1. The problem statement, all variables and given/known data In PIXE (Proton-Induced X ray Emission), if you increase the beam energy from 2 MeV to 4 MeV, do you expect that the factor of increase in the cross section for the K X ray production from Ni is higher than that from S, or vice-versa? 2. Relevant equations 3. The attempt at a solution The professor didn't give us any equation, just a conceptual explanation of the process. It makes sense that the cross section increases with the energy of the incident particle but I am not sure how this increase depends on different elements. So I am not sure how to approach this problem. Any idea?