Low-energy neutron cross section for monoisotopic material

Dear FLUKA experts

I’m interested in the yields from (gamma,n), (n, 2n), (n,n+p), etc. reactions in titanium at energies below 20 MeV. The situation is as follows: I can run my simulation with natural titanium as my target material and the simulation runs without problems. However, when I want to disentangle the contributions from the different stable isotopes, e.g. only from Ti-48, FLUKA complains because it has no low-energy neutron cross section for monoisotopic titanium (I just define Z and A in the MATERIAL card). I can fix this by using the same natural titanium low-energy neutron cross section also for the monoisotopic case (with the LOW-MAT card). However, I’m not sure if this affects only the neutron transport (according to the JANIS database the total neutron cross sections for different titanium isotopes is almost identical) or if the computed yields will have some systematic error (e.g. the measured (n,p) cross section varies by almost a factor 5 for different titanium isotopes around 13 MeV). Any help is very appreciated!


Dear @lorenzo.mercolli,
for what concerns neutron induced reactions below 20 MeV (not talking then about (gamma,n)), you meet a structural limitation of FLUKA that is related to the multi-group treatment based on the cross section library built for specific materials. Among the latter ones, as you observed, there is natural Titanium but not Ti-48. This simply means that you cannot disentangle the contributions from the different stable isotopes of Ti, because, for every low energy neutron reaction in natural Ti, FLUKA will sample the resulting residual nucleus from the global distribution referring to natural Ti, with no correlation with respect to the number of secondary neutrons emitted (while charged particles such as protons are not even explicitly generated at that point). If you set Ti-48 as material (be careful that - as indicated by the manual - the mass number A should be set as WHAT(6) and not WHAT(2)) and associate to it the natural Ti cross sections (that is the only choice you have for Ti), nothing will change for the n-induced reactions, with respect to the original natural Ti material, and by construction you will not be able to get any further insight on the residual dependence on the target isotope, since this differential information is just not there, but is lost in the composite distribution.

Dear @ceruttif

Thanks for the clarifying answer!