Speaker
Description
Isobaric interferences have restricted the applicability of Accelerator Mass Spectrometry (AMS) for ultra-trace measurements to certain long-lived radionuclides up to the mid-mass range of fission products or whose isobars do not form negative ions.
We investigated two methods to extend isobar separation capabilities to the actinides based on the formation of and interactions with negative ion beams. Anion Formation Isobar Analysis (AFIA) utilizes element-specific AnF$_4^-$/AnF$_5^-$ (An = U, Np, Pu, Am) formation ratios in a Cs-sputter ion source. These ratios differ by an order of magnitude between adjacent actinides; significant interference thus results in a measurable deviation. While AFIA provided a first isobar screening capability, it is susceptible to false positives from complex isobar mixtures. By contrast, Ion-Laser InterAction Mass Spectrometry (ILIAMS) provides higher sensitivity and reliable results independent of sample composition. The anion beam is decelerated in a gas-filled radiofrequency quadrupole ion guide for residence times of milliseconds. Then, high-powered lasers or reactive gases can be used to selectively suppress isobars. We demonstrated selective UF$_4^-$ removal by a 637 nm laser. Alternatively, adding 3% O$_2$ to the He buffer gas suppresses both UF$_4^-$ and NpF$_4^-$, but not PuF$_4^-$. This approach enabled the preliminary analysis and validation of a $^{236}$Np isotopic spike, facilitating AMS measurements of the anthropogenic actinide $^{237}$Np. Further isobar separation schemes have been explored for the measurements of $^{241}$Pu, $^{241}$Am and – if combined with robust chemical separation of $^{238}$U - even $^{238}$Pu. These radionuclides can provide important source term or age information for environmental samples.
The research was supported by the Austrian Science Fund grant I 4803-N and a Dimitrov Fellowship of the Austrian Academy of Sciences.