Critical concentration of the exotic nuclei in the 232Th chain for the alpha analysts

Tóm tắt

The 232Th (Thorium) isotope and its α-decay daughters are ubiquitous in soil, rock, and water environments that are harmful to living organisms if accidentally up-taking. Therefore, it is necessary
to detect these isotopes and measure their concentrations in the environments in order to protect
living organisms from the contaminated areas. In the measurements using alpha analyzers, the estimation of the critical concentration of the isotopes in the samples is highly demanded to confirm
the feasibility of the analysis. In this work, we evaluated the critical concentrations of the radioactive
nuclei in the 232Th chain based on their half-lives and radioactivities for the alpha analysis. In particular, we examined the α-decay half-lives of the mentioned isotopes by using the semi-empirical
formulae proposed by Viola-Seaborg, Royer, and Poenaru to estimate their radioactivities and decay constants. The predicted half-lives were then normalized by their average values and compared
with the NuDat data of the National Nuclear Data Center (Brookhaven National Laboratory). The
relationships between the estimated half-lives and the NuDat data are deduced as linear functions.
The critical concentration of the isotopes is determined based on the decay constants following
the decay rule. The results show that most of decay half-lives are in consistent with those obtained
from the NuDat data except for the multi-decay-mode isotopes. However, there is a large variation
in the average values, 15% - 95% (in decimal logarithmic scale), of the half-lives evaluated by different formulae, that consequently leads to large decay constants deviations of 1% — 120% from the
average values. From these results, we confirmed that the large radioactivity uncertainty because
of the half-life deviations should be concerned for the preparation of the environmental samples
when using alpha spectrometers. By assuming an efficiency of 100%, we found that the critical
concentration for the alpha analysts of the 232Th nucleus is in a range of 1.5 − 2.5 µ g/(l or kg) in
the samples.