Neutron diffraction study of quantum effects on the pair correlationfunction of low-density 4He

An extensive neutron diffraction investigation on low-density (n < 4.35nm−3) states of helium gas along the 6-K isotherm has been performed by means of both wideand small-angle experiments, allowing for the extraction of the zero- and first-order density coefficients of c(k), the Fourier transform of the direct correlation function, in a very wide wavevector region extending from k = 1nm−1 to k = 160nm−1. The two independent measurements provide quantitatively consistent results, and the experimental quantities show a good agreement with the thermodynamic (i.e. k = 0) data. The comparison of the experimental pure two-body correlation with the corresponding result of a classical calculation clearly indicates the need of a quantum-mechanical approach.