Measurement of radon exhalation rate from the surface of cavity and effect analysis of waterproof and radon suppression project in China Jinping Underground Laboratory phase II

<p indent="0mm">Radon is a major contributor to environmental radiation in underground laboratories. The Deep Underground and Ultra-low Radiation Background Facility for frontier physics experiments (DURF), a National Major Science and Technology Infrastructure Project, is being built based on phase II of the China Jinping Underground laboratory (CJPL-II). Through the installation of a new ventilation system and a waterproof radon-suppression layers, radon in the underground lab air is effectively controlled. To assess the effectiveness of the radon suppression project, including the impact of the waterproof radon-suppression layers on radon exhalation from the surrounding rock, as well as its overall effectiveness in reducing air radon concentrations, a surface radon exhalation rate detector was employed to measure radon exhalation rates from various surfaces of the waterproof radon-suppression layers. Additionally, radon concentrations and gamma background from radon progeny were measured using the AlphaGuard and a portable high-purity germanium spectrometer before and after the radon suppression project. The results revealed that following the installation of the waterproof radon-suppression layers, the surface radon exhalation rate dropped to less than 1% of the rock exhalation rate, with all experiment halls showing exhalation rates below 0.1 mBq m⁻² s⁻¹. The count rates of the characteristic peaks of ²¹⁴Pb and ²¹⁴Bi in the <italic>in-situ</italic> gamma spectrum were reduced by about 60%, and the contribution of radon progeny ²¹⁴Pb and ²¹⁴Bi to the characteristic peaks in the spectrum decreased by approximately 20%–40%. The radon concentration in the experiment halls decreased to <sc>(20.3±0.1) Bq m^‒³,</sc> roughly one-tenth of the pre-construction level of <sc>(208.0±0.1) Bq m⁻³.</sc> These findings demonstrate that the radon suppression project successfully lowered radon and its progeny concentrations, laying a strong foundation for maintaining a low-background environment in the Phase II laboratory.