What energies are needed to observe interference maxima in a crystal?

[dswatson]

A certain crystal has a planar spacing of .25nm. What energies are necessary to observe up to three interference (diffraction) maxima for a)electrons and b)neutrons?

 

[gabbagabbahey]

What have you tried? What do you know about diffraction through a crystal? What equations do you know that give the interference maxima for this type of diffraction?