STRUCTURE OF HELIUM AT 4 AND 8 K FROM NEUTRON-DIFFRACTION

We determine by neutron diffraction the static structure factor S(q) o f fluid helium at 4 K and 8 K at number density n = 19.5 nm(-3) for wa ve numbers 0.5 < q < 7 nm(-1). S(q) decreases very rapidly from its th ermodynamic value S(0) at q = 0 to about S(0)/2 at q similar or equal to 5 nm(-1) confirming the behavior seen in X-ray diffraction experime nts. We combine the experimental data with previous larger-q results a nd compare them with classical model calculations that indicate long r ange correlations similar to 1/r(4) between the helium particles. By m eans of computer simulations we show that the structure is incompatibl e with any Lennard-Jones interaction.