Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Laser photodetachment and molecular dissociation processes of anions provide unprecedented isobar suppression factors of >10¹⁰ for several established AMS isotopes like ³⁶Cl [1] or ²⁶Al [2] and give access to new AMS isotopes like ⁹⁰Sr [3], ¹³⁵Cs [4] or ¹⁸²Hf [5] at environmental levels. Five years ago, a setup for Ion-Laser InterAction Mass Spectrometry (ILIAMS) was coupled to the Vienna Environmental Research Accelerator (VERA) five years ago. Its potential and applicability as a new means of isobar suppression in AMS has since been explored at this state-of-the-art 3 MV tandem facility [6]. Over this time, ILIAMS has been proven to meet AMS requirements regarding efficiency, reliability and robustness with a typical reproducibility of results of 3%.

The benefits of the ILIAMS technique are in principle helpful for any AMS machine, irrespective of attainable ion beam energy. ILIAMS 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, although the possibility of reverse reactions may cause some plateau effect not observed with laser photodetachment. In order to achieve the required ion-laser interaction times or ion-gas collision energies, the anion beam is decelerated to almost thermal energies within a gas-filled radiofrequency quadrupole.

Since isobar suppression via ILIAMS is so efficient, there is often no need for any further element separation in the detection setup. Hence, highly-populated charge states can be selected after the accelerator, which in combination with 100% efficient ion detection in an ionization chamber more than compensates for transmission losses in ILIAMS, which are on the order of 20-50%. Thus, counting statistics with ILIAMS are typically similar or better than with conventional AMS means, e.g. 500 cts of ²⁶Al in a 10 min run on a sample with 26Al/Al = 10−¹².

Recent test measurements also demonstrated that, owing to the virtually complete suppression of isobars, ²⁶Al (extraction of AlO−) and ⁴¹Ca (extraction of CaF₃−), can be measured directly from stony meteorite samples as little as 1-2 mg without doing any chemical sample preparation [7]. There is also potential for ILIAMS measurements of terrestrial cosmogenic ²⁶Al in in-situ-dating quartz originating from high altitudes. Last but not least, ³⁶Cl may even become accessible without the need for sulfur reduction by chemical treatment and, maybe, without accelerator at all.

[1] Lachner et al., NIMB 92 (2019) 146.
[2] Lachner et al., IJMS 465 (2021) 116576.
[3] Marchhart et al., this meeting
[4] Wieser et al., this meeting
[5] Martschini et al., EPJ Web of Conferences 232 (2020) 02003.
[6] Martschini et al., NIMB 456 (2019) 213.
[7] Merchel et al., this meeting