Muon and Neutron detector of KIMS experiment 2003-09-23 Jungwon Kwak and JingJun Zhu DMRC 2003-09-23

Backgrounds in KIMS experiment External background Sources Cosmic rays – muon Reduce in Deep underground ( 10-5 level of ground in 700 m underground ) Radioactive source in Environmental material – Gamma, Neutron Lead, Mineral Oil, Copper shield ( mainly related with U238, Th232, K40) Rn in air – Alpha N2 flowing Internal background sources CSI crystal – Cs137,134 Rb R&D for ultra low background CSI crystal ( currently 10 cpd level) PMT background – glass window, noise signal Quartz window PMT with low background Main shielding - Stainless steel, lead, copper Pure copper, low background lead MUD CSI NMD 2003-09-23

Diagram of KIMS detector Active Detector CSI Cs(Tl) Crystal Detector MUD NMD Neutron Monitor Detector PE shield (5cm) Pb shield (15cm) MUD Muon Detector - LS 5% Copper shield (10cm) CSI Passive shield NMD Copper : 10 cm PE : 5 cm Lead : 15 cm Mineral Oil : 30 cm Mineral Oil shield (30cm) 2003-09-23

NMD Neutron background To understand neutron background well is important in dark matter search experiment because neutron signals mimic the WIMP signals. Neutron background source: cosmic ray induced neutron spontaneous fission of U238 (α,n) reaction of U & Th in the rock Neutron flux at underground laboratory is very small, so we need a large detector and a long term measurement. MUD CSI 2003-09-23

NMD Neutron Monitor Detector Liquid Scintillator BC501A 20 liter ( 16 cm diameter x 100 cm ) 10 liter ( 16 cm diameter x 50 cm ) 2 x 5” PMT 10-3 times of Neutron rate inside main shield MUD CSI inside Pb 5 cm shield inside main shield 2003-09-23

NMD DCC method MUD CSI 20liter BC501A Neutron Detector DCC(Digital Charge Comparison) method: tail of signal / total signal for BC501A scintillator neutron and gamma signal separation using pulse shape MUD CSI 2003-09-23

NMD Neutron background inside main shield 2003-09-23

NMD time variation Fitting function = lifetime (3.8 days for Rn) + constant Constant flux : 27 counts / day /liter Data taken just after windows are sealed 2003-09-23

NMD FADC DAQ system 10liter BC501A Neutron Detector used 1G Hz data sampling using digital oscilloscope Data taken two month later window sealing Scatter plot of part/full for each PMT Energy spectrum of neutron candidates (MeV) Flux for Neutron candidate = 29 counts / day / liter 2003-09-23

MUD MUon Detector/Neutron Shield 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 2003-09-23

MUD Muon detection Efficiency CSI Trigger Muon using two other scintillator detectors in the Ground lab Use one(MUD2) of muon modules 2003-09-23

MUD Attenuation length CSI Fitting function : two exponential decay function Fitting results : fast term - 50 cm , slow term – need more length 2003-09-23

MUD Muons in Y2L and CPL MUD CSI Ground level : ~ 8.64 x 106 muons / day.m2 = Fluxg CPL(-350m) : ~ 1.45 x 103 muons /day.m2 = 1.6 x 10-4 Fluxg Y2L(-700m) : ~ 2.20 x 102 muons /day.m2 = 2.5 x 10-5 Fluxg 2003-09-23

MUD Hit position reconstruction CSI MUD8 module used Trigger Muon using Plastic scintillator and MUD2 Divide 64 x 64 cells and calculate c2 values for each cell choose one cell with minimum c2 Use only ADC information ( in progress for TDC ) 2003-09-23

Conclusion MUD CSI NMD (Neutron Monitoring Detector) Upper limit of Neutron flux ~ 30 counts/day/liter Few ppt level of U, Th can cause comparable Alpha rate Need Checking U, Th level of NMD MUD (Muon Detector) Muon efficiency ~ 98 % Muon flux in Y2L ~ 2.5 x 10-5 of ground level Muon track reconstruction in progress MUD CSI 2003-09-23

NMD Neutron background outside main sheild 2003-09-23