Direct Detection Of Dark Matter Based On A Point Contact HPGe Detector Immersed In Liquid Nitrogen

The astronomical observations have consistently indicated that nearly 85%of the matter content of our universe is long-lived,non-luminous dark matter.Further research on dark matter is the frontier and hot topic in the fundamental physics research for it is helpful for us to understand the origin of matter and the formation and evolution of the Universe.Weakly Interacting Massive Particle(WIMP)is one of the most promising candidate of dark matter.The l ow mass WIMP(m_?<10GeV/c~2)detection is not only very competitive,but also some contradictions in the results given by different experimental groups.Therefore,the detection of low mass WIMP is a hot topic in the detection of dark matter.CDEX experiment leading by Tsinghua University,is a dark matte r direct detection experiment using point contact germanium(PCGe)detectors.As a part of CDEX-10 experiment,this dissertation tried and successfully used a PCGe detector which is encapsulated in a copper vaccum tube and immersed directly into liquid nitrogen to detect the dark matter for the first time in the world.It is an important step in the CDEX experiment from the single detector with cold finger cooling system to the detector array with liquid nitrogen direct cooling system.As the first stage of CDEX-10 experiment,this work achieved the stable operation of C10B-Ge1 detector and C10C-Ge1 detector in the liquid nitrogen and set up a direct detection system based on the C10B-Ge1 and C10C-Ge1detectors.Through the research and comparison of detecto r performance,this work obtained very low threshold(160eVee)and very low background level(background level reached 2cpkkd at energy range of 2-4 keVee)on C10B-Ge1detector,which has excellent detection sensitivity for low mass dark matter.Because of its lower energy threshold and lower background level,the C10B-Ge1 detector has a better sensitivity to the low mass dark matter detection.A complete and effective data analysis method is set up for the C10B-Ge1detector.With this detector,this dissertation work observed and verified a kind of Extremely Fast Events in the low energy region(0-12keVee).With the fine structure of time distribution,this dissertation work optimize and expand the bulk/surface events discrimination ability of the ratio method.And the energy theshold of the C10B-Ge1 detector is pushed successfully to 160eVee and count rate is reduced from 34cpkkd to~2.5cpkkd at the 160eVee energy range.The first phase of the CDEX-10 experiment was stable and the data were obtained for 150.5 days.After data and events selections,a 102.8 kg·day exposure data based on the C10B-Ge1 detector were obtained.Through the understanding and analysis of the background,and considering the WIMP elastic scattering model and the least square method,this dissertation work obtained a competitive experimental results of low mass dark matter.For spin-independent cross section and spin-dependent cross section,the most stringent limits were given with the same detection technology.And especially for spin-independent cross section,the most stringent limits at the energy range of 4-5GeV were given.