MEG 実験 2009 液体キセノン検出器の性能 II 西村康宏, 他 MEG コラボレーション 東京大学素粒子物理国際研究セン ター 第 65 回年次大会 岡山大学

Contents Monitoring the liquid xenon (LXe) detector during physics data taking – Light yield – Detector gain Performance in 2009 – Energy – Position – Detection efficiency 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 2

LXe detector proton muon beam line Monitoring tools of LXe detector Cockcroft-Walton accelerator – Nuclear reaction by protons –Li(p,  )Be 14.6, 17.6MeV –B(p,  )C 4.4, 11.7MeV –Useful to monitor the light yield and to check the uniformity of detector response LED, 241 Am  source and cosmic ray – Monitor in both beam on and off PMT gain affected by beam current, which is corrected but to be confirmed. – Alpha events triggered even in  + beam since 2009  -  separation improved by the change of the scintillation property Checked t he stability of the light yield and of the detector gain 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 3   2008 start / end 2009

Monitoring light yield Reached full intensity in 2009 Purification finished before 2009 physics run Checked the stability of the light yield – Use 17.6MeV Li peak by CW accelerator (3times / week) 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 4 1% Stable within ~1% during physics run in 2009 The light yield history in 2009 by Li 17.6MeV The light-yield history Li 17.6MeV Cosmic ray Li 17.6MeV peak

Monitoring the detector in   beam The light yield was stable but PMT gain shifted with  + beam The shift is measured using LED and it's corrected depending on beam status We confirmed that the detector was stable by monitoring LED, , cosmic ray 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 5  from 241 Am Cosmic ray LED Stable within the precision of each monitoring  from 241 Am Cosmic ray LED

Energy reconstruction Energy The number of scintillation photons =  (weight x PMT charge / gain / Q.E.) x energy scale x correction factor Energy scale determined at 55MeV in  0 run near signal 53MeV  ray Correction factor for the change of the light yield and non-uniformity The light yield was stable so not used in 2009 Non-uniformity of the scintillation-photons detection measured by Li peak – Intrinsic non-uniformity exists due to the current reconstruction method – Observed the change with light yield increased in 2008 – Non-uniformity differs from 2008 results after light yield completely increased – Make correction factor for 2009 run 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス MeV peak [a.u.] Uniformity within 0.14%  after correction 0.14%  CW w/o correction CW (corrected) Uniformity before/after correction Detector inner face Peak [a.u.] Non-uniformity measured by 17.6MeV 

Energy resolution around signal E  55MeV  from  0 decay – Gaussian for upper part and exponential for a lower tail 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 7 Preliminary  distribution on inner face Detector inner face  upper along depth from inner face Energy resolution in  at 55MeV 2.0%  upper (averaged) –The same result as 2008 even after the light yield improved. Events along depth Entries for fit

Energy linearity / resolution Checked linearity and energy resolution by various energies – B (4.4MeV, 12.0MeV), Li (17.6MeV),  0 decay (54.9MeV, 83.0MeV),  - p → n  (129MeV) 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 8 CEX CW AmBe determination of energy scale  CEX CW pedestal Preliminary Enable the extrapolation to signal 53MeV region Signal

Position measurement Reconstruct position by the light distribution on inner face Position resolution in 2009 was measured with a new lead collimator – Long slits along the beam axis are prepared – Estimate a bias of absolute position on a long line – 1cm and 5mm slits with 1.8cm thickness are prepared – Put in front of the detector in  0 run 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 9 Detector inner face Scanning peaks along beam axis PMT

1cm slit Peak position Performance of position reconstruction Position determination : 0.7mm bias – Enough small compared with resolution 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 10 Width of peak 1cm slit 5mm slit Averaged width of peaks 10mm slit : 6.8mm , 5mm slit : 6.4mm  Position resolution is the same as in 2008 : 5mm  – Obtained after subtracting effects of projected slits size and beam spread of ~ 8mm  excluded Central 1cm slit All slits Central 1cm slit

Detection efficiency Lower energy tail of energy distribution is taken into account – Interaction with material before  reaches acceptance of LXe detector – Leakage of photons near the detector's face Absolute detection efficiency obtained from Monte Carlo simulation – Signal  with position dependence : 67% (E  > 46MeV) Consistency check by  0 → 2  between measurement and MC – Counts coincident 2  decay in LXe detector tagged by the NaI – Set energy region, no analysis cut – 67% in 2009 with twice amount of data in 2008 Consistent with 66% in 2008 – 4% difference between data and MC 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 11 data MC Neutron from  - p→n  is subtracted Preliminary ? 83MeV  55MeV 

Conclusions Checked the stable operation during physics data taking in 2009 Performance in 2009 with the increased light yield and upgraded waveform digitizer is almost the same as 2008 result (preliminary) – Energy resolution to be 2.0%  average – 67ps  LXe timing and 180ps  of  -e + timing in previous talk – 5mm  with 0.7mm bias of reconstructed position 67% detection efficiency in  0 run is consistent with 2008 Investigating more proper calibration and analysis 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 12

Non-uniformity in 2008 and 2009 Observed the change of non-uniformity by changing the scintillation property 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス /Nov /Nov MeV peak [a.u,] 17.6MeV peak [a.u.] /Nov /Nov - purification started peak on the inner face purification was done

Performance evaluation in  0 run Principle 2010/3/23 日本物理学会 第６５回年次大会 岡山大学津島キャンパス 14 n  00 --   p (98.8%) ~60% ~40% 8.9MeV MeV 54.9 ~ 83.0MeV Setup Evaluate detector performance around signal 53MeV energy  0 decay provides MeV  ray Monochromatic  obtained by selecting back-to-back opening angle Movable NaI detector opposite to LXe detector and hydrogen target for a charge- exchange reaction 9 NaI(Tl) crystals and APDs 2 plastic counters in front of the NaI detector for timing measurement Scan whole acceptance of LXe detector by moving the NaI detector  0 run in Oct before physics run started