No abstract

Accurate measurement of thoron activity concentration is an important issue for both thoron exposure evaluation and reducing its influence on radon measurement. For radon monitors based on electrostatic collection technique and alpha spectrometry analyzer, air humidity and sampling flowrate are key factors influencing the sensitivity of thoron activity concentration measurement. For the purpose of improving thoron measurement sensitivity and stability, theoretical derivation and experimental studies were systemically performed in this study. Results show that the thoron measurement sensitivity decreases as a negative exponential function with absolute humidity increasing, and the sensitivity of thoron is much lower than that of radon at the same condition, which is mainly caused by the small value of the concentration ratio of thoron inside to outside of the chamber. When the air exchange rate of the measurement chamber (sampling flowrate/inner volume) increases, the measurement sensitivity of thoron gas first increases rapidly and then decreases slowly after reaching its maximum at the air exchange rate of 0.24s-1. In practice, in normal air exchange rate range (for example <0.05s-1), increasing the sampling flowrate could improve the thoron measurement sensitivity greatly, which suggests an effective way for updating thoron measurement under the present condition of the monitor, consequently.