Three-Body Effects from the Density Dependence of the Neutron Structure Factor of Gaseous 36 Ar at 140 K

We present an analysis of the density dependence of the static structure factor in low density 36 Ar gas at T = 140 K, from which we derive the three-body contribution. Within the experimental accuracy, the three-body contribution in the linear density expansion for the Fourier transform of the direct correlation function C(k) agrees, for 3 < k < 100 nm -1 , with theoretical calculations based on a pair potential and the Axilrod-Teller three-body potential, eventually modified for short-range effects; for k < 3 nm -1 the neutron diffraction data show a behaviour significantly different with respect to the theoretical predictions.