Development of a SIMS method for isotopic measurements in nuclear forensic applications

Methodologies based on secondary ion mass spectrometry (SIMS) for isotopic measurements in nuclear forensic applications relevant to the age determination of Pu particles and isotopic composition of oxygen for geolocation assignment are described. For the age determination of Pu particles, a relative sensitivity factor (RSF) to correct for the different ionisation efficiencies of U and Pu, was obtained by analysing standard Pu materials with known ages. An RSF of 2.41 +/- 0.05 was obtained for PuO2 from measurements on samples with different Pu/U ratios. In a sample of known origin, using this RSF value, the age calculated from the Pu-238/U-234 and Pu-240/U-236 ratios agreed well with the reported age of 2.3 years. For geolocation assignment, a new approach based on the measurement of differences in the natural abundance of O-18 and O-16 isotopes and their ratio was developed. The instrumental mass discrimination of the O-18/O-16 ratio was determined using three O-isotope samples of different chemical composition. The measured precision (the standard error of 100 cycles/analysis) obtained for the oxygen isotopic measurement on the samples was typically +/-1.1parts per thousand.