Final state effects in liquid 4He: An experimental test

Inelastic neutron scattering at high momentum transfers can provide direct information on the atomic momentum distribution n(p) when the Impulse Approximation (IA) is valid. In isotropic systems, the scattering in the IA is directly proportional to the longitudinal momentum distribution which is a function of a single scaling variable Y /triple bond/ (M/Q)(ω /minus/ ω/sub r/), where M is the mass of the scatterer, Q is the momentum transfer, and ω/sub r/ = Q2/2M is the recoiled energy. However, the experimentally attainable Q's are not large enough to reach the IA limit. Deviations from the IA due to final state scattering by neighboring atoms, known as final state effects, will distort the observed scattering from the IA prediction. Thus, an understanding of deviations from the IA is essential to accurate determination of n(p). Liquid helium provides an excellent testing ground for studying FSE in a dense, strongly interacting system for two reasons. First, theoretical calculations of the momentum distributions are available in both the normal liquid, and superfluid phases. These calculations are believed to be quite accurate, since they agree well with several other measured properties of the liquid. In addition, n(p) in the superfluid exhibits a very sharp feature, the Bose condensate peak, which should be very sensitive to FSE. Comparison of the predicted scattering obtained from the theoretical n(p) using the IA to the experimentally observed scattering can be used to study deviations due to FSE. 14 refs., 7 figs