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Neutron Spectrum from a Uranium-235 Sphere Bombarded by 14-MeV Neutrons

C. E. Ragan III, G. F. Auchampaugh, A. Hemmendinger, M. G. Silbert

Nuclear Science and Engineering

Volume 61 / Number 1 / September 1976 / Pages 33-39


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A benchmark measurement of the neutron leakage spectrum from a pulsed 38-kg uranium (93.5% 235U) sphere has been made using time-of-flight techniques. The sphere had a multiplication of ∼11 for 14-MeV neutrons, and a neutron hold-up time of ∼40 nsec. The centrally located source of 14.1 ± 0.8-MeV neutrons, produced by bombarding a tritium gas target with pulses of low-energy deuterons, was isotropic to ±7.7%. Neutrons in the 0.180- to 16.0-MeV energy range were detected at the end of a 39-m flight path by an Ne-213 liquid scintillator employing pulse-shape discrimination. The detector efficiency was measured over this same energy range using monoenergetic neutrons from the T(p,n) T(d,n), and D(d,n) reactions. The measured neutron flux as a function of energy is compared with the results of Monte Carlo calculations performed with the MCN code. Uranium cross sections from ENDF/B-IV and an older set from Lawrence Livermore Laboratory were used in these calculations. The results calculated using the ENDF/B-IV cross sections are in good 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.

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