1. Muon flux at Y2L and reconstruction of muon tracks
JingJun Zhu
Tsinghua University & KIMS collaboration
2004 Jan TEXONO-KIMS Joint Workshop

2. 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.

3. 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.

4. MUD CSI Liquid Scintillator 5 % Mineral Oil 95 %
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

5. 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

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

7. Muon spectra & Flux YangYang ( ~ 700m underground) :
~ 380 /day.m2 = 4.4 x /s.cm2
CheongPyoung ( ~ 350m underground) :
~1450 /day.m2 = 1.7 x 10-6 /s.cm2

8. 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

9. 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

10. Hit reconstruction on Muon Detector
Plastic scintillator position
and the calculated result
Hit position projected to x-axis

11. Reconstructed Hit Position of Muon
Calculated result of background data (without plastic scintillator as trigger) .

12. 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

13. 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 2π angle).
Muon generated area
Detector area

14. Optical photon collected by PMT
MD8
MD7
MD4
MD6
MD5
MD3
MD2
MD1

15. Energy spectra of muon from simulation

16. 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.

17. Summary
We measured muon flux at 700m underground laboratory, it is about 380 /day.m2 ( equal to 4.4 x /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.