PEAK FITTING APPLIED TO LOW-RESOLUTION ENRICHMENT MEASUREMENTS*

primarily of weight and uranium enrichment measurements. Most LEU processing facilities draw separate materials balances for each enrichment handled at the facility. The enrichment measurement determines the isotopic abundance of the 235U, thereby determining the proper enrichment strata for the item, while the weight measurement generates the primary accounting value for the item. Enrichment measurements using the passive gamma radiation from uranium were developed for use in U.S. facilities a few decades ago.[1] In the U.S., the use of low-resolution detectors was favored because they cost less, are lighter and more robust, and do not require the use of liquid nitrogen. When these techniques were exported to Europe, however, difficulties were encountered. Two of the possible root causes were discovered to be inaccurate knowledge of the container wall thickness and higher levels of minor isotopes of uranium introduced by the use of reactor returns in the enrichment plants. The minor isotopes cause an increase in the Compton continuum under the 185.7 keV assay peak and the observance of interfering 238.6-keV gamma rays. The solution selected to address these problems was to rely on the slower, more costly, high-resolution gammaray detectors when the low-resolution method failed. Recently, these gamma-ray-based enrichment measurement techniques have been applied to Russian-origin material.[2] The presence of interfering gamma radiation from minor isotopes was confirmed. However, with the advent of fast portable computers, it is now possible to apply more sophisticated analysis techniques to the low-resolution data in the field. Explicit corrections for Compton background, gamma rays from 236U daughters, and the attenuation caused by thick containers can be part of the least-squares fitting routine. Preliminary results from field measurements in Kazakhstan will be discussed.