Quantum momentum distribution and kinetic energy in solid 4He.
We present measurements of neutron scattering from solid 4He at high momentum transfer. The solid is held close to the melting line at molar volume 20.87 cm3/mol and temperature T=1.6 K. From the data, we determine the shape of the momentum distribution, n(k), of atoms in the solid and the leading final state contribution to the scattering. We show that n(k) in this highly anharmonic, quantum solid differs significantly from a Gaussian. The n(k) is more sharply peaked with larger occupation of low momentum states than in a Maxwell-Boltzmann distribution, as found in liquid 4He and predicted qualitatively by path integral Monte Carlo calculations. The atomic kinetic energy is =(24.25+/-0.30) K. If n(k) is assumed to be Gaussian, as is usually the practice, a 10% smaller is obtained.
Quantum effects in deep inelastic neutron scattering.
Presentation de l'approximation des impulsions (IA) qui est utilisee pour interpreter les mesures de diffusion inelastique profonde de neutrons. La diffusion predite est identique a celle qui pourrait etre obtenue a partir d'un gaz d'atomes non interactifs ayant une distribution de quantite de mouvement des atomes dans le systeme cible. Hypotheses fondamentales et validite de l'IA. Non validite de l'hypothese de l'IA dans un systeme quantique a la temperature zero. On montre que les effets de l'etat initial causes par la rupture de l'IA a basses temperatures, rendent compte des asymetries observees et du deplacement des pics par rapport aux predictions. On montre que les effets des etats finaux sont negligeables. Discussion des conditions pour lesquelles les effets de l'etat initial sont negligeables
Resolution in deep inelastic neutron scattering using pulsed neutron sources
In recoil scattering with neutrons the impulse limit [1,2] is only reached by utilising large incident neutron energies generally in excess of 1eV, which are now readily available on spallation neutron sources. Since the property of direct interest is the atomic momentum distribution, n(p), (where p represents the atomic momentum), it is important to present and compare the recoil scattering data in terms of this quantity and not in terms of the measured scattering function S(q,ω) [3]. This transformation is relatively easy to undertake and, moreover, offers several distinct advantages over a comparison in time-of-flight or energy transfer.