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Exposure age calculators — update to Be-10 decay constants

March 23, 2010

Last week I received the following email:

Dear Greg,

Could you, please, update your web-update to the article and to the calculator to the new 10Be half-life of 1.39 Ma? Your latest re-calculations of Be-10 PR are made with Kuni’s 1.36 Ma half-life, and this contradicts to 1.39 Ma. Since several people known to me refer to your web-update as to the latest Be-10 PR, to keep the old Be-10 PR is to add more to the already existing confusion in the community. The update should include, naturally, Heisinger’s constants (*1.39/1.51). I guess this will be a quick effort for you, and the community will be very grateful to you for keeping the calculator and its reference paper updated.

The part about the half-life made sense to me, but not the part about the production rates. Fortunately , it was quickly followed by this email:

Hi Greg,

sorry for messing things up myself…

I realized that the Be-10 production rates (PR) must stay as you cite them in the version 2.2 update, because they are normalized to an absolutely calibrated Be-10 standard, and it does not matter if Kuni has calculated 1.36 Ma or whatever.

Just to be sure — I assume that you have used 1/1.096 scaling (ETH->Kuni) of the ETH data that were used in calculation of the Be10 PR in your update 2.2?

If this is correct, then the only new changes for the calculator and update would be a new decay constant (ln(2)/1.39) and 1/1.096 scaling (ETH->Kuni) of old Be10 production constants from muogenic reactions.

What do you think?

This unnamed person is correct, at least in his second email, on all counts. First, the great value of the Nishiizumi et al., 2007 paper is that they calibrated AMS standards without reference to an activity measurement, thus decoupling measurements of AMS standard isotope ratios from measurements of the decay constant. Second, all this stuff has recently been published — which is the main criterion for inclusion in the calculators — so should be included in the calculators.

Thus, I have updated the constants file used by version 2.2 of the calculators to use the Chmeleff/Korschinek value of the Be-10 decay constant (discussed at greater length here) as well as to update the values for the muon interaction cross-sections to reflect a new ETH-KNSTD intercomparison. It is important to note that these two changes will have a negligible effect on exposure ages and erosion rates in nearly all situations. There is more information about the update on the documentation page for version 2.2 of the calculators.

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