The Study of Muon-induced Neutrons in The KIMS Experiment

1. The Study of Muon-induced Neutrons in The KIMS Experiment 2005. 10. 22. The Korean Physical Society J.H. Choi

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

3. Muon Spectrum in Laboratory Geant4 Simulation dN / dE Energy [MeV] • 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-2s-1 at 2000 mwe in KIMS lab. KIMS P.Lipari et al., Phys.Rev.D44(1991)3543

4. Detector • 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.

5. Time Coincidence • 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 Δt [sec]

6. Experiment Result 0.025 cnt/day/liter Events of neutron energy region Coincident events of neutron energy region Energy [MeV] Muon-induced Neutron Gamma Neutron Energy [Mev] • 2 candidates of muon-induced neutrons are found • The size of data is too small PID [MeV/MeV]

7. Geant4 Simulation Length [cm] Theta Angle [degree] Energy [MeV] Phi Angle [degree] • Total 282,000 muons are fired with angle distributions. -170.8 days(MC) / 67.4 days(Exp)

8. Generated Neutrons in Detector Total Length [cm] Lead Energy [MeV] Length [cm] • Most of neutrons are generated in the lead. - 2.45 x 10-10 cnt/cm-3s-1

9. 1.46x10-4 /muon/(g/cm2) By Geant4 IGEX : 8.8x10-5 /u/(g/cm2) By Fluka Simulation 4.6x10-4 /u/(g/cm2) By Geant4 IGEX : 1.7x10-3 /u/(g/cm2) By Fluka Simulation Generated Neutrons – 1cm and 35cm Total Secondary Primary 16 % 84 % Energy [MeV] Energy [MeV] Energy [MeV] 78 % 22 % Energy [MeV] Energy [MeV] Energy [MeV] • 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.

10. Deposited Coincedence Energy - MC • The coincidence events rate with muon detector is 38.94 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 170.8 days MC simulation data • We need more statistics • We need study of MC model dependence 0 ~ 150MeV ToTal : 38.94 evt/day Muon : 4.32 evt/day E.M. : 34.62 evt/day

11. Coincedence Energy –Exp. • The coincidence events rate with muon detector is 14.64 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 2.5 MeV ~ Exp. : 5.21 evt/day M.C. : 8.30 evt/day

12. Coincidence Low Energy • The coincidence events rate with muon detector is 16.31 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. 150KeV~2.5MeV MC 16.31 evt/day Exp. 9.36 evt/day

13. Summary & Plan • The flux of cosmic muon at 2000 mwe in KIMS lab is 2.7x10-7 cnt/cm-2s-1 • The rate of generated neutrons by Geant4 MC simulation is 2.45 x 10-10 cnt/cm-3s-1in 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.) : 16.31 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