum h3 (AmBe source) 4.4 MeV from γ 9 MeV from n capture MeV

16 BACKUP 16

17 μ eγ search 17 We search for charged lepton flavor violating decay of muon, μ->eγ. Prohibited in SM, detectable branching ratio in some BSM model Main background is the accidental background. Detector resolutions, especially energy resolution of γ-ray, are important to effectively distinguish the signal event from the accidental background. Signal Background Signal decay Accidental background Radiative muon decay E=52.8MeV back-to-back coincident Dominant background E < 52.8MeV not back-to-back E<52.8MeV not back-to-back coincident

18 MEG II experiment 18 Upgrade of MEG experiment μ + stopping rate will be doubled μ/s μ/s Detection efficiency will improve. Resolutions of all detectors will become half. New detector for background tagging will be introduced e + drift chamber e + Liquid Xe γ-ray detector γ Gradient magnetic field μ + beam e + timing counter Expected sensitivity: One order of magnitude better than MEG DAQ with muon beam Engineering run

19 .9aa Energy resolution (%) all depth depth <2cm depth >2cm Energy resolution vs. MPPC PDE MPPC PDE (%) Expected Position resolution (mm) Position resolution vs. MPPC PDE u v w MPPC PDE (%) Energy resolution (%) Energy resolution with Estimation error of PDE, QE No error Error in MPPC all depth depth <2cm depth >2cm Error in MPPC, PMT

20 Motivation 2 検出器コミッショニングの 1step として 光センサーの較正を実施 検出器較正手法の確認 問題の洗い出しをする 検出器性能には光センサーの性能の正しい理解が必要 光センサー特性 (Gain, PDE) の個体差 二種類の光センサーの PDE の比 Position resolution (mm) Position resolution with Estimation error of PDE u v w Energy Resolution (%) Energy resolution by assuming several PDE for MPPC Graph true PDE:22% Red:all depth Blue:depth>2cm Green:depth<2cm Estimation error of MPPC PDE(%) (Assumed PDE)/ (True PDE) (Assumed PDE ratio)/ (True PDE ratio)

21 MPPC Gain 21 MPPC gain has been estimated from 1p.e.peak. Reasonable gain has been over voltage 7V MPPC gain (@54V) vs. Over voltage mean :.82M sigma : 5% Gain 7V

22 LXe light yield 22 aa nsum Graph MEG I equivalent time (day)

23 Average MPPC waveform 23 aa [a.u.] XEC Template MPPC waveform Waveform Black : 1 p.e. from LED Red : Alpha [sec] 6

24 MPPC Gain Graph vs Serial Graph vs Serial vs Vhama

25 MPPC PDE 25 MPPC PDE QEnearest*.22 {QEnearest> && ipm<492} MPPC PDE vs index QEnearest*.22:iPM {QEnearest> && ipm<492} 25 2 htemp Entries 3765 Mean.1814 Std Dev.2852 QEnearest* QEnearest* ipm

26 Absolute PDE / QE QEnearest*.22 MPPC {QEnearest> PDE&& ipm<492} htemp Entries 3765 Mean.1814 Std Dev QEnearest* htemp QEnearest*.16 {QEnearest> PMT QE && ipm>492} Entries 398 Mean.1867 Std Dev QEnearest*.16