Neutron-Scattering Study of the Momentum Distribution ofHe4

Neutron inelastic scattering at a momentum transfer of about 15 ${\mathrm{\AA }}^{-1\mathrm{}}$ has been used to measure the scattering law $S(Q,\omega )$ with high accuracy for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at 1.2 and 4.2°K. At this momentum transfer the impulse approximation is nearly valid and momentum distributions for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ can be determined directly from the data. The momentum distributions for the two temperatures show features not previously observed and provide new information to compare with theoretical results.