CsI crystal for WIMP Search Sun Kee Kim Seoul National University For the KIMS Collaboration 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 Korea Invisible Mass Search

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) 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 ICHEP 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) Decay Time(ns) ~1050 ~230 Peak emission(nm) 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 137 Cs(  1/2 ~30y), 134 Cs(  1/2 ~2y) may be problematic

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Photoelectron yield Full size crystal (7x7x30) : ~ 4 p.e./keV Small crystal(3x3x3) : 5~7 p.e./keV RbCs(green extended) PMT(D726UK) 70% larger p.e. than the normal bialkali PMT

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 : 3 H(p,n) 3 He  2.62 MeV n

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Quenching factor QF = E measured / E recoil 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

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Quality factor S B SS BB cut Ideal detector  ~ 1,  ~ 0 K << 1

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) CsI(Na) 1 st measurement of CsI(Na)

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) ~ 2 hours from SNU Power plant Homyung Mt.( 虎鳴山 ) Reservoir Laboratory Access tunnel(1.4km) 350m Pukhan River( 北漢江 ) Underground Lab. at Cheongpyung Storage Water Power Plant ~1000 mwe

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) External background Cosmic rays : ~ relative to the sea level Expected at ~400 m underground The rock composition : 238 U ~ 4.8 ppm, 232 Th ~ 6 ppm 40 K ~ 4 ppm (ICP-Mass) Gamma background With a shielding of 15cm Pb + 10cm Cu(OFHC)  Can be controlled < cpd (counts/keV/kg/day) Neutron background Photonuclear interaction of cosmic ray muon Fission of 238 U, (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.) W/o shielding 10cm Pb + 10 cm Cu (16 days data taking) 100% HPGe installed in CPL Gamma Background 222 Rn_up( 226 Ra)/ 222 Rn_down( 214 PB, 214 Bi)=

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Neutron Background ~ 4x10 -5 /cm 2 /sec Input to the GEANT4 simulation Measured with 0.5 liter BC501A liquid scintillator

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Internal background 137 Cs :  1/2 = year  decay to 137 Ba* (Q = keV)  2 min life time, emitting keV gamma Hard to reject => serious background at low energy 134 Cs :  1/2 = year  decsy to 134 Ba* (Q= keV)  immediate gamma emission Can be rejected easily => not a serious problem Radioisotopes in the crystal - most important background after the proper shielding 87 Rb :  1/2 half life = 4.75 x year (27.8% natural abundance)  Beta deacy to 87 Sr (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   rays and  rays of 87 Rb, 134 Cs, and 137 Cs + GEANT 60 cpd 137 Cs : 13.3mBq/kg, 134 Cs : 54.2mBq/kg, 87 Rb :~203ppb Another crystal 137 Cs : ~155mBq/kg, 134 Cs : ~35mBq/kg, 87 Rb : 3.9 ppb 0.63 cpd/1ppb 87 Rb 0.3 cpd/1mBq 137 Cs cpd/1mBq 134 Cs

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) 238 U, 232 Th 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 137 Cs, 134 Cs : direct measurement (fit to the MC prediction) Rb : direct measurement + chemical analysis U, Th : direct measurement of alphas ~ 15 ppt Powder 137 Cs, 134 Cs : 100% HPGe detector Rb : chemical analysis (ICP Mass) CsOH, CsNO3, Pollucite, Water 137 Cs, 134 Cs : 100% HPGe detector 137 Cs(mBq/kg) 134 Cs(mBq/kg)

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) Background measurement of powders Crystals CsMnO 4 ~ 3mBq/kg CsI Powders 137 Cs ~14mBq/kg Rb ~ 21 ppb CsOH CsNO 3

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) 137 Cs in Pollucite ? 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 < 5.8 mBq/kg

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.) CsI (30-50 keV) : 14.3 kg days NaI(Tl) (35-40 keV) : 330 kg days Surface effect ? V.A. Kudryavtsev et al., hep-ex/ kg days exposure

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 PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg Quartz 20cm BC501A  tagging efficiency > 75%

Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 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  -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