Highly sensitive 26Al measurements by Ion-Laser-InterAction Mass Spectrometry

Johannes Lachner,Martin Martschini,Andreas Kalb,Michael Kern,Oscar Marchhart,Felix Plasser,Alfred Priller,Peter Steier,Alexander Wieser,Robin Golser

doi:10.1016/j.ijms.2021.116576

Abstract

The method of Ion-Laser InterAction Mass Spectrometry (ILIAMS) offers new options for the determination of 26Al by Accelerator Mass Spectrometry (AMS) and improves the sensitivity and efficiency for the detection of this isotope in artificial and environmental samples. In ILIAMS, a laser is overlapped with the ion beam during its passage through a radiofrequency quadrupole ion cooler. Those ions with electron affinity lower than the energy of the photons are selectively neutralized in a photodetachment process. Because the electron affinity of MgO is lower than that of AlO, ILIAMS can suppress the isobar 26Mg by 14 orders of magnitude. No further isobar suppression on the high-energy side of the spectrometer is necessary, so that the more prolific AlO− beam can now also be used at facilities with terminal voltages < 5 MV. At the 3 MV Vienna Environmental Research Accelerator (VERA) routine 26Al AMS measurements assisted by ILIAMS are performed utilizing AlO− extracted from the ion source and charge states 2+ and 3+ for the Al ions after the accelerator on the high-energy side of the spectrometer. The most efficient generation of AlO− currents (in the range of several μA) is realized when mixing the Al2O3 sample material with Fe powder. Blank materials are measured down to 26Al/27Al ratios of 5⋅10−16. The efficiency relative to the use of Al− extraction is improved typically by a factor 3–5 and thus the new method is useful for measurements with highest sensitivity and down to very low 26Al/27Al ratios.

Highlights

We describe the implementation of Ion-Laser InterAction Mass Spectrometry (ILIAMS) for measurements of 26Al at the Vienna Environmental Research Accelerator (VERA)
The intensity of the ion beam currents extracted from the Accelerator Mass Spectrometry (AMS) Cs sputter ion source is affected by the metal matrix mixed to the Al2O3
The cathodes contained a known amount of Al2O3 (z2 mg) and were sputtered for 5e7 h, which represents long sputter durations during a measurement aiming at consuming most of the sample material

Summary

Introduction

Sputter targets prepared in the chemical extraction of Al from the original sample (e.g. rock or sediment) normally consist of Al2O3. They are mixed with a metal to boost the extraction of Al¡ with an electron affinity (EA) of 0.43 eV [4] from the ion source. As Mg does not form atomic negative ions, the final detection of ions at mass 26 is not affected by the isobar 26Mg. Extraction of AlO¡ (EA 1⁄4 2.60 eV [8]) from the same target results in much higher efficiency of Al usage. The isobar 26Mg is extracted from the ion source in form of the molecule MgO¡

Methods

Results

Conclusion