Internal background of CsI(Tl) crystal detectors for dark matter search Tae Yeon Kim Seoul National University For the KIMS Collaboration Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim, S. Y. Kim, T.Y.Kim*, H.S.Lee, H.S.Park, I.H.Park, E.I.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.Han, E.K.Lee SeongKyunKwan Univ. : I.S.Cho, D.H.Choi, S.H.Noh, I.T.Yu Chonbuk National Univ. : S.Y.Choi KAIST : 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, China : J.J.Zhu (Korean Invisible Mass Search)

- Location Underground Pumped Storage Power Plant of Korea South Electric Company 80 km northeast from Seoul 2 hours from Seoul by car - Depth Minimum : 350 m Access tunnel : 1.4 km Temperature : o C Relative humidity : more than 90 % - Experimental hall 1. Underground laboratory

2. External background - Cosmic rays Measured with 3 plastic scintillation(BC408) counters of 1 cm thickness in coincidence. AB A : spectrum at sea level for 1 day B : spectrum at underground lab. for 2 weeks Count rate B / count rate A = (1.4±0.15)x Neutron Measured with 0.5 liter BC501A liquid scintillator for 13 days 3675 counts/13days Fast neutron flux = 4x10 -5 /cm 2 /sec in the energy range between 1.5 MeV and 6.0 MeV.

- Gamma ray The rock composition (ICP-Mass analysis) : 238 U = 9 ppm 232 Th = 3 ppm can be controlled < cpd 40 K = 4 ppm in the above shielding. GEANT4 simulation : The shieilding of (Pb 15 cm + Cu 10 cm) can reduce gamma ray background of 1.5 MeV by a factor of Pb 210 Bi 210 Po 206 Pb (stable) Beta-decay Alpha-decay Gamma ray : 46.5 keV Beta ray(Q-value) : 16.5, 63.0, KeV In lead shield

CsI crystal ABCD Dimension Front surface 70  70 mm 2 55  55 mm 2 60  60 mm 2 Rear surface 70  70 mm 2 65  65 mm 2 62  62 mm 2 60  60 mm 2 length300 mm 230 mm300 mm Mass(kg) Exposure(kg  day) CsI(Tl) crystal detector -Crystal ; Commercially available one and stored at underground laboratory for a few month. - Shielding Lead 15 cm OFHC copper 10 cm N2 gas flow : 1 liter/min. Temperature : o C Humidity : %

- Photoelectron yield ( 7x7x30 cm 3 ) 55 Fe source (5.9 KeV X-ray) (4.5±0.9) photoelectorns /keV - Energy resolution- Position dependence ( 60 Co) Relative standard deviation = 1.7%

- Background spectra Isotope CsI crystal ABCD 134 Cs 38.5     Cs 74.4     ±5.1 Counts/keV/kg/day - GEANT4 MC simulation Internal 662 keV gamma-ray ; Photo-peak efficiency ~ 38 % Total detection efficiency ~ 58% Internal beta-ray ; ~ 100 % - Contamination of cesium radioisotopes 4. Measurement Results

CsI crystal ABCD Li< Na1646< Mg Al K<327 - Cr Fe Rb Sr< Ba La Sm Lu Tl Th< U < ICP-Mass analysis (Unit in ppb)- Alpha-ray spectrum 1 ppb Rb = 0.6 cpd at a few keV region Crystal B is closed to this value. 238 U = 0.39 ± 0.09 ppt 232 Th = 4.5 ± 0.5 ppt

-HPGe measurement - An expected spectrum (GEANT4) 134 Cs = 1mBq/kg 137 Cs = 1mBq/kg Rb = 1ppb 238 U = 0.39 ± 0.09 ppt 132 Th = 4.5 ± 0.5 ppt Pollucite < 5.8 mBq/kg 63.0 ±3.7 mBq/kg Start WIMP Search !!