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

Luisa F. Hansen, Calvin Wong, Ted T. Komoto, Bertram A. Pohl, Eugene Goldberg, Robert J. Howerton, Walter M. Webster

Nuclear Science and Engineering / Volume 72 / Number 1 / Pages 35-51

October 1979

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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.

 
 
 
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