Reaction Mechanisms with Exotic Nuclei

This talk examines a number of reaction mechanisms for scattering initiated by an exotic projectile. Comparisons are made with recent experimental data, in order to extract information on the peculiarity of the nuclear structure under extreme conditions and to test the accuracy of the available theoretical methods. Predictions for future experiments are also made.Comment: 13 Latex pages, 2 table, 2 ps figures. Invited talk given at the Symposium on Nuclear Clusters, Rauischholzhausen, Germany, 5-9 August 200

Optical potentials of halo and weakly bound nuclei

The optical potential of halo and weakly bound nuclei has a long range part due to the coupling to breakup that damps the elastic scattering angular distributions at all angles for which the effect of the nuclear interaction is felt. In charge exchange reactions leading to a final state with a halo nucleus, the surface potential is responsible for a strong reduction in the absolute cross section. We show how the halo effect can be simply estimated semiclassically and related to the properties of the halo wave function. Assuming an exponential tail for the imaginary surface potential we show that the most important parameter is the diffusness $\alpha$ of the potential which is directly related to the decay length $\gamma_i$ of the initial wave function by $\alpha\approx(2\gamma_i)^{-1}$Comment: 18 Latex pages, 1 table, 1 eps figures, 3 ps figures. accepted to Nucl. Phys.

On the eikonal approach to nuclear diffraction dissociation

The study of nuclear breakup of halo and weakly bound particles has been one of the key ingredients in the understanding of exotic nuclei during the last thirty years. One of the most used methods to analyse data, in particular absolute breakup cross sections, has been the eikonal approximation. Here we revise critically the formalisms used for calculating the diffraction dissociation part of nuclear breakup and show that there is a formula that can be applied to breakup on any target, while a most commonly used formula must be restricted to light targets as it contains also the effect of Coulomb breakup calculated to first order in the sudden approximation which is well known for not being accurate.Comment: 5 pages, 1 figur

Dynamical effects in proton breakup from exotic nuclei

We study dynamical effects in proton breakup from a weakly bound state in an exotic nucleus on a heavy target. The Coulomb interactions between the proton and the core and the proton and the target are treated to all orders, including also the full multipole expansion of the Coulomb potential. The dynamics of proton nuclear and Coulomb breakup is compared to that of an equiva- lent neutron of larger binding energy in order to elucidate the differences with the well understood neutron breakup mechanism. A number of experimentally measurable observables such as parallel momentum distributions, proton angular distributions and total breakup cross sections are calculated. With respect to nuclear breakup it is found that a proton behaves exactly as a neutron of larger binding energy. The extra 'effective energy' is due to the combined core-target Coulomb barrier. In Coulomb breakup we distinguish the effect of the core-target Coulomb potential (called recoil effect), with respect to which the proton behaves again as a more bound neutron, from the direct proton-target Coulomb potential. The latter gives cross sections about an order of magnitude larger than the recoil term. The two effects give rise to complicated interferences in the parallel momentum distributions. They are instead easily separable in the proton angular distributions which are therefore suggested as a very useful observable for future experimental studies.Comment: 10 pages, 8 figures, 1 table. Accepted for publication on Phys. Rev.

Coulomb breakup effects on the optical potentials of weakly bound nuclei

The optical potential of halo and weakly bound nuclei has a long range part due to the coupling to breakup that damps the elastic scattering angular distributions. In order to describe correctly the breakup channel in the case of scattering on a heavy target, core recoil effects have to be taken into account. We show here that core recoil and nuclear breakup of the valence nucleon can be consistently taken into account. A microscopic absorptive potential is obtained within a semiclassical approach and its characteristics can be understood in terms of the properties of the halo wave function and of the reaction mechanism. Results for the case of medium to high energy reactions are presented.Comment: 25 latex pages, 4 tables, 6 figures. Submitted to Nucl. Phys.

The optical potentials and nuclear reaction cross sections for the nn-12 ^{12}C and NN-12 ^{12}C scattering

In this work we extend a previously derived $n$- $^9$Be optical potential up to 500 MeV and apply it to the system $n$- $^{12}$C, finding excellent results for the energy dependence of the total cross sections. Results obtained with a standard optical model calculation are compared to those from the eikonal formalism in order to asses the accuracy of the latter as a function of the nucleon incident energy. For comparison, single folded (s.f.) nucleon-target potentials are also obtained using $^{12}$C densities from different models. These potentials are sensitive to the density used and none of them reproduce the characteristics of the phenomenological potential nor the cross section results. We then calculate nucleus-nucleus ($NN$) potentials and total reaction cross sections for some "normal" and exotic projectile nuclei on $^ {12}$C within the eikonal formalism. We find that single folded (S.F.) projectile-target imaginary potentials and double folded (D.F.) potentials can produce similar energy dependence of the reaction cross sections but the S.F. results agree better with experimental data provided the radius parameter of the phenomenological $n$-target potential is allowed to be energy dependent. We conclude that the results previously obtained for a $^9$Be target are quite general, at least for light systems, and that a S.F. $NN$ potential built on a phenomenological nN potential can constitute an interesting and useful alternative to D.F. potentials.Comment: 11 pages, 8 figure

Status of art of reaction models for projectiles far from stability

This talk will review the status of art of nuclear and Coulomb breakup theories and their relation to optical models of elastic scattering of exotic projectiles. The effect of the final state interactions between the breakup particle and the core and target nuclei will be clarified and some typical numerical calculations for the relevant observables will be presented and compared to experimental data. Finally new results will be shown to demonstrate the feasibility of a novel type of experiment involving heavy projectiles far from stability on heavy targets.Comment: 10 pages, 4 figures. Proceedings of NN2007, Rio de Janeir