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We now know the Be-10 half-life

July 17, 2009

Perhaps the most impressive presentation at the cosmogenic-nuclide-fest that was the recent Goldschmidt meeting was the unveiling of two new measurements of the Be-10 half-life, by a European group spearheaded by Friedhelm von Blanckenburg. This is described in two Goldschmidt abstracts, this one by Gunther Korschinek and this one by Jerome Chmeleff.

This is important because it appears to finally resolve a complicated and embarrassing problem with Be-10 measurements and the exposure ages derived from these measurements. Basically, past measurements of the Be-10 half-life, although individually precise, differed by approximately 10%. This was an unfortunate situation for cosmogenic-nuclide applications that rely on radioactive decay of Be-10 — burial dating in particular — but in itself of limited importance for the main application of Be-10 measurements, exposure dating of surfaces that are very young with respect to the Be-10 half-life. A more important problem for exposure dating came from the link between half-life measurements and the Be isotope ratio standards used for AMS measurement of exposure dating samples. AMS measurement of sample Be-10/Be-9 ratios relies on comparison between samples and Be standards whose 10/9 ratio is absolutely known — and the way one determines the absolute amount of Be-10 in one of the source materials used to make these standards is by decay counting. Thus, if you start with an incorrect value for the half-life, you get the wrong isotope ratio in the standard and then the wrong isotope ratio in the sample. This means that if one were to take a particular sample and measure it against two AMS standards whose assumed isotope ratios were derived from different values of the half-life, you would infer different isotope ratios and hence Be-10 concentrations. In principle this is not a problem if i) all measurements on calibration samples used to determine nuclide production rates, and ii) all measurements on unknowns, are referenced to the same AMS standards, but in practice it is difficult to ensure this, and the existing Be-10 literature is rife with inconsistent standardizations.

Two steps were needed to fix this problem. First, an absolute determination of the isotope ratios of commonly used AMS standards that did not rely on an existing half-life measurement. Second, a measurement of the half-life that did not rely on any assumed concentrations of existing isotope ratio standards.

Kuni Nishiizumi and a handful of co-workers, mostly from LLNL, accomplished the first of these a couple of years ago (in a 2007 paper noted below). They used the LLNL accelerator/detector array to implant a precisely known number of Be-10 atoms in a silicon wafer, then added a measured amount of Be-9 to obtain a sample of Be with an absolutely known 10/9 ratio. They could then use this as a primary standard to measure the true 10/9 ratio of many of the commonly used AMS Be standards. This solved the problem of Be-10 concentration measurement by uncoupling the assumed isotope ratios of AMS standards from any particular value for the Be-10 half life.  This in turn made it possible to determine how many atoms of Be-10 were present in a sample without any assumptions about the Be-10 half-life.

Chmeleff, Korschinek, and their co-authors have now solved the second half of the problem by determining the Be-10 half-life without recourse to any of the existing stocks of Be-10-enriched Be. They began by manufacturing a new Be-10 enriched Be solution. In order to determine the Be-10 half-life from such a solution, one must i) measure the Be-10 activity by decay counting, and ii) determine the amount of Be-10 present, which typically involves measuring both the total Be concentration and the absolute 10/9 ratio. The activity is straightforward to measure by liquid scintillation counting, and each group made a separate activity measurement using this method. The difficult part of the overall project is to measure the absolute 10/9 ratio.

The Chmeleff group accomplished this by ICP-MS, a technique which is straightforward except for the problem that the instrument has a large mass fractionation. They first attempted to account for the mass fractionation using a Be-7 spike; for a variety of interesting reasons this proved to be an extremely instructive radiochemical parable — if you have the opportunity to get this story from someone involved in this project, ask them — but not an successful measurement.  Fortunately they could fall back on determining the mass fractionation characteristics of the ICP-MP by  analysing samples of numerous other elements with known isotope ratios and establishing an atomic mass – mass fractionation relationship.

The Korschinek group used a completely different method known as heavy ion elastic recoil detection. To the extent that I understand how this works, one coats a silicon wafer with the Be solution to be analysed, and then bombards it with a high-energy ion beam. Be atoms are scattered from the surface with sufficient energy to be individually identified in an energy-loss detector similar to that used for AMS measurements.

The impressive part of these two experiments is that they involved two completely independent measurements of the quantity of Be-10 and its activity, but both obtained equivalent results for the Be-10 half-life, the mean of which is 1.387 +/- 0.012 Ma. This value is i) consistent with the value inferred by retroactively applying the Nishiizumi et al. (2007) restandardization to the Be solutions from which the half-life was originally determined, but ii) significantly more precise.

To summarize, even though Kuni Nishiizumi made an excellent point at the meeting — that early on in the development of radiocarbon dating, the C-14 community thought they had measured the C-14 decay constant accurately, but they were wrong —  in my opinion the issue of the Be-10 half-life is now for all practical purposes solved.

The combination of this result and the Nishiizumi et al. (2007) restandardization of AMS standards has two important effects on cosmogenic-nuclide geochemistry. First, production rate calibration measurements and exposure-dating measurements in the existing and future literature, even if made against different AMS standards, can now be restandardized to a common basis. This makes it possible to accurately compare the results of different exposure-dating studies. The ability to compare two exposure-dating studies on a common basis may seem like a trivial thing, but to the embarrassment of the entire community it has not in general been possible in the past. We can stop looking away and mumbling about the weather when questioned on this topic by other geochemists.

Second, a much more precise determination of the Be-10 half-life significantly increases the precision of cosmogenic-nuclide burial dating. This method is becoming a lot more popular — in large part for purposes of dating hominin fossils and stone tool assemblages in Plio-Pleistocene sections without volcanic ashes — and half-life uncertainties are a significant fraction of the total uncertainty in this method.

Only two challenges remain. First, the difficulty of determining which AMS standards were used, and what isotope ratios were assumed for these standards, in studies from the existing exposure-dating literature is pathetic and appalling. No matter how much we know about the true isotope ratios of AMS standards, if a paper doesn’t document what standard and assumed ratio was used – and most do not — the exposure ages in that paper are a useless waste of time and money. Authors, editors, and reviewers must do a better job of making sure that all exposure-dating publications contain enough information to unambigously define how many Be-10 atoms are present in each sample. Sure, fully understanding this issue is complicated, but it is not optional. Second, we now must persuade von Blankenburg, Chmeleff, and Korschinek to measure the Al-26 half-life with equal precision.

Reference:

K. Nishiizumi, M. Imamura, M. Caffee, J. Southon, R. Finkel, and J. McAnich. Absolute calibration of Be-10 AMS standards. Nuclear Instruments and Methods in Physics Research B, 258:403–413, 2007.

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