Decay Corrections in Isotopic Ratio Estimation Based on the Activity Concentration Profile During Sampling for CTBT-Relevant Radionuclides with Short Half-Life

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, play a crucial role in characterizing release events under assumed scenarios. This analysis involves 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 requires considering the post-shot radionuclide evolution of an assumed underground nuclear explosion, the simulations of atmospheric transport modelling, and, finally, the sample measurement. 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 for radionuclides with relatively longer half-lives, that is, exceeding 7 times the sampling duration, challenges arise for the isotopes with short half-lives, such as 135 Xe with half-life of 9.14 hours compared to the 12-hour collection duration for some noble gas systems. Therefore, this study investigates the 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 of 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.