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

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.

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.

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

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 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

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

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

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

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

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

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

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

Energy spectra of muon from simulation

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.

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.