M.M. Coimbra

Fusion of 16O + 144Sm at sub-barrier energies

Abstract Fusion cross sections have been measured for the system 16 O+ 144 Sm at bombarding energies in the range 63 MeV ⩽E lab ( 16 O )⩽72 MeV by observation of delayed X-rays emitted by the evaporation residues. Comparison of the present results for the magic nucleus 144 Sm with those already existing for the fusion of 16 O with other samarium isotopes shows that the enhancement of the cross sections at sub-barrier energies can be explained in terms of a one-dimensional barrier-penetration model that incorporates the quadrupole deformation parameter, s 2 . There is no need to invoke the neck degree of freedom.

Fusion and peripheral processes in the 9Be + 10,11B reactions

Abstract Measurements of total fusion cross sections for 9 Be + 10,11 B have been performed over the energy range from 2 to 4.5 times the Coulomb barrier. Elastic scattering and peripheral processes have also been investigated. No significant inhibition of the maximum fusion cross section is observed in spite of the low binding energy of the collision participants.

Fusion of 59Co with light projectiles at near barrier energies

Abstract Fusion cross sections have been measured for the 14 N, 16 O+ 59 Co systems at bombarding energies from 32 to 56 MeV. Total transfer cross sections to excited states were simultaneously measured for the stripping of one proton and stripping of one neutron channels in the 14 N induced reactions, and their upper limits for other transfer channels could be estimated. The elastic-scattering angular distribution was measured at E lab = 40 MeV for the 16 O+ 59 Co system. The effects of collective properties of the nuclei, included in the Wong model or the alternative approach of coupling inelastic channels, were found to be too small to explain the large sub-barrier fusion cross section enhancement. In order to perform simplified coupled-channel calculations involving transfer channels, a semiclassical formalism was used to derive the transfer form factors. Within the uncertainties of the calculations, the behavior of the sub-barrier fusion could be understood in terms of the coupling of charged-particle transfer channels, particularly the stripping of multi-nucleons, which act as doorways to fusion. An alternative approach, the liquid drop model, which accounts for the neck formation, also leads to satisfactory and compatible results.

Fusion of 16O + 46,50Ti near and below the coulomb barrier

Abstract Fusion excitation functions for the systems 16O + 46,50Ti at bombarding energies in the range 30 ≦ Elab(16O) ≤ 65 MeV were measured by means of the γ-ray spectroscopy technique. The fusion excitation functions are analysed using a semi-classical one-dimensional barrier-penetration model with different nuclear potentials describing the heavy-ion interaction. Fusion barrier parameters were extracted. The observed fusion cross sections at sub-barrier energies are larger than predicted by this conventional model. The fusion enhancement at sub-barrier energies for these systems can be explained by the simple approach of the Wong model, considering explicity the quadrupole deformation parameter of the target nucleus.

The validity of the compound nucleus mechanism in the 12C(16O, α) reaction at high energies☆

Abstract Excitation functions for several discrete states in 24 Mg excited in the 12 C( 16 O, α) reaction are studied from E( 16 O ) = 40 MeV to 150 MeV . A Hauser-Feshbach analysis is performed for states with J π = 0 + through 10 + and a consistent interpretation of these data is obtained in a compound nucleus model with reasonable critical angular momenta. We show that the energy dependence of the high spin selectivity is in agreement with the absence of selectivity recently reported for the 14 N( 14 N, α) reaction at high energy. Contrary to recent speculations, no compelling evidence for preequilibrium or direct processes in 16 O + 12 C reactions is found.