Pre-equilibrium emission surface effects in activation reactions

Improved cross-section calculations are reported in this work, based on updated particle-hole level density formalism including energy-dependent single-particle level densities as well as surface effects. The often low predictive power of the statistical and pre-equilibrium (PE) nuclear reaction models has recently been underlined by IRMM studies of both absolute cross sections and shape of excitation functions of fast neutron-induced reactions on medium mass nuclei. A better description of the high-energy side of activation excitation functions, most sensitive to PE modeling is obtained especially within the geometry-dependent hybrid (GDH) model by means of averages of the imaginary potential and nuclear density over projectile trajectories for various partial waves in order to determine the intranuclear transition rates. At the same time it is assumed that the reactions are localized within spherical shells determined by the projectile impact parameter. Actually, the discussion of the nuclear surface localization of preequilibrium reactions at low energy, by means of the semiclassical method, has confirmed the geometry effect taken into account within the GDH model. Moreover, Kalbach's recent phenomenological re-analysis of the surface localization of the initial target-projectile interaction provides an increased effect for lower energy neutrons, in close agreement with the results obtained by using the improved particle-hole level density within the GDH formalism. (authors)