Important Note Concerning 10Be Results
Basic Information
All AMS laboratories calibrate their 10Be/9Be ratios by using a standard. The 10Be/9Be ratio which PRIME Lab reports for a submitted sample is thus directly proportional to the value assigned to the standard material. AMS standards are usually diluted from parent solutions of known isotopic ratio or known activity. There are two sets of 10Be standards widely recognized and used in the AMS community. One set is derived from a parent solution produced by NIST (National Institute of Standards and Technology) that has a certified 10Be/9Be ratio. Another set was diluted by Nishiizumi from a parent solution produced by ICN that has a certified activity. To convert activity into 10Be/9Be ratios, Nishiizumi used a 10Be half-life of 1.5 My. Some labs have in-house standards, although these are usually cross-calibrated with one or both of the NIST and ICN standards. A problem that has been recognized for some time is that the two main sets of standards, NIST and ICN, are not consistent with each other. To compare sample values for 10Be/9Be ratio (measured at different labs, or at the same lab at different times) one must know the standards used for the measurements. This is also important for production rates, since they are determined by AMS measurements of samples of known age. A recent series of measurements has been made that determines the 10Be/9Be ratio of the ICN standard, in a way which is independent of the assumed half-life (see Nishiizumi et al, NIMB B258 p.403, 2007). This new work indicates that the original ratios assigned to the ICN set of standards are incorrect. These new measurements furnish new values, and also put the Revised ICN values into much better agreement with the NIST values. One implication is that the half-life of 10Be can also be inferred from the certified activity and the measured 10Be/9Be ratio
A History of the 10Be Standards Used at PRIME Lab.
- For measurements made prior to June 2005, we used a standard derived from NIST 4325. The certified ratio for this standard is (2.68+/-0.14)x10 -11.
- From June 2005 until July 2007 we have used the ICN standards. These standards were diluted from a solution of certified activity, so their ratio depends on the half-life assumed for the conversion. Nishiizumi assumed a half-life of 1.5 My. These standards are widely known to differ from the NIST standards by approximately 14% (c.f. Fink and Smith, NIMB B259, p.600, 2007).
- Beginning in September 2007, we still use the ICN standards, but with revised assigned values. These revised values are taken from Nishiizumi et al., 2007. The absolute ratios of the ICN standards are revised downwards by a factor of 0.90. The revised value of the half-life is 1.36 +/- 0.07 My.
Conversion Table
In this table we provide multiplicative factors to convert a ratio calibrated with one standard to the ratio which would be obtained with a different standard. For example, to convert a ratio normalized to NIST to a ratio normalized to the revised ICN, multiply the NIST-normalized ratio by 1.04.| From \ To | NIST | 6/2005-7/2007 Original ICN | Revised ICN |
|---|---|---|---|
| NIST | 1 | 1.15 | 1.04 |
| 6/2005-7/2007 Original ICN | 0.87 | 1 | 0.90 |
| Revised ICN | 0.96 | 1.11 | 1 |
Discussion:
- As the values in the Table show, the NIST and the Revised ICN standard are in reasonably close agreement.
- The widely used sea-level, high latitude production rate in quartz of 10Be (~5 at/g/yr; e.g. Stone, 2000) is normalized to the Original (unrevised) ICN standard. When using 10Be ratios measured against the Revised ICN values, one should use a production rate that is correspondingly lower, about 4.5 at/g/yr.
- An important implication of the revised ICN standard is that the half-life of 10Be must be adjusted to be consistent with the measured activity of the standard parent solution. This implies that the half-life of 10Be determined from the Revised ICN standards is 1.36 +/- 0.07 My. This is similar to the 10Be half-life determined by NIST of 1.34 +/- 0.07 My. It is significantly different from the half-life of 1.5 My used by many investigators.
Practical Advice:
- At present time, our judgment is that the best choice is to assume that the Revised ICN standard is correct. This choice means using a sea level, high latitude production rate in quartz of about 4.5 at/gr/yr, and a half-life of about 1.36 My. Using the NIST standard will give very similar results.
- For short exposure ages (ages much less than the half-life), using the Original ICN and a production rate in quartz of about 5 at/gr/yr will give the same results.
- In their publications, researchers should indicate which standard was used in arriving at their 10Be/9Be ratios, and what half-life and production rate were used in calculating their exposure ages, erosion rates, and burial ages.
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