Status of WIMP search in KIMS experiment Kwak, Jungwon ( KIMS Collaboration ) The dark Side of the Universe KIAS-APCTP-DMRC Workshop in KIAS May 26th 2005

KIMS Collaboration Korea Invisible Mass Search experiment since 2000 H.C.Bhang, J.H.Choi, S.C.Kim, S.K.Kim, S.Y.Kim, J.W.Kwak, J.H.Lee H.S.Lee, S.E.Lee, J. Lee, S.S.Myung, H.Y.Yang Seoul National University Y.D.Kim, J.I. Lee 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, Q.Yue, X. Lee Tsinghua University 2005-05-26 KIAS-APCTP-DMRC Workshop

Yangyang Underground Laboratory, Y2L Korea Middleland Power Co. Yangyang Pumped Storage Power Plant Minimum depth : 700 m / Access to the lab by car (~2km) Completion of the power plant(2006) Construction of the laboratory buildings done (2003) 2005-05-26 KIAS-APCTP-DMRC Workshop

Environment Parameters in Y2L Depth Minimum 700 m Temperature 20 ~ 25 oC Humidity 35 ~ 60 % Rock contents 238U less than 0.5 ppm 232Th 5.6 A 2.6 ppm 40K 270 A 5% ppm Muon flux 2.7 x 10-7 /cm2/s ( 1 x 10-2 /cm2/s ) Neutron flux 8 x 10-7 /cm2/s ( 1.4 x 10-3 /cm2/s ) 222Rn in air 1~2 pCi/liter ( 4 pCi/liter ) Blue,Green : Gneiss (2 Gyr ) Red : Igneous Rock ( 200 Myr ) have more radioactivity 238U lifetime = 4.4 Gyr 232Th lifetime = 14.05 Gyr 40K lifetime = 1.277 Gyr 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop KIMS Main shield in Y2L Mineral oil 30cm Pb 15cm : 30t OFHC Cu 10cm : 3t PE 5cm HPGe detector measurement 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop External backgrounds External gamma Isotopes in surrounding materials (Rock) Decay chain of U238 and Th232 Isotopes (K40, …) Rn222 in air Shielding structure made of pure and high Z materials Partially or fully distinguishable from WIMP signal by PSD N2 flowing to remove air contaminated by Rn222 Neutron background Undistinguishable from WIMP signal (Nuclear recoil) (a, n) reaction Nuclear fission Induced by cosmic muon ( E mean ~ 230 GeV ) - possible to veto with muon detector Neutron moderator made of material with High Hydrogen density Veto system using Muon detector Underground experimental facility is necessary 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Muon detector (MUD) 2 x 2” PMT for each channel muon modules 28 signal channels ( 6 + 4 x 4 + 3 x 2 ) Liquid Scintillator 5 % PC 1 liter + PPO 4 g + POPOP 15 mg Mineral Oil 95 % - Neutron moderator 10-5 times of ground Muon rate at 700 m deep underground 2005-05-26 KIAS-APCTP-DMRC Workshop

Muon detection efficiencies Plastic scintillator MUD8 Bottom modules Trigger : Use Plastic scintillator and Bottom modules Muon trigger Trigger in MUD8 is issued or not… Muon trigger efficiency of single module : 97.0 % Muon veto efficiency : 99.9 % Muon Tagging efficiency : 94.0 % - Required coincidence triggers in different modules 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Muon flux measurement Tagging efficiency : 94.0 % Area of top module = 70220 cm2 Measured Muon flux : 2.7 x 10-7 /cm2/s DAQ rate of Muon detector = 1 ~ 3 Hz Real Muon rate of KIMS Muon detector = 4.5 x 10-2 Hz 2005-05-26 KIAS-APCTP-DMRC Workshop

Neutron Monitoring Detector (NMD) 1liter BC501A liquid scintillator - Teflon container - Quartz window - Low background 3” PMT n/g separation using PSD method - 500MHz sampling FADC 2005-05-26 KIAS-APCTP-DMRC Workshop

Alpha background study – 238U and 232Th chain 214Bi -decay  214Po -decay Coincidence time [ms] 222Rn -decay  218Po -decay Coincidence time [m] 220Rn -decay  216Po -decay 2005-05-26 KIAS-APCTP-DMRC Workshop Coincidence time [m]

Neutron flux measurement in Y2L Inside main shield < 1.8 neutrons/day/liter @90%CL, E threshold = 300 keV Tag a events using a-a and b-a coincidences in 238U & 232Th chain. All neutron candidates are consistent with alphas from internal sources Outside main shield = 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV ) Subtract energy spectrum inside shield to reject internal background Expected 10-3 times of neutron flux outside shield 5 x 10-3 cpd level of background contribution on CsI(Tl) detector 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Muon induced neutron Energy [MeV] Log10(Dt) Dt = min(Abs(t Muon event – t Neutron event)) Require Dt < 1 ms for coincidence events Dtmean = 130 ns delay cable for muon and more electronics sDt = 32 ns 16ns clock pulse used High energy events of neutron detector are mostly from muons. E mean ~ 230 GeV of muons at 2000m w.e. underground 3.5 muons /day in neutron detector ~ 2.7 x 10–7 /cm2/s muon flux 2005-05-26 KIAS-APCTP-DMRC Workshop

Muon induced neutron (cont’d) Energy [MeV] 2 events of Muon induced neutron during 67.4 days DAQ run ~ 0.03 counts/day/liter Expected muon induced neutron rate = 0.06 counts/days/liter 50% neutron detection efficiency - Lower efficiency for Higher Energy of neutron Neutron yield for muon = 2 x 10-4 (m g/cm2)-1 for 15cm-thick lead Muon flux = 2.7x10-7 /cm2/s Muon veto efficiency – 99.9% Expected non-vetoed muon = 2.7x10-10 /cm2/s non-vetoed Neutron rates = 1.2 x 10-4 counts/day/liter 2005-05-26 KIAS-APCTP-DMRC Workshop

Radon Monitoring detector Electrostatic alpha spectroscopy : 70 liter stainless container Use Si(Li) photodiode : 30 x 30 mm Estimate 222Rn amount with energy spectrum of a from 218Po & 214Po. Photodiode calibration : 210Po, 241Am 222Rn in air = 1 ~ 2 pCi/liter Absolute efficiency calibration done with 226Ra 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Why CsI(Tl) Crystal ? Advantage High light yield ~60,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) Relatively easy to get large mass with an affordable cost NaI(Tl) CsI(Tl) Disadvantages Emission spectra does not match with normal bi-alkali PMT => Effectively reduce light yield 137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic CsI(Tl) NaI(Tl) Photons/MeV ~60,000 ~40,000 Density(g/cm3) 4.53 3.67 Decay Time(ns) ~1050 ~230 Peak emission(nm) 550 415 Hygroscopicity slight strong 2005-05-26 KIAS-APCTP-DMRC Workshop

Internal background of CsI(Tl) crystal detector 137Cs : 10 mBq/kg 134Cs : 20 mBq/kg 87Rb : 10 ppb 87Rb 137Cs 134Cs keV Geant Simulation 137Cs (artificial) serious background at low energy 134Cs (artificial+133Cs(n,gamma)) 87Rb (natural) Hard to reject  reduction technique in material is known Best available Crystal at Market Powder Selection Cs137 Reduction Using Pure water Rb87 Reduction by Re-crystallization Single Crystal (8.7 kg) background @ ~10keV 87Rb 1.07 cpd/1ppb 137Cs 0.35 cpd/1mBq/kg 134Cs 0.07 cpd/1mBq/kg 2005-05-26 KIAS-APCTP-DMRC Workshop

Further reduction of internal background New CsI powder produced with ultra pure water 2mBq/kg  0.7 cpd internal background 2005-05-26 KIAS-APCTP-DMRC Workshop

WIMP search using CsI(Tl) CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) 3” PMT (9269QA) Quartz window, RbCs photo cathode 5 Photo-electron/keV DAQ 500MHz Home Made FADC 5 photo-electron within 2μsec trigger condition total 32μsec window Now on DAQ run with 3 modules – 24 kg - 6.3 cpd background level Typical Fe55 Signal Crystal Data # of p.e. Size 0406 237 kg days 5.5 8x8x23 cm3 6.7 kg 0501A Running - 8x8x30 cm3 8.7 kg 0501B 2005-05-26 KIAS-APCTP-DMRC Workshop

Data analysis for WIMP Limit Single photon clustering Mean time definition i = cluster number ti = time of i th cluster Ai = charge of i th cluster Pulse Shape Discrimination 2005-05-26 KIAS-APCTP-DMRC Workshop

Neutron Calibration facility in SNU In-house Am/Be source 9Be ( a , n ) 12C ~ 50% 9Be ( a , n ) 12C* , 12C*  12C + g (4.4 MeV) ~ 50% Activity = 300 mCi = 1.1 x 1010 Bq - neutron rate = 7 x 105 neutrons /sec - a few 100 neutrons/s hit 3X3x3 cm3 CsI(Tl) crystal 46o CsI BC501a LSC 90o 17.4o n Am/Be Tag γ(4.4MeV) to measure TOF and energy of neutrons BC501A Neutron detectors - Tag the scattered neutron from CsI at various angles - Calculate the scattering angle between nuclear and neutron Measure time of flight between LSC and CsI(Tl) detector - Energy of Incident neutron 3x3x3 cm3 CsI(Tl) crystal - Nuclear recoil energy 2005-05-26 KIAS-APCTP-DMRC Workshop

Neutron Calibration facility in SNU Energy of neutrons from Am/Be source [MeV] @Energy = 4 ~ 5 keV a) Comparison between mean time distibutions of neutron data and compton data for test crystal b) Comparison between mean time distributions of compton data for test crystal and full sized crystal 2005-05-26 KIAS-APCTP-DMRC Workshop

Summary CsI(Tl) Data taking Data for WIMP search 430 kg days for 8x8x30 cm3 crystal of 14 cpd background level 2004 spring 237 kg days for 8x8x23 cm3 crystal of 6.3 cpd background level MC data using Geant4 simulation Calibration data Neutron data from Neutron calibration facility Reference distribution for nuclear recoil events 57Co ( 122.06 keV g ), 241Am (59.54 keV) and 55Fe(5.899 keV) Compton data Check cut efficiency, Single photon calibration 137Cs ( 661.657 keV g ) Compton data Reference distribution for electron recoil events 2005-05-26 KIAS-APCTP-DMRC Workshop

Analysis Cut Efficiency Analysis Cuts To remove PMT noise and surface events PMT noise - Originate from PMT structure ( spark in dinode) - Cluster due to PMT noise is abnormally big Qc/Nc cut – mean charge of single cluster Efficiency curve for PMT Noise cut Compton data neutron reference data The difference of efficiencies is taken into account as systematic error. 2005-05-26 KIAS-APCTP-DMRC Workshop

Log Mean Time distribution of Neutron & 137Cs Compton & Data 3~4 keV 4 ~5 keV 5~6 keV 6~7 keV 7~8 keV 8~9 keV 9~10 keV 10~11 keV 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop WIMP candidates Log mean time distribution of background events are fitted to distributions of Compton events and neutron events Filled circle : w/o PMT noise cut Open square : with PMT noise cut - efficiency corrected Open circle : fitted the number of nuclear recoil events 55Fe energy distribution solid line for MC and dotted points for data Simulated energy distributions of WIMP for different masses 20 GeV,50 GeV,100 GeV,1 TeV 2005-05-26 KIAS-APCTP-DMRC Workshop

WIMP Proton Cross Section WIMP Nucleus Scattering ρχ=galatic halo density vE=earth velocity in galatic frame v0=sun velocity in galatic frame Spin Independent Spin Dependent 2005-05-26 KIAS-APCTP-DMRC Workshop

WIMP Limit curve ( spin independent ) 3879 kg days exposure of NAIAD - NaI(Tl) 237 kg days exposure of KIMS - CsI(Tl) 4123 kg days exposure of DAMA - NaI(Tl) Solid line for DAMA region by annual modulation Although the amount of data is less than 1/10 times of NaI(Tl) experiments, lower limit thanks to the better PSD power of CsI(Tl) crystal and lower recoil energy threshold. sW-A combined WIMP-neucleon X-section sW-A (EK) WIMP-neucleon X-section for E k energy bin 2005-05-26 KIAS-APCTP-DMRC Workshop

WIMP Limit curve ( spin dependent ) WIMP – Proton <Sp>Cs = 0.370 <Sp>I = 0.309 WIMP – Neutron <Sn>Cs = 0.003 <Sn>I = 0.075 For I , SM(Bonn A), M.T.Ressell, D. J. Dean Phys. Rev. C58 (1997) 535 For Cs, IBFM, F. Iachello, L.M. Krauss, G. Maino Phys. Lett. B254 (1991) 220 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Summary External background - Gamma background ~ less than 10-4 reduction rate by shield - Neutron background Neutron from environmental radioactive sources ~ neutron moderator = 30cm of Mineral oil and 5cm of PE ~ expected rate inside shield 10-9 /cm2/s ~ 5 x 10-3 cpd on CsI(Tl) Neutron induced by cosmic muon ~ 700m deep underground lab. in Y2L ~ muon flux in 700m underground 2.7 x 10-7 /cm2/s ~ 99.9% of veto efficiency ~ expected rate inside shield 10-8 /cm2/s for 15cm thick lead layer 2. Internal background - Achieve 6.3 cpd level internal background level ~ 24 kg CsI(Tl) crystal detector is running - Successful powder R&D ~ Purification of process water and re-crystallization - PMT noise reduction with pulse shape analysis of signal photon cluster 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Prospects Reduction of internal background of CsI successful  Expect ~ 1cpd crystal 600 kg powder production was finished 1cpd level CsI(Tl) crystal one prototype crystal ready to run Current shield structure can house 250kg of CsI(Tl) crystal detector 237 kg days data of 5.5 cpd background level DAMA result by annual modulation Current best limit curve by CDMS experiment 1 year data taking with 250 kg CsI(Tl) crystals of 2cpd background level 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Prospects WIMP - Proton WIMP - Neutron 2005-05-26 KIAS-APCTP-DMRC Workshop

Low mass WIMP search in KIMS Collaboration with China and Taiwan ULE HPGe detector Limited by threshold 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Status of ULE HPGe detector setup 5g 1 cpd level detector Tested at Academia Cinica, Taiwan To be delivered to SNU in Dec. If successful  upgrade to 1kg mass CsI(Tl) crystal Compton veto  Built by TU  Delivered to SNU 2005-05-26 KIAS-APCTP-DMRC Workshop

2005-05-26 KIAS-APCTP-DMRC Workshop Thanks !!! 2005-05-26 KIAS-APCTP-DMRC Workshop