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

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.