GEM Algorithm Advancements to
Improve Reprocessed Uranium Enrichment Measurements

A lanthanum bromide (LaBr3(Ce)) detector coupled to a digital multichannel analyser and gamma enrichment measurements (GEM) spectrum analysis software currently is the IAEA “workhorse” medium-resolution gamma spectrometer employed for uranium enrichment verification. As the associated GEM spectrum fitting algorithm provides a high-quality fitting with low systematic error, it makes only a minor contribution to the total measurement uncertainty across various enrichments and material types, including materials with moderate content of reprocessed uranium (RepU). Recent tests on blends containing elevated amounts of RepU (232U content higher than 0.5 ppb) showed a degradation of the spectrum fitting quality that led to a noticeable systematic error and, hence, biased results overestimating the 235U enrichment. The issue stemmed from spectral interferences caused by gamma emissions of 232U decay daughters in the spectrum analysis range from 130 to 290 keV. Apart from a set of discrete-energy gamma rays, out of which only the most intense line of 212Pb at 238.6 keV was considered in the standard GEM algorithm, a substantial interference was found to be also due to a cumulative backscatter peak of intense gamma rays of 208Tl and 212Bi with energies from 0.51 to 2.61 MeV. As a result of this work, the GEM algorithm was upgraded to include a complete set of discrete low-energy lines and a cumulative backscatter peak from the higher energy lines of 232U decay daughters. MCNP simulations were used to determine the backscatter peak shape for the IAEA standard LaBr3(Ce) detector and collimator setup. The simulated backscatter continuum was approximated analytically and its amplitude was linked to the area of the 238.6 keV peak, thereby forming a combined 232U spectrum fitting component. The modified GEM algorithm was field tested using a set of RepU material items containing substantial amounts of 232U. The testing results showed a significant improvement of the spectrum fitting quality and unbiased uranium enrichment results.