It’s all about $\mu$
By far the biggest pain in the neck in the overall field of cosmogenic-nuclide geochemistry is the need to compute nuclide production rates due to muon interactions. Production rates due to muons are quite small relative to total production rates at the surface, but they’re not small enough that you can ignore them completely. In the subsurface, they’re the majority of production, so are actually important for applications such as burial dating and depth profile dating, but they’re complex to calculate and hard to verify due to a general lack of subsurface calibration data. It’s much more difficult to obtain useful calibration data for production rates due to muons than for surface production rates due to spallation, mainly because there aren’t really any geologic processes that completely remove the entire preexisting muon-produced nuclide inventory, which extends to tens of meters depth, all at once, leaving a blank slate. And if there were, production rates are really low, so the blank slate would have to sit around for a really long time before you could do the measurements accurately, in which case something else bad would probably happen to it. So the only hope for decent calibration data is to collect tens-of-meters-long cores from bedrock surfaces that have been sitting around at extremely low erosion rates for millions of years, and there aren’t a whole lot of such things.
The last couple of years, largely due to work in the CRONUS-Earth and CRONUS-EU projects, has either improved or unimproved this situation by generating some new methods of computing muon production rates and also by collecting several new bedrock cores. This increases the overall complexity of the situation as a whole, but also helps a bit by providing enough data to make a decent start at evaluating which methods of computing muon production rates work best. In the last few months I’ve gotten tired of being perpetually confused on this subject, so I’ve waded through all these data and put together an extraordinarily long, tedious, and pedantic document on the subject of benchmarking muon production rate calculation methods against the available calibration data set. I imagine that I will submit this to an actual journal at some stage, but for now it is here (the PDF file). Warning: it is long, tedious, and pedantic. However, if you want my best shot at a synoptic view of all reasonably accessible knowledge that is relevant to production by muons, it might be potentially useful. Of course, it hasn’t been reviewed in detail by anyone else, so it might also be wrong. The MATLAB code that does all the calculations is also available at the same address (the zip-file).
Presumably, about six people on Earth will be interested in this document. However, I imagine that all six of those folks are also regular readers of this blog. You know who you are. As noted, this will probably be sent off to a journal at some stage, in which case some of you will also have the truly unpleasant and unfortunate duty of reviewing it. Enjoy!
Oh yeah. The 2%-of-production-98%-of-suffering logo above is also in that directory, in case you want to put it on a T-shirt. For example, from CafePress: