Activity ratios of paired isotopes detected at radionuclide stations in the international monitoring system (IMS) network can be used to not only discriminate a nuclear test from civil nuclear releases, but also determine the explosion time under assumed scenarios. The released activities, the activity concentrations in a plume and the activities collected in a sample could be related together through the assumptions of decay evolutions, atmospheric transport modelling (ATM) simulations, the sampling and the measurement. Only the activities in the sample are determined directly by spectra analysis. It is assumed that there is a linear relationship between activities released at a source location and activity concentrations at a measurement station in ATM simulations. For each pair of isotopes, a function of the isotopic ratio with time from the explosion time up to the stop of collection can be derived, based on Bateman equations of given decay chains. However, the activities collected in the duration of sampling are depended on not only exponential decay, but also exponential integration of activities accumulated in the sample. As a consequence, it is not a linear relationship between activities collected in the sample and activity concentrations in the plume during sampling. The functions of the isotopic ratio with time are different before and after the stop of collection. This will impact discrimination of the nuclear test and estimation of the explosion time.
Year
2020
Abstract