Application of automated particle screening for effective analysis of individual uranium particles by thermal ionization mass spectrometry

Abstract

The isotope ratio analysis of individual uranium particles in environmental samples taken at nuclear facilities is important to clarify their origins for nuclear safeguards. Secondary ion mass spectrometry (SIMS) is often used for this purpose. An automated particle measurement (APM) screening software was recently developed for SIMS instruments, which enables us to obtain scanning ion images of uranium isotopes over the sample in short duration. The positions and approximate isotope ratios of each uranium particle can be determined from the images. This makes SIMS more effective because a few uranium particles with irregular isotopic compositions among thousands of uranium particles with normal isotopic compositions can be screened prior to precise isotope ratio analysis. However, the formation of molecular and/or hydride ions often leads to spectral interferences and inaccurate results in SIMS. In the present study, APM screening was applied to select uranium particles prior to precise isotope ratio analysis by thermal ionization mass spectrometry (TIMS). As a result, it was demonstrated that the APM-TIMS method eliminated molecular ion interferences in the uranium mass region in the analysis of real inspection samples, while higher and unreasonable 234U and 236U atomic ratios for some particles were obtained by APM-SIMS.