Physics - Old Questions

9. In neutron spectroscopy a beam of monoenergetic neutrons is obtained by reflecting reactor neutrons from a beryllium crystal. If the separation between the atomic planes of the beryllium crystal is 0.732 Å  , what is the angle between the  incident neuton beam and the atomic planes that will yield a monochromatic beam of neutrons of wavelength of wavelength 0.1  Å?