Guy Reidemeister

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.

Microscopic study of the 16O+16O quasimolecule

An angular momentum projected microscopic calculation, including the Coulomb force, is performed with accuracy for the 16O+16O system. The projected energy curves form an almost pure rotational structure, but with an effective reduced mass which is a function of the distance. A critical number Nc = 24 is shown to exist. The recent ORNL experimental results seem to confirm this value. Simple parametrizations make clear the essential characteristics of the projection and allow calculation of the effective reduced mass. Emphasis is put on the quasibound states of the system, both in the projected curves and in a generator coordinate calculation. Some confirmation of their energies is found in experimental results on related channels. The rotational band characterized by a constant 62 keV is due to the existence of a quasimolecule whose excitation energy in the spectrum of 32S is estimated to be 24.5 MeV.

Deformation of the oxygen heavy ions in a molecular description of the 16O + 16O system

Abstract A microscopic description of the system formed by two interacting 16 O nuclei is presented. The total wave function is a Slater determinant built up with the molecular orbitals of a two-centre shell model. The deformation of the nuclei due to their nuclear interaction is determined through a variational calculation. This calculation suggests the existence of a very deformed prolate state in 32 S.

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

P4 deformation in an independent-particle model of light nuclei

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

Centre-of-mass motion for a system of two unequal fragments

Ca and

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

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

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

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.