How to get proton nuclear cross section for a single atom?

Hi, everyone , I’m studying on the cross section for proton.
And I want to know the cross section for proton-oxygen elstic/inelastic cross section in water, how can I get this?
I have found that G4EmCalculator has a ComputeCrossSectionPerAtom() method, but it need processName, I want to know how can I get this.

Thank you very much!