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Dark Matter Search with CsI crystal

Dark Matter Search with CsI crystal. For the KIMS Collaboration. 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. Dark Matter ?.

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Dark Matter Search with CsI crystal

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  1. Dark Matter Search with CsI crystal For the KIMS Collaboration 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

  2. Dark Matter ? * 60 years ago Fritz Zwicky first claim missing mass in galaxy *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

  3. Evidence for the existence of Dark Matter Rotational curves of galaxies NGC6503

  4. 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

  5. How to detect WIMP directly ? Elastic Sacttering of WIMP off a nuclues in the crytsal WIMP 10-6 ~ 10-10pb Cs Expected event rate ~ 1/kg/day or less I Recoiled nucleus Energy loss by ionization and lattice vibration

  6. Direct detection strategies 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 • 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)

  7. Annual modulation WIMP hitting rate depends on season 30km/s Earth Sun 232km/s

  8. Pulse Shape Discrimination Method Event by event separation (Low temp)

  9. 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

  10. Status of WIMP Search CDMS limit 2000 DAMA 1st positive result based on annual modulation KIMS 100kg year

  11. CsI Crystal Advantages High light yield ~50,000 photons/MeV Pulse shape discrimination Easy fabrication and handling High mass number Good spin dependent interaction coupling CsI(Tl) NaI(Tl) Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1050 ~230 Peak emission(nm) 550 415 Hygroscopicity slight strong Disadvantages Emission spectra does not match with normal bialkali PMT  effectively reduce light yield 137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic

  12. 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)

  13. CsI(Tl) Detector Unit CsI(Tl) crystal PMT PMT Amplification & self coincidence Oscilloscope or digitizer coincidence Trigger Signal

  14. 660 keV g Typical signals from CsI(Tl) 10 keV g 660 keV a

  15. Photoelectron yield Full size crystal (7x7x30) : ~ 4 p.e./keV Small crystal(3x3x3) : 5~10 p.e./keV

  16. Neutron Beam Test KIGAM(Korea Institute of Geology and Material) 3.2 MeV p => 2.4 MeV n

  17. Quenching factor QF = Emeasured / Erecoil

  18. Mean Time Distribution

  19. Quality Factor Separation of n/gamma Ideal case: QF<<1 CsI(Na) CsI(Tl)

  20. 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 –87Rb, 137Cs, 134Cs … •  Material study – chemistry, low background • powder selection

  21. Underground Lab. at Cheongpyung Homyung Mt.(虎鳴山) Reservoir Access tunnel(1.4km) 350m Pukhan River(北漢江) Laboratory Power plant

  22. Phase II ( 2000,4 - 2001,4 ) Used for the null background and the sample measurements with HPGe detector. Size : External volume = 75x75x85 cm3 Internal volume = 10x15x23 cm3

  23. 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

  24. Neutron gamma separation with BC501A

  25. Neutron background at underground • Measured with 0.5 liter BC501A liquid scintillator • after unfolding ~ 4x10-5 /cm2/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

  26. Sources of internal background In acrystal currently available 60 cpd g-rays and b-rays of 87Rb, 134Cs, and 137Cs + GEANT 137Cs : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb 137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg,87Rb :~203ppb 238U : 5.5mBq/kg , 232Th : 12.4mBg/kg Another crystal

  27. 137Cs in Pollucite In CsI : 0.063 Bq/kg In Pollucite : < 0.008 Bq/kg Suspected to be inserted during extraction of Cs powder  measurement of 137Cs contamination in water 137Cs ~ 0.01Bq/liter * a lot of water is used in the process of extracting Cs powder ~ 145 liter/ 1kg Cs (from a company)

  28. Shielding Design

  29. Detector Design CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg Quartz PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg

  30. Sensitivity of Spin Independent Interaction After 1 year data taking with 50 kg CsI(Tl) assume 2 keV threshold

  31. 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

  32. 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

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