Dear @uchow
I might not be the most qualified person to answer, but unfortunately FLUKA does not have the low-energy neutron cross sections for Ra-226 implemented (see this similar question of mine Missing neutron transport cross section ). If your simulation stops with the message
**** Low energy neutron xsec not found for some media 5 6 ****
in the output file, then this is the problem.
If you are only interested in the yields of a thin Ra-226 target, you could still get a running simulation with meaning full results if you set the low-energy neutron transport threshold high enough with the PART-THR card.
Cheers
Lorenzo