Nuclear analytical application of thermionic mass spectrometry

The relative uncertainty on the isotope abundance ratio measurements of uranium and plutonium samples by means of thermionic mass spectrometry at the Central Bureau for Nuclear Measurements (CBNM) in Geel, Belgium, has decreased to a level of about 2x10-4. The improvement was mainly achieved through the preparation of synthetic isotope mixtures of uranium and plutonium, to a relative uncertainty of 0.01% (computed on a 2s basis) on the ratios of isotopes with major abundances. This allowed to determine some error sources more precisely, such as: isotope fractionation, and non-linearity of the ion beam current measuring system. As a consequence CBNM is able to prepare certified uranium isotopic reference materials (U IRM's) for distribution with a relative uncertainty of 0.07% (computed on a 2s basis) on the 235U isotope abundance and to provide reference values on samples for the (European) Interlaboratory Measurement Evaluation Programmes (REIMEP). (orig.)