The Study of Muon- induced Neutrons in The KIMS Experiment The Korean Physical Society J.H. Choi

Underground Laboratory Yangyang KIMS Lab. Depth : 700m 2000m(mwe)

P.Lipari et al., Phys.Rev.D44(1991)3543 The high energy muon can pass through an underground in depth. Neutrons can be generated by the muon when the muon through out material – mimic WIMP The everage energy of muons is 216 GeV at 2500 mwe in IGEX lab. The flux of cosmic muon is 2.7x10 -3 cnt/m -2 s -1 at 2000 mwe in KIMS lab. Muon Spectrum in Laboratory Geant4 Simulation dN / dE Energy [MeV] KIMS

CsI(Tl) crystal is shielded by 30cm mineral oil, 15cm lead, 5cm PE, 10cm Cu All of other detectors are surrounded by the muon detectors Neutron detector is located inside PE and outside Cu CsI crystal detector is in the copper box. Detector

We expected that we can find the coincidence of neutron and muon detector We computed the time difference Δt between muon and neutron detector’s signal Δt = Min(Muon trigger time – Neutron trigger time) The pick shows the information of coincidence Time Coincidence Δt [sec]

candidates of muon-induced neutrons are found The size of data is too small Muon-induced Neutron Energy [MeV] cnt/day/liter Events of neutron energy region Coincident events of neutron energy region Experiment Result Energy [Mev] PID [MeV/MeV] GammaNeutron

Geant4 Simulation Total 282,000 muons are fired with angle distributions days(MC) / 67.4 days(Exp) Energy [MeV] Length [cm] Theta Angle [degree] Phi Angle [degree]

Generated Neutrons in Detector Most of neutrons are generated in the lead x cnt/cm -3 s -1 Total Lead Length [cm] Energy [MeV]

Generated Neutrons – 1cm and 35cm TotalSecondar y Primary 84 % 22 % 16 % 78 % The rate of neutron production of Fluka is at most 2 times more than Geant4 over 100GeV. The rate of neutron production of Geant4 depend on physics model at least 2 times. Energy [MeV] 1.46x10 -4 /muon/(g/cm 2 ) By Geant4 IGEX : 8.8x10-5 /u/(g/cm 2 ) By Fluka Simulation 4.6x10 -4 /u/(g/cm 2 ) By Geant4 IGEX : 1.7x10-3 /u/(g/cm 2 ) By Fluka Simulation

Deposited Coincedence Energy - MC The coincidence events rate with muon detector is evt/day under 150 MeV The coincidence events rate with muon detector is 8.30 evt/day over 2.5 MeV In the high energy, most of evnets are from muon In the low energy, most events are from E.M. Neutron is not detected in the days MC simulation data –We need more statistics –We need study of MC model dependence ToTal : evt/day Muon : 4.32 evt/day E.M. : evt/day 0 ~ 150MeV

The coincidence events rate with muon detector is evt/day from 150KeV to 10 MeV The coincidence events rate with muon detector is 5.21 evt/day over 2.5 MeV In the 67.4 days data, two candidate events of muon- induced neutrons are detected Exp. : 5.21 evt/day M.C. : 8.30 evt/day 2.5 MeV ~ Coincedence Energy – Exp.

Coincidence Low Energy 150KeV~2.5MeV MC evt/day Exp evt/day The coincidence events rate with muon detector is evt/day from 150KeV to 2.5 MeV in M.C. The coincidence events rate with muon detector is 9.36 evt/day from 150KeV to 2.5 MeV in Exp.

Summary & Plan The flux of cosmic muon at 2000 mwe in KIMS lab is 2.7x10 -7 cnt/cm -2 s -1 The rate of generated neutrons by Geant4 MC simulation is 2.45 x cnt/cm -3 s -1 in lead of KIMS detector. The rate of generated neutrons in 35cm Pb 4.6x10 -4 /muon/(g/cm2) (KIMS-Geant4) : 1.7x10 -3 /muon/(g/cm2) (IGEX-Fluka) Total Coincidence events (150KeV~2.5MeV) 9.36 evt/day(Exp.) : evt/day(M.C.) 2 candidates of muon-induced neutrons are found in KIMS Experiment We need more statistics for M.C. These result will be also tested with adequate physical model of Geant4 We are studying the coincidence between the muon detector and the CsI detector