Momentum distributions and the impulse approximation in helium

The momentum distribution n(p) of atoms in condensed matter may be determined from neutron inelastic scattering measurements. However, sattering at high momentum transfers h-bar Q (high enough that the impulse approximation holds) is required. A model of solid helium is used to assess the error introduced in n(p) by assuming the impulse approximation holds at several Q values. At Q = 20 angstrom super(-1) the error in n(p) is similar to 5-10%. This error can be reduced by a factor of ten using a symmetrization procedure.