Design of a cold moderator for total cross section measurements of moderator materials at sub-thermal energies

Abstract

In order to extend the neutron cross section measurement capability to below 0.001 eV, the sub-thermal neutron flux of the existing enhanced thermal target needed to be improved at the Rensselaer Polytechnic Institute (RPI) linear accelerator (LINAC). To meet this need, a novel polyethylene based cold moderator capability was designed and constructed. When coupled to the enhanced thermal target, the neutron flux was increased by up to a factor of 8 below 20 meV after background subtraction. A new neutron producing target was also designed in order to handle higher electron beam intensities that would be present with the new proposed upgrade to the RPI LINAC.This cold moderator capability was used for transmission measurements of various moderating materials, including yttrium hydride at a hydrogen concentrations of 1.85 and 1.68, polyethylene, polystyrene and Plexiglas from 0.0005 - 3 eV. These measurements were compared directly to thermal scattering library (TSL) evaluations and existing experiments where applicable. Generally good agreement was found, but some discrepancies were noticed and will be discussed. The polyethylene measurements validated the use of the cold moderator system and its method, while extending the measured cross section to 0.0005 eV. For yttrium hydride, the measured range was extended above 0.8 eV and below 0.05 eV, representing the first measurements to encompass the entire thermal energy range. The Plexiglas measurements represented the first where the material was well known and extended the measured cross section above 1 eV and below 0.002 eV. These measurements are the first total thermal cross section measurements for polystyrene.