Elevation - atmospheric pressure relationships

Cosmogenic-nuclide production rates change with elevation because the atmosphere absorbs some of the cosmic-ray flux before it reaches ground level. The lower the elevation of your sample site, the more of the cosmic-ray flux is absorbed, so the lower the production rate. The important thing that controls the production rate at your site, therefore, is not the elevation itself but the average atmospheric pressure at the site -- that is, how much atmospheric mass is above the site to act as an absorber. This means that, in order to calculate the production rate at a sample site, it's necessary to convert the site elevation into an atmospheric pressure. The standard elevation-pressure relationship used in meteorology is a formula known as the ICAO standard atmosphere, and in nearly all cases this is also adequate for computing production rates. The primary exception is in Antarctica, where atmospheric pressure is generally lower than elsewhere on earth and a different formula does a better job of describing the height-pressure relationship. Thus, we provide two height/pressure relations, one for Antarctica, and one for the rest of the world. If neither one of these approximations is good for your site, we also allow input of average atmospheric pressure rather than elevation. This is described in detail in a paper by John Stone which is referenced here.

AMS measurement standards

AMS measurements of beryllium or aluminum isotope ratios depend on an analytical standard to which the isotope ratio of the sample is normalized. This is because it's hard to measure the absolute ratio of atoms of, for example, Be-10 and Be-9 in a sample. This isn't necessary, however, if one makes an analytical standard by combining known amounts of Be-10 and Be-9. It turns out to be much easier to measure the difference between the isotope ratio of the analytical standard and that of the sample, and then if the true 10/9 ratio of the standard is known one can compute the true 10/9 ratio of the sample. What this means is that every measurement of Be-10 concentration has been normalised to a particular standard. In turn, when calculating an exposure age from that Be-10 concentration, it's necessary to make sure that the Be-10 production rate you are using is normalised to the same standard. In this calculator, we use production rates that are normalised to the Lawrence Livermore National Laboratories (LLNL) Be-10 and Al-26 standards. This means that your Be-10 concentrations must also be normalised to the LLNL standards, or you will obtain the wrong exposure age. If you made your measurements at LLNL, no problem. If you didn't, you need to make sure that your measurements are normalised to the LLNL standards. Some but not all AMS facilities use these standards. For example, PRIME Lab at Purdue measures Be-10 against the NIST standards. If your Be-10 measurements are from PRIME, therefore, you must multiply your Be-10 concentrations by 1.14 to renormalize them to the LLNL standards before entering them into the calculator. In future, we will provide separate input forms for different accelerator facilities to account for this; until then, check with the AMS facility where your measurements were made to determine how to renormalise your measurements.