Alpha-particle mean field and statistical emission

Available as short communication only. All direct, pre-equilibrium (PE) and compound emission of nucleons as well as α-particles which occur in fast neutron reactions have to be described in a consistent way, e.g. by using the same independent input parameter sets within semiclassical reaction models. The (n, α) reaction cross section in the first few MeV above the threshold is determined by the compound nucleus mechanism alone, so that it can indeed be a test of the α-particle optical model potential (OMP), a set of global OMP for α-particles with energies above 80 MeV was extended to lower energies and proved appropriate to describe (n,α) reactions in the A=50 region. The higher energy (n,α) data, including the 14 MeV neutron induced reactions, comprise also PE as well as direct processes. a generalized version of the Geometry-Dependent-Hybrid (GDH) model has been developed by assuming in addition to the angular momentum and parity conservation: (i) the α-particle pre-formation probability (Φ)α; (ii) the energy-dependent single-particle state density for nucleus as well as for α-particles; (iii) the Fermi energy for the α-particle cluster of the form εFα=4 εF+Bα and the α-particle state density gα(εFα)=A/10.36 MeV-1; (iv) the intranuclear transition rates given by the average imaginary OMP. The (n, α) reaction excitation function has been calculated and compared with the experimental data for the targets 51 V, 52 Cr, 55 Mn, 54 F, and 59 Co. The analysis involved also the competing reaction channels, i.e. the (n,p) and (n,2 n) reactions, as well as a cross reaction with the same residual nucleus but in the neutron ar proton channel, so that no level density could effect the agreement with experiment. (author) 2 Figs., 8 Refs