Departures from the impulse approximation in deep inelastic neutron scattering

A new approach to the formulation of the impulse approximation (IA) in deep inelastic neutron scattering (DINS) is developed. It is shown that observed departures from the IA are due to the quantum nature of the initial state of the scatterer, rather than final-state effects (FSE) as has been previously assumed and that these effects are small at high temperature. It is shown that previous interpretations of FSE calculations are unjustified. Finally it is shown that DINS is a reliable method of measuring atomic-momentum distributions in most systems, but that in quantum fluids such as superfluid 4He, the IA cannot be reached at attainable momentum transfers