A method of evaluating fast-neutron differential scattering cross sections with short experimental runs
The method utilized a pulsed neutron source with a smooth, broad spectrum, making it possible to measure neutron scattering from 0.5 to 10 MeV in a single run of 1 to 2 h duration for each scattering angle θ. Scattering samples of carbon, aluminum, iron, lithium hydride and concrete about 5 cm in dia. were located about 1/3 m from the pulsed neutron source, and neutrons scattered through an angle θ were directed down a 55-m flight path to a liquid organic scintillator used to measure the time-of-flight spectrum of scattered neutrons. The energy-dependent neutron intensity thus obtained was compared with Monte Carlo calculations whose inputs were the measured neutron source term and neutron elastic, inelastic, and reaction cross sections compiled from the existing literature. The comparison provides direct corrections to the cross sections for the lighter elements because single scattering is about ten times more probable than multiple scattering with the sample sizes chosen. These corrections can be further improved by repeating the calculation with the improved cross-section values. Inelastic scattering into any given energy region was small compared with elastic scattering for low Z scatterers because of the shape of the source spectrum. In this case, the method provides an unambiguous check on the elastic differential scattering cross sections used as input for the Monte Carlo calculations, and good first-order corrections to these cross-sections. Several modifications of both experiment and calculation are proposed for reducing the statistical uncertainty, for improving the experimental and calculational efficiencies, and for increasing the range of energy covered.