Some subjects on the the measurement of thoron progeny by passive methods

In the evaluation of the exposure dose by radon and thoron in houses, the passive method using solid track detectors has accomplished the important role as the method of measuring mean dose for long period. But in the places where thoron concentration is high, the measurement of the concentration of thoron daughter nuclide is important because of the correction of the measured values of radon concentration and the evaluation of the exposure due to thoron daughter nuclide. In this report, the high sensitivity, electrostatic collection type monitor using a solid track detector CR39, which has been developed by the author, and the problems when the daughter nuclides of radon and thoron are measured simultaneously but separately by utilizing the properties of discriminating alpha ray energy are described. The change of the shape of etch pits due to alpha ray energy and incident angle, the conditions in the discrimination of alpha ray energy, and the separate measurement of the concentrations of radon and thoron daughter nuclides are reported. The structure of the cup for electrostatic collection and the passive monitor are described, and the notice for measuring daughter nuclide concentrations by passive method is given. (K.I.)