Matrix-assisted production of actinide molecular anions for AMS
Dr Barbara Francisco1, Dr Xiaolei Zhao1
1University Of Ottawa, Ottawa, Canada
The multiple probable oxidization states of actinide elements can lead to their molecular anions to result in complicated yield patterns from a Cs+ sputter ion source. For actinides AMS, two methods can be practiced in order to concentrate the yields into just one or two anion species. The present common practice has been to deprive the availability of oxygen in the sputter targets, so that the mono-oxide anions are produced with good or applicable ionization efficiencies. However, this method requires the typical 860-type Cs+ sputter ion sources to be operated with relatively high Cs+ fluxes, a condition that not all such sources in use can be operated with a lasting stability. On the contrary, with low Cs+ fluxes, good or applicable ionization efficiencies can also be achieved for producing element-specific poly-atomic oxide, oxyfluoride, or fluoride anions of actinides, if the sputter targets are aided with dominantly oxidizing or fluorinating matrices. We describe a systematic study of such phenomena using ZnO and FeF₃ to form the respective oxidizing and fluorinating matrices. It was found that using element-specific polyatomic atomic fluoride anions and FeOxHy+FeF₃ sputter targets, not only all rare actinides could be measured with low detection limits matching those by mono-oxides, but also with the available ionization efficiencies fully utilized within a shorter time. In addition: sample chemistry is simplified without the need for high temperature calcination; the lack of triply charged hydride ions and mono-isotopic fluorine help in achieving better ²³⁶U/²³⁸U abundance sensitivity using just one large analyzing magnet in the high energy system of AMS; scatter interferences of ²³²ThF₆- to ²³¹PaF₆- detection, and ²³⁸UF₄- to ²³⁷NpF₄- & ²³⁹PuF₄- detection, are naturally lower by an order of magnitude because Th and U molecular anions are concentrated into ThF₅- and UF₅-; and finally, measurement background is two orders of magnitude less sensitive to any long-lasting ion source memories. In conclusion, the use of element-specific poly-atomic fluoride anions and FeOxHy+FeF₃ sputter targets is a viable alternative approach to actinides AMS, especially when low-output Cs+ sputter ion sources are used.
Key Words: actinides AMS; matrix-assisted sputtering; FeF₃ matrix; poly-atomic fluoride anions; ionization efficiency; detection limits; Cs+ sputter ion source memory effects;
Research Scientist and Laboratory Coordinator at AEL-AMS, University of Ottawa. Research focus in Actinides and Fission Products.