Comparison of methods for the detection of 10Be with AMS and a new approach based on a silicon nitride foil stack

Abstract

Natural 10Be (t1/2 = 1.387 ± 0.012 Ma) is produced by cosmic rays and is present on Earth's surface only at ultratrace concentrations (typically 104 to 1010 atoms/g). Its cosmogenic origin makes it an important tracer for many applications in Earth and environmental sciences. An improved accelerator mass spectrometry (AMS) method has been developed at the Vienna Environmental Research Accelerator (VERA) at the University of Vienna to detect the long-lived radionuclide 10Be and separate it from its isobar 10B. Recently installed and projected AMS facilities mainly apply a degrader foil followed by an electrostatic or magnetic separator to remove 10B from the ion beam. This provides the highest suppression of 10B, but suffers from significant transmission losses of 10Be ions. The new technique described here achieves comparable 10B suppression with a passive absorber, consisting of a stack of silicon nitride foils. Compared to a gas absorber, the smaller energy straggling in foils allows separation at lower energies. For a tandem accelerator operated at 3 MV, the charge state 2 + instead of 3 + can be used, with a stripping yield as high as 55%. This way, a high overall efficiency is gained. The setup is simple to operate, and provides good precision and accuracy. We compare this new approach with other methods used at VERA and at other AMS facilities. The foil stack setup was fully characterized with artificial samples from chemically and isotopically well-defined reagents, and is now routinely applied to real samples in various research projects at VERA. The new method is straightforward to be implemented, and was already adopted at another AMS facility with higher terminal voltage, the potential use at tandem accelerators with lower terminal voltage is under exploration.