Advancements in plutonium radio-nuclear counting
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This work integrates analysis techniques common to destructive nuclear forensics and non-destructive safeguards in the radio-nuclear analysis of plutonium. This work accomplishes this task by applying deconvolution methodologies typical in alpha spectrometry to the analysis of LaBr₃:Ce spectra. It further integrates and applies uncertainty methodology common to destructive chemistry laboratories to the nondestructive analysis of plutonium. In pursuit of this aim, an alpha spectrometry deconvolution algorithm is developed capable of determining the ²⁴¹Am/Pu mass fraction in plutonium with about a 1% uncertainty with a 95% coverage factor. Results agree very well with published alpha spectrometry and mass spectrometry mass ratios. Model ages calculated from the ²⁴¹Pu/²⁴¹Am chronometer also agree very well with the historical separation dates for reference materials that have been well purified chemically. A sister program is developed capable of determining plutonium mass ratios in spectra obtained with LaBr₃:Ce scintillation detectors. This method is capable of determining the mass ratios with total measurement uncertainties of about 16%, 26%, 10%, and 4% (k = 2) for the mass ratios (relative to ²³⁹Pu) for ²³⁸Pu, ²⁴⁰Pu, ²⁴¹Pu, and ²⁴¹Am, respectively for 15 minute duration spectra of reactor grade plutonium obtained at Los Alamos National Laboratory. Results for weapons-grade plutonium are somewhat worse with uncertainties for the mass ratios ²⁴⁰Pu, ²⁴¹Pu, and ²⁴Am of about 30%, 80% and 10% (k = 2), respectively. The primary issue with this work is a somewhat higher systematic uncertainty for ²⁴⁰Pu content in reactor grade plutonium for spectra taken by the International Atomic Energy Agency (IAEA). Further, a bottom up uncertainty methodology for plutonium non-destructive γ-ray analysis is developed. Unlike the two primary plutonium analysis codes Fixed Energy Response Function Analysis with Multiple Efficiencies (FRAM) and Multi-Group Analysis (MGA), this methodology propagates uncertainty from many different contributors according to the Joint Committee for Guides in Metrology (JCGM) Guide to the Expression of Uncertainty in Measurement (GUM) supplement 1. It is demonstrated this method yields very high fidelity uncertainties for ²³⁸Pu, ²³⁹Pu, and ²⁴⁰Pu content in unshielded coaxial spectra in high statistics spectra when compared to a top down analysis. Finally, this work develops a method to determine much improved plutonium photon emission intensities markedly different from the prior art. It is planned to incorporate these improved emission intensity values and derived uncertainties into the GUM uncertainty model for FRAM.