Abstract
Experiments on small-angle scattering of neutrons on liquid helium at temperatures of 1.0–5 K at the ISIS pulsed neutron source (England) are reported. Detailed measurements of the angular distribution of the scattered neutrons reveal a significant change in the temperature dependence of the second moment of the pairwise correlation function (the first derivative of the angular distribution for small scattering angles). At high temperatures the angular distribution of the scattered neutrons follows the classical description for small-angle scattering, but at temperatures below the λ-point, quantum mechanical behavior is observed (scattering of neutrons on quantum fluctuations). It is confirmed experimentally that over the entire temperature range the neutron scattering cross section at zero angle is determined by classical thermodynamic fluctuations in the density.
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