Stopping power of charged particles in heavy elements I am performing an experiment which requires knowledge of the stopping power function of roughly 5 MeV alpha particles in heavy elements, e.g. californium. I have not been able to find such data anywhere. For example, NIST (https://physics.nist.gov/PhysRefData/Star/Text/ASTAR.html) only has data for up to Z=92 (Uranium), same goes for SRIM. I believe both of those are based on measured results.
Does anyone know if these measurements have actually never been made before? If not, is it safe to use uranium stopping power data to approximate californium stopping power (Z=98)? Or am I better off just using the Bethe formula? 
 A: With the advent of practical solid-state detectors with good (~4keV) energy resolution in the late 1950's/early 1960's, the use of Rutherford Backscattering Spectrometry (RBS) for composition analysis took off. To convert detected energy to depth required knowing the stopping power vs energy for the incident and backscattered light ions (usually protons and $\alpha$). There are a variety of tabulations, combining theory and experiment, but as you noted the tables generally stop at uranium. This includes, say, the Handbook of Modern Ion Beam Materials Analysis (ed. J.R. Tesmer and M. Nastasi, MRS 1995).
The theory of light ion stopping goes way back, including Bethe, Bragg, Bohr, Lindhard, Ziegler, and many more. Over the years the models have been refined. Quantitative experiments have been performed on a variety of ion/target combinations, with stopping powers generally good to a few percent or so (perhaps). A major source of uncertainty is just how many atoms/cm$^{2}$ the ions in the incident beam actually passed by. For silicon, perhaps the most widely RBS'ed substrate in the world, there are several internally consistent experimental measurements of stopping powers that differ from each other over most of the normal RBS energy range.
To measure the stopping powers for Cf, one would need a thin film target and an accelerator. One article I found (from phys.org) indicates that 5 milligrams of Cf cost them $1.4 million to obtain from the Department of Energy. I won't even begin to think of the hassles of the safety paperwork I'd have to do to make thin films with it and play with it in an accelerator...
So, what to do in your case? Fortunately, a 5MeV $\alpha$ seems unlikely to penetrate a Cf nucleus, so you can likely discount induced nuclear reactions and non-Rutherford nuclear cross sections. But, you want the stopping powers. I suggest two ways to contemplate it. 


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*Plot up the tabulated stopping powers and think how you would extrapolate them to Cf. This is not straightforward, since they are not monotonic but oscillate due to the electronic structure of the target atoms. This then requires applying some level of theory to guesstimate correctly.

*Go to Ziegler's semi-empirical formula and extrapolate from that, taking into account what you know from above. This combination is about as good as you are going to get.
Without too much effort, I think you could come up with a value that you could argue to be good to within $\pm 5\%$ or so. By doing both steps you would have a clear argument on the value chosen, and the impact of the exact stopping power on your results.
