Francis Michel

Local Potential Approach to the Alpha-Nucleus Interaction and Alpha-Cluster Structure in Nuclei

We review the merits of the local potential approach in the description of elastic alpha-particle scattering and of alpha-cluster structure in medium-weight nuclei. We recall how optical model analyses of elastic scattering angular distributions, displaying an anomalous large angle enhancement, yielded unique optical potentials, whose real part is determined with good accuracy up to small internucleus distances, and we investigate their properties; we then analyze the general compatibility of these potentials with the microscopic requirements resulting from the constraints due to antisymmetrization, in the case of a system which was thoroughly investigated from a microscopic point of view, and whose spectroscopy is well agreed upon, that is, the 20Ne = α + 16O system. The local potential approach is then applied to the 44Ti = α + 40Ca system - which is the analogue of 20Ne in the fp-shell -whose α-cluster spectroscopy remained unclear for a long time and for which microscopic calculations led to contradictory interpretations; not only does this simple model give a nice account of the energy location and intraband electromagnetic transition probabilities for the members of the 44Ti ground state band, but it predicts the existence of excited alpha-cluster bands, which were subsequently identified experimentally in alpha-transfer experiments. The same approach is applied to the 40Ca = α + 36Ar system, where similar predictions have recently been confirmed experimentally, and to other systems close to the sd-shell closure region. Finally the persistence of alpha-cluster structure in medium-weight and heavy nuclei is discussed within the local potential approach.

A unique deep potential for the 16O+16O system

We have reanalysed the recently observed nuclear rainbow data for the 16O+16O system and have demonstrated that there are three optical potentials which fit the data equally well. This discrete ambiguity originates from the limited angular range of the data. By taking into account the results of a recent analysis of low-energy resonant phenomena in this system and the dispersion relation between the real and imaginary parts of the optical potential, a unique selection of the potential set can be made.

Direct evidence for rapid energy dependence of the nucleus-nucleus interaction near the barrier radius at low energy

Abstract We show that a consistent description of 16 O(α, α 0 ) scattering close to the Coulomb barrier can be achieved only if (1) the interaction is made slightly angular momentum dependent; (ii) the surface of the potential is significantly less diffuse than that needed to describe the scattering data at higher incident energies. The second feature agrees qualitatively with recent theoretical predictions invoking either antisymmetrization or dispersion relation effects.

Barrier and internal wave contributions to the quantum probability density and flux in light heavy-ion elastic scattering

We investigate the properties of the optical model wave function for light heavy-ion systems where absorption is incomplete, such as

Interpretation of Airy minima in 16O + 16O and 16O + 12C elastic scattering in terms of a barrier-wave/internal-wave decomposition

\ensuremath{\alpha}{+}^{40}

A geometric approach to strong coupling effects in heavy-ion collisions. Deviation from the Fresnel diffraction pattern

Ca and

Evidence for a parity dependence of the α+20Ne interaction

\ensuremath{\alpha}{+}^{16}

Unexpected transparency in the scattering of fragile 6Li and 6He Nuclei

O around 30 MeV incident energy. Strong focusing effects are predicted to occur well inside the nucleus where the probability density can reach values much higher than that of the incident wave. This focusing is shown to be correlated with the presence at back angles of a strong enhancement in the elastic cross section, the so-called ALAS (anomalous large angle scattering) phenomenon; this is substantiated by calculations of the quantum probability flux and of classical trajectories. To clarify this mechanism, we decompose the scattering wave function and the associated probability flux into their barrier and internal wave contributions within a fully quantal calculation. Finally, a calculation of the divergence of the quantum flux shows that when absorption is incomplete, the focal region gives a sizable contribution to nonelastic processes.

Connection of Kukulin's nucleon-nucleon deep potential with realistic repulsive core interactions

The observation of refractive effects in 16O+16O and 16O+12C elastic scattering data has definitively established the fact that the optical potential for some light heavy-ion systems is relatively transparent and that its real part is deep. Most of the interpretations of the rainbow features of these data rely on the so-called nearside-farside decomposition of the scattering amplitude. Starting from recent optical model analyses of 16O+16O and 16O+12C elastic scattering around 100 MeV incident energy as an example, we present an alternative interpretation based on the barrier-wave/internal-wave decomposition first proposed by Brink and Takigawa. This method, which complements the nearside-farside approach, demonstrates clearly the exceptional transparency of the 16O+16O, and to a lesser extent 16O+12C, interactions at the investigated energies and makes possible the extraction of the two contributions whose interference explains the Airy oscillations seen in the farside amplitude.

Evidence for alpha-particle clustering in the /sup 44/Ti nucleus

We offer a novel interpretation of the deviation of the experimental angular distribution for elastic 18O+184W scattering at 90 MeV from the standard Fresnel diffraction pattern in terms of an interference between scattering amplitudes corresponding to different orientations of the target nucleus. The importance of Coriolis coupling, which is initially neglected in this geometric approach, is discussed in the case where long-range Coulomb coupling is active.