Mean kinetic energy of helium atoms in fluid 3He and 3He-4He mixtures

Momentum distributions and mean kinetic energies of helium atoms, in pure fluid 3He and 3He-4He mixtures, at T = 2 K and 3He concentrations of x = 0.20 and x = 1.00, are presented. The experimental technique employed is deep inelastic neutron scattering measurements in the eV energy range, with wavevector transfer of typically 100 Å-1<q<250 Å-1. Single-particle dynamical properties of 3He-4He mixtures are discussed in the context of previous results on mixtures at different concentrations and pure 3He and 4He. In the pure fluids, the kinetic energy of 3He and 4He are remarkably similar for molar volumes above 25 cm3 mol-1, while for smaller molar volumes, upon approaching the liquid-solid transition, the kinetic energy is larger in 3He than 4He. On the other hand, the short-time dynamics of the helium mixture reveal quite a different picture with respect to the pure 3He and 4He: the momentum distribution and mean kinetic energy of the light helium component are independent of the molar volume and concentration.