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A CsI(Tl) Dark Matter Search Experiment - KIMS -

A CsI(Tl) Dark Matter Search Experiment - KIMS -. Korean Invisible Mass Search. Yeongduk Kim Sejong University, Seoul, Korea IDM 2002 meeting, 2002. Sep 5. Collaborators. Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*, T.Y.Kim ,

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A CsI(Tl) Dark Matter Search Experiment - KIMS -

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  1. A CsI(Tl) Dark Matter Search Experiment- KIMS - Korean Invisible Mass Search Yeongduk Kim Sejong University, Seoul, Korea IDM 2002 meeting, 2002. Sep5

  2. Collaborators Seoul National Univ.: J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*,T.Y.Kim, H.S. Lee, S.E. Lee, H..Park, H.Y.Yang, M.S.Yang Sejong Univ. : W.K.Kang, J.I. Lee, D.S.Lim, Y.D.Kim, Yonsei Univ. : J.Hwang, H.J.Kim, Y.J.Kwon Iwha Womans Univ. : I.S.Han, E.K.Lee, I.H. Park SeongKyunKwan Univ. : I.Yu Chonbuk National Univ. : S.Y.Choi KAIST : P.Ko Univ. of Maryland :M.H.Lee, E.S.Seo National Taiwan Univ., : H.B.Li, C.H.Tang, M.Z.Wang Academia Cinica : W.P.Lai, H.T. Wong Inst. Of High Energy Physics : J.Li, Y.Liu, Q.Yue Inst. Of Atomic Energy : B.Xin, Z.Y.Zhou Tsinghua University : J. Zhu * PI

  3. Outline • CsI(Tl) crystals • Underground site • Studies on background reduction • Perspectives • Summary

  4. Why CsI(Tl) Crystal ? Advantage High light yield ~50,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) SI + SD CsI(Tl) NaI(Tl) Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1000 ~230 Peak emission(nm) 550 415 Hygroscopicity slight strong Disadvantages Emission spectra does not match with normal bi-alkali PMT 137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic

  5. Low energy signal with CsI(Tl) 3” Green Extended RbCs PMT (Electron Tubes) Digital Oscilloscope with 10ns bin Large crystal (7x7x30cm) : ~ 4.5 p.e./keV Small crystal(3x3x3cm) : ~ 6 p.e./keV

  6. Response of CsI(Tl) with elastically scattered neutron • CsI(Na) has spurious events due to surface effect • 2 keV threshold  ~ 10 keV recoil energy

  7. Pulse shape discrimination at ~ keV energy • Nuclear recoil vs gamma events • Mean time for each events for each photoelectrons in an event 4<E<10 keV

  8. cut B S NaI(Tl) S B CsI(Tl) Comparison of PSD power Ideal detector  ~ 1,  ~ 0 K << 1

  9. Underground Site • Location : minimum 350 m underground Access tunnel(1.4km) 350m Laboratory Power plant

  10. Background of CsI(Tl) • 137Cs (artificial) • 134Cs (artificial+133Cs(n,gamma)) • 87Rb (natural) Single Crystal (~10 kg) background @ ~10keV 87Rb0.63 cpd/1ppb HR ICP-MASS 137Cs0.35 cpd/1mBq/kg HPGe 134Cs 0.07 cpd/1mBq/kg “ Pollucite(raw material for Cs) contains < 8 mBq/kg of 137Cs

  11. Crystals w/o selection of CsI powder (1) 137Cs Dominating crystal 8.9 kg day data Geant 4 Simulation 137Cs 155mBq/kg 134Cs ~35mBq/kg 87Rb 3.9 ppb (ICP-MASS)

  12. Crystals w/o selection of CsI powder (2) 87Rb Dominating crystal CsI(Tl) from IHEP(China) 137Cs 13.3mBq/kg 134Cs 54.2 mBq/kg 87Rb 203 ppb (ICP-MASS)

  13. Selection of CsI powder from various vendors Crystals CsOH CsNO3 CsMnO4 ~ 3mBq/kg CsI Powders Small samples 137Cs ~14mBq/kg Rb ~ 21 ppb Chemetall Selected 137Cs 87Rb

  14. Crystals with selection of CsI powder 1st Demonstration of Reducing Bacground of CsI(Tl) by selecting powder. Should reduce further. Powder  Crystal 137Cs 15.5 ± 2.619.8 ±2.5 mBq/kg 134Cs 27.4 ± 4.634.0 ± 4.4 87Rb 20.0 ppb 23.2 (?) BG(~10 keV) 20.0 cpd 21 cpd

  15. Water Samples A large amount of water used for extraction Of Cs (Chemetall) Water samples with HPGe –Precipitation with AMP (Ammonium Molybdophosphate) 137Cs(“Normal water”) >> 137Cs(“Purified”) ~ 20 times “Purified” “Normal” Water is main source ! “Ultra-pure”

  16. CsI powder with “Purified” water • CsI powder with only “purified” water in a production scale. CsI powder Crystal “Normal” water  15.5 ±2.6  7cpd (5.4 cpd expected) “Purified” water  5.3 ± 1.0  2.4 cpd(Expected) • Factor 3 reduction of 137Cs with “Purified” water

  17. Rb reduction by Recrystallization • CsI solubility in water is very high. • Recrystallization is done at slightly lower temperature from saturation point. • 20 ppb powder  ~ 1 ppb (< 1cpd) Crystal growing by Bridgmann reduced Rb by about 25%

  18. P P C C Summary of Internal Background Reduction W Crystals W/O Selection Purified Water Normal Water

  19. External background • Cosmic rays : ~ 10-4 relative to the sea level • The rock composition (ICP-MASS) • 238U ~ 4.8 ppm, 232Th ~ 6 ppm, 40K ~ 4 ppm • With a shielding of 15cm Pb(Boliden) + 10cm Cu(OFHC) •  Can be controlled < 0.005 cpd • based a MC simulation study (GEANT4)

  20. Neutron Background at underground BC501A liquid scintillator Neutron Flux ~ 4x10-5 /cm2/sec Mainly from (alpha,n) reaction GEANT4 simulation  Can be controlled <0.001 cpd 30cm LSC (Outside Shielding) + 20cm LSC(Inside Shielding)

  21. Shielding Structure Cosmic Muon Veto

  22. Neutron detector inside Copper shielding Po-Be neutron source 20cm BC501A  Neutron tagging efficiency > 75%

  23. Sensitivity (Spin-Independent) After 1 year data taking with 100 kg CsI(Tl) 2 keV threshold 3 count/(kev kg day) CDMS Limit DAMA

  24. Summary • Extensive R&D on CsI(Tl) crystal has been carried out • Pulse shape discrimination from -rays • Main source of 137Cs contamination due to impure water. • Rb reduction down to ~1ppb achieved. •  < 5cpd from internal background. • Shielding capable of 250 kg of CsI(Tl) under construction. • Environmental background : small enough • Large (n,gamma) separable LSC inside shielding is tested. • Perspectives • ~100 kg CsI(Tl) crystal within 1 year • 1 year data taking will cover DAMA region

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