1. 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)

2. - 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 : 11 - 16 o C Relative humidity : more than 90 % - Experimental hall 1. Underground laboratory

3. 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)x10 -4 - 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.

4. - Gamma ray The rock composition (ICP-Mass analysis) : 238 U = 9 ppm 232 Th = 3 ppm can be controlled < 0.005 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 10 6. 210 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, 1161.4 KeV In lead shield

5. 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)6.634.873.564.87 Exposure(kg  day) 13.08.922.527.4 3. 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 : 26-29 o C Humidity : 30-40 %

6. - 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%

7. - Background spectra Isotope CsI crystal ABCD 134 Cs 38.5  2.834.9  2.9537.2  10.251.4  3.2 137 Cs 74.4  4.8177.2  3.8192.5  9.8161.1  3.2 64.7±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

8. CsI crystal ABCD Li<1.7 10.6 Na1646<120 21290 Mg33.528.327.4661.6 Al50.578.6139.8503.5 K<327 - Cr0.82.02.5121.3 Fe218.3488.0253.4705.8 Rb816.03.2205.1202.8 Sr<0.08 298.9 Ba1052.71062.41042.7654.4 La18.718.818.612.2 Sm2.22.32.51.3 Lu0.0150.00970.00762.5 Tl269500554600216400265500 Th<0.02 0.5 U0.050.00860.0066<0.002 - 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

9. -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 !!