Research On Cosmogenic Activation And Key Performance Of High-Purity Germanium Detector In Dark Matter Experiment

Dark matter detection is one of the most important frontier topics in recent decades.Direct detection of dark matter can be achieved by measuring the nuclear recoil energy generated by the interaction of dark matter and standard model particles.With significant low threshold and high energy resolution,High-Purity Germanium(HPGe)detectors play an important role in the direct detection of light dark matter.The China Dark matter EXperiment(CDEX)aims at direct detection of light dark matter with a tonne-scale HPGe detector array.For rare event experiments,background level has always been the focus of attention and the key point of experiments.How to correctly understand the background components and effectively control the background level will directly affect the experimental sensitivity.Based on the background requirement of 10-3 cpkkd around 1 keV proposed by CDEX future tonne-scale experiment,as well as the detail analysis of the background sources and contribution levels,this dissertation focus on the research of cosmogenic activation in germanium to evaluate the cosmogenic background level in germanium detectors.The production rates of long-lived radionuclides were calculated with Geant4 and CRY.After studying on the production mechanism,decay laws and the spectrum characteristics of those radionuclides,a total of 10 nuclides were finally taken into consideration,namely,68Ge,68Ga,65Zn,63Ni,57,60Co,55Fe,54Mn,49V,and 3H.Results were validated by comparing the simulated and experimental spectra of CDEX-1B detector.Once produced,the cosmogenic radionuclides in the germanium crystal are difficult to remove effectively within a short period of time.The control of cosmogenic background needs to trace the entire process from the preparation of germanium materials to the development of experiment.By studying the effects of altitude,latitude,and isotopic abundance on the cosmogenic radionuclides production rates,the quantitative analysis of cosmogenic background level was carried out to meet the background requirement of future tonne-scale experiment.68 Ge and 3H are the main contributors in the energy region below 2 keV,and 68Ga and 60Co are that around 2 MeV.Underground crystal growth and detector fabrication is an effective method to suppress the cosmogenic background level,and it is also an important trend in the development of future experiments.This research provides an important reference for the preliminary preparation of CDEX tonne-scale dark matter experiment,and the experimental potential for new physics including solar neutrino detection and neutrinoless double-beta decay detection were discussed.The corresponding research on the key performance and parameters of the HPGe detector were also carried out to fully understand its working principle.The characteristics of the surface dead layer of a p-type point-contact germanium detector were studied.A charge collection efficiency function was developed according to the comparison between simulation spectra and experimental data.Knowing the properties of the dead layer helps in discriminating bulk/surface events and detailed modeling of the energy spectra at lower energy range.Due to the fact that germanium detector array will be immersed in liquid nitrogen in the future tonne-scale experiment,a temperature difference exists between the liquid nitrogen environment and current cold-finger cooling cryostat.The temperature dependence of the pulse rise time was measured,which provides crucial parameters for data analysis and pulse shape simulation.