Environmental dependence of geometrical efficiency for the scintillation cell in radon and thoron measurement

²²²Rn is recognized as a matter of international concern for human health risk. Because ²²⁰Rn as well as ²²²Rn coexist in the natural environment, thoron sometimes influences the experiment for radon measurement. It is important to measure radon and thoron separately to evaluate the risk of the exposure to ²²²Rn. As a discriminative measurement method for ²²²Rn and ²²⁰Rn, a simple technique with a single scintillation cell is well known. However, in recent years, the influence of atmospheric environment on the geometrical efficiency of the scintillation cell has not yet been investigated. In this study, environmental dependence of geometrical efficiency for the scintillation cell in ²²²Rn and ²²⁰Rn measurement was investigated using the Lucas type scintillation cell and Monte Carlo particle simulation. It was found that the influence of temperature and pressure on the geometrical efficiencies were larger than that of relative humidity.