Negative ionisation efficiencies for 10Be, 26Al and Pu with MC-SNICS at ANSTO
Dr Klaus Wilcken1, David Child1, Dr. Michael Hotchkis1, Michael Mann1, Krista Simon1, Dominik Koll2,3, Prof. Anton Wallner2,3, Thilo Hauser4, Richard Kitchen4
1ANSTO, Lucas Heights, Australia, 2Australian National University, Canberra, Australia, 3HZDR, Dresden, Germany, 4National Electrostatic Corporation, Middleton, USA
Low overall detection efficiency for actinides and cosmogenic isotopes (Al, Be) is the limiting factor affecting precision and sensitivity for applications where the amount of available sample material is small and/or rare isotope concentration is low. Due to low ionisation efficiencies for these isotopes it is not uncommon that more than 99% of the rare isotopes in the sample do not contribute to the statistical precision of the measurement. Optimising ion transmission and detection efficiency in the AMS measurement offers some room for improvement but these avenues are already close to their theoretical limits.
On the other hand, optimising the performance and operation of the negative ion Cs-sputter sources has significant scope for improvement but is challenging. One often needs to compromise between competing requirements, for example, maintaining high sputtering rate to allow expedient consumption of the sample material but at the same time keeping the source insulators clean for longevity. The lack of a well-understood theoretical model for the negative ionisation process adds to the engineering challenges.
Negative ionisation efficiencies above 30% have been demonstrated for radiocarbon  but remain often more than an order of magnitude lower for Be, Al and actinides. This is sometimes taken to be an inherent limitation of the technique, rather than a challenge to be addressed.
Here we present details of the modified MC-SNICS sources at ANSTO, including engineering modifications that have improved longevity and stability. With attention to a combination of ion source running conditions, sample masses and sample binders the total efficiency for Pu measurements was increased up to 1.5%, corresponding to a negative ionisation yield of 4%. For Al- and BeO- negative ion source yields are 0.2% and 3%, respectively.
S.J. Fallon, et al., Nuclear Instruments and Methods in Physics Research B 259 (2007) 106–110
Biographies to come