The basic approximations of the Griffin model for nuclear reactions are reexamined. Proton-neutron distinguishability and the Pauli exclusion principle are treated in an improved way. Several improvements in the model input values are suggested. Direct reaction mechanisms are systematically considered, and semi-empirical descriptions are proposed for those not included in the Griffin model calculations (knockout and inelastic processes involving alpha particles and stripping and pickup). When all of these points are considered, good agreement with experimental energy spectra for emitted particles are obtained without arbitrary and unphysical factors in the particle emission rates and without the artificial projectile dependence previously needed for the average residual two-body matrix element.

The Griffin model, complex particles and direct nuclear reactions

Complex-particle emission in the pre-equilibrium exciton model

Pre-equilibrium emission

Nuclear data sheets for A = 136

Proton-induced α- and Τ-emission at 72 MeV

Energy spectra of α-particles emitted under bombardment of (20÷30) MeV protons have been studied in many nuclei of masses ranging from 150 to 210. It is found that the α-spectrum is mainly due to the pre-compound emission mechanism, except for the nuclei near lead, where a strong component of direct effect is found. As the pre-compound emission mechanism here considered assumes the pre-formation of α-like clusters in the target nucleus, the reduction of this effect in nuclei near lead is attributed to a reduction of the α cluster pre-formation coefficient. In the frame of the pre-compound theory, the α cluster pre-formation coefficient is extracted from the absolute values of the reaction cross-section and compared with other results.

G. M. Braga-Marcazzan

Papers

Further measurements of the probability of α cluster pre-formation by means of (p, α) reactions in heavy elements