Momentum Distribution of4He across the Melting Transition

We have measured the momentum distribution of liquid and solid ${}^4$He by means of neutron Compton scattering, using the high energy neutrons of the pulsed neutron source ISIS. The experiment has been carried out on the isotherm at 4.35 K in the pressure interval between 35 and 500 bars. We have obtained the value of the kinetic energy, as a function of density, at four thermodynamic points in the liquid phase and eleven thermodynamic points in the solid phase. The data of the present experiment extend considerably the previous density range and reveal a gap in the density behavior crossing the melting transition.