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Determination of muon hit position 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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Determination of muon hit position for the KIMS experiment

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  1. Determination of muon hit position for the KIMS experiment JingJun Zhu Tsinghua University & KIMS collaboration 2003 Oct. 24th KPS meeting

  2. KIMS experiment • 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.

  3. Diagram of the power plant

  4. External Background in KIMS • The major external background of KIMS experiment is gamma, neutron and Muon. • For gamma, we can use PSD (Pulse Shape Discrimination) method to reject. Heavy shielding material can protect gamma from environment well. • 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. • 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.

  5. 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.

  6. MUD 7 MUD 1 MUD 4 MUD 6 MUD 8 MUD 2 MUD 3 MUD 5 MUD CSI 2x2” PMT for each channel 8 muon modules , 28 signal channels Liquid Scintillator5 % PC 1 liter + PPO 4 g + POPOP 15 mg Mineral Oil95 % 10-5 times of ground Muon rate at Y2L

  7. 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

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

  9. Muon Energy 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

  10. 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

  11. Calibration of Muon Detector • 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

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

  13. Reconstructed Hit Position of Muon • Calculated result without plastic scintillator as trigger.

  14. 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

  15. 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 finished and Track Reconstruction is in progress. The track reconstruction give the information to reject muon events from WIMP candidate data.

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