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

Momentum distributions and mean kinetic energies of helium atoms, in pure fluid He-3 and He-3-He-4 mixtures, at T = 2 K and He-3 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 angstrom(-1) < q < 250 angstrom(-1). Single-particle dynamical properties of He-3-He-4 mixtures are discussed in the context of previous results on mixtures at different concentrations and pure He-3 and He-4. In the pure fluids, the kinetic energy of He-3 and He-4 are remarkably similar for molar volumes above 25 cm(3) mol(-1), while for smaller molar volumes, upon approaching the liquid-solid transition, the kinetic energy is larger in He-3 than He-4. On the other hand, the short-time dynamics of the helium mixture reveal quite a different picture with respect to the pure He-3 and He-4: the momentum distribution and mean kinetic energy of the light helium component are independent of the molar volume and concentration.