Residual Nuclei Production below 6 MeV/A

Dear FLUKA Experts,
I am calculating residual nuclei produced by an accelerator beam incident upon a target and am running into some issues with calculations that I am performing below 10 MeV/A. The attached files are a simple example with a 208Pb beam incident upon a copper target. At 6 MeV/A (beam-test-2 in the flair file), I am seeing some production of 64Cu.


At 5 MeV/A, (beam-test-1), this calculation shows no residual nuclei or activity in the target. However, if I use a smaller nucleus at 5 MeV/A, e.g., 56Fe (beam-test-3), my calculation shows a handful of products as shown here:


Is there a non-physical reason or an issue with my input that could result in no production of residual nuclei by 208Pb at 5 MeV/A?

I have tried varying the physics inputs and parameters to track down this issue without success. For the 208Pb ions, it appears that residual nuclei generation starts between 5.6 MeV/A and 5.7 MeV/A.

I greatly appreciate any help or guidance that you can provide to help me understand this issue. Thank you in advance.

beam-test.flair (3.5 KB)
beam-test.inp (3.0 KB)

Hi John,
nuclear reactions have a threshold, namely they can physically take place only above a given energy, which in case of ions is linked to the respective Coulomb barrier. For lighter projectiles (of lower atomic number), the latter is smaller and explains your findings.
Nonetheless, especially at such low energies in the close vicinity of the reaction threshold, one should not blindly trust the code and assume that at 5.6 A MeV no activity at all can be generated, rather take a sensible safety margin, e.g. based on estimates at slightly higher energies (or relevant data if any).