Year
2008
Abstract
The enrichment of items containing thick homogeneous uranium materials of known chemical composition is routinely determined non-destructively by applying the Enrichment Meter Principle. For radio-pure materials in secular equilibrium (aged so that the decay rate of Th-234 is equal to that of U-238 which in the case of reprocessed material also implies the complete decay of U-237 from Pu-241) the shape of the spectrum beneath the U-235 peaks at around 186 keV is effectively constant and so low resolution gamma spectroscopy may be applied over the whole range of interest, that is from depleted uranium (circa 0.2wt% U-235/U) to very highly enriched uranium (> 98wt% U-235%). A simple two region-of-interest method of spectral analysis is practical: one region covers the 186keV peaks from U-235 and the other, located at higher energies, is used to scale the underlying U-238 related background and continuum. The spectral interpretation involves two parameters; a constant of proportionality and a scale factor for the underlying region. In principle two standards are sufficient to extract the two parameters. In the special case of only two standards the solution is also analytical and the uncertainties may be propagated in a straight forward way (in other words multiple samples and elaborate curve fitting is not required). In this work we develop the necessary solution including the treatment of covariance terms and apply it to a test data set taken on sources up to nominally 4.5wt% enrichment.