Transmission of Slow Neutrons by Liquid Helium

Abstract

We have determined the total scattering cross section of liquid helium for neutrons of wavelengths between 3 and 16 angstroms and at six helium temperatures between 1.25 and 4.6°K. The neutrons were obtained from the thermal beam of a reactor and were monochromatized by a low resolution velocity selector. The total scattering cross section decreases with temperature at all wavelengths studied. At the shortest wavelengths the cross section approximates the free atom value; it exhibits a rapid drop with increasing wavelength. For the 4.6° liquid it passes through a shallow minimum at about 10 A. At lower helium temperatures, the fall from the free atom cross section is steeper; the existence of a minimum has not been established. The results are discussed qualitatively on the basis of several models of liquid helium. The only one predicting the overall features of the change of cross section with wavelength and temperature is the solid model.