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
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.