10Be/9Be ratios of phenakite and beryl measured via direct Cs sputtering: Implications for selecting suitable Be carrier minerals for the measurement of low-level 10Be

Preparing Be carrier solutions with low Be-10/Be-9 ratios is essential for the applications of in-situ-produced cosmogenic Be-10 in geochronology. This is because commercially available Be carriers are non-negligibly contaminated by Be-10. Recently, in-house Be carriers have been successfully applied to samples that contain small amounts of in-situ-produced Be-10. The first step in preparing in-house Be carriers is selecting suitable Be-bearing minerals that contain less Be-10. Here, we present a simple method for selecting appropriate raw minerals for in-house Be carriers. That is, measuring the Be-10/Be-9 ratios of Be-bearing minerals by direct Cs sputtering. Analyses of the Be-10/Be-9 ratios of phenakite (B2SiO4) and beryl (Be3Al2Si6O18) obtained from a mineral collection at the Geological Survey of Japan indicate that phenakite generally contains more B-10, interfering isobar of Be-10, than beryl. In addition to the necessity of finding raw materials that contain low Be-10, our results indicate that it is preferable to select a starting material with a low B concentration. Fragments of Be-bearing minerals from target samples were directly packed, and showed effective beam currents. Therefore, we anticipate that the direct packing method can also potentially be used to measure Be-10 in Be-bearing minerals for geological applications. The measurement background of accelerator mass spectrometry was evaluated using a B-removed Be carrier solution. While B is partly adhered to hydroxide gel, we demonstrate that the hydroxide gel wash reduces B in a Be carrier solution. To highlight the Be-10/Be-9 ratios of Be-bearing minerals against commercially available Be solutions, we also investigated the Be-10/(9)Beratios of commercially available Be solutions.