The Temperature Dependence of Neutron Inelastic Scattering from Water

The double differential neutron scattering cross section for water has been measured at temperatures of 27, 170, and 270°C, The RPI linear electron accelerator provided the pulsed neutron source for a time-of-flight analysis of neutron energy. By phasing the LINAC with a high speed chopper, incident neutron energies ranging from 0.04 to 0.632 eV were selected. Energy distributions of scattered neutrons were obtained at scattering angles of 10, 14, 25, 40, 60, 90, 120, and 150 deg. The relatively wide range of incident energies with good resolution made it possible to observe clearly the molecular energy levels. The structure was seen to broaden considerably in going from 27 to 170°C but was little changed by the further temperature increase. A model has been developed in which the water scattering system is considered as consisting of aggregates of molecules. These clusters have temperature-dependent sizes, and diffuse according to a temperature-dependent diffusion constant. The energy levels of the molecules bound in the cluster are represented by a multi-Gaussian frequency distribution which varies with temperature. The internal vibrations of the individual water molecules are represented by delta functions. A new approach has been taken in comparing this and other models with the data, involving the application of recent improvements in methods of treating resolution and multiple scattering: a Monte Carlo technique has been used to impose these conditions on the models. These comparisons, and comparisons with integral data, indicate that the new model should have advantages for reactor calculations.