CDEX暗物质实验中液氩反符合探测器的关键技术研究

发布时间：2018-11-28 07:04

【摘要】：暗物质探测已成为21世纪物理学界最重要的课题之一。弱相互作用重粒子(Weakly Interacting Massive Particles,WIMPs)被认为是最有可能的暗物质粒子候选者。因它与物质作用截面极小,直接探测暗物质要求极低的本底环境。中国暗物质实验(CDEX)计划利用吨量级低能量阈高纯锗(HPGe)探测器开展低能量暗物质直接探测工作。为了进一步降低实验本底,CDEX第一阶段CDEX-1实验的质量为1kg的高纯锗探测器使用碘化钠反符合探测器。随着HPGe探测器质量增大,固态的NaI反符合探测器已无法满足实验要求。作为CDEX项目的第二阶段,CDEX-10实验使用10 kg HPGe探测器作为主探测器,并计划利用液氩(LAr)作为HPGe探测器的反符合探测器。这是世界上首个以LAr探测器作为反符合探测器的暗物质直接探测实验。本课题作为CDEX-10 LAr反符合探测器的预研课题,主要研究工作有:(1)设计、研制了一套小型LAr探测器实验平台。掌握了LAr探测器的实验原理和实验流程,建立了一套LAr探测器数据处理方法。这些方法与经验将为CDEX-10 LAr反符合探测器的运行提供借鉴。(2)小型LAr探测器光电子产额的实验测量与蒙卡模拟研究。研究了PMT的关键性能,测量得到了PMT的绝对增益、分辨率和峰谷比等关键参数。测量了小型LAr探测器对不同入射位置662 keVγ射线的能谱,得到了对应的光电子产额为0.051~0.079 p.e./keV,并结合蒙卡模拟确定了小型LAr探测器的闪烁光产额LY、LAr对闪烁光的吸收长度L和Teflon表面的反射率R等关键参数。这些参数被应用于CDEX-10 LAr反符合探测器的模拟计算,评估反符合探测器的预期性能。(3)氩闪烁光信号波形的研究。研究了未经纯化的氩的波形甄别特性,观察到了γ粒子事件和α粒子事件的快成分比重参数Fprompt有显著的差异,为进一步研究LAr反符合探测器的粒子甄别效率提供了可能性。还研究了闪烁光慢成分时间常数与氩纯度的关系,可作为实时监测氩纯度的手段。(4)通过蒙卡模拟评估CDEX-10 LAr反符合探测器的预期性能,并对CDEX-10探测器系统的设计和结构提出优化建议。模拟得到了当前实验条件下,CDEX-10 LAr反符合探测器的能量阈值和反符合效率,预测了经过反符合后可将CDEX-10的本底降至0.1 cpkkd的物理目标以下。
[Abstract]:Dark matter detection has become one of the most important topics in physics in the 21st century. The weakly interacting heavy particle (Weakly Interacting Massive Particles,WIMPs) is considered the most likely candidate for dark matter particles. Because of its minimal cross section with matter, direct detection of dark matter requires very low background environment. China dark matter experiment (CDEX) plans to use low energy threshold high purity germanium (HPGe) detector to carry out direct detection of low energy dark matter. In order to further reduce the experimental background, the high purity germanium detector with the quality of 1kg in the first stage of CDEX CDEX-1 experiment uses sodium iodide anti-coincidence detector. With the quality of HPGe detector increasing, the solid state NaI counter-coincidence detector can not meet the experimental requirements. As the second phase of the CDEX project, the 10 kg HPGe detector is used as the main detector in the CDEX-10 experiment, and the liquid argon (LAr) is planned to be used as the anti-coincidence detector for the HPGe detector. This is the world's first direct detection of dark matter using a LAr detector as an anti-coincidence detector. As a pre-research task of CDEX-10 LAr anti-coincidence detector, the main research work is as follows: (1) A set of experimental platform of small LAr detector is designed and developed. The experiment principle and flow chart of LAr detector are mastered, and a set of data processing method of LAr detector is established. These methods and experiences will provide a reference for the operation of CDEX-10 LAr anti-coincidence detectors. (2) Experimental measurement and Monka simulation of photoelectron yield of small LAr detectors. The key performance of PMT is studied and the key parameters of PMT, such as absolute gain, resolution and peak-to-valley ratio, are obtained. The energy spectra of small LAr detectors for 662 keV 纬 -rays at different incident positions were measured. The corresponding photoelectron yield was 0.051 ~ 0.079 p.e.keV, and the scintillation light yield LY, of the small LAr detector was determined by combining with Monka simulation. The key parameters such as the absorption length L of LAr and the reflectivity R of Teflon surface are obtained. These parameters are applied to the simulation calculation of CDEX-10 LAr anti-coincidence detector to evaluate the expected performance of the anti-coincidence detector. (3) the study of argon scintillation signal waveform. The waveform discrimination characteristics of unpurified argon have been studied. It has been observed that there are significant differences in the fast component specific gravity parameters Fprompt between 纬 particle events and 伪 particle events, which provides a possibility for further study on the particle discrimination efficiency of LAr anticoincidence detectors. The relationship between the time constant of slow component of scintillation and the purity of argon is also studied, which can be used as a means to monitor the purity of argon in real time. (4) the expected performance of CDEX-10 LAr anti-coincidence detector is evaluated by Monka simulation. The design and structure of CDEX-10 detector system are optimized. The energy threshold and anti-coincidence efficiency of the CDEX-10 LAr anti-coincidence detector are obtained under the current experimental conditions, and it is predicted that the background of CDEX-10 can be reduced to below the physical target of 0.1 cpkkd after the anti-coincidence.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P145.9

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