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

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LXe detector

protonmuon

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, 241Am 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 the stability of the light yield and of the detector gain

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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)

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1%

2009

2008

• Stable within ~1% during physics run in 2009

The light yield history in 2009 by Li 17.6MeV

The light-yield history 2008-2009Li 17.6MeVCosmic 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

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• Stable within the precision of each monitoring

from 241Am Cosmic rayLED

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

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17.6

MeV

pea

k [a

.u.]

• Uniformity within 0.14% after correction

0.14% CW w/o correction

CW (corrected)Uniformity before/after correctionDetector

inner face

Peak [a.u.]

• Non-uniformity measured by 17.6MeV

Energy resolution around signal E• 55MeV from 0 decay

– Gaussian for upper partand exponential for a lower tail

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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

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)

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CEX

CW

AmBe

determination determination of energy scaleof energy scale

CEX

CW

pedestalPrelim

inary 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

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Detectorinner face

Scanning peaks along beam axis

PMT

1cm slitPea

k po

sitio

nPerformance of position reconstruction

• Position determination : 0.7mm bias– Enough small compared with resolution

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Wid

th o

f pea

k

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

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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

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