Dear FLUKA experts,
We are carrying out a FLUKA simulation to determine the pion decay photon spectrum expected from energetic ions impacting a thick target source with solar atmosphere composition. A beam of protons is directed at a cuboid composed of a substance which is mostly hydrogen, with small quantities of helium, carbon, oxygen, nitrogen, etc. The simulations run fine and produce understandable results for moderate proton energies, up to a few GeV, but if we go to 10 GeV we start to find an event that causes a crash with the error message
Abort called from STCPPR reason INVALID WHYDR Run stopped!
STOP INVALID WHYDR
If we run the simulation with a single species target source there is no error. So, we guess this is something to do with transport in our composite medium. We can work around it by doing several runs with modest numbers of primaries. Not all of them will crash and we can use results from the successful ones in the usual way. But this is slow and fiddly, and we would like to understand what causes the error in case it highlights something we should be aware of in our results.
Example .flair, .inp and .err files are attached. Thanks in advance for help.