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CsI crystal for WIMP Search Sun Kee Kim Seoul National University For the KIMS Collaboration Korea Invisible Mass Search Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*, T.Y.Kim, H.S.Lee, H.Park I.H.Park, E.Won, H.S.Won, 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, J.H.Lee, Y.J.Kwon, Iwha Womans Univ. : I.S.Hahn, E.K.Lee SeongKyunKwan Univ. : I.S.Cho, D.H.Choi, S.H.Noh, 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, Tsing Hua University : J.J.Zhu Brief History of KIMS • 1997 Summer : First discussion on WIMP search in Korea • 1997 Fall : Started R&D on CsI(Tl) for WIMP search • 1998 Summer : Demonstrated that CsI(Tl) can be used as a WIMP detector - presented at ICHEP98 • 1999 Spring : Started background measurement at Cheongphyung • 2000 Summer : Project funded, KIMS collaboration formed • 2001 August : TDR, Project Review Committee Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) CsI(Tl) Crystal CsI(Tl) NaI(Tl) Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1050 ~230 Peak emission(nm) 550 415 Hygroscopicity slight strong Advantages High light yield ~50,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) Disadvantages Emission spectra does not match with normal bialkali PMT => effectively reduce light yield 137Cs(t ~30y) ,134Cs(t ~2y) 1/2 1/2 may be problematic Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Photoelectron yield RbCs(green extended) PMT(D726UK) 70% larger p.e. than the normal bialkali PMT Full size crystal (7x7x30) : ~ 4 p.e./keV Small crystal(3x3x3) : 5~7 p.e./keV Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Neutron Beam Test KIGAM(Korea Institute of Geology and Material) 3.2 MeV p : 3H(p,n)3He 2.62 MeV n Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Quenching factor QF = Emeasured / Erecoil n n Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Pulse Shape Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Mean Time t At A i i i Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Quality factor (1 ) K ( ) 2 cut S Ideal detector ~ 1, ~ 0 K << 1 S K B S B B MT Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) CsI(Na) 1st measurement of CsI(Na) Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Underground Lab. at Cheongpyung Storage Water Power Plant ~1000 mwe ~ 2 hours from SNU Homyung Mt.(虎鳴山) Reservoir 350m Access tunnel(1.4km) Pukhan River(北漢江) Laboratory Power plant Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) External background Cosmic rays : ~ 10-4 relative to the sea level Expected at ~400 m underground The rock composition : 238U ~ 4.8 ppm, 232Th ~ 6 ppm 40K ~ 4 ppm (ICP-Mass) Gamma background With a shielding of 15cm Pb + 10cm Cu(OFHC) Can be controlled < 0.005 cpd (counts/keV/kg/day) Neutron background Photonuclear interaction of cosmic ray muon Fission of 238U, (alpha,n) reaction With 30cm polyethylene + 20 cm Liquid scintillator Can be controlled <0.001 cpd Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Gamma Background 100% HPGe installed in CPL W/o shielding 10cm Pb + 10 cm Cu (16 days data taking) 222Rn_up(226Ra)/222Rn_down(214PB,214Bi)=0.22+-0.06 Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Neutron Background Measured with 0.5 liter BC501A liquid scintillator ~ 4x10-5 /cm2/sec Input to the GEANT4 simulation Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Internal background Radioisotopes in the crystal - most important background after the proper shielding 137Cs : t 1/2= 30.07 year decay to 137Ba* (Q = 1175.6 keV) 2 min life time, emitting 661.6 keV gamma Hard to reject => serious background at low energy 134Cs : t 1/2 = 2.065 year decsy to 134Ba* (Q=2058.7 keV) immediate gamma emission Can be rejected easily => not a serious problem 87Rb : t 1/2 half life = 4.75 x 1010 year (27.8% natural abundance) Beta deacy to 87Sr (Q=282.3 keV) no gamma emission Hard to reject => potentially a serious problem => reduction technique in material is known Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Internal background In a crystal currently available 60 cpd grays and rays of 87Rb, 134Cs, and 137Cs + GEANT 0.63 cpd/1ppb 87Rb 0.3 cpd/1mBq 137Cs 0.005 cpd/1mBq 134Cs 137Cs Another crystal : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb 137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg, 87Rb :~203ppb Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) 238U, 232Th in the crystal 10~20 alphas/kg/day ~3 ppt (U+Th) Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Measurement of Internal Background Crystal 137Cs, 134Cs : direct measurement (fit to the MC prediction) Rb : direct measurement + chemical analysis U, Th : direct measurement of alphas ~ 15 ppt 134Cs(mBq/kg) Powder 137Cs, 134Cs : 100% HPGe detector Rb : chemical analysis (ICP Mass) 70 60 50 40 30 CsOH, CsNO3, Pollucite, Water 137Cs, 134Cs : 100% HPGe detector 20 10 0 0 Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 20 40 60 80 100 120 140 160 180 200 137Cs(mBq/kg) Sun Kee Kim(Seoul National Univ.) Background measurement of powders 16 Crystals 15 CsOH CsNO3 14 13 12 CsMnO4 ~ 3mBq/kg 11 10 9 8 7 6 CsI Powders 5 4 3 137Cs ~14mBq/kg Rb ~ 21 ppb 2 1 0 50 100 Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 150 Sun Kee Kim(Seoul National Univ.) 137Cs in Pollucite ? < 5.8 mBq/kg In CsI powder : 10~100 mBq/kg In Pollucite : < 5.8 mBq/kg Processing water : 10.1 4.8 mBq/liter Extraction of CsMnO4 from Pollucite with pure water 2.05 3.16 mBq/kg Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Surface effect ? NaI(Tl) (35-40 keV) : 330 kg days CsI (30-50 keV) : 14.3 kg days 62 kg days exposure V.A. Kudryavtsev et al., hep-ex/0109013 Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Shielding Design Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Detector Design CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg Quartz PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 20cm BC501A tagging efficiency > 75% Sun Kee Kim(Seoul National Univ.) Sensitivity After 1 year data taking with 100 kg CsI(Tl) 2 keV threshold Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Summary & Prospect Underground Lab. at CheongPyung is established Environmental background : low enough Comfortable place for long term experiment Extensive R&D on CsI(Tl) crystal has been carried out Low energy threshold ~ 2keV Pulse shape discrimination of g-rays : promising Intrinsic background : almost understood Shielding design/Simulation/Electronics/DAQ Design almost at final stage Prospect ~100 kg CsI(Tl) crystal within this year 1 year data taking will cover DAMA region Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)