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WIMP Search with CsI(Tl) Crystals – Status and Future

WIMP Search with CsI(Tl) Crystals – Status and Future. The Future of Dark Matter Detection. Y.D. Kim ( KIMS collaboration ). KIMS Collaboration K orea I nvisible M ass S earch experiment. H.C.Bhang, S.C.Kim, S.K.Kim*, S.Y.Kim, J.W.Kwak, H.S.Lee J. Lee, S.S.Myung Seoul National University

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WIMP Search with CsI(Tl) Crystals – Status and Future

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  1. WIMP Search with CsI(Tl) Crystals – Status and Future The Future of Dark Matter Detection Y.D. Kim ( KIMS collaboration )

  2. KIMS CollaborationKorea InvisibleMassSearchexperiment H.C.Bhang, S.C.Kim, S.K.Kim*, S.Y.Kim, J.W.Kwak, H.S.Lee J. Lee, S.S.Myung Seoul National University Y.D.Kim, J.I. Lee, D.S.Lim Sejong University H.J.Kim Kyungpook National University M.J.Hwang, Y.J.Kwon Yonsei University I.S.Hahn, I.H.Park Ewha Womans University M.H.Lee, E.S.Seo Univ. of Maryland J.Li Institute of High Energy Physics J.J.Zhu, D.He Tsinghua University

  3. YangYang Underground Laboratory(Y2L) [1] Location • Located in mountain area of east Korea • 3.5 hours by car from Seoul • Only one underground lab in Korea Yangyang Laboratory Run by DMRC (Dark Matter Research Center) @ Seoul National University http://dmrc.snu.ac.kr Yangyang airport 3.5 hours by car Seoul

  4. Top Lake 700m entrance Access Tunnel(2km) • [2] Profile of YangYang Underground Laboratory(Y2L) • located besides of Pumped Storage Power Plant • 700 m minimum depth, 2km Access tunnel by car

  5. CsI(Tl) crystal Neutron shield / Muon det.(30cm) Lead shield(15cm) Polyethylene(5cm) Copper shield(10cm) KIMS Neutron flux are measured in realtime as actual data

  6. Environment Parameters Temperature stability of CsI(Tl) crystal detector 21.7 +/- 0.2 oC N2 flowing rate = 4 liter / min For Rn Reduction and low humidity

  7. Muon Detector • 4 coverage muon detector : 28 channels • Liquid Scintillator(5%) + Mineral Oil (95%) = 7 ton • Measured Muon flux = 4.4 x 10 –7 /cm2/s • Position resolution : sx, ~ 8 cm • Reconstructed muon tracks with hit information

  8. Neutron Monitoring Detector • 1liter BC501A liquid scintillator • n/g separation using PSD • Measured Neutron flux • Inside of main shield < 3 neutrons/day/liter @90%CL, E threshold = 300 keV • Tag a events using a-a and b-a coincidences in 238U & 232Th chain. • 0 consistent result of Neutron rate • Expect 10-3 of neutron flux outside of main shield • No problem with current CsI background level • outside of main shield = 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV ) • Subtract energy spectrum inside of main shield to reject internal background

  9. Neutron region Energy [MeV] Energy [MeV] Neutrons induced by muons Coincidence between muon and neutron detector Log10(Dt) • Two strong neutroncandidates induced by muon • 2 events for 67.41 days • 0.025 +- 0.017 cnts/liter/day • Needs more statistics • It seems low enough compared other background

  10. E=2.62MeV Pulse Shape Discrimination : Reference spectra of Recoil Previous test with mono-energetic neutron beam Mean Time 6<E<7 keV Quenching Factor

  11. New Facility for PSD measurement • Motivation • Neutron beam is usually limited by beam time. • Moving detector to accelerator place is sometimes difficult. • Idea • Use Intense Am-Be source for both PSD and Quenching factor • 9Be(α,n)12C (~50%)  neutron only (higher energy) • 9Be(α,n)12C* (~ 50%)  lower energy neutron + γ(4.43MeV) • Have to use TOF to get the En 300 mCi(1.1x1010 Bq) Am/Be source  emits 7 x 105n/sec (70 neutrons/106 α’s)  ~a few 100 neutrons/sec hit 3cmX3cm crystal

  12. TOF  En neutron detectors 55o BC501A 10MeV g(4.4MeV) CsI θ n 5MeV LSC Am/Be neutron detectors BC501A 1MeV 90o Neutron Calibration Setup at SNU • Am-Be Source + • 20cm Liquid Scintillation Counter(LSC) • 5cm Pb shielding + • 10cm Borated Paraffin

  13. 2-4keV 4-6keV 6-8keV 8-10keV 10-12keV Reference Recoil Spectrum :Recoil vs.Cs137

  14. ToF functioning for quenching factor 2γ’s (1.173 & 1.333 MeV) 60Co σToF = 1.9ns neutron γ (4.43MeV) Qmax γ γ n n Qtot

  15. 90º 55º ECsI(keV) En(MeV) Quenching Factor Neutron Energy

  16. cpd 6cpd 137Cs & Rb87 Reduction • full size crystal ~ 5 p.e./keV • Trigger Threshold less than 2 keV ( 5 p.e. within 2 ms ) Background data and limits • CsI(Tl) Crystal 8x8x23 cm3 (6.6 kg) • 3” PMT (9269QA) Quartz window, RbCs photo cathode • DAQ 500MHz FADC • Contributions • Cs137 ~ 3.0 cpd • Cs134 ~ 1.8 cpd • Rb87 ~ 1.0 cpd • Total ~ 6 cpd @ 10keV 237 kg days data

  17. T1/2 = 0.138+/-0.012 sec (T1/2 = 0.145 sec) Th : 0.411 +/- 0.078 ppt of Th232 if it is equilibrium Coincidences U Chain : 218Po214Pb(145 msec) T1/2=4.23+-1.10 min (T1/2 = 3.10 min) After subtraction of Th232 294.8 +/- 49.4 events Th Chain : 216Po212Pb(145 msec) U : 1.16 +/- 0.19 ppt Accidentals

  18. CsI Data Taking • Data for WIMP search • 237 kg days for 8x8x23 cm3 crystal of 6 cpd background level • MC data using Geant4 simulation • Calibration data • Neutron data from Neutron calibration facility • Reference distribution for recoil events • - equivalent to 1175 kg days of underground data • PMT background data • Determine the cut values • 57Co ( 122.06 keV g ) and 55Fe (5.9 keV) Compton data • Check Surface events, Single photon calibration • 137Cs ( 661.657 keV g ) Compton data • Reference distribution for g events • - equivalent to 350 kg days of underground data • Cut efficiency calculation

  19. Raw Data Underground data PMT noise PMT related noises The low energy events are PMT related. Most likely, radioactivities(K-40,U,Th) in glass Scintillates weakly, propagated through crystal. UKDMC NaI(Tl) crystal

  20. Analysis cuts for WIMP search • Cut conditions for reduction of PMT background • Numbers of photoelectron • red marker : PMT noise • black marker : 137Cs Compton • # of p.e. > 4 for both channels • Charge & # of p.e. ratio • Energy threshold > 3 keV Preliminary

  21. Cut efficiency correction Before Cut Underground data PMT noise After Cut Efficiency curve After efficiency correction Preliminary

  22. Reference PSD distribution of Neutron & 137Cs Compton & Data 3~4 keV 4 ~5 keV 5~6 keV 8~9 keV 7~8 keV 6~7 keV 9~10 keV 10~11 keV 11~12 keV Preliminary

  23. Background level Log Mean Time fitting using Distribution ofNeutron & 137Cs Compton WIMP mass 20 GeV/c2 60 GeV/c2 130 GeV/c2 250 GeV/c2 Preliminary

  24. Preliminary Limit curve 1 - SI • Dark matter density at the solar system rD = 0.3 GeV c-2 cm -3 • Use annual average parameters V0 = 220 km s-1, VE = 232 km s-1, VEsc = 650 km s-1 • Preliminary results • Recoil energy threshold > 20 keV • Comparable limit with NAIAD with ten times more data

  25. I Cs All Preliminary Limit curve 2 - SD spin values of I-127 & SD form factor M. T. Ressel and D. J. Dean , Phys. Rev. C 56 535 (1997) Spin values of Cs-133 Iachello et al., PLB 254(1991) Method by Tovey et al, used.

  26. Plan - Projected Limit • 1.7 tons of CsI powder with 2mBq/kg of 137Cs produced (Chemetall, Germany) • and crystallization starts. • 137Cscontribution < 1 cpd is assured. • 250 kg(25 crystals) may start in 2005 with < 2 cpd background level. • DAMA data can be tested with similar crystal detector containing Iodine. •  should be helpful to identify current discrepancy Current powder still contains a little 137Cs. Further reduction anticipated.

  27. Other exp. in Yangyang: low mass WIMP Limited by threshold Collaboration with China and Taiwan : HPGe installation in Dec. ULE HPGe detector(5g) 0.1 keV threshold

  28. Summary • Established an underground laboratory (700 m deep) • Examined Environment parameters in underground laboratory Detailed MC study is in progress. • CsI R&D • 5 p.e./keV , Eth = 2 keV, E recoil > 20 keV • PSD technique works at low energy • successful reduction of internal background ( 6 cpd level ) • Shielding structure and other detectors in operation • Preliminary SI and SD limits obtained with 1 crystal ( 6.6 kg ) of 6 cpd background. • 250 kg detector with < 2 cpd will start 2005.

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