I am calculating the cross-section of alpha particles bombarding thorium to produce 223Ra and 225Ra. I used the method outlined in the following link ( Ac225 and Ac227 cross sections for proton beam on Th232 target - Scoring - FLUKA User Forum (cern.ch)) to perform the calculations, and the results are shown in the figure below. As you can see, the cross-section is relatively high below 100 MeV/u, and I am not sure if this result is correct. Why would the cross-section be higher at lower energies?
Both because the reaction cross section as a whole (and so the nuclear interaction probability) has typically a maximum at low energies (see for instance https://doi.org/10.1016/j.asr.2011.11.029) and because the likelihood of a specific channel may typically decrease for increasing energy in favor of other channels benefiting from the energy increase.
Nonetheless, the minimum at 100 MeV/u and the increase up to 150 MeV/n should be recognized as artificial, reflecting the progressive transition from the BME to the RQMD ion interaction model adopted by FLUKA.
