# Elastic X-ray diffraction: Where does the Photon Momentum go to?

lately I was discussing the following situation. Let's suppose you are doing X-Ray diffraction experiments on a crystal. So what you basically do is shooting a high energy photon with a wave vector $$k$$ into the crystal where it will undergo diffraction according to the Bragg condition.

That is, an outgoing photon of wave vector $$k'$$ will emerge if there exists a reciprocal lattice vector $$G$$, such that $$\Delta k = G$$ Now, we were asking ourselves the question: Where does the momentum change actually go? The crystal momentum that is produced in this process will be $$\hbar G$$, but "who" carries this momentum? Will the photon induce phonons ( or excitons or whatever quasi-particle ) in the crystal carrying away the momentum? Or will the crystal as a whole take the momentum? Or a mixture of that? Or something completely different?

Thanks for any hints :)