Equilibrium and pre-equilibrium processes in the 55Mn(6Li,xp) and 57Fe(α,xp) reactions

Spectra of outgoing neutrons and protons from the Li-6 + Mn-55 reaction and protons from the alpha + Fe-57 reaction have been measured with beams of 15-MeV Li-6 ions and 30-MeV alpha particles. These reactions proceed through the same Ni-61 nucleus at the same excitation energy, thus allowing the difference in reaction mechanisms to be studied. It is shown that spectra from the first reaction measured at backward angles are due to emission from a traditional compound nucleus reaction, in which the intermediate nucleus has reached statistical equilibrium; the spectra from the second reaction contain a significant fraction of pre-equilibrium emission at all angles. Level density parameters of the residual nucleus Co-60 have been obtained from the first reaction. Both emission spectra and angular distributions have been measured for the second reaction. It was found that the pre-equilibrium component exhibits a forward-peaked angular distribution, as expected, but with a steeper slope than predicted and with an unusual slight rise at angles above 120 degrees. The backward-angle rise is explained qualitatively by the dominance of the multistep compound mechanism at backward angles.