Absolute calibration of 10Be AMS standards

The increased detection sensitivity offered by AMS has dramatically expanded the utility of 10Be. As these applications become more sophisticated attention has focused on the accuracy of the 10Be standards used to calibrate the AMS measurements. In recent years it has become apparent that there is a discrepancy between two of the most widely used 10Be AMS standards, the ICN 10Be standard and the NIST 10Be standard. The ICN (ICN Chemical and Radioisotope Division) 10Be AMS standard was calibrated by radioactive decay counting. Dilutions, ranging from 5 x 10-13 to 3 x 10-1110Be/Be, have been prepared and are extensively used in many AMS laboratories. The NIST 10Be standard, prepared at the National Institute of Standards and Technology (NIST), is calibrated by mass spectrometric isotope ratio measurements. To provide an independent calibration of the 10Be standards we implanted a known number of 10Be atoms in both Si detectors and Be foil targets. The 10Be concentrations in these targets were measured by AMS. The results were compared with both the ICN and NIST AMS standards. Our 10Be measurements indicate that the 10Be/9Be isotopic ratio of the ICN AMS standard, which is based on a 10Be half-life of 1.5 x 106 yr, is 1.106 ± 0.012 times lower than the nominal value. Since the decay rate of the ICN standard is well determined, the decrease in 10Be/9Be ratio requires that the 10Be half-life be reduced to (1.36 ± 0.07) x 106 yr. The quoted uncertainty includes a ±5% uncertainty in the activity measurement carried out by ICN. In a similar fashion, we determined that the value of the NIST 10Be standard (SRM4325) is (2.79 ± 0.03) x 10-1110Be/9Be, within error of the certified value of (2.68 ± 0.14) x 10-11. The Lawrence Livermore National Laboratory (LLNL) internal standards were also included in this study. We conclude that the 9Be(n, γ) neutron cross section is 7.8 ± 0.23 mb, without taking into account the uncertainty in the neutron irradiation