Neutron background measurement is always very important for dark matter detection due to almost the same effect for the recoiled nucleus scattered off by the incident neutron and dark matter particle. For deep under-ground experiments, the flux of neutron background is so low that large-scale detection is usually necessary. In this paper, by using Geant4, the relationship between detection efficiency and volume is investigated, meanwhile, two geometrical schemes for this detection including a single large-sized detector and arrayed multi-detector are compared under the condition of the same volume. The geometrical parameters of detectors are filtrated and detection efficiencies obtained under the similar background condition of China Jingping Underground Laboratory （CJPL）. The results show that for a large-scale Gd-doped liquid scintillation detector, the detection efficiency increases with the size of detector at the beginning and then trends toward a constant. Under the condition of the same length and cross section, the arrayed multi-detector has almost similar detection performance as the single large-sized detector, while too much detector number could cause degeneration of detection performance. Considering engineering factors, such as testing, assembling and production, the 4 × 4 arrayed detector scheme is flexible and more suitable. Furthermore, the conditions for using fast and slow signal coincidence detection and the detectable lower limit of neutron energy are evaluated by simulating the light process.
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.
The CDEX （China Dark matter EXperiment） Collaboration will carry out a direct search for WIMPs （Weakly Interacting Massive Particles） using an Ultra-Low Energy Threshold High Purity Germanium （ULE-HPGe） detector at the CJPL （China JinPing deep underground Laboratory）. A complex shielding system was designed to reduce backgrounds and a detailed GEANT4 Monte Carlo simulation was performed to study the achievable reduction of γ rays induced by radionuclides and neutron backgrounds by D（γ,n）p reaction. Furthermore, the upper level of allowed radiopurity of shielding materials was estimated under the constraint of the expected goal. Compared with the radiopurity reported by other low-background rare-event experiments, it indicates that the shielding used in the CDEX can be made out of materials with obtainable radiopurity.
The CDEX collaboration has been established for direct detection of light dark matter particles, using ultra-low energy threshold point-contact p-type germanium detectors, in China JinPing underground Laboratory （CJPL）. The first 1 kg point-contact germanium detector with a sub-keV energy threshold has been tested in a passive shielding system located in CJPL. The outputs from both the point-contact P＋ electrode and the outside N＋ electrode make it possible to scan the lower energy range of less than 1 keV and at the same time to detect the higher energy range up to 3 MeV. The outputs from both P＋ and N＋ electrode may also provide a more powerful method for signal discrimination for dark matter experiment. Some key parameters, including energy resolution, dead time, decay times of internal X-rays, and system stability, have been tested and measured. The results show that the 1 kg point-contact germanium detector, together with its shielding system and electronics, can run smoothly with good performances. This detector system will be deployed for dark matter search experiments.
The China Dark Matter Experiment （CDEX） is located at the China Jinping Underground Laboratory （CJPL） and aims to directly detect the weakly interacting massive particles （WIMP） flux with high sensitivity in the low mass region. Here we present a study of tile predicted photon and electron backgrounds including the background contribution of the structure materials of the germanium detector, the passive shielding materials, and the intrinsic radioactivity of the liquid argon that serves as an anti-Compton active shielding detector. A detailed geometry is modeled and the background contribution has been simulated based on the measured radioactivities of all possible components within tile GEANT4 program. Then the photon and electron background level in the energy region of interest （〈10-2events-kg1·day 1·keV-1 （cpkkd）） is predicted based on Monte Carlo simulations. The simulated result is consistent with the design goal of the CDEX-10 experiment, 0.1cpkkd, which shows that the active and passive shield design of CDEX-10 is effective and feasible.
暗物质是当今物理学最基本也是最吸引人的前沿研究课题之一,对认识宇宙起源、演变和结构以及物质的本源等基本科学问题具有十分重要的意义.暗物质的理论研究和实验探测经过几十年的积累和发展已经取得了长远的进步.实验上有多种方法可以进行暗物质粒子的探测,直接探测是一种非常重要的手段.本文评述了暗物质直接探测方法的原理和当今国际国内采用直接探测法的不同实验的研究现状,着重介绍了中国暗物质实验(China Dark matter Experiment,CDEX)合作组的研究历程、探测技术和数据分析方法、以及研究取得的重要成果和未来规划.
