Neutron and Gamma-Ray Spectra from ²³²Th, ²³⁵U, ²³⁸U, and ²³⁹Pu After Bombardment with 14-MeV Neutrons

The neutron and gamma-ray leakage spectra from pulsed spheres having a radius equal to 1.0 mean-free-path (mfp) for 14-MeV neutrons in ²³²Th, ²³⁵U (with radii of 0.7 and 1.5 mfp), ²²⁸U (with radii of 0.8 and 2.8 mfp), and ²³⁹Pu (with radii of 0.7 and 1.25 mfp) were measured by using time-of-flight techniques. The neutron spectra were measured between 0.9 and 15 MeV using stilbene and NE-213 scintillators. For the gamma rays, the electron recoil spectra were measured between 0.35 and 8 MeV with the NE-213 detector only. Pulse-shape discrimination and flight paths of ∼10 m were used in these measurements. The measured spectra are compared with calculations carried out with TARTNP, a coupled neutron-photon Monte Carlo transport code. The Lawrence Livermore Laboratory ENDL (neutron and photon cross sections) and ENDF/B-IV libraries were used in these calculations. In the region between 10 and 15 MeV, the calculated neutron spectra are in good agreement with the measurements (±5%). The maximum discrepancy observed for the neutrons between 0.9 and 10 MeV is 30%. The notable exception is ²³²Th, where calculations carried out with the ENDF/B-IV underestimate by a factor of 2 the neutron emission between 5 and 10 MeV. The gamma-ray emission calculations were carried out only with ENDL because of its overall better representation of the neutron measurements; ENDL overestimates the gamma-ray production, with discrepancies ranging between 5 and 20%. However, use of the measured detector efficiency would reduce the maximum discrepancy to <13%. To facilitate the use of the neutron data for calculational purposes by other laboratories, an “experimental” one-dimensional neutron energy spectrum is given for each measurement.