The quest for AMS of ¹⁸²Hf – why poor gas gives pure beams

The trace isotope ¹⁸²Hf (T_1/2 = 8.9 Ma) is of high astrophysical interest as its potential abundance in environmental archives would provide rare insight into heavy element nucleosynthesis in recent r-process events in the vicinity of our planet. Despite substantial efforts, however, it could not be measured at its natural abundance level with conventional AMS so far due to strong isobaric interference from stable ¹⁸²W.
The new Ion Laser InterAction Mass Spectrometry (ILIAMS) technique at VERA tackles the problem of elemental selectivity in AMS with a novel approach. It achieves near-complete suppression of isobar contaminants via selective laser photodetachment of decelerated anion beams in a gas-filled radio frequency quadrupole (RFQ). The technique exploits differences in electron affinities (EA) within elemental or molecular isobaric systems neutralizing anions with EAs smaller than the photon energy. Alternatively, these differences in EA can also result in anion separation via chemical reactions with the buffer gas.
In this contribution, we present first results with this approach on AMS-detection of ¹⁸²Hf. With He +O₂ mixtures as buffer gas in the RFQ, suppression of ¹⁸²WF₅^- vs ¹⁸⁰HfF₅^- by >10⁵ has been demonstrated. Mass analysis of the ejected anion beam identified the formation of oxyfluorides as an important reaction channel.
The overall Hf-detection effciency at VERA presently is 1.4 x 10^-3 and the W-corrected blank value is ¹⁸²Hf/¹⁸⁰Hf = (3.4 + 2.1) x 10^-14. In addition, a survey of several sputter materials for highest negative ion yields of HfF₅^- has been conducted.