Muon flux at Y2L and reconstruction of muon tracks

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Muon flux at Y2L and reconstruction of muon tracks
JingJun Zhu
Tsinghua University KIMS collaboration
2004 Jan. 29-31 TEXONO-KIMS Joint Workshop
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Muon background at underground

• In WIMP search experiment, muon is one of the
  main background.
• Energetic muon can easily penetrate rocks to deep
  underground.
• When muon penetrate through the shielding
  materials, the interaction of them can induce
  neutron inside shielding. This is very harmful.
  Because the events induced by neutron in CsI
  crystal is undistinguishable with that induced by
  WIMP.
• In order to monitor the muon background at
  underground lab, we constructed veto detector for
  muon.

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Structure of Muon Detector

• 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.

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MUD
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
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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
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Detection efficiency of muon detector
Trigger Muon using two other scintillator
detectors in the Ground lab Use one(MUD2) of
muon modules
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Muon spectra Flux

• YangYang ( 700m underground)
• 380 /day.m2 4.4 x 10-7 /s.cm2
• CheongPyoung ( 350m underground)
• 1450 /day.m2 1.7 x 10-6 /s.cm2

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Determination of position of muons

• Besides flux, another important thing is to
  determine the position where muon hit the
  detector and reconstruct the track of muons.
• Minimum square method
• Choose one point, calculate the energy response
  according to distance to PMT and attenuation
  length of liquid scintillator
• Compare the calculated result to the measured
  one, get a square value
• Change the assumed position and calculated again,
  until found the point which has minimum square

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Calibration of muon hit position

• 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
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Hit reconstruction on Muon Detector
Plastic scintillator position and the calculated
result
Hit position projected to x-axis
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Reconstructed Hit Position of Muon

• Calculated result of background data (without
  plastic scintillator as trigger) .

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Tracking and veto

• 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
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Monte Carlo simulation

• Generate muon from a rectangular area above the
  detector, the size of this area is 2 times of
  that of the top detector. Muon is generated at
  random position inside of this area and in
  random direction (downward 2p angle).

Muon generated area
Detector area
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Optical photon collected by PMT
MD8
MD7
MD4
MD6
MD5
MD1
MD2
MD3
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Energy spectra of muon from simulation
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More about simulation

• Try more amount of muon events to get better
  energy spectra.
• Try graphic mode to show muon track in 3
  dimensional mode.

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Summary

• 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 tested and
  Track Reconstruction is in progress. The track
  reconstruction give the information to reject
  muon events from WIMP candidate data.
• We performed Monte Carlo simulation for muon
  detector. For next step, we will try
• More amount of muon events to get better energy
  spectra
• Graphical mode to show muon track in
  3-dimensional mode.