Status of Neutron flux Analysis in KIMS experiment Jungwon Kwak Seoul National University

Korea Invisible Mass Search experiment International collaboration - 5 nations, 8 institutes Dark matter , missing mass in Universe Universe = missing ( ~ 95%) + Visible Mass ( ~ 5 %) Dark Matter means matter whose existence has been inferred only through its gravitation effects … Particle Data Group WIMP (Weakly Interacting Massive Particle) Excellent CDM candidate Super partner of neutral gauge particle and higgs Underground Laboratory To avoid neutrons induced by cosmic ray - Neutron signal is identical to the WIMP signal in CsI(Tl) crystal YangYang underground Laboratory In YangYang Water pumping power plant 700 m underground : 4.4 x 10-7 /cm2/s cosmic rate Main-shield : 30cm Mineral Oil, 15cm Lead, 5cm PE, 10cm Cu - Gamma (10-4 ) and Neutron background(10-3) rate of outside CsI(Tl) crystal detector, 4p full coverage Muon detector, Neutron monitoring detector and Rn monitoring detector 2018-12-26 2004 10 22 KPS meeting

Backgrounds of Neutron Detector Neutron monitoring detector 1.2 liter BC501A liquid scintillator Quartz window and Teflon (CF2) container (10f x 16 cm3 ) D7265 3 “ PMT – ultra low background g Background External radioactive source and internal radioactive impurity PSD make enable to reject out gamma background - Develop new PSD method using probability function for g and neutron a Background - Mimic neutron signal in neutron detector Source : 238U and 232Th impurities in Liquid Scintillater Tagging a – a coincidence events - Lifetime Limit < 4.3 m Tagging b – a coincidence events Lifetime Limit < 0.1 s 2018-12-26 2004 10 22 KPS meeting

Probability function for g and neutron pulses Pg i and Pn i Normalized distribution of accumulated pulse shape Pn i - Pg i Bin by bin difference of probability function Bin number i ( 1 bin = 2 ns ) Bin number i ( 1 bin = 2 ns ) During DATA run period, never changed DAQ and Detector system to keep the same probability functions of pulse shapes. Blue and Pink line indicate the start points of partial pulses for simple DCC method. (Blue 20ns , Pink 60ns delayed points from peak point of pulse) 2018-12-26 2004 10 22 KPS meeting

Simple DCC and Weighted DCC method Simple DCC patial (60ns) / Full ratio Weighted DCC A+/A- ratio Energy [MeV] Energy [MeV] if (Pn i - Pg i > 0 ) A+ = Sum( (Pn i - Pg i ) x Pulse i ) if (Pn i - Pg i < 0 ) A- = - Sum( (Pn i - Pg i ) x Pulse i ) Better Pulse Shape Discrimination Applied the more Weighting factor to bins in which the more difference between n and g probability functions 2018-12-26 2004 10 22 KPS meeting

Comparison between two DCC methods FOM Energy [MeV] FOM = ( Peak n – Peak g ) /sqrt( sn2 + sg 2 ) Worse PSD of 20 ns DCC at low energy more contamination of short component term Worse PSD of 60 ns DCC at high energy smaller statistics of long component term Weighted DCC shows better PSD than simple DCC method 2018-12-26 2004 10 22 KPS meeting

Data analysis cuts A+/A- Use the data of 67.4 days DAQ period Energy [MeV] Data taking time [Day] Use the data of 67.4 days DAQ period Energy Threshold 300 keV Ratio A+/A- > 0.085 for Neutron region Require 390 ns < peak of pulse < 430 ns 2018-12-26 2004 10 22 KPS meeting

Energy spectrum Inside of shield for 67.41 days Rate: 0.1 counts/liter/s Energy [MeV] Energy [MeV] Inside of shield for 67.41 days Suspect the most of the events in neutron region are a backgrounds Check the coincidence of 238U and 232Th decay chains Lifetime limit of a – a coincidence < 4.3 m - 1 a event per every 42.8 m ( 33.65 cnts/liter/day ) Lifetime limit of b – a coincidence < 1 s - 1 g event per every 10 s ( 0.100 cnts/liter/s) 2018-12-26 2004 10 22 KPS meeting

238U decay chain Family in 238 U Isotope Lifetime Decay Q-value (MeV) 234 Th 234 Pa* 234 Pa 4.468 E+9 y 24.10 d 1.18 m 6.70 h Alpha Beta Beta (IT) 4.270 0.274 2.117 (0.08) 2.197 234 U 2.455 E+5 y 4.859 230 Th 7.538 E+4 y 4.770 226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 1600 y 3.824 d 3.10 m 26.8 m 19.9 m 164.3 ms 4.871 5.590 6.115 1.024 3.272 7.833 210Pb 210 Pb 210 Bi 210 Po 22.4 y 5.013 d 138.376 d 0.064 1.163 5.407 2018-12-26 2004 10 22 KPS meeting

232Th decay chain Family in 232 Th Isotope Lifetime Decay Q-value (MeV) 232 Th 228 Ra 228 Ac 1.405 E+10 y 5.75 y 6.15 y Alpha Beta 4.083 0.046 2.127 228 Th 224 Ra 220 Rn 216 Po 212 Pb 1.9116 y 3.66 d 55.6 s 0.145 s 10.64 h 5.520 5.789 6.405 6.906 0.574 212 Bi 212 Po 208 Tl 60.55 m 299 ns 3.057 m Beta (64.06%) Alpha (35.94%) 2.254 6.207 8.954 5.001 2018-12-26 2004 10 22 KPS meeting

b – a coincidence candidates 238U chain 214Bi : 3.327 MeV b-decay g 214Po : 7.833 MeV a-decay Lifetime of 214Po = 0.1643 ms … 4 ms dead time 97.6% efficiency 100 keV Energy threshold … 90.3 % efficiency of b-spectrum 232Th Chain 212Bi : 2.254 MeV b-decay g 212Po : 8.954 MeV a-decay Lifetime of 212Po = 299 ns … 4 ms dead time 2.3 E-4 % efficiency Require two pluses within 2 ms window … 99.9 % efficiency About 50 keV Hardware threshold … more than 90% efficiency 2018-12-26 2004 10 22 KPS meeting

214Bi b-decay g 214Po a-decay Coincidence time [ms] 2018-12-26 2004 10 22 KPS meeting

a – a coincidence candidates 238U chain 222Rn : 5.590 MeV a-decay g 218Po : 6.115 MeV a-decay Lifetime of 218Po = 3.10 m 232Th Chain 224Ra : 5.789 MeV a-decay g 220Rn : 6.405 MeV a-decay Lifetime of 220Rn = 55.6 s 220Rn : 6.405 MeV a-decay g 216Po : 6.906 MeV a-decay Lifetime of 216Po = 0.145 s 2018-12-26 2004 10 22 KPS meeting

222Rn a-decay g 218Po a-decay Coincidence time [m] 2018-12-26 2004 10 22 KPS meeting

220Rn a-decay g 216Po a-decay Coincidence time [m] 2018-12-26 2004 10 22 KPS meeting

Summary of Internal background a-a coincidence 553 events of 222Rn a-decay g 218Po a-decay . 553 +- 44 events ( 1341 events of background ) 26 events of 220Rn a-decay g 216Po a-decay . 26 +- 5 events b-a coincidence 471 events of 214Bi b-decay g 214Po a-decay Efficiency correction 534=471/0.976/0.903 . 534 +- 25 events 230Ra dominant contamination in 238U chain . 6.66 +- 0.27 cnts/liter/day – 1.5 x10-6 ppt of 230Ra level 232Th dominant contamination in 232Th chain . 0.32 +- 0.06 cnts/liter/day - 0.63 ppt of 232Th level 2722(all events) – 2851(expected alpha) = -121 +- 114 . -1.5 +- 1.4 neutron cnts/liter/day less than 1.8 cnts/liter/day (90 % confidence level ) of neutron background inside of shield. 2018-12-26 2004 10 22 KPS meeting

Neutron flux outside of shield Energy [MeV] Neutron rate outside of shield 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV ) Subtract energy spectrum inside of shield to reject internal background from real neutron 2018-12-26 2004 10 22 KPS meeting

Neutron flux induced by muon Log10(Dt) Energy [MeV] Dt = Min(Muon trigger time – Neutron trigger time) High energy events ( > 3 MeV) of Neutron detector is mostly from cosmic muon Time offset of muon and neutron detector = 133 ns - 20m delay cable for muon detector and more electronics Time resolution = 25 ns - Use clock pulse of 16ns width 2018-12-26 2004 10 22 KPS meeting

Coincidence neutron signal Gamma region Neutron region Energy [MeV] Energy [MeV] Two events is strong candidates of neutron induced by muon - 2 events for 67.41 days = 11 +- 8 cnts/liter/year Also Tagged low energy gamma induced by cosmic muon Gamma rate = 0.9 cnts/liter/day 2018-12-26 2004 10 22 KPS meeting

Summary Estimated internal background of Neutron detector - 33.3 cnts/liter/day of alpha’s from 238U chain - 1.92 cnts/liter/day of alpha’s from 232Th chain Upper limit of neutron rate inside of shield - less than 1.8 cnts/liter/day @90% CL Neutron flux outside of shield - 8x10-7/cm2/s - 15 cnts/liter/day Flux of neutron induced by cosmic muon - 11 +- 8 cnts/liter/year 2018-12-26 2004 10 22 KPS meeting