That sounds more like an engineering problem. As long as you can reliably produce grains or pellets of the metal with enough lattice locations to produce a net

That sounds more like an engineering problem. As long as you can reliably produce grains or pellets of the metal with enough lattice locations to produce a net positive amount of energy then the problem becomes simply how do you move those grains through the x-ray beam at the required rate while capturing the energy. Perhaps suspending them in a carrier fluid that could also act as a medium for heat transfer.