The single atom kinetic energy kappa of high purity solid hcp 4He has been measured by neutron Compton scattering, at temperatures between 0.07 and 0.4 K and a pressure of 40 bar. Within statistical error of approximately 2% no change in kappa was observed. The values of kappa at approximately 0.07 K were the same in a single crystal and a polycrystalline sample and were also unaffected (within statistical error) by the addition of 10 ppm of 3He. The lattice constant was also found to be independent of temperature to within 1 part in 2000. These results suggest that the supersolid transition in 4He has a different microscopic origin to the superfluid transition in the liquid.

We present neutron scattering measurements of the atomic momentum distribution, n(k), in solid helium under a pressure p = 41 bars and at temperatures between 80 mK and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, Tc = 200 mK where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k = 0 state below Tc, we find a condensate fraction of n0 = (-0.10 ±1.20)% at T = 80 mK and n0 = (0.08±0.78)% at T =120 mK, consistent with zero. The shape of n(k) also does not change on crossing Tc within measurement precision.

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Neutron scattering measurements of the dynamic structure factor, S(Q, ω), of liquid 4 He as a function of pressure at high momentum transfer, Q, are presented. At high Q the dynamics of single atoms in the liquid is observed. From S(Q, ω) the atomic momentum distribution, n(k), the Bose-Einstein condensate fraction, n0, and the Final State broadening function are obtained. The shape of n(k) differs from a classical, Maxwell-Boltzmann distribution with higher occupation of low momentum states in the quantum liquid. The width of n(k) and the atomic kinetic energy, K , increase with pressure but the shape of n(k) remains approximately independent of pressure. The present observed and Monte Carlo calculations of K agree within error. The condensate fraction decreases from n0 = 7.25 ± 0.75% at SVP (p ≃ 0) to n0 = 3.2 ± 0.75% at pressure p = 24 bar, a density dependence that is again reproduced by MC calculations within observed error. The Final State function is the contribution to S(Q, ω) arising from the interaction of the struck atom with its neighbors following the scattering. The FS function broadens with increasing pressure reflecting the increased importance of FS effects at higher pressure.

Today almost a quarter of all neutron scattering experiments performed at neutron scattering facilities require sample temperatures below 2 K. However, a global shortage of helium gas can seriously jeopardise low temperature experimental programmes at neutron scattering laboratories.Luckily the progress in cryo-cooler technology offers a new generation of cryogenic systems with significantly reduced consumption and in some cases nearly a complete elimination of cryogens.Here we discuss design and test results of a new cryogen free top-loading cryostat developed by the through an ISIS and Oxford Instruments collaborative project. The cryostat provides neutron scattering sample environment in the temperature range 1.4 -300 K. High cooling power (0.23 W at temperature less than 2 K) achieved at the cryostat's variable temperature insert heat exchanger allows operation of a standard dilution refrigerator insert in a continuous regime. From a user perspective, the system offers operating parameters very similar to those of an Orange cryostat but without the complication of cryogens.