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

The method of Ion-Laser InterAction Mass Spectrometry (ILIAMS) offers new options for the determination of Al-26 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 Mg-26 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 Al-26 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 mA) is realized when mixing the Al2O3 sample material with Fe powder. Blank materials are measured down to Al-26/Al-27 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 Al-26/Al-27 ratios. (C) 2021 The Authors. Published by Elsevier B.V.