Pre-equilibrium excitation of nuclei in (p,α) and (p,3He) reactions induced by 120, 160 and 200 MeV protons

The study of pre-equilibrium reactions induced by medium-energy light ions have become of increasing importance in recent years as they represent the bulk of the measured spectrum. On a nuclear time scale these reactions are known to take place after a one-step process of exciting discrete states in the residual nucleus but long before the process whereby the projectile distributes its kinetic energy equally over all target nucleons to form a compound nucleus which decays statistically. In this work the systematic behaviour of the continuum of complex ejectiles was investigated as a function of incident proton energy and target mass. The continuum energy spectra for the reactions 27Al(p,X), 59Co(p,X) and 197Au(p,X), with X = α and 3He were measured using a five element detector telescope. Data was acquired at scattering angles between 10-160 degrees at incident proton energies of 120 and 200 MeV, and at scattering angles between 20-140 degrees for an incident proton energy of 160 MeV. Experimental angle-integrated cross sections were compared with calculated angle-integrated cross sections using the geometry dependent hybrid (GDH) model. The comparison between the experimental data and the calculations were used to quantitatively estimate the contribution of the evaporation process to the pre-equilibrium spectrum. The experimental angular distributions from the reaction (p,α) and (p,3He) on 27Al, 59Co and 197Au were compared with calculated angular distributions using the phenomenological parametrization of Kalbach. In general good agreement between the experimental data and the calculations were obtained. Minor discrepancies were observed with regard to target mass and incident proton energy dependence. It is believed that the characteristics of these discrepancies could provide guidance to the future attempts to develop a proper quantum mechanical description of the reaction mechanism of complex-ejectile production induced by energetic protons. (author). 39 refs., 52 figs., 3 tabs