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External Backgrounds Muon flux in Yangyang underground lab. Neutron background study

Muon and Neutron Backgrounds at Yangyang underground lab. 2005. 2. 18. Muju Workshop Kwak, Jungwon Seoul National University. External Backgrounds Muon flux in Yangyang underground lab. Neutron background study Conclusion. External backgrounds. External gamma

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External Backgrounds Muon flux in Yangyang underground lab. Neutron background study

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  1. Muon and Neutron Backgroundsat Yangyang underground lab.2005. 2. 18. Muju WorkshopKwak, Jungwon Seoul National University External Backgrounds Muon flux in Yangyang underground lab. Neutron background study Conclusion

  2. External backgrounds • External gamma • Isotopes in surrounding materials (Rock) • Decay chain of U238 and Th232 • Isotopes (K40, …) • Rn222 in air • Shielding structure made of pure and high Z materials • Improve PSD power of CsI crystal detector • N2 flowing to remove air contaminated by Rn222 • Neutron • Undistinguishable with WIMP (Nuclear recoil) • (n,a) 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 2005-02-18 MuJu workshop

  3. MUD PE shield (5cm) Pb shield (15cm) Copper shield (10cm) CSI NMD Mineral oil 30cm Pb 15cm : 30t Mineral Oil shield (30cm) OFHC Cu 10cm : 3t HPGe detector measurement PE 5cm KIMS Main shield in Y2L 2005-02-18 MuJu workshop

  4. MUD 7 MUD 1 MUD 4 MUD 6 MUD 8 MUD 2 MUD 3 MUD 5 Muon flux in Yangyang underground lab. (1) MUD • 2 x 2” PMT for each channel muon modules • 28 signal channels ( 6 + 4 x 4 + 3 x 2 ) • Liquid Scintillator5 % • PC 1 liter + PPO 4 g + POPOP 15 mg • Mineral Oil95 % - Neutron shield • 10-5 times of ground Muon rate at 700 m deep underground 2005-02-18 MuJu workshop

  5. 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 ~ 10-2 cm-2s-1 • Coincidence of two trigger detectors 2005-02-18 MuJu workshop

  6. Muon flux in Yangyang underground lab. (3) Muon flux [cm-2 m-1] Time [days] Energy [MeV] Net DAQ time = 154.5 days Total 403227 muon events Muon flux = 4.30 x 10-7 cm-2 s-1 Muon flux [cm-2 m-1] 2005-02-18 MuJu workshop

  7. Muon flux in Yangyang underground lab. (4) mud5 mud6 mud4 mud7 Lab is here 2005-02-18 MuJu workshop

  8. Neutron background study (1-1) • Use 1 liter BC501A Liquid Scintillator • Teflon container ( 10 f 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 • 238U and 232Th impurities in Liquid Scintillator • Coincidence study of decay chains 2005-02-18 MuJu workshop

  9. Pi and Pn i Normalized distribution of accumulated pulse shape Pni - P i Bin by bin difference of probability function Neutron background study (1-2) Bin number i ( 1 bin = 2 ns ) Bin number i ( 1 bin = 2 ns ) FOM Old New 2005-02-18 MuJu workshop

  10. Use the data of 67.4 days DAQ period • Energy Threshold 300 keV • Stable PSD power Neutron background study (2) 2005-02-18 MuJu workshop

  11. Inside of shield for 67.41 days • Suspect the most of the events in neutron region are  backgrounds • Check the coincidence of 238U and 232Th decay chains Neutron background study (3) Energy [MeV] Energy [MeV] 2005-02-18 MuJu workshop

  12. 238U decay chain Family in 238 U Isotope Lifetime Decay Q-value (MeV) 238 U 238 U 234 Th 234 Pa* 234 Pa 4.468 E+9 y 24.10 d 1.18 m 6.70 h Alpha Beta Beta (IT) Beta 4.270 0.274 2.117 (0.08) 2.197 234 U 234 U 2.455 E+5y Alpha 4.859 230 Th 230 Th 7.538 E+4 y Alpha 4.770 226 Ra 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 us Alpha Alpha Alpha Beta Beta Alpha 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 Beta Beta Alpha 0.064 1.163 5.407 2005-02-18 MuJu workshop

  13. 232Th decay chain Family in 232 Th Isotope Lifetime Decay Q-value (MeV) 232 Th 232 Th 228 Ra 228 Ac 1.405 E+10 y 5.75 y 6.15 y Alpha Beta Beta 4.083 0.046 2.127 228 Th 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 Alpha Alpha Alpha Alpha Beta 5.520 5.789 6.405 6.906 0.574 212 Bi 212 Bi 212 Po 208 Tl 60.55 m 299 ns 3.057 m Beta (64.06%) Alpha (35.94%) Alpha Beta 2.254 6.207 8.954 5.001 2005-02-18 MuJu workshop

  14. 214Bi -decay 214Po -decay 222Rn -decay 218Po -decay 220Rn -decay 216Po -decay Coincidence time [m] Neutron background study (4) –238U and 232Th chain Coincidence time [ms] Coincidence time [m] 2005-02-18 MuJu workshop

  15. - coincidence • 553 events of 222Rn -decay 218Po -decay • - 553 +- 44 events ( 1341 events of background ) • 26 events of 220Rn -decay 216Po -decay • - 26 +- 5 events • -coincidence • 471 events of 214Bi -decay 214Po -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 /day/liter (90 %CL) of neutron background inside of shield. Neutron background study (5) 2005-02-18 MuJu workshop

  16. Neutron background study (6) • Quenching factor using tagged Alpha events • Decay chain in equilibrium from 226Ra Gamma equivalent Alpha Energy [MeV] Real Alpha Energy [MeV] Equilibrium from 238U Equilibrium from 226Ra 2005-02-18 MuJu workshop

  17. Neutron background study (7) 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 2005-02-18 MuJu workshop

  18. Kims(Korea) Conclusion • Muon Flux measurement • 4.3 x 10-3 m-2 s-1 • Stable flux - no modulation • Neutron Flux measurement • Inside shield < 1.8 cnts/liter/day (90% CL) • Outside shield = 8 x 10-7 cm-2 s-1 (1.5 ~ 6 MeV ) • Alpha (226Ra decay chain) dominant internal background 2005-02-18 MuJu workshop

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