Research on precise measurement methods for radon concentration and its progeny particle size distribution

Ubiquitous natural radionuclides, radon and its progeny, are the largest contributors to public radiation exposure from natural sources and the second leading cause of lung cancer, closely linked to radiation safety. Simultaneously, radon is the most significant source of radiation background in rare-event experiments conducted in deep underground laboratories, which are at the forefront of physics research today. Therefore, studies on environmental or ultra-low-level radon measurements hold substantial academic value and practical application significance. This paper presents the progress and achievements of the Radiation Protection Group, School of Physics at Peking University, in radon and progeny measurement research, detailing high-sensitivity radon measurement results for deep underground laboratories and studies on the particle size distribution of radon progeny for dose estimation purposes. According to the test results of the four radon meters with different volumes developed by our team, the overall trend indicates that the sensitivity and average background value of the electrostatic radon meter increase with the expansion of the measurement chamber volume, while the detection limit and decision threshold decrease accordingly. These findings are consistent with theoretical analysis and the conclusions of international peers, demonstrating that this study has successfully achieved the expected technical indicators. For the second part, due to the presence of various forms of radon progeny (222Rn/220Rn) in the environment, such as unattached, nucleation, accumulation, and coarse particle modes, each contributing differently to radiation dose, on-site measurements require distinct separation and sampling methods based on the physical properties of these progeny modes. The progeny aerosols are drawn by a pump into an eight-stage sampling device, with a sampling flow rate of 4 liters per minute (lpm) at each stage. The first stage employs a filter membrane, which collects nearly 100% of particles across all size ranges. The second stage consists of a filter membrane combined with an impactor, designed to capture coarse particle mode progeny with diameters larger than 2.5 μm. The third stage utilizes an impactor paired with a single-layer wire screen, which filters both coarse particle mode and unattached progeny. Stages four through eight incorporate impactors combined with optimally selected SDB wire screen assemblies, enabling size-resolved sampling of nucleation and accumulation mode progeny aerosols. The activity concentration of unattached and coarse particle mode progeny in the environment is determined by successively subtracting the measurement results from the first three stages. For the latter six stages, the activity size distribution parameters, such as the activity median diameter (AMD) and geometric standard deviation (σ), are derived through inversion using the expectation-maximization (EM) algorithm and the Twomey algorithm.