Transcript Determination of position of muons for the KIMS experiment

Determination of muon hit position
for the KIMS experiment
JingJun Zhu
Tsinghua University &
KIMS collaboration
2003 Oct. 24th KPS meeting
KIMS experiment
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The KIMS (Korean Invisible Mass Search) experiment is
aimed at searching WIMPs (Weakly Interacting Massive
Particles) --- one of the most possible dark matter
candidates.
Principle: when WIMPs pass the main detector material --CsI crystal, they have the chance (although it's very small)
to hit the nuclei and produce a recoil signal that can be
detected by the PMTs attached to both sides of the crystal.
Difficulty: the cross section of WIMPs and nuclei interaction
is very small ( < 10-41 cm2 ), so a low background
environment is required. For this reason, the experiment is
carried on at a water power plant of YangYang which is
placed in 700 m underground.
Diagram of the power plant
External Background in KIMS
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The major external background of KIMS experiment is gamma,
neutron and Muon.
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For gamma, we can use PSD (Pulse Shape Discrimination)
method to reject. Heavy shielding material can protect
gamma from environment well.
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Neutron signal is very similar to WIMP signal. Only Offline
analysis and MC simulation enable to estimate. 30cm thick
mineral oil and 5cm PE layer is nice shielding to reduce
external neutron background.
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Energetic Muon can easily go pass whole shielding material
and induce neutrons via interacting with environmental
materials. Even if we use PSD method, that’s not perfect to
reject muon background. We need to construct veto detector
for muon.
Muon Detector
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To moniter the muon backgound we constructed muon
detectors surronding the main detector as active shielding,
whick is 30cm thick, filled with liquid scintillator, using PMT
to read out.
MUD 7
4
MUD 28
MUD
MUD
MUD
MUD MUD 1
6
MUD 3
MUD 5
CSI
2x2” PMT for each channel
8 muon modules , 28 signal channels
Liquid Scintillator 5 %
PC 1 liter + PPO 4 g + POPOP 15 mg
Mineral Oil 95 %
10-5 times of ground Muon rate at Y2L
Attenuation length of muon
detector
Use small scintillator for trigger muon events in specific position
Fitting function : two exponential decay function
Fitting results : fast term - 50 cm
Detection efficiency of muon
detector
Trigger Muon using two other scintillator detectors in the Ground lab
Use one(MUD2) of muon modules
Muon Energy spectra & Flux
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YangYang ( ~ 700m underground) :
~ 380 /day.m2 = 4.4 x 10-7 /s.cm2
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CheongPyoung ( ~ 350m underground) :
~1450 /day.m2 = 1.7 x 10-6 /s.cm2
Determination of position of muons
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Besides flux, another important thing is to determine the
position where muon hit the detector and reconstruct the
track of muons.
Minimum square method :
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Choose one point, calculate the energy response
according to distance to PMT and attenuation length of
liquid scintillator
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Compare the calculated result to the measured one, get
a square value
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Change the assumed position and calculated again, until
found the point which has minimum square
Calibration of Muon Detector
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To verify the effect of this method, we put a plastic scintillator at
the center of top detector to choose the muon events only
around center.
A plastic scintillator ( 85 x 20 cm2 )
has been put at the top as trigger
Hit reconstruction on Muon
Detector
Plastic scintillator position
and the calculated result
Hit position projected to x-axis
Reconstructed Hit Position of
Muon
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Calculated result without plastic scintillator as trigger.
Tracking and veto
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After finished position determination for all the detector, we
can get the track of each muon event, and then we can
reject the muon events which pass through the CsI crystal.
Muon detector
Copper box for
CsI crystal
Muon track
Summary
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We measured muon flux at 700m underground
laboratory, it is about 380 /day.m2 ( equal to 4.4 x
10-7 /s.cm2 )
Hit reconstruction of Muon has been finished and
Track Reconstruction is in progress. The track
reconstruction
give the information to reject muon events from
WIMP candidate data.