Structure of helium at 4 and 8 K from neutron diffraction

The authors determine by neutron diffraction the static structure factor S(q) of fluid helium at 4 K and 8 K at number density n = 19.5 nm-3 for wavenumbers 0.5 < q < 7 nm -1. S(q) decreases very rapidly from its thermodynamic value S(0) at q = 0 to about S(0)/2 at q ≅ 5 nm-1 confirming the behavior seen in X-ray diffraction experiments. The authors combine the experimental data with previous larger-q results and compare them with classical model calculations that indicate long range correlations ∼ 1/r4 between the helium particles. By means of computer simulations they show that the structure is incompatible with any Lennard-Jones interaction