Small-angle neutron scattering from heavy water in the vicinity of the critical point

Small-angle neutron scattering spectra from supercritical heavy water were recorded along a near-critical isochore at temperatures between (Tc,exp+1.63 K) and (Tc,exp+22.02 K), Tc,exp being the experimental critical temperature. At low values of the wave number transfer q, the scattered intensity shows a pronounced increase due to the divergence of density fluctuations at the critical point. In the studied q range, from 0.07 to 0.36 Å−1, all the spectra can be accurately described by the Fisher–Langer correlation function provided that a multiplicative additional term describing short-range correlations between water molecules is taken into account. The value found for the amplitude of the critical correlation length is ξ0=(1.36±0.06) Å, in fair agreement with the value predicted by the two-scale hypothesis.