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

The inorganic CsI(Tl) crystal scintillator is a candidate anti-compton detector for the China Dark matter Experiment. Studying the intrinsic radiopurity of the CsI(Tl) 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 359×3 kg-days data from three prototypes of CsI(Tl) crystals were taken at China Jinping Underground Laboratory (CJPL), which offers a good shielding environment. The contamination levels of internal isotopes from ¹³⁷Cs, ²³²Th and ²³⁸U series, as well as the upper bounds of ²³⁵U series are reported. Identification of the whole α peaks from U/Th decay chains and derivation of those corresponding quenching factors are achieved.     

Limits on light WIMPs with a 1 kg-scale germanium detector at 160 eVee physics threshold at the China Jinping Underground Laboratory

We report results of a search for light weakly interacting massive particle(WIMP) dark matter from the CDEX-1 experiment at the China Jinping Underground Laboratory(CJPL). Constraints on WIMP-nucleon spin-independent(SI) and spin-dependent(SD) couplings are derived with a physics threshold of 160 eVee, from an exposure of 737.1 kg-days. The SI and SD limits extend the lower reach of light WIMPs to 2 GeV and improve over our earlier bounds at WIMP mass less than 6 GeV.     

Measurement of cosmic ray flux in the China JinPing underground laboratory

The China JinPing underground Laboratory (CJPL) is the deepest underground laboratory running in the world at present. In such a deep underground laboratory, the cosmic ray flux is a very important and necessary parameter for rare-event experiments. A plastic scintillator telescope system has been set up to measure the cosmic ray flux. The performance of the telescope system has been studied using the cosmic rays on the ground laboratory near the CJPL. Based on the underground experimental data taken from November 2010 to December 2011 in the CJPL, which has an effective live time of 171 days, the cosmic ray muon flux in the CJPL is measured to be (2.0±0.4)×10-10/(cm2 ·s). The ultra-low cosmic ray background guarantees an ideal environment for dark matter experiments at the CJPL.     

低本底液闪中子探测器的材料测试研究

为测量中国锦屏地下实验室（China JinPing underground Laboratory,CJPL）极低通量的中子本底,需要建造大型快中子液体闪烁体探测器。建造探测器需要选用低放射性本底材料,并且化学性质稳定,不与液体闪烁体发生反应而影响液体闪烁体（EJ-335）的性能。为此,建造了小型液闪探测器,在其中分别放入聚四氟乙烯、无氧铜、氟橡胶、全氟醚橡胶这4种材料进行长期浸泡,并分别进行了如下测试:液闪长时间浸泡各种材料前后液闪的吸收光谱测试;用60Co γ源检测小型液闪探测器光产额随时间的变化情况;用Am-Be中子源检验探测器在浸泡前后的n-γ甄别性能。最终实验结果表明,聚四氟乙烯和全氟醚这2种材料与液闪有很好的兼容性,可作为大型快中子液闪探测器的容器和密封圈的备选材料。In order to measure the flux and spectrum of neutron background which is extremely low in China JinPing underground Laboratory (CJPL), liquid scintillator detector with large scale used to detect fast neutron need to be fabricated. Surrounding materials used in detector require the excellent performance, such as low radioactivity as well as stable chemical properties, which do not react with liquid scintillator, so as to avoid affecting the performance of detector. Polytetrafluoroethylene(PTFE), highly purified no-oxygen copper, fluororubber and perfluoroelastomer had been put in a small detector and soaked for a long time. The change of absorption spectrum and light yield along with time as well as the performance of n-γ discrimination after soaked were tested. The results show that PTFE and Perfluoroelastomer are compatible with liquid scintillator, which offers a reference for the selection of material used in container and seal of liquid scintillator detector used to detect fast neutron.