Decay Corrections in Isotopic Ratio Estimation for Short-Lived CTBT-Relevant Radionuclides Based on Activity Concentration Profiles During Sampling

Abstract Isotopic activity ratios of the relevant radionuclides detected at radionuclide stations within the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), are crucial for characterizing release events under assumed scenarios. This analysis involves considering radioactive gases initially released from an underground nuclear event, resulting in radionuclide concentrations in a plume of air passing over an IMS station that are subsequently sampled by that station. Modeling this process requires considering the post-detonation radionuclide evolution of an assumed underground nuclear explosion, simulating atmospheric transport, and finally, collecting and measuring samples. Activities collected in the samples are determined through spectrum analysis of sample measurements, and the activity concentrations are then estimated by assuming a constant concentration during sampling. While this assumption holds true for radionuclides with relatively longer half-lives, exceeding 7 times the sampling duration, challenges arise for isotopes with short half-lives, such as 135 Xe with half-life of 9.14 h compared to the 12-h collection duration for some noble gas systems. Therefore, this study investigates decay correction during sampling using two approaches: 1) Interval constant concentration: the collection duration is divided into multiple intervals, with a constant concentration assumed in each interval; 2) Decaying concentration: the activity collected in the sample is derived based on an analytical solution to the ordinary differential equations governing the activity decay and ingrowth. The impact of these approaches on isotopic activity ratios is demonstrated in three cases with short half-lives: 140 Ba/ 140 La, and 133m Xe/ 133 Xe, and 135 Xe/ 133 Xe. The decay correction on the ratio 135 Xe/ 133 Xe might be approximately 2 for 24-h sampling, and 1.2 for the shortest collection duration of 6 h. For the pair of 133m Xe to 133 Xe, the ingrowth correction might be negligible compared to the relatively large value of the ratio 133 Xe/ 133m Xe, which exceeds 10 in a few days after the detonation. For the ratio 140 La/ 140 Ba, the contribution from the ingrowth of parent-daughter decay is approximately 0.35 for a collection duration of 24 h.