Technical note: Accelerator mass spectrometry of ¹⁰Be and ²⁶Al at low nuclide concentrations

Abstract. Accelerator Mass Spectrometry (AMS) is currently the standard technique to measure cosmogenic ¹⁰Be and ²⁶Al concentrations, but the challenge with measuring low nuclide concentrations is to combine high AMS measurement efficiency with low backgrounds. The current standard measurement setup at ANSTO uses the 3+ charge state with Ar stripper gas at 6 MV for Be and 4 MV for Al, achieving ion transmission through the accelerator for ¹⁰Be³⁺ and ²⁶Al³⁺ of around 35 % and 40 %, respectively. Traditionally, ²⁶Al measurement uncertainties are larger than those for ¹⁰Be. Here, however, we show that ²⁶Al can be measured to similar precision as ¹⁰Be even for samples with ²⁶Al / ²⁷Al ratios in the range of 10⁻¹⁵, provided that measurement times are sufficiently long. For example, we can achieve uncertainties of 5 % for ²⁶Al / ²⁷Al ratios around 1 × 10⁻¹⁴, typical for samples of late-Holocene age or samples with long burial histories. We also provide empirical functions between the isotope ratio and achievable measurement precision, which allow predictive capabilities for future projects and serve as a benchmark for inter-laboratory comparisons. For the smallest signals, not only is understanding the source of ¹⁰Be or ²⁶Al background events required to select the most appropriate blank correction method but also the impact of the data reduction algorithms on the obtained nuclide concentration becomes pronounced. Here we discuss approaches to background correction and recommend quality assurance practices that guide the most appropriate background correction method. Our sensitivity analysis demonstrates a 30 % difference between different background correction methods for samples with ²⁶Al / ²⁷Al ratios below 10⁻¹⁴. Finally, we show that when the measured signal is small and the number of rare isotope counts is also low, differing ²⁶Al or ¹⁰Be concentrations may be obtained from the same data if alternate data reduction algorithms are used. Differences in the resulting isotope concentration can be 50 % or more if only very few ( 10) counts were recorded or about 30 % if single measurement is shorter than 10 min. Our study presents a comprehensive method for analysis of cosmogenic ¹⁰Be and ²⁶Al samples down to isotope concentrations of few thousand atoms per gram of sample, which opens the door to new and more varied applications of cosmogenic nuclide analysis.