Muon and Neutron Backgrounds at Yangyang underground lab Muju Workshop Kwak, Jungwon Seoul National University 1.External Backgrounds 2.Muon flux in Yangyang underground lab. 3.Neutron background study 4.Conclusion

MuJu workshop2 External backgrounds 1.External gamma  Isotopes in surrounding materials (Rock) Decay chain of U 238 and Th 232 Isotopes (K 40, … ) Rn 222 in air  Shielding structure made of pure and high Z materials  Improve PSD power of CsI crystal detector  N 2 flowing to remove air contaminated by Rn Neutron  Undistinguishable with WIMP (Nuclear recoil) (n,  ) reaction Nuclear fission Induced by cosmic muon (~ 200GeV) In shield - possible to veto In Rock - impossible to veto  Neutron shield of High Hydrogen density material  Veto using Muon detector  Underground experimental lab

MuJu workshop3 KIMS Main shield in Y2L CSI Pb shield (15cm) PE shield (5cm) Copper shield (10cm ) Mineral Oil shield (30cm) NMD MUD Mineral oil 30cm Pb 15cm : 30t OFHC Cu 10cm : 3t PE 5cm HPGe detector measurement

MuJu workshop4 Muon flux in Yangyang underground lab. (1) MUD MUD 2MUD 3MUD 1 MUD 7 MUD 4 MUD 6 MUD 5 MUD 8  2 x 2 ” PMT for each channel muon modules  28 signal channels ( x x 2 )  Liquid Scintillator 5 % PC 1 liter + PPO 4 g + POPOP 15 mg  Mineral Oil 95 % - Neutron shield  times of ground Muon rate at 700 m deep underground

MuJu workshop5 Muon flux in Yangyang underground lab. (2)  Attenuation length  Fast components ~ 50 cm ~ Distance between PMTs  Slow components ~ 250 cm ~ Size of Muon detector  Muon Efficiency = 98.1 %  Cosmic Muon rate at Ground level ~ cm -2 s -1  Coincidence of two trigger detectors

MuJu workshop6 Muon flux in Yangyang underground lab. (3) Muon flux [cm -2 m -1 ] Time [days] Net DAQ time = days Total muon events Muon flux = 4.30 x cm -2 s -1 Energy [MeV]

MuJu workshop7 Muon flux in Yangyang underground lab. (4) mud5 mud4mud4 mud7 mud6mud6 Lab is here

MuJu workshop8 Neutron background study (1-1)  Use 1 liter BC501A Liquid Scintillator  Teflon container ( 10  x 18 cm )  Quartz window and low background 3 “ PMT  Backgrounds of Neutron detector  Very small neutron flux measurement  Gamma  External radioactive source and internal radioactive impurity Development new PSD method  Alpha  238 U and 232 Th impurities in Liquid Scintillator  Coincidence study of decay chains

MuJu workshop9 P  i and P n i Normalized distribution of accumulated pulse shape P n i - P  i Bin by bin difference of probability function Neutron background study (1-2) Bin number i ( 1 bin = 2 ns ) FOM OldNew

MuJu workshop10 Neutron background study (2)  Use the data of 67.4 days DAQ period  Energy Threshold 300 keV  Stable PSD power

MuJu workshop11 Energy [MeV]  Inside of shield for days  Suspect the most of the events in neutron region are  backgrounds  Check the coincidence of 238 U and 232 Th decay chains Energy [MeV] Neutron background study (3)

MuJu workshop U decay chain Beta Alpha 22.4 y d d 210 Pb 210 Bi 210 Po 210 Pb Alpha Beta Alpha 1600 y d 3.10 m 26.8 m 19.9 m us 226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 226 Ra 4.770Alpha7.538 E+4 y 230 Th 4.859Alpha2.455 E+5 y 234 U (0.08) Alpha Beta Beta (IT) Beta E+9 y d 1.18 m 6.70 h 238 U 234 Th 234 Pa * 234 Pa 238 U Q-value (MeV) DecayLifetimeIsotopeFamily in 238 U

MuJu workshop Th decay chain Beta (64.06%) Alpha (35.94%) Alpha Beta m 299 ns m 212 Bi 212 Po 208 Tl 212 Bi Alpha Beta y 3.66 d 55.6 s s h 228 Th 224 Ra 220 Rn 216 Po 212 Pb 228 Th Alpha Beta E+10 y 5.75 y 6.15 y 232 Th 228 Ra 228 Ac 232 Th Q-value (MeV) DecayLifetimeIsotopeFamily in 232 Th

MuJu workshop14 Neutron background study (4) – 238 U and 232 Th chain 214 Bi -decay  214 Po -decay 222 Rn -decay  218 Po -decay Coincidence time [m] Coincidence time [ms] 220 Rn -decay  216 Po -decay Coincidence time [m]

MuJu workshop15 Neutron background study (5) - coincidence 553 events of 222 Rn -decay  218 Po -decay events ( 1341 events of background ) 26 events of 220 Rn -decay  216 Po -decay events -coincidence 471 events of 214 Bi -decay  214 Po -decay Efficiency correction 534=471/0.976/ events 230 Ra dominant contamination in 238 U chain cnts/liter/day – 1.5 x10 -6 ppt of 230 Ra level 232 Th dominant contamination in 232 Th chain cnts/liter/day ppt of 232 Th level 2722 (all events) – 2851 (expected alpha) = neutron cnts/liter/day less than 1.8 cnts /day/liter (90 %CL) of neutron background inside of shield.

MuJu workshop16 Neutron background study (6) Real Alpha Energy [MeV] Gamma equivalent Alpha Energy [MeV] Equilibrium from 238UEquilibrium from 226Ra  Quenching factor using tagged Alpha events  Decay chain in equilibrium from 226 Ra

MuJu workshop17  Neutron rate outside of shield 8 x 10 –7 /cm 2 /s ( 1.5 < E neutron < 6 MeV )  Subtract energy spectrum inside of shield to reject internal background from real neutron Energy [MeV] Neutron background study (7)

MuJu workshop18 Conclusion 1. Muon Flux measurement  4.3 x m -2 s -1  Stable flux - no modulation 2. Neutron Flux measurement  Inside shield < 1.8 cnts/liter/day (90% CL)  Outside shield = 8 x cm -2 s -1 (1.5 ~ 6 MeV )  Alpha ( 226 Ra decay chain) dominant internal background Kims(Korea)