Isobar separation of actinides in mass spectrometry by the combination of laser photodetachment with ion-gas interactions

Isobaric interference is a major limitation of mass spectrometric measurements of trace radionuclides. For accelerator mass spectrometry (AMS), isobaric separation is only available up to the mass range of fission products. The present work explores the potential of ion-laser interaction mass spectrometry (ILIAMS) for trace analysis of anthropogenic actinides with isobaric interference. Such capabilities are crucial for characterizing a highly sought-after isotopic spike material for 237Np measurements and for accessing additional anthropogenic actinides with AMS, which could serve as environmental tracers, emission source signatures, or for determining the age of nuclear materials. ILIAMS is a novel low-energy isobar separation technique that combines a gas-filled ion cooler with reactive gases or high-power lasers to suppress isobars selectively. In this study, we demonstrate that UF4– can be selectively suppressed by two orders of magnitude using a 637 nm laser without affecting NpF4–. Initial results indicate the potential for the selective suppression of AmF5– to measure PuF5– or, in reverse, the suppression of PuF4– to measure AmF4– using a 355 nm laser. The admixture of O2 with the buffer gas of the ion cooler can be used to suppress UF4– by up to seven and NpF4– by up to three orders of magnitude against PuF4–. The first application of these separation schemes for the characterization of a prototype 236Np spike demonstrated the successful chemical removal of the co-produced isobars 236U and 236Pu, and similar measurements can now be performed for other prospective Np spike materials. The isobar separation schemes developed here can also enable the measurement of 241Pu without chemically removing 241Am or 242mAm in the presence of 242Pu. Even measuring 238Pu using AMS has become feasible for suitable sample matrices, despite the presence of the primordial isobar 238U. These are important isotopic signatures for attributing environmental contamination to potential sources of emissions.