The distance between two atoms in a crystal is 0.2 nm. Diffraction would be ideal (specular reflection occurring,as pictured in my extra notes slides for Bragg's Law) if the wavelength of the interfering wave was the same as the distance between obstacles that cause the scattering.
Calculate what speed a neutron would need to have to be suitable for this task ,to have a de Broglie wavelength equal to the distance between the two atoms.