Neutron spectrum from an oralloy sphere

The neutron spectrum from a 38-kg oralloy sphere, pulsed by a centrally located 14-MeV neutron source, has been measured using time-of-flight (TOF) techniques. The nearly isotropic source of 14-MeV neutrons was produced by bombarding a tritium gas target with low-energy deuterons. Neutrons in the 0.180- to 15.0-MeV energy range were detected at the end of a partially evacuated, 39-m flight path by a NE-213 liquid scintillator that used pulse shape discrimination to distinguish neutron events from gamma-ray events. Detector thresholds of 0.15 and 0.85 MeV were used to cover the above energy range, and the detector efficiency was measured over this range using monoenergetic neutrons from the T(p,n), T(d,n), and D(d,n) reactions. Several measurements of the TOF spectrum were used to determine the neutron flux at the detector, and a comparison of this flux with the results of Monte Carlo calculations is presented. These calculations were performed with the code MCN using two cross-section sets, those from the Lawrence Livermore Laboratory (LLL) and those from the ENDF/B-IV library. The results calculated using the ENDF/B-IV cross sections are in excellent agreement with the measurements, especially in the 1- to 6-MeV energy region, where the uncertainties in both the calculated and experimental results are the smallest. (5 tables, 15 figures) (U.S.)