A series of high-sensitivity radon detection systems and their applications in nitrogen as well as the boil-off liquid nitrogen measurement.

In modern rare-event search experiments such as neutrino experiments and dark matter search experiments, radon is one of the most important radiation backgrounds since it can emanate from nearly all the materials containing radium and migrate freely in the experiment system. To support the China Dark Matter Experiment (CDEX) at China Jinping Underground Laboratory (CJPL), a series of high-sensitivity radon detection systems with different electrostatic collection chambers were designed, and radon in nitrogen as well as boil-off liquid nitrogen was measured after accurate calibration and enrichment. Results showed that the calibration factors were 2.1 ± 0.2 (counts/h)/(Bq/m3), 21.1 ± 0.7 (counts/h)/(Bq/m3), 186.2 ± 2.2 (counts/h)/(Bq/m3), 387 ± 7 (counts/h)/(Bq/m3) and the 90% confidence level detection limits were 27.22 mBq/m3, 1.89 ∼ 3.06 mBq/m3, 0.41 ∼ 0.68 mBq/m3, 0.44 mBq/m3 for CJPL-HR2, CJPL-HR20, CJPL-HR140 and CJPL-HR300 measurement systems, respectively. Combined with an enrichment system consisting of twenty g CarboACT activated charcoals in a cold trap, the lower level detection limit (LLD) of typical No.1 CJPL-HR140 could reach 1.8 μBq/m3 with three days' enrichment time and three days' measurement time at 20 L/min sampling flowrate. For verification and application, the radon activity concentrations in nitrogen were 0.6 ∼ 1.9 mBq/m3 with an average of 1.1 ± 0.1 mBq/m3.While in boil-off liquid nitrogen, the radon activity concentrations ranged from 0.04 to 0.62 mBq/m3, and they were significantly lower in old-decayed liquid nitrogen compared to newly-filled liquid nitrogen, with a nearly five-fold decrease.