On the Origin of the Intensity Deficit in Neutron Compton Scattering
  Neutron Compton Scattering measurements in a variety of materials have shown a relative deficit in the total signal from hydrogen compared to deuterium and heavier ions. We show here that a breakdown in the Born-Oppenheimer approximation in the final states of the scattering process leads to such a deficit, and may be responsible for the effect. 
G. F. Reiter, P. M. Platzman