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

Abstract

The long-lived radioisotope 182Hf (T1/2 = 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 182W. 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 182Hf. With He +O2 mixtures as buffer gas in the RFQ, suppression of 182WF5− vs 180HfF 5− 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 efficiency at VERA presently is 1.4% and the W-corrected blank value is 182Hf/180Hf = (3.4 ± 2.1)×10−14. In addition, a survey of different sample materials for highest negative ion yields of HfF 5− with Cs-sputtering has been conducted.

5 Years of Ion-Laser Interaction Mass Spectrometry—status and Prospects of Isobar Suppression in Ams by Lasers

“…However, ISA isobar suppression seems to be ultimately limited by a plateau effect (Eliades et al 2015). We observe similar behavior in our setup when running the system without lasers and using reactive gases instead of (Martschini et al 2020). In our interpretation, this plateau effect is determined by the likelihood of reverse reactions when the chemical equilibrium inside the ion cooler is reached.…”

“…182 Hf Attempts to measure 182 Hf of astrophysical origin at environmental concentrations with VERA have started early on but were hindered by isobaric background from 182 W (Vockenhuber et al 2004;Forstner et al 2011). The status of 182 Hf AMS with ILIAMS has been recently summarized in (Martschini et al 2020). Both HfF 5 − and its isobaric species WF 5…”

r-Process Radioisotopes from Near-Earth Supernovae and Kilonovae

“…Combining these numbers, we find an upper limit on the fraction by weight of 182 Hf of 8 × 10 −12 . Martschini et al (2020) showed that these limits could in principle be improved dramatically using a laser technique to remove stable isobars. They find 182 Hf/ 180 Hf = (3.4 ± 2.1) × 10 −14 , on the basis of which they estimate a sensitivity of ( 182 Hf/ 180 Hf) min ≈ 6 × 10 −14 .…”

Deep-Sea and Lunar Radioisotopes from Nearby Astrophysical Explosions

“…Recent progress in AMS for the detection of 182 Hf seems promising (173)(174)(175). However, the present measurement background in AMS for other nuclides as identified by Ellis et al (60), such as 107 Pd or some p-process nuclides (such as 92 Nb, 146 Sm) remains too high compared to the expected interstellar signature.…”