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
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Monitoring tools of LXe detector
• Cockcroft-Walton accelerator
LXe detector
– Nuclear reaction by protons
– Li(p, g)Be 14.6, 17.6MeV
– B(p, g)C 4.4, 11.7MeV
– Useful to monitor the light yield and
to check the uniformity of detector response
muon
proton
beam line
• LED, 241Am a 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 m+ beam since 2009
• a-g separation improved by the change of the scintillation property
• Checked the 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
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Monitoring light yield
The light-yield history
2008-2009
Li 17.6MeV
Cosmic ray
2009
• 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)
2008
The light yield history in 2009 by Li 17.6MeV
1%
Li 17.6MeV
peak
• Stable within ~1%
during physics run
in 2009
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
Monitoring the detector in m+ beam
• The light yield was stable but PMT gain shifted with m+ 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, a, cosmic ray
LED
a from 241Am
Cosmic ray
• Stable within the
precision of each
monitoring
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
Energy reconstruction
• Energy
• The number of scintillation photons = S (weight x PMT charge / gain / Q.E.)
x energy scale x correction factor
• Energy scale determined at 55MeV in p0 run near signal 53MeV g 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
• Non-uniformity
measured by 17.6MeV g
Detector
inner face
17.6MeV peak [a.u.]
–
–
–
–
• Uniformity within 0.14%s
after correction
Uniformity before/after correction
CW (corrected)
CW w/o
0.14% s
correction
Peak [a.u.]
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
6
Energy resolution around signal Eg
• 55MeV g from p0 decay
– Gaussian for upper part
and exponential for a lower tail
Energy resolution in s at 55MeV
supper
along depth from inner face
Events along depth
Detector inner face
• 2.0% supper (averaged)
s distribution
on inner face
– The same result as 2008 even
after the light yield improved.
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
Energy linearity / resolution
• Checked linearity and energy resolution by various energies
– B (4.4MeV, 12.0MeV), Li (17.6MeV),
p0 decay (54.9MeV, 83.0MeV), p- p → n g (129MeV)
CEX
a
CW
CW
Signal
determination
of energy scale
CEX
pedestal
AmBe
• Enable the extrapolation to signal 53MeV region
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
Position measurement
• Reconstruct position by the light distribution
on inner face
• Position resolution in 2009 was measured
with a new lead collimator
PMT
Detector
inner face
– 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 p0 run
Scanning peaks
along beam axis
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
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Performance of position reconstruction
Width of peak
All slits
Central
1cm slit
5mm
slit
•
•
1cm
slit
Averaged width of peaks 10mm slit : 6.8mm s, 5mm slit : 6.4mm s
Position resolution is the same as in 2008 : 5mm s
Peak position
– Obtained after subtracting effects of projected slits size and beam spread of ~ 8mm s
excluded
•
Central
1cm slit
1cm slit
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
Detection efficiency
• Lower energy tail of energy distribution is taken into account
– Interaction with material before g reaches acceptance of LXe detector
– Leakage of photons near the detector's face
• Absolute detection efficiency obtained from Monte Carlo simulation
– Signal g with position dependence : 67% (Eg > 46MeV)
• Consistency check by p0→2g between measurement and MC
– Counts coincident 2g 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
data
MC
2010/3/23
?
83MeV g
55MeV g
Neutron
from p-p→ng
is subtracted
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
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% s average
– 67ps s LXe timing and 180ps s of g-e+ timing in previous talk
– 5mm s with 0.7mm bias of reconstructed position
• 67% detection efficiency in p0 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
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