SLAC-PUB-95-6764 (October 1995) CALCULATIONS OF THE GIANT-DIPOLE-RESONANCE PHOTONEUTRONS USING A COUPLED EGS4-MORSE CODE

CALCULATIONS OF THE GIANT-DIPOLE-RESONANCE PHOTONEUTRONS USING A COUPLED EGS4-MORSE CODE J. C. Liu, W. R. Nelson, K. R. Kase and X. S. Mao Stanford Linear Accelerator Center, P. 0. Box 4349, Stanford, CA 94309 Abstract The production and transport of the photoneutrons from the giant-dipoleresonance reaction have been implemented in a coupled EGS4-MORSE code. The total neutron yield (including both the direct neutron and evaporation neutron components) is calculated by folding the photoneutron yield cross sections with the photon track length distribution in the target. Empirical algorithms based on the measurements have been developed to estimate the fraction and energy of the direct neutron component for each photon. The statistical theory in the EVAP4 code, incorporated as a MORSE subroutine, is used to determine the energies of the evaporation neutrons. These represent major improvements over other calculations that assumed no direct neutrons, a constant fraction of direct neutrons, monoenergetic direct neutron, or a constant nuclear temperature for the evaporation neutrons. It was also assumed that the slow neutrons (< 2.5 MeV) are emitted isotropically and the fast neutrons are emitted anisotropically in the form of l+Csin%, which have a peak emission at 900. Comparisons between the calculated and the measured photoneutron results (spectra of the direct, evaporation and total neutrons; nuclear temperatures; direct neutron fractions) for materials of lead, tungsten, tantalum and copper have been made. The results show that the empirical algorithms, albeit simple, can produce reasonable results over the interested photon energy range.