A neutron beam of energy E scatters from atoms
Question:

A neutron beam of energy E scatters from atoms on a surface with a spacing d = 0.1nm. The first maximum of intensity in the reflected beam occurs at θ = 30°. What is the kinetic energy E of the beam in eV?

Solution:

From Bragg’s law, 2d sin θ = nλ

p = h/λ = 6.6 × 10-24 kg m/s

E = 1/2 mv2 = p2/2m

E = 0.085 eV