Measurement of intrinsic radioactive backgrounds from Cs-137 and U/Th chains in CsI(Tl) crystals

The inorganic CsI(T1) crystal scintillator is a candidate anti-compton detector for the China Dark matter Experiment. Studying the intrinsic radiopurity of the CsI(T1) crystal is an issue of major importance. The timing, energy and spatial correlations, as well as the capability of pulse shape discrimination provide powerful methods for the measurement of intrinsic radiopurities. The experimental design, detector performance and event-selection algorithms are described. A total of 359 x 3 kg-days data from three prototypes of CsI(T1) crystals were taken at China Jinping Underground Laboratory (CJPL), which offers a good shielding environment. The contamination levels of internal isotopes from Cs-137, Th-232 and U-238 series, as well as the upper bounds of U-235 series are reported. Identification of the whole alpha peaks from U/Th decay chains and derivation of those corresponding quenching factors are achieved.