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, 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

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

Why CsI(Tl) Crystal ? Advantage Disadvantages 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

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

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

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

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

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

134Cs (artificial+133Cs(n,gamma)) 87Rb (natural) Background of CsI(Tl) 137Cs (artificial) 134Cs (artificial+133Cs(n,gamma)) 87Rb (natural) Single Crystal (~10 kg) background @ ~10keV 87Rb 0.63 cpd/1ppb HR ICP-MASS 137Cs 0.35 cpd/1mBq/kg HPGe 134Cs 0.07 cpd/1mBq/kg “ Pollucite(raw material for Cs) contains < 8 mBq/kg of 137Cs

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)

CsI(Tl) from IHEP(China) 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)

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

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 ± 2.6 19.8 ±2.5 mBq/kg 134Cs 27.4 ± 4.6 34.0 ± 4.4 87Rb 20.0 ppb 23.2 (?) BG(~10 keV) 20.0 cpd 21 cpd

Water Samples “Purified” “Normal” Water is main source ! “Ultra-pure” 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”

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

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%

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

Cosmic rays : ~ 10-4 relative to the sea level 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)

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)

Shielding Structure Cosmic Muon Veto

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

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

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