Dark Matter Direct Detection With Ultra-Low Energy Threshold High Purity Germanium Detectors

Dark matter is one of the most fundamental subjects of physics in the21st century.Research of dark matter holds great significance to the extent that it can helpunderstanding the origin and evolution of our universe, as well as the basis andinteraction of matter. The direct detection of dark matter searches in the low massregion under10GeV·c-2becomes a new research topic of focus in recent years.As the first step of China Dark matter EXperiment (CDEX) project, thisdissertation work is focused on dark matter searching in the low mass region (below10GeV·c-2) with High-purity Germanium detector.To lower the background event rate, we set up our experiment in the world’sdeepest rock overburden China JinPing underground Laboratory, constructed a superiorpassive shielding system containing polyethylene, lead, copper, etc., and applied ananti-Compton active shielding system with a NaI detector. To practice low energythreshold, a20g Ultra-Low Energy threshold Germanium detector (20g-ULEGe) wasused as the essential part of the experiment setup, which included the detector and liquidnitrogen filling system, the electronics system of signal processing and data taking, andthe DAQ software system. Through the commissioning and testing, the experimentsetup realized an energy nonlinearity of less than0.2%below9keV, an energyresolution (full width at half maximum) of around200eV, and a noise level of roughly120eV which can provide potential merit of220eV energy threshold. These goodperformances are able to meet the demand of dark matter research below10GeV·c-2low mass region.A data analysis method based on time relationship and pulse shape characteristicswas established to filter noise and microphonics events. Data quality check, physicsevent selection and corresponding efficiency correction are carried out to derive thespectrum of selected physics events.With the experiment data of42days’ livetime, physics interpretation was carefullycarried out using elastic scattering theory and the maximum gap statistical method. Theexclusion curve in the parameter space of cross section and dark matter mass was plotted.The result shows the most sensitive behavior of dark matter searches at2-4GeV·c-2in the world.On the other hand, CDEX is upgrading more massive germanium detectors toachieve better sensitivity. A1kg P-type Point Contact Germanium detector was set upand tested. Several new analysis methods were summarized for the larger detector,which could be a good reference for its upcoming dark matter research.The attempt of dark matter detection using20g-ULEGe in the low mass regionincluded in this dissertation is the first dark matter direct detection in China. The mostsensitive result within2-4GeV·c-2was achieved with0.207kg·day’s effective data.This work underlies further dark matter research of the CDEX new phase project.