Abstract
The scattering of slow neutron beams provides unique, non-destructive, quantitative information on the structure and dynamics of materials of interest in physics, chemistry, materials science,
biology, geology, and other fields. Liquid hydrogen is a widely-used neutron moderator medium, and
an accurate knowledge of its slow neutron cross section is essential for the design and optimization
of intense slow neutron sources. In particular the rapid drop of the slow neutron scattering cross
section of liquid parahydrogen below 15 meV, which renders the moderator volume transparent to
the neutron energies of most interest for scattering studies, is therefore especially interesting and
important. We have placed an upper bound on the total cross section and the scattering cross section
for slow neutrons with energies between 0.43 meV and 16.1 meV on liquid hydrogen at 15.6 K using
neutron transmission measurements on the hydrogen target of the NPDGamma collaboration at the
Spallation Neutron Source at Oak Ridge National Laboratory. At 1 meV this new upper bound is a
factor of 3 below the data from previous work which has been used in the design of liquid hydrogen
moderators at slow neutron sources. We describe our measurements, compare them with previous work, and discuss the implications for designing more intense slow neutron sources.
