APPI 2002, HongJoo Kim, Feb 15/2002 Dark Matter Search with CsI crystal APPI2002 Feb 15/2002 HongJoo Kim, Yonsei University. Introduction Characteristics of CsI crystal Neutron beam test External background Internal background Prospect of KIMS Summary Contents For the KIMS Collaboration

APPI 2002, HongJoo Kim, Feb 15/2002 *The phrase "dark matter" means matter whose existence has been inferred only through its gravitational effects ( Particle data group) *It is believed that 90% of universe is filled with unknown matter and about 30% could be from dark matter Dark Matter ? * 60 years ago Fritz Zwicky first claim missing mass in galaxy

APPI 2002, HongJoo Kim, Feb 15/2002 Evidence for the existence of Dark Matter Rotational curves of galaxies NGC6503

APPI 2002, HongJoo Kim, Feb 15/2002 Weakly Interacting Massive Particle (WIMP) Certain classes of SUSY model predict Neutralino as LSP : Stable, weak interaction scale annihilation cross section gives proper relic density for dark matter  Excellent CDM candidate (proposed by M.Goodman, E.Witten PRD 31, 1985) Neutralino : super-partner of neutral gauge bosons and higgs partciles

APPI 2002, HongJoo Kim, Feb 15/2002 Elastic Sacttering of WIMP off a nuclues in the crytsal How to detect WIMP directly ? Energy loss by ionization and lattice vibration WIMP Cs I Recoiled nucleus ~ pb Expected event rate ~ 1/kg/day or less

APPI 2002, HongJoo Kim, Feb 15/2002 Direct detection strategies We need Large mass and long time measurement Low background Low threshold and further background rejection method ( K factor) Annual modulation Nuclear recoil discrimination (PSD) Nuclear recoil elastic scattering : exponential spectrum in target material B : background rate in cpd(counts/keV/kg/day) M : Mass of the detector in kg, T : Days of data accumulation in days, K : Quality factor Lower value gives better sensitivity

APPI 2002, HongJoo Kim, Feb 15/2002 Annual modulation WIMP hitting rate depends on season 232km/s 30km/s Sun Earth

APPI 2002, HongJoo Kim, Feb 15/2002 Pulse Shape Discrimination Method Event by event separation (Low temp)

APPI 2002, HongJoo Kim, Feb 15/2002 DAMA experiment DAMA NaI(Tl) crystal ~100 kg Gran sasso underground lab. Low threshold ~ 2keV Low background ~ 1 cpd Annual modulation method Found a evidence of the annual modulation

APPI 2002, HongJoo Kim, Feb 15/2002 Status of WIMP Search DAMA 1st positive result based on annual modulation CDMS limit 2000 KIMS 100kg year

APPI 2002, HongJoo Kim, Feb 15/2002 CsI 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 photons/MeV Pulse shape discrimination Easy fabrication and handling High mass number Good spin dependent interaction coupling 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

APPI 2002, HongJoo Kim, Feb 15/2002 KIMS collaboration (Korean Invisible Mass Search) H.J.Ahn, J.M.Choi, R.K.Jain,S.C.Kim, S.K.Kim, T.Y.Kim, H.S.Lee,S.E.Lee, H.Park, I.H.Park, E.I.Won, H.S.Won,H.Y.Yang,M.S.Yang (Seoul National Univ., Korea) W.K.Kang, Y.D.Kim (Sejong Univ., Korea) M.J.Hwang, H.J.Kim, J.H.Lee, Y.J.Kwon (Yonsei Univ., Korea) I.S.Han, E.K.Lee (Ehwa Womans Univ., Korea ) I.S.Cho, D.H.Choi, S.H.Noh, I.T.Yu (SeongKyunKwan Univ., Korea) S.Y.Choi (Chonbuk National Univ., Korea) P.Ko (KAIST, Korea) M.H.Lee, E.S.Seo (Univ. Maryland, USA) H.B.Li, C.H.Tang, M.Z.Wang (National Taiwan Univ., Taiwan) W.P.Lai, H.T. Wong (Academia Cinica, Taiwan) J.Li, Y.Liu, Q.Yue (Inst. Of High Energy Physics, China) B.Xin, Z.Y.Zhou (Inst. Of Atomic Energy, China) J.J.Zhu(Tsing Hua University, China)

APPI 2002, HongJoo Kim, Feb 15/2002 CsI(Tl) Detector Unit CsI(Tl) crystal PMT Amplification & self coincidence coincidence Oscilloscope or digitizer Trigger Signal

APPI 2002, HongJoo Kim, Feb 15/2002 Typical signals from CsI(Tl) 660 keV  660 keV  10 keV 

APPI 2002, HongJoo Kim, Feb 15/2002 Photoelectron yield Full size crystal (7x7x30) : ~ 4 p.e./keV Small crystal(3x3x3) : 5~10 p.e./keV

APPI 2002, HongJoo Kim, Feb 15/2002 Neutron Beam Test KIGAM(Korea Institute of Geology and Material) 3.2 MeV p => 2.4 MeV n

APPI 2002, HongJoo Kim, Feb 15/2002 Quenching factor QF = E measured / E recoil

APPI 2002, HongJoo Kim, Feb 15/2002 Mean Time Distribution

APPI 2002, HongJoo Kim, Feb 15/2002 Quality Factor CsI(Na) CsI(Tl) Separation of n/gamma Ideal case: QF<<1

APPI 2002, HongJoo Kim, Feb 15/2002 Background External Background Cosmic ray muons – produce n,   Underground laboratory Environmental radio-isotopes – U, Th, K, …  Heavy shielding with low background(Cu, Pb ) Neutron background -> Most serious problem with PSD method (n and WIMP scattering is the same)  Heavy shielding (Polyethylene), Active shielding with BC501A (n background measurement) Radon background  Pure nitrogen gas flowing Internal background Within crystals – 87 Rb, 137 Cs, 134 Cs …  Material study – chemistry, low background powder selection

APPI 2002, HongJoo Kim, Feb 15/2002 Power plant Homyung Mt.( 虎鳴山 ) Reservoir Laboratory Access tunnel(1.4km) 350m Pukhan River( 北漢江 ) Underground Lab. at Cheongpyung

APPI 2002, HongJoo Kim, Feb 15/2002 Phase II ( 2000, ,4 ) Used for the null background and the sample measurements with HPGe detector. Size : External volume = 75x75x85 cm 3 Internal volume = 10x15x23 cm 3

APPI 2002, HongJoo Kim, Feb 15/2002 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)=

APPI 2002, HongJoo Kim, Feb 15/2002 Neutron gamma separation with BC501A

APPI 2002, HongJoo Kim, Feb 15/2002 Neutron background at underground Measured with 0.5 liter BC501A liquid scintillator after unfolding ~ 4x10 -5 /cm 2 /sec  ~ 5 cpd in CsI detector expected w/o neutron shielding GEANT4 simulation with measured neutron flux  with 30cm Polyethylene + 20 cm liquid scintillator active shieling < 0.05 cpd can be achieved

APPI 2002, HongJoo Kim, Feb 15/2002 Sources of internal background 137 Cs : ~155mBq/kg, 134 Cs : ~35mBq/kg, 87 Rb : 3.9 ppb 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 238 U : 5.5mBq/kg, 232 Th : 12.4mBg/kg Another crystal

APPI 2002, HongJoo Kim, Feb 15/ Cs in Pollucite In CsI : Bq/kg In Pollucite : < Bq/kg Suspected to be inserted during extraction of Cs powder  measurement of 137 Cs contamination in water  137 Cs ~ 0.01Bq/liter * a lot of water is used in the process of extracting Cs powder ~ 145 liter/ 1kg Cs (from a company)

APPI 2002, HongJoo Kim, Feb 15/2002 Shielding Design

APPI 2002, HongJoo Kim, Feb 15/2002 Detector Design CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg Quartz

APPI 2002, HongJoo Kim, Feb 15/2002 Sensitivity of Spin Independent Interaction After 1 year data taking with 50 kg CsI(Tl) assume 2 keV threshold

APPI 2002, HongJoo Kim, Feb 15/2002 Spin Dependent WIMP search Prospect Spin dependent interaction sensitivity : If WIMP interact only with spin of nucleus Spin factor of Cs is bigger than I After 1 year data taking with 100 kg CsI(Tl) : Sensitivity is better than DAMA ’ s limit

APPI 2002, HongJoo Kim, Feb 15/2002 Summary & Prospect Underground Lab. at CheongPyung is being established Environmental backgrounds : Understood. Small enough after the shielding Extensive R&D on CsI(Tl) crystal has been carried out Low energy can be measured with good resolution Pulse shape discrimination of  -rays : promising Intrinsic background : almost understood Shielding design/Simulation/Electronics/DAQ Design almost at final stage Prospect Started engineering design of the detector ~ 100 kg CsI(Tl) crystal 1 year data taking would confirm or reject DAMA result