The Design and Validation of a Zero Boil-Off LAr System for CDEX-10 Experiment

The China Dark Matter Experiment (CDEX) is an experiment of detecting light weakly interacting massive particles by using the p-type point contact germanium (pPCGe) detector. The underground laboratory of the CDEX is situated in Jinping Mountain in Sichuan province of China. In this experiment, a 1200-L liquid argon (LAr) scintillator is employed as anti-coincidence veto detector to maximally rule out the background radiation, and to provide a stable low-temperature surrounding for the pPCGe detector. A self-circulation argon liquefaction system with three Pulse Tube Refrigerators is developed to satisfy the zero boil-off condition of the LAr detector; bubbles formed during system operation are prevented by LAr sub-cooling condition. Meanwhile, an actively cooled LAr shield outside the LAr detector is used to prevent the external heat radiation from propagating inside to keep the required 1.0 K temperature gradient inside the LAr detector. The structure of the cryogenic system and the testing results of this cryogenic system by using liquid nitrogen are presented in the paper. The testing results indicate that the cryogenic system satisfies the dark matter experiment requirements.