Temperature and voltage dependence of electrostatic collection efficiency for radon and thoron progeny in hemispherical chambers

Although electrostatic collection systems based on alpha spectrometry are widely used for radon monitoring, their response to environmental conditions and applicability to thoron progeny measurements are not well understood. In particular, the combined influence of electric field strength, chamber configuration, and temperature on collection efficiency requires further investigation through experimentation. This study experimentally evaluated electrostatic collection efficiency using a reference chamber and a larger hemispherical chamber operated under controlled radon and thoron atmospheres. Measurements were performed as a function of applied high voltage and temperature, considering configurations with and without a diffusion barrier. Collection efficiency increased with applied voltage for both radon (218Po) and thoron (216Po) progeny, confirming the key role of electric field strength in promoting ion transport towards the detector. Temperature-dependent measurements revealed configuration-dependent and nonlinear behavior. For 216Po, opposite trends were observed depending on the presence of the diffusion barrier, whereas for 218Po, a reproducible U-shaped dependence was observed, with a minimum at intermediate temperatures. This behavior can be explained by competing processes affecting ion transport and charge survival, including ion drift, neutralization, and decay losses. A simplified thermodynamic framework was used to qualitatively relate temperature variations to possible changes in relative humidity. However, the absence of direct humidity measurements limited the possibility of a quantitative interpretation. These results provide experimental insight into how operating conditions and chamber design coupled together affect electrostatic collection efficiency. This highlights the need for environment-specific calibration strategies in radon and thoron monitoring applications.