Experimental determination of the linear term in the density expansion of the dynamic structure factor by means of neutron scattering

The linear term in the density expansion of the dynamic structure factor has been determined experimentally for the first time. We used neutron-scattering data at very low momentum transfer (0.3<k<1.2 nm-1) on room-temperature 36Ar at low density (0.06-0.25 times the critical). Only for k<0.95 nm-1, the experimental data agree qualitatively with a corresponding hard-spheres system. We conclude that the linear term is a very sensitive probe for studying the two-particle interaction potential in dilute gases.