  High energy vibrational scattering in the binary systems UC and US is
measured using time-of-flight inelastic neutron scattering. A clear set of
well-defined peaks equally separated in energy is observed in UC, corresponding
to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The
scattering is much weaker in US and only a few oscillator peaks are visible. We
show how the difference between the materials can be understood by considering
the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray
tracing is used to simulate the scattering, with near quantitative agreement
with the data in UC, and some differences with US. The possibility of observing
anharmonicity and anisotropy in the potentials of the light atoms is
investigated in UC. Overall the observed data is well accounted for by
considering each light atom as a single atom isotropic quantum harmonic
oscillator.
