Fast-Photoneutron Spectra due to 55-85-MeV Photons

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Abstract
Energy spectra of fast photoneutrons from eighteen elements (Li, Be, B, C, N, O, Al, S, Fe, Cu, Zn, In, Sn, Ta, Tl, Pb, Bi, and U) were measured at 67½°, using thin-target bremsstrahlung from the Rensselaer linac. The neutron detection system utilized a 100-meter flight path and had an over-all time resolution of 0.4 nsec/m. Measurements of the spectra from each target at bremsstrahlung energies of both 55 and 85 MeV permitted extraction of neutron difference spectra effectively due to photons between 55 and 85 MeV. Absolute normalization of the spectra was effected by comparison with the experimentally measured photoneutron spectrum of deuterium. Difference spectra were obtained for neutron energies greater than 10 MeV. Cross sections for production of these neutrons ranged from 59 μb/sr for lithium to 1.28 mb/sr for uranium. The experimental data were compared to the predictions of the quasideuteron model as recently modified by Levinger, with secondary-neutron effects estimated by Monte Carlo calculations. Good agreement was obtained for the absolute-neutron-production cross sections and the shapes of the neutron energy spectra over the range of targets studied.