Dark Matter Direct Detection Research with CDEX-1B Point-Contact High Purity Germanium Detector

L. T. Yang , Tsinghua University(2017)

        Compelling evidences from astronomical observations have demonstrated the existence of dark matter in the Universe, which plays a very important role in understanding the origin of mass and the evolution of the Universe. Weakly Interacting Massive Particle (WIMP) is one kind of the most popular dark matter candidates. In recent years, several direct detection experiments have claimed the discovery of suspected darkmatter signals around 10 GeV/c2, which leaded to conflicts with other experiments. Therefore, the detection of low mass dark matter has become a hot topic in dark matter research. This dissertation work focuses on direct detection of dark matter with CDEX-1B, which is an upgraded point contact high purity germanium detector (PCGe) based on the first one 1 kg-scale mass PCGe prototype in the world(CDEX-1A).

This dissertation work aims to establish a direct detection system with a kg-scale PCGe with extremely low energy threshold for dark matter searching, and significantly improve the sensitivity on the low mass dark matter. It is of great significance that the designing and study of the extremely low energy threshold PCGe will laid an important foundation for the future detectors array.

Based on CDEX-1B PCGe detector, a new direct detection system for dark matter searching was designed and developed, including the NaI(Tl) Anti-Compton detector, data acquisition system,shielding system and monitoring system. Then the performance of CDEX-1B system has been studied. For the first time, an energy threshold lower to 160 eVee with a kg-scale PCGe was achieved. The system has run for 3.3 years continuously and stably, with 783 day’s valid background data obtaining.

This dissertation work establishes a whole set of data analysis processing, especially develops a Bulk and Surface events discrimination method (Ratio Method) based on the probability density function (PDF) of rise time distribution and the count ratios, which not only lower the energy analysis threshold to 160 eVee but also greatly suppress the surface events background level. With 783 day’s valid background data, the physics energy spectrum corresponding to an exposure of 737.1kg·days was obtained through the data analysis. There s idual energy spectrum was obtained by subtracting the contributions of characteristic X-rays from the cosmogenic nuclides inside PCGe, showing an obvious rise at energy below 2 keVee.

The exclusion plots at 90% confidence level (90% C. L.) for spin-independent (SI) and spin-dependent(SD) were obtained by considering the WIMP elastic scattering model and Binned Poisson Method, pushing the detection limit of dark matter searching with a single kg-scale PCGe down to 2 GeV/c2. For spin-independent cross section, the most sensitive result at WIMP mass less than 6 GeV/c2 achieved during experiments with the same detection technology. And for spin-dependent cross section, the most stringent limits at the mass less than 4 GeV/c2 were given.

With very low energy threshold and better rise time resolution, many fine features have been observed on CDEX-1B, including the Compton steps at low energy, ultra-fast events (Very-Bulk Events), etc., especially for the first time that Compton steps at low energy was seen in a Germanium detector. The appearance of these fine features is particularly important for the background understanding in the direct detection of dark matter.
Keywords: point-contact high-purity germanium detector; low energy threshold; dark matter;directdetection


Dark Matter Direct Detection Research with CDEX-1B Point-Contact High Purity Germanium Detector.pdf