Improved precision for 10Be-based chronology by optimizing carrier addition

Alianora Walker, Alan J. Hidy, Susan H. Zimmerman, Scott J. Tumey, Thomas A. Brown, Sandra M. Braumann, Roseanne Schwartz, & Joerg M. Scheafer

Submitted September 11, 2022, SCEC Contribution #12011, 2022 SCEC Annual Meeting Poster #085

We conducted a series of carrier mixing and target packing experiments to optimize 10Be measurement precision for samples analyzed at the CAMS-LLNL (Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory)—the primary facility used by SCEC for 10Be analyses. Improving precision for count-limited samples has the potential to be invaluable in fault studies when dating particularly young tectonic landforms or when quartz mass is limited. In our experiments, a quartz standard was dissolved and then split to prepare targets identical in 10Be content, but with variable 9Be (30 - 280 μg). Samples were bulked with Fe or Al carrier to replace the reduced 9Be. Packing experiments were performed with oxide-Nb mixtures to determine volume-equivalent masses needed to keep each target packed to a consistent depth. Our experiments show up to a 40% increase in 10Be counts for samples prepared with ~90 μg 9Be vs. the typical 250 - 280 μg. This result is substantiated by increased total 10Be counts observed on targets analyzed to exhaustion, implying significant gain in total system efficiency with the lower carrier mass. We further tested this approach with multiple low-level (~10^4 atoms 10Be g-1) quartz samples prepared using both typical and lower carrier amounts and analyzed under normal AMS operating conditions. In each case, significantly better counting statistics were achieved with the lower-carrier mass while reproducing the 10Be concentration obtained from the typical-mass duplicate. Similar improvements were also observed in targets prepared with lower carrier from an external sample prep lab (LDEO Columbia).
Collectively, our results indicate that optimal measurement efficiency for 10Be AMS targets analyzed at CAMS is obtained using 75 - 100 μg of 9Be carrier while replacing the reduced mass with Fe, at a Fe/Be replacement ratio of ~0.4. This reduction in per-sample carrier use—by a factor of 3.2x—offers substantial benefits to both analysis quality and cost (improved counting statistics, shorter, cheaper analyses, and longer lasting carrier bottles) for count-limited samples, without modification to established AMS operations for 10Be analysis at CAMS. Prepared by LLNL under Contract DE-AC52-07NA27344.

Key Words
Accelerator mass spectrometry, CAMS, 10Be, sample preparation

Walker, A., Hidy, A. J., Zimmerman, S. H., Tumey, S. J., Brown, T. A., Braumann, S. M., Schwartz, R., & Scheafer, J. M. (2022, 09). Improved precision for 10Be-based chronology by optimizing carrier addition. Poster Presentation at 2022 SCEC Annual Meeting.

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