The scattering of cold neutrons by liquid helium

The differential cross-section and the average energy loss of 45°K neutrons for scattering by liquid helium have been measured at angles of scatter near 0 and 180° and for liquid temperatures from 1∙6 to 5∙2°K. From these data the second moments of the radial distribution function and the zero-angle cross-sections have been calculated. The latter are close to those given by the theory of density fluctuations in a liquid. The energy transfers cannot be explained in the absence of a liquid model. The backward scattering can be explained as either a free atom process or one in which phonons are excited. Data on the scattering of 45°K neutrons by the saturated helium vapour at 4∙2°K are given and it is concluded that the moments of the radial distribution function are larger for the gas than the liquid.