Coherent Inelastic Scattering of Slow Neutrons by a Polyatomic Liquid Element

Abstract

An expression for the coherent inelastic-scattering cross section of slow neutrons by a diatomic liquid element has been determined by extending Vineyard's approximation to systems with internal degrees of freedom. The range of validity of the expression obtained has been discussed, taking into account also the energy resolution of the experimental setup. The energy spectrum of neutrons scattered by a sample of liquid bromine has been measured with a triple-axis spectrometer for a fixed value of the modulus of the momentum exchange. Reasonable experimental evidence for the existence of a peak associated with the excitation of the first vibrational level in liquid bromine was obtained in the neighborhood of an energy transfer $\ensuremath{\hbar}\ensuremath{\omega}=0.04$ eV; this is approximately the value established by measurements of conventional spectroscopy in the gaseous phase.