Study of a low-background detection system for surface radon emanation measurement for the CDEX-50 experiment

Abstract: Rare-event searches such as dark matter detection and neutrinoless double beta decay demand extremely stringent control of ambient backgrounds. Radon and its progeny are among the dominant background sources in CDEX-50: radon emanating from the inner walls of the experiment chamber can enter the liquid nitrogen shielding medium, and the decays of the resulting progeny contribute a radiation background that must be accurately evaluated. To this end, we developed a highly sensitive radon monitor and a sample radon emanation measurement system based on the electrostatic collection method, for monitoring the radon concentration in the liquid nitrogen and measuring the radon emanation rate of materials, respectively. The high-sensitivity radon monitor has a background count rate of $(0.88 \pm 0.09)$~cph in the $^{214}\mathrm{Po}$ channel, corresponding to a sensitivity of $0.09\ \mathrm{mBq/m^3}$ at 95\% C.L. The sample radon emanation measurement system achieves a 95\% C.L. upper limit of $170\ \mu\mathrm{Bq}$ on the radon emanation rate for a measurement time of 61.9~h. With these systems, the surface radon emanation rate of the CDEX-50 experiment chamber was measured to have a 95\% C.L. upper limit of $10.65\ \mathrm{mBq}$. Assuming uniform distribution of the emanated radon in the liquid nitrogen, the corresponding radon concentration is $4.6\ \mu\mathrm{Bq/kg}$, which accounts for approximately 2.0\% of the total background level. This indicates that the background contribution from radon emanation of the experimental chamber is negligible for CDEX-50.