Estimation Of Isotopic Ratios Of Fission Products By Using The Monte-Carlo Method

Activity ratios of fission products detected at radionuclide stations of the international monitoring system (IMS) are used to not only discriminate a nuclear test from civil nuclear releases, but also determine the explosion time under assumed scenarios. 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. Activities collected in a sample are determined by spectrum analysis. It is not a linear relationship between activities collected in the sample and activity concentrations in the air plume passing the IMS station. Non-linear relationships of isotopic ratios could also be caused by the division algorithm when the activity concentration in a denominator has a larger measurement uncertainty. Covariances between isotope concentrations might be another reason for non-linearity. Correlations of two concentrations could be caused by decay corrections of a parent-daughter chain, interference corrections between two isotopes and subtractions of the same detector background measurement. This presentation demonstrates Monte-Carlo procedures estimating the probability distributions of isotopic ratios, based on input distributions related to spectrum measurements, calibration data and decay chains. Then the isotopic ratio, its uncertainty and associated limits of the coverage interval are estimated by using numerical or analytical solution. The methods and results for three typical isotope pairs are given, e.g. Ba-140 and La-140, Zr-95 and Nb-95, and Xe-133 and Xe-131m. Furthermore, the explosion time and its uncertainty can be estimated in the same way, if applicable.