XRays offer an intriguing irradiation agent to safely substitute for atomics, after all, X- and Gamma- rays differ only in energy, the electromagnetic spectrum gradually fades from one into the other, and they are both ionizing radiation lethal to living things, like microbes.

But if there is no difference in kind, there certainly is in degree. Xrays are formed from interactions in the electron shells of atoms, and have energies in the order of KeV. Gammas come from disintegrating atomic nuclei and typically have energies in the MeV range. I don't now if it is even possible to build an XRAY machine that would be the radiation equivalent of a few kilograms of a highly radioactive gamma-emitting substance. Maybe some kind of particle accelerator could do it, but you'd still have to plug it in to the wall socket. Radio-isotopes give off energy naturally, for free.

We also need high energy fluxes to process food on an industrial scale. In the lab, you might be able to throw a lamb chop in an XRAY machine for a few minutes and treat it, but the kind of food processing facility we are talking about has a conveyor belt moving thousands of pounds of produce through it every day. You need an extremely powerful source to treat that much food with only a brief exposure.

I haven't done any research to back up my suspicions, but my instincts tell me XRays probably don't have the ability to replace nuclear isotopes in these applications.