【作者】 赵伟；

【导师】 李元景；

【作者基本信息】 清华大学 ， 核科学与技术， 2016， 博士

【摘要】 众多强有力的证据显示暗物质是宇宙的重要组成部分,但是迄今为止,其本质仍然未知。物理学家构建了许多模型来描述暗物质,其中弱作用大质量粒子是最流行的候选者之一。由于暗物质对物质起源和宇宙演化等具有十分重要的意义,因此暗物质已经成为当今基础物理研究的前沿和热点课题之一。直接探测法是研究暗物质最直接和有效的方法之一。有多个实验已经在10 GeV/c2质量附近发现了疑似暗物质事例,因此这个质量区域已经成为暗物质研究的焦点。本课题利用世界上第一个公斤级的点电极高纯锗探测器设计和研制了暗物质直接探测研究实验系统,旨在对<10 GeV/c2的质量区间进行探测,给出暗物质在“质量-反应截面”参数空间中的灵敏度曲线。为了降低点电极高纯锗探测器的本底水平,实验在世界最深的中国锦屏地下实验室开展,搭建了有效的被动屏蔽体系统,并且利用低本底的NaI(Tl)反符合探测器作为主动屏蔽体进一步降低本底。针对实验系统的不同噪声来源,发展了基于时间和波形信息的信号甄别方法。特别地,针对本课题中脉冲反馈电荷灵敏前放的一种特定电子学噪声,发展了一种时间关联性甄别技术,将实验系统的能量阈值从1 keVee(1)降低到了400 eVee,达到了国际同类型探测器的最低阈值;同时,发展了反康普顿事例甄别技术和针对P型点电极高纯锗探测器的体/表面事例甄别技术,极大地降低了本底水平,其中在3-6 keVee能量区间的本底水平达到了约4 cpkkd(2)。这两个特性使得此实验系统对低质量(<10 GeV/c2)暗物质的探测灵敏度具有很强的国际竞争力。本课题利用365天的有效本底数据,经过数据处理得到物理能谱,再扣除掉点电极高纯锗探测器自身的宇生放射性核素和高能γ射线在低能区的贡献,最后得到可能由暗物质贡献的能谱。通过暗物质与靶核的弹性散射作用模型和最小二乘法的统计分析方法,得到了暗物质的灵敏度曲线。对于自旋无关反应,在低质量区域的实验结果达到了国际一流的水平,排除了10 GeV/c2质量附近的大部分疑似暗物质区域,特别是完全排除了采用类似探测技术的CoGeNT实验组给出的信号区域。对于自旋相关反应,完全排除了DAMA实验组给出的10 GeV/c2质量附近的信号区域,并且在4-7 GeV/c2的质量区域,达到了国际最灵敏的水平。更多还原

【Abstract】 A great many of compelling evidences have revealed that dark matter is one important component of the Universe,however,its nature remains unknown so far.The physicists have constructed enormous models to describe dark matter,and the weakly interacting massive particle(WIMP)is one kind of the most popular candidates.Since dark matter is significant to understand the origin of mass and the evolution of the Universe,it has become one of the frontier and hot topics in the fundamental physics research.The direct detection of dark matter is thought to be the one of the most promising and significant methods.Until now,some direct detection experiments have announced that some signal regions of dark matter have already been found around 10GeV/c2.Therefore,it makes this mass region to be the most attractive one.This dissertation work aims at direct detection of dark matter with point-contact high purity germanium detector(PCGe)which is the first one at1kg-scale mass in the world,focusing on verifying the dark matter signal regions around 10 Ge V/c2 and giving the exclusion limits on the parameter space of the mass and cross section of dark matter.To suppress the contributions from background,the experiment located in China Jin Ping underground laboratory(CJPL)which is the deepest in the world,and one effective passive shielding system was constructed.In addition,the Na I(Tl)anti-compton detector was applied as active shielding system to further decrease the background level.Different methods based on timing information and pulse shape characteristics were developed to discriminate various sources of noise in the experiment.In especially,an innovative method originating from the timing correlations of events was derived to eliminate one kind of the specific electronic noise induced by the pulsed reset feedback preamplifier,which reduced the energy threshold of the PCGe from 1 ke Vee down to 400 e Vee.Besides,the anti-compton events and bulk/surface events for p-type PCGe discrimination methods were developed to suppress the background level,leading to about 4 cpkkd at 3-6 ke Vee.These advantages make the system to bestrong international competitive on sensitivity for low mass(<10 Ge V/c2)dark matter search.Based on the 365 day’s valid background data,the possible recoil spectrum of dark matter was achieved through the data analysis and known background subtraction including the budget from the cosmogenic nuclides of the PCGe itself and the contribution induced by high energy gamma rays.The exclusion plots in the parameter space of dark matter mass and cross section were carried out by combining the elastic scattering model and minimal chi-square statistical method.For spin-independent cross section,the sensitivities get to the first class level in the world,excluding most of the signal region around 10 Ge V/c2,especially the whole signal region proposed by Co Ge NT whose detection technology is similar.As to sin-dependent cross section,the results totally exclude the DAMA allowed region around 10 Ge V/c2,and give the most stringent limits at the mass region of 4-7 Ge V/c2.更多还原