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

Abstract

The long-lived radioisotope Hf-182 (T-1/2 = 8.9 Ma) is of high astrophysical interest as its potential abundance in environmental archives would provide insight into recent r-process nucleosynthesis in the vicinity of our solar system. Despite substantial efforts, it could not be measured at natural abundances with conventional AMS so far due to strong isobaric interference from stable W-182. Equally important is an increase in ion source efficiency for the anions of interest. 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) ion cooler. 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 facilitate anion separation via chemical reactions with the buffer gas. We present first results with this approach on AMS-detection of Hf-182. With He + O-2 mixtures as buffer gas in the RFQ, suppression of (WF5-)-W-182 vs (HfF5-)-Hf-180 by >10(5) has been demonstrated. Mass analysis of the ejected anion beam identified the formation of oxyfluorides as an important reaction channel. The overall Hf-detection efficiency at VERA presently is 1.4 parts per thousand and the W-corrected blank value is Hf-182/Hf-180 = (3.4 +/- 2.1)x10(-14). In addition, a survey of different sample materials for highest negative ion yields of HfF5- with Cs-sputtering has been conducted. Show more