【作者】 王力；

【导师】 康克军；

【作者基本信息】 清华大学 ， 核科学与技术， 2017， 博士

【摘要】 暗物质直接探测实验和无中微子双贝塔衰变实验是当前物理领域具有重大意义的前沿课题。由于具有低本底、低阈值及高能量分辨率等特点,高纯锗探测器尤其是点电极高纯锗探测器在这两类实验中都有非常重要的应用并占有重要的地位。这些实验的关键均在于建立尽可能低的本底环境,对实验结果的解释也依赖于对实验谱本底成分及贡献的理解。对实验测量谱高能区的本底分析是无中微子双贝塔衰变实验的重要内容,而基于低能区与高能区的密切联系,高能区本底分析对暗物质能区（<12 keV）本底理解也具有重要参考意义。本课题主要内容是基于CDEX-1的1 kg级点电极高纯锗探测器,对高能区（500³000 keV）本底进行分析。首先研究并建立了高能区数据处理方法,根据探测器前放输出信号特点,分析了Inhibit周期带来的效率问题并给出了相应的修正方法,通过波形信息提取、信号甄别、能量刻度和效率修正分别给出了两个测量阶段的高能区能谱。鉴于对高能区本底理解的需求,本课题根据CDEX-1探测系统结构特点和伽马本底的分布特点,使用Geant4模拟给出了宇生核素以及天然放射性核素在探测器系统不同位置的响应能谱。通过实验数据与模拟数据的对比分析,根据宇生核素的特征X射线峰或γ峰确定了相应宇生核素的本底贡献,根据拟合的方法确定了来自不同位置的天然放射性核素的本底贡献,并在此基础上分别建立了两个测量阶段的高能区（500³000 keV）本底模型,分析了本底构成特点。对于中子本底,一方面,本课题研制了基于波形甄别和符合测量技术相结合的载钆液闪探测器,并成功应用于地下实验室极低计数率的快中子本底测量;另一方面,使用²⁵²Cf中子源研究了CDEX-1探测器的中子响应能谱特点;结合两方面的研究,对CDEX-1实验谱中子本底进行了评估。基于对高能区的本底理解,本课题根据CDEX-1第二阶段测量的304 kg·day数据,对⁷⁶Ge的Q_ββ能区进行了分析,使用拟合的方法给出了对0ββ衰变的测量结果N^0ν=0,使用Profile Likelihood方法给出了测量结果所对应的0ββ衰变半衰期下限T^0ν_1/2≥6.43×10^{2 2}yr,90%C.L.。更多还原

【Abstract】 The direct detection of dark matter and the search for neutrinoless double beta decay（0ββ）are both important experiments in modern astro-particle physics.Due to its low activity,low energy threshold and good energy resolution,high purity germanium detector was widely used in the two kinds of experiments.The key to these experiments is to establish an extremely low background environment as far as possible and the explanation of the experiment result depends on the understanding of background components and their contributions in the experiment spectrum.The background analysis of the measured spectrum in high energy region is one of the main work in the 0ββexperiment.Since the intimate connection bettween the low energy region and the high energy region,the background analysis in high energy region is also useful to the background understanding in low energy region（<12 keV）for dark matter detection.Based on the measured data of CDEX-1 point-contact high purity germanium detector,this dissertation aims at the background analysis in high energy region（500³000 keV）.The data processing methods for heigh energy region were established.According to the features of signal from the detector’s preamplifier,the efficiency problem induced by the inhibit period was analyzed.The energy spectrum of the two phases were abtained by pulse information extraction,signal discrimination,energy calibration and efficiency correction.According to the configuration features CDEX-1 system and the distribution characteristics of the radioactive impurities,the response spectrums of cosmogenic isotopes and natural radioactive nuclides at different location were obtained by Geant4 simulation.Comparing the simulated spectra with the experiment spectrum,the background contribution of cosmogenic isotopes were derived according to the corresponding Kx peaks or gamma peaks and the background contribution of natural radioactive were obtained by fitting the experiment spectrum with the simulated spectra.The background models for the two phases were established and analyzed respectively.For the neutron background,A Gd-loaded liquid scintillator detector has been developed based on the pulse shape discrimination and the coincidence measurement.The fast neutron background in CJPL has been measured by the neutron detector.Besides,the characteristics of the neutron response spectrum was studied by using a ²⁵²Cf neutron source.Combined these results,the neutron background contribution in the experient spectrum was evaluated.On the basis of the background analysis in high energy region,the 304 kg·day data from the second phase were used to evaluate the 0ββsignal strength and the half-life limit of 0ββdecay has been derived asT^0ν_1/2≥6.43×10^{2 2}yr,90%C.L.with a profile likelihood method.更多还原