Hello,
You can download different/special cross section data for a few applications, which you can use instead of the standard corss section libraries.
There are many Evaluated neutron cross section Labraries at
https://www-nds.iaea.org/geant4/
and there is the TENDL library at
https://geant4.web.cern.ch/support/download
What is your beam energy? For low-energy proton reactions (around 20 MeV) the TENDL library could improve your results.
I recommend you take a look into the advanced example ‘STCyclotron’ because this example seems to fit your problem pretty well.
Regards