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CsI crystal for WIMP Search

CsI crystal for WIMP Search. 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

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CsI crystal for WIMP Search

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  1. CsI crystal for WIMP Search 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.ChoiKAIST : 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 Sun Kee Kim Seoul National University

  2. 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

  3. CsI(Tl) Crystal Advantages High light yield ~50,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) 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

  4. 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

  5. Neutron Beam Test KIGAM(Korea Institute of Geology and Material) 3.2 MeV p : 3H(p,n)3He  2.62 MeV n

  6. Quenching factor n n QF = Emeasured / Erecoil

  7. Pulse Shape

  8. Mean Time

  9. Quality factor cut B S S B Ideal detector  ~ 1,  ~ 0 K << 1

  10. CsI(Na) 1st measurement of CsI(Na)

  11. Underground Lab. at Cheongpyung Storage Water Power Plant ~1000 mwe ~ 2 hours from SNU Homyung Mt.(虎鳴山) Reservoir Access tunnel(1.4km) 350m Pukhan River(北漢江) Laboratory Power plant

  12. 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

  13. 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

  14. Neutron Background Measured with 0.5 liter BC501A liquid scintillator ~ 4x10-5 /cm2/sec Input to the GEANT4 simulation

  15. Internal background Radioisotopes in the crystal - most important background after the proper shielding 137Cs : t1/2= 30.07 year b 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 : t1/2 = 2.065 year b decsy to 134Ba* (Q=2058.7 keV)  immediate gamma emission Can be rejected easily => not a serious problem 87Rb : t1/2 half life = 4.75 x 1010 year (27.8% natural abundance) b Beta deacy to 87Sr (Q=282.3 keV)  no gamma emission Hard to reject => potentially a serious problem => reduction technique in material is known

  16. In acrystal currently available 60 cpd g-rays and b-rays of 87Rb, 134Cs, and 137Cs + GEANT Another crystal 137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg,87Rb :~203ppb Internal background 0.63 cpd/1ppb 87Rb 0.3 cpd/1mBq 137Cs 0.005 cpd/1mBq 134Cs 137Cs : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb

  17. 238U, 232Th in the crystal 10~20 alphas/kg/day  ~3 ppt (U+Th)

  18. 134Cs(mBq/kg) 137Cs(mBq/kg) 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 Powder 137Cs, 134Cs : 100% HPGe detector Rb : chemical analysis (ICP Mass) CsOH, CsNO3, Pollucite, Water 137Cs, 134Cs : 100% HPGe detector

  19. Background measurement of powders Crystals CsOH CsNO3 CsMnO4 ~ 3mBq/kg CsI Powders 137Cs ~14mBq/kg Rb ~ 21 ppb

  20. 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

  21. 62 kg days exposure Surface effect ? CsI (30-50 keV) : 14.3 kg days NaI(Tl) (35-40 keV) : 330 kg days V.A. Kudryavtsev et al., hep-ex/0109013

  22. Shielding Design

  23. Detector Design CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg Quartz PMT (3 inch RbCs) 5x5 = 25 crystals ~ 275 kg 20cm BC501A  tagging efficiency > 75%

  24. Sensitivity After 1 year data taking with 100 kg CsI(Tl) 2 keV threshold

  25. 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

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