Position-Dependent Neutron Spectra in a Multiplying Medium

Position-dependent thermal spectra have been measured in a paraffin-moderated  ²³⁵U multiplying assembly using two independent methods. Steady-state spectra were obtained with the time-of-flight technique employing a 24.40-m flight in conjunction with the Rensselaer Polytechnic Institute linear accelerator; asymptotic (in time) spectra were obtained with the pulsed-LINAC chopper technique.  A new technique was developed to monitor the source intensity with a tracking reliability of ± 1.5%. The measured spectra are compared to a series of DTF-IV transport calculations using a P₁ kernel and P₁ source; the spatial source was obtained from a transport calculation. The spectrum calculations were performed with a polyethylene  kernel utilizing either the Goldman model or the Koppel-Young four-oscillator model. The experimental spectra are generally in closer (better than 5%) agreement with calculations using the Goldman kernel than with those using the Koppel-Young kernel. This result is consistent with the total cross section of polyethylene measured by Armstrong.