掺钆液闪中子探测器的研制

中子在探测器中能产生与暗物质粒子(WIMPs)类似的核反冲信号,对于低背景的暗物质直接探测实验,精确测量和排除中子的干扰尤其重要。为测量中国锦屏地下实验室(CJPL)中子本底的通量,研制了高效的、具有很强"中子-伽马"分辨的掺钆液闪快中子探测器。介绍了掺钆液闪中子探测器的研制和性能,包括探测器形状和尺寸的设计,液体闪烁体类型、光电倍增管型号以及液闪容器材料的选择,探测器的伽马能量刻度以及中子与伽马信号的甄别。用Am-Be中子源对探测器进行探测效率刻度,得到阈值为0.2 MeV等效电子能量的中子探测效率为(6.30±0.30)%,满足中国锦屏地下实验室对中子通量测量的要求。

Discrimination of neutrons and γ-rays in liquid scintillator based on Elman neural network

In this work, a new neutron and γ （n/γ） discrimination method based on an Elman Neural Network （ENN） is proposed to improve the discrimination performance of liquid scintillator （LS） detectors. Neutron and γ data were acquired from an EJ-335 LS detector, which was exposed in a 241Am-9Be radiation field. Neutron and γ events were discriminated using two methods of artificial neural network including the ENN and a typical Back Propagation Neural Network （BPNN） as a control. The results show that the two methods have different n/γ discrimination performances. Compared to the BPNN, the ENN provides an improved of Figure of Merit （FOM） in n/γ discrimination. The FOM increases from 0.907 4- 0.034 to 0.953 4- 0.037 by using the new method of the ENN. The proposed n/γdiscrimination method based on ENN provides a new choice of pulse shape discrimination in neutron detection.

CDEX-1 1 kg point-contact germanium detector for low mass dark matter searches

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.

Characterization of large area photomultiplier ETL 9357FLB for liquid argon detector

The China Dark Matter Experiment （CDEX） Collaboration will carry out a direct search for weakly interacting massive particles with germanium detectors. Liquid argon will be utilized as an anti-Compton and cooling material for the germanium detectors. A low-background and large-area photomultiplier tube （PMT） immersed in liquid argon will be used to read out the light signal from the argon. In this paper we have carried out a careful evaluation on the performance of the PMT operating at both room and cryogenic temperatures. Based on the single photoelectron response model, the absolute gain and resolution of the PMT were measured. This has laid a foundation for PMT selection, calibration and signal analysis in the forthcoming CDEX experiments.

Measurement of photoelectron yield of the CDEX-10 liquid argon detector prototype

The China Dark Matter Experiment （CDEX） is a low background experiment at China Jinping Under- ground Laboratory （CJPL） designed to directly detect dark matter with a high-purity germanium （HPGe） detector. In the second phase, CDEX-10, which has a 10 kg germanium array detector system, a liquid argon （LAr） anti- Compton active shielding and cooling system is proposed. To study the properties of the LAr detector, a prototype with an active volume of 7 liters of liquid argon was built and operated. The photoelectron yields, as a critically important parameter for the prototype detector, have been measured to be 0.051-0.079 p.e./keV for 662 keV γ rays at different positions. The good agreement between the experimental and simulation results has provided a reasonable understanding and determination of the important parameters such as the surviving fraction of the Ar2 excimers, the absorption length for 128 nm photons in liquid argon, the reflectivity of Teflon and so on.

Study of the material photon and electron background and the liquid argon detector veto efficiency of the CDEX-10 experiment

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.