Study on a Method for Measuring Ultra-Low Concentrations of 222Rn and 220Rn Based on Enrichment of Activated Carbon by an Electrically Cooled System

In neutrino and dark matter research, 222Rn and its progeny are among some of the main sources of radioactive backgrounds in underground laboratories. In areas with high 232Th backgrounds, the interference from 220Rn and its progeny must also be considered. This study presents a highly sensitive system for measuring ultra-low concentrations of 222Rn and 220Rn, utilizing the electrostatic collection method with a Si-PIN semiconductor detector. To achieve the required sensitivity, a cryogenic enrichment stage using activated carbon in an electrically cooled device (replacing conventional liquid-nitrogen cooling) is incorporated. The calibration factor for 222Rn is (67.93 ± 2.76) cph/(Bq·m−3), while for 220Rn, it is (46.08 ± 0.59) cph/(Bq·m−3). The detection limits of the complete system are 5.63 mBq·m−3 for 222Rn and 3.56 mBq·m−3 for 220Rn. The system was successfully applied to measure 222Rn contamination in commercial high-purity nitrogen cylinders, revealing that internal rusting of the cylinder walls is a dominant factor enhancing radon emanation. This electrically cooled, liquid-nitrogen-free design offers a practical and convenient solution for ultra-low-background radon measurements in underground physics experiments and industrial gas quality-control applications.