1. 2003-04-25 79th KPS meeting 1 WIMP Search with CsI(Tl) Crystals – Status and Future The Future of Dark Matter Detection Y.D. Kim ( KIMS collaboration )

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

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

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

6. Environment Parameters DepthMinimum 700 m Temperature20 ~ 25 o C Humidity35 ~ 60 % Rock contents 238 U less than 0.5 ppm 232 Th 5.6 +/- 2.6 ppm K 2 O 4.1 % Muon flux 4.4 x 10 -7 /cm 2 /s Neutron flux 8 x 10 -7 /cm 2 /s 222 Rn in air 1 ~ 4 pCi/liter Temperature stability of CsI(T l ) crystal detector 21.7 +/- 0.2 o C N 2 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 /cm 2 /s Position resolution :  x, ~ 8 cm Reconstructed muon tracks with hit information

8. Neutron Monitoring Detector 1liter BC501A liquid scintillator n/  separation using PSD Measured Neutron flux  Inside of main shield < 3 neutrons/day/liter @90%CL, E threshold = 300 keV  Tag  events using  and  coincidences in 238 U & 232 Th 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 /cm 2 /s ( 1.5 < E neutron < 6 MeV )  Subtract energy spectrum inside of main shield to reject internal background

9. Neutrons induced by muons Log 10 (t) Energy [MeV] Neutron region Energy [MeV] Two strong neutron candidates 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 Coincidence between muon and neutron detector

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

11. 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 9 Be(α,n) 12 C (~50%)  neutron only (higher energy) 9 Be(α,n) 12 C * (~ 50%)  lower energy neutron + γ(4.43MeV) Have to use TOF to get the E n New Facility for PSD measurement 300 mCi(1.1x10 10 Bq) Am/Be source  emits 7 x 10 5 n/sec (70 neutrons/10 6 α’s)  ~a few 100 neutrons/sec hit 3cmX3cm crystal

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

13. Reference Recoil Spectrum : Recoil vs. Cs137 2-4keV4-6keV6-8keV 10-12keV8-10keV

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

15. Quenching Factor Neutron Energy 90 º 55 º E CsI (keV) E n (MeV)

16. Background data and limits  CsI(Tl) Crystal 8x8x23 cm 3 (6.6 kg)  3 ” PMT (9269QA) Quartz window, RbCs photo cathode  DAQ 500MHz FADC cpd  full size crystal ~ 5 p.e./keV  Trigger Threshold less than 2 keV ( 5 p.e. within 2  s ) 137Cs & Rb87 Reduction 6cpd Contributions 1. Cs137 ~ 3.0 cpd 2. Cs134 ~ 1.8 cpd 3. Rb87 ~ 1.0 cpd Total ~ 6 cpd @ 10keV 237 kg days data

17. Coincidences T 1/2 = 0.138+/-0.012 sec (T 1/2 = 0.145 sec) Th : 0.411 +/- 0.078 ppt of Th232 if it is equilibrium Th Chain : 216 Po  212 Pb(145 msec) T 1/2 =4.23+-1.10 min (T 1/2 = 3.10 min) After subtraction of Th232 294.8 +/- 49.4 events U : 1.16 +/- 0.19 ppt Accidentals U Chain : 218 Po  214 Pb(145 msec)

18. CsI Data Taking  Data for WIMP search  237 kg days for 8x8x23 cm 3 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 57 Co ( 122.06 keV  ) and 55 Fe (5.9 keV) Compton data Check Surface events, Single photon calibration 137 Cs ( 661.657 keV  ) Compton data Reference distribution for  events - equivalent to 350 kg days of underground data Cut efficiency calculation

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

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

21. Cut efficiency correction Before CutAfter Cut Efficiency curveAfter efficiency correction Underground data PMT noise Preliminary

22. Reference PSD distribution of Neutron & 137 Cs Compton & Data 3~4 keV4 ~5 keV 11~12 keV10~11 keV9~10 keV 8~9 keV 7~8 keV 6~7 keV 5~6 keV Preliminary

23. Log Mean Time fitting using Distribution of Neutron & 137 Cs Compton Energy [keV] Recoil events [cpd] 3~40.44 +/- 1.23 4~50.08 +/- 0.75 5~60.46 +/- 0.58 6~7-0.42 +/- 0.35 7~8-0.37 +/- 0.30 8~9-0.16 +/- 0.24 9~100.11 +/- 0.18 10~110.21 +/- 0.18 Background level WIMP mass 20 GeV/c 2 60 GeV/c 2 130 GeV/c 2 250 GeV/c 2 Preliminary

24. Preliminary Limit curve 1 - SI  Dark matter density at the solar system   D = 0.3 GeV c -2 cm -3  Use annual average parameters V 0 = 220 km s -1, V E = 232 km s -1, V Esc = 650 km s -1  Preliminary results  Recoil energy threshold > 20 keV  Comparable limit with NAIAD with ten times more data

25. Preliminary Limit curve 2 - SD I Cs All 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 137 Cs produced (Chemetall, Germany) and crystallization starts. 137 Cs contribution < 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 137 Cs. Further reduction anticipated.

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