Measurements of Neutron Spectra in Water, Polyethylene, and Zirconium Hydride

An experimental arrangement designed for accurate measurements of low-energy neutron spectra has been assembled and tested. A pulsed high-current electron linear accelerator is used to produce short bursts of fast neutrons which are introduced into a moderating and absorbing assembly. The steady-state energy spectrum of neutrons in the assembly is determined by pulsed-beam time-of-flight techniques. Hydrogen-moderated systems poisoned with a number of common neutron absorbers (boron, cadmium, samarium) have been studied, and the resulting spectra compared with theoretical predictions using both free and bound hydrogen scattering kernels. In general, a marked difference exists between measured spectra and spectra calculated using a free hydrogen kernel. In the case of water where a detailed scattering kernel is available for room temperature, theory and experiment are in reasonable agreement.