Thoron daughter working level measurements by one and two gross alpha-count methods

Abstract

A study has been conducted on the determination of the thoron daughter working level, WL(Tn), by one gross α-count, and of WL(Tn) and the concentrations of ThB, [ThB], and ThC, [ThC], by two gross α-counts. For the one gross α-count the relationship between the gross α-count rate per unit of volume of air sampled and WL(Tn), denoted the F-factor, has been investigated as a function of sampling time, elapsed time from the end of the sampling period, i.e., waiting time, and the thoron daughter disequilibrium ratio [ThC]/[ThB]. It has been found that F depends on both the waiting time and [ThC]/[ThB]. If α-count measurements are made at least 300 minutes after the end of sampling, F changes by less than 10% over the full range of theoretical values of [ThC]/[ThB], i.e., from 0 to 1. The F-factor is independent of [ThC]/[ThB] at approximately 215 min after the end of sampling. This feature can be used to determine WL(Tn) with higher accuracy and at least 1.5 h earlier than is commonly done using other one gross α-count methods reported in the literature. An optimization method based on standard statistical techniques has been followed for the two gross α-count method. Optimum combinations of sampling, waiting and counting times compatible with experimental constraints have been obtained. A sampling time of 10 min and equal counting times of 15 min for the first and second α-counts situated 2 min and 92 min, respectively, after the end of sampling have been used in conjunction with a thoron daughter calibration facility. Comparison of the two gross α-counts method investigated here with other methods shows excellent agreement.