Negative ionisation efficiencies for 10Be, 26Al and Pu with MCSNICS at ANSTO
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Date
2021-11-17
Journal Title
Journal ISSN
Volume Title
Publisher
Australian Nuclear Science and Technology Organisation
Abstract
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 [1] 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 Aland BeO- negative ion source yields are 0.2% and 3%, respectively.
Description
Keywords
Ions, Beryllium 10, Aluminium 26, Plutonium, ANSTO, Actinides, Isotopes, Mass spectroscopy
Citation
Wilcken, K., Child, D., Hotchkis. M., Mann, M., Simon, K., Kill, D., Wallner, A., Hauser, T., & Kitchen, R. (2021). Negative ionisation efficiencies for 10Be, 26Al and Pu with MCSNICS at ANSTO. Poster plus presentation to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021, (pp. 210). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf