Speaker
Description
Authors: Boxue Liu, Richard Britton, Seokryung Yoon, Ashley Vaughan Davies, Nikolaus Hermanspahn, Gohla Herbert, Jonathan Bare, Peter Jansson, Martin Kalinowski
Affiliation: CTBTO, Vienna, Austria
The current address is Swedish Defence Research Agency, Stockholm, Sweden
The accurate measurement of radioxenon isotopes plays an important role in the radionuclide component of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). There are four isotopes relevant to the CTBT verification regime, 133Xe, 133mXe, 131mXe and 135Xe. Ratios of four radioxenon isotopes can be used to discriminate nuclear explosion sources from releases of nuclear facilities such as medical isotope production as well as to determine the time of detonation under an assumed scenario. The determination of the concentrations of these isotopes relies on a robust calibration method. This paper outlines a calibration method based on four radioxenon spikes. For the beta/gamma coincidence spectrum analysis of the four radioxenon isotopes, the gross number of counts in a Region of Interest (ROI) includes contributions from the radionuclide associated with that ROI, the detector background, interference contributions from other radionuclides (including radon), and the activity from previous samples remaining in the detector cell. The net numbers of counts in ROIs and their associated uncertainties are estimated by the net count calculation (NCC) method. An alternative approach is the regression analysis, such as, the least squares fitting of ROI counts, enabling the deconvolution of X-ray contributions from the different radioxenon isotopes and radon. A combined calibration procedure between the NCC method and least squares fitting is investigated, which could also be used in the maximum likelihood fitting of ROI counts. For this method, spike measurements of four xenon isotopes are performed. The output of the beta-gamma detector system can be read out as three measurement channels: beta-gamma coincidences, beta singles and gamma singles. All three channels detect the same number of radioactive decays but differ in the number of detected emissions in 4π measurement geometry. The detection efficiencies are absolutely determined by comparison among the numbers of the ROI counts from three measurement channels, without the need for a reference value of radioxenon activity. The efficiency of 30 keV X-rays is determined by the 131mXe spike first, then applied to the other radioxenon spikes for determining the gamma efficiencies of associated ROIs. One of the key challenges is to estimate the uncertainty of the net number of counts, particularly considering correlations between beta-gamma coincidences and beta/gamma singles. In this work, the activity values of radioxenon standard sources, e.g., radioxenon spikes, are estimated based on the NCC method first, and then used for calibration of the regression analysis method.
