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A number of calculator questions

July 20, 2012

Dear Greg ,

I am taking the liberty of writing to you for asking some help. It is convenient to use the online calculator written by you. In order to understand better, I am also trying to calculate the age by the corresponding formula.

Unfortunately, I have some trouble in doing this. Would you mind helping me? Here are some questions I have.

1) the sea level, high latitude production rate: in the document “Al-26-Be-10 exposure age/erosion rate calculators: update from v.2.1 to v.2.2”, Table 1 shows the reference Be-10 production rate due to spallation. If i used the Be-10 standard NIST_certified, how should I modify the production rate? Use 4.49, and just multiply the concentration by the coefficient 1.0425?

 If you measured Be-10 concentrations using the certified value for the NIST standard, you have two options.

First, you can correct the measured Be-10 concentration to the “07KNSTD” standardization by multiplying by 1.0425. Then you could apply the production rates normalized to 07KNSTD.

Second, you can correct the production rate to the “NIST_certified” standardization by dividing by 1.0425. Then calculate the age using your concentration measured against NIST and this corrected production rate.

If you use the online calculator, of course, you should not do either of these things: you should enter the measured concentration with the “NIST_Certified” standardization code, and the calculator will do the conversion internally before computing the age.

2) the percent production by muons at sea level…2.2%(1-0.978)?
The calculator does not calculate the production by muons as a percentage of sea level production. Instead, it computes the absolute production rates by muons using the method of Heisinger et al. This method is complicated and is described in complete detail here:

3) erosion rate. on the website, it is said the erosion rate must be inferred from independent evidence. if I have both the Be-10 and Al-26 concentration, can i obstain the erosion rate by setting age(10Be)=age(26Al)?
Yes, in theory, if you measure both Be-10 and Al-26, you can solve for both the age and the erosion rate. However, when you consider the uncertainties you will find that a large range of erosion rates will be consistent with the data. There is a detailed discussion of this (using Be-10 and Cl-36) in the following paper:

Gillespie, A.R. and Bierman, P.R, 1995, Precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ: Journal of Geophysical Research, B, Solid Earth and Planets, v. 100, p. 24,637-24,649.

In practice, measurements of Al-26 are not precise enough to accomplish this in the majority of cases. It may be possible to accomplish this with very old surfaces where very precise measurements are possible.
4) with Al-26/Be-10: the Al-26/Be-10 is bigger than 6.1…how to explain?
Two things to think about:

1. The value of the 26/10 production ratio depends on what standardization is used for Be-10 measurements. If one uses the “07KNSTD” standardization, the 26/10 production ratio is 6.75. The online calculator uses this standardization, so if you have a sample with a short exposure age and no erosion, it will report a 26/10 ratio of 6.75.

2. If you observe a 26/10 ratio that is higher than the production ratio once the Be-10 standardization is accounted for, then it is most likely that there is an analytical problem with the Al-26 measurements. Al-26 is much more difficult to measure than Be-10, and there are several ways to erroneously measure too much Al-26.
5) what is the difference between the “time-dependent” and “scaling scheme for spallation?” The former is basd on the latter and adds geomagnetic calibration?
I am not sure I understand your question. A “time-dependent” scaling scheme considers changes in the magnetic field, whereas a “non-time-dependent” scaling scheme assumes that production rates are constant through time.

6) how to calculate the internal uncertainty?

That is a bit complicated. The method is described in detail here:

regards,

–greg
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