Enhancing cross section of a process for primary particle only

Dear Geant4 experts,

I am trying to do a simulation of interaction between a mono-energetic proton beam hitting an Aluminium target (O(um) thick) and emitting neutrons. I want to bias the primary (proton) track such that more no. of neutrons are emitted from the target. To my understanding, the primary event biasing only enables to sample a particular primary track having particular value of a parameter (like energy, angle, co-ordinate etc.). How do I implement the biasing on the physics process of the primary track only?

Thanks & Regards
Shubham Dutta