29 research outputs found
Gamow-Teller response and its spreading mechanism in doubly magic nuclei
The scope of the paper is to apply a state-of-the-art beyond mean-field model
to the description of the Gamow-Teller response in atomic nuclei. This topic
recently attracted considerable renewed interest, due, in particular, to the
possibility of performing experiments in unstable nuclei. We study the cases of
Ca, Ni, Sn and Pb. Our model is based on a fully
self-consistent Skyrme Hartree-Fock plus random phase approximation. The same
Skyrme interaction is used to calculate the coupling between particles and
vibrations, which leads to the mixing of the Gamow-Teller resonance with a set
of doorway states and to its fragmentation. We compare our results with
available experimental data. The microscopic coupling mechanism is also
discussed in some detail.Comment: 27 pages, 10 figure
The Giant Dipole Resonance as a quantitative constraint on the symmetry energy
The possible constraints on the poorly determined symmetry part of the
effective nuclear Hamiltonians or effective energy functionals, i.e., the
so-called symmetry energy S(rho), are very much under debate. In the present
work, we show that the value of the symmetry energy associated with Skyrme
functionals, at densities rho around 0.1 fm^{-3}, is strongly correlated with
the value of the centroid of the Giant Dipole Resonance (GDR) in spherical
nuclei. Consequently, the experimental value of the GDR in, e.g., 208Pb can be
used as a constraint on the symmetry energy, leading to 23.3 MeV < S(rho=0.1
fm^{-3}) < 24.9 MeV.Comment: 5 pages, 2 figures, submitte
Unified description of structure and reactions: implementing the Nuclear Field Theory program
The modern theory of the atomic nucleus results from the merging of the
liquid drop (Niels Bohr and Fritz Kalckar) and of the shell model (Marie
Goeppert Meyer and Axel Jensen), which contributed the concepts of collective
excitations and of independent-particle motion respectively. The unification of
these apparently contradictory views in terms of the particle-vibration
(rotation) coupling (Aage Bohr and Ben Mottelson) has allowed for an ever
increasingly complete, accurate and detailed description of the nuclear
structure, Nuclear Field Theory (NFT, developed by the Copenhagen-Buenos Aires
collaboration) providing a powerful quantal embodiment. In keeping with the
fact that reactions are not only at the basis of quantum mechanics (statistical
interpretation, Max Born) , but also the specific tools to probe the atomic
nucleus, NFT is being extended to deal with processes which involve the
continuum in an intrinsic fashion, so as to be able to treat them on an equal
footing with those associated with discrete states (nuclear structure). As a
result, spectroscopic studies of transfer to continuum states could eventually
use at profit the NFT rules, extended to take care of recoil effects. In the
present contribution we review the implementation of the NFT program of
structure and reactions, setting special emphasis on open problems and
outstanding predictions.Comment: submitted to Physica Scripta to the Focus Issue on Nuclear Structure:
Celebrating the 1975 Nobel Priz
Momentum distributions in halo nuclei
From the analogy between the break-up of weakly bound, neutron-rich nuclei and the phenomenon of optical diffraction, it is possible to formulate a model for the momentum distribution of both the core and the valence neutrons of halo nuclei which displays a simple dependence on nuclear structure parameters. The model is applied to the analysis of reactions where11Be,11Li and14Be impinge on12C, providing an overall account of the experimental findings and predictions for further measurements
Reaction mechanism of two-neutron transfer in DWBA
We present a brief introduction to the second order DWBA reaction formalism which we have used
to perform the theoretical analysis of two鈥搉ucleon transfer reactions induced both by heavy and light ions. We
also show an example of such a calculation, emphasizing the connection between the structure aspects of the prob-
lem and the resulting predicted two鈥搉eutron transfer cross section. The calculations were carried out making use
of software specifically developed for this purpose. It includes sequential, simultaneous and non鈥搊rthogonality
contributions to the process. Microscopic form factors are used which take into account the relevant structure
aspects of the process, such as the nature of the single鈥損article wavefunctions, the spectroscopic factors, and the
interaction potential responsible for the transfer. Overall agreement with the experimental absolute values of the
differential cross section is obtained without any free paramete
Quantitative study of coherent pairing modes with two-neutron transfer: Sn isotopes
Pairing rotations and pairing vibrations are collective modes associated with a field, the pair field, which changes the number of particles by two. Consequently, they can be studied at profit with the help of two-particle transfer reactions in superfluid and in normal nuclei, respectively. The advent of exotic beams has opened, for the first time, the possibility to carry out such studies in medium heavy nuclei, within the same isotopic chain. The case studied in the present paper is that of the Sn isotopes [essentially from closed (Z=N=50) to closed (Z=50, N=82) shells]. The static and dynamic off-diagonal, long-range order phase coherence in gauge space displayed by pairing rotations and vibrations, respectively, leads to coherent states which behave almost classically. Consequently, these modes are amenable to an accurate nuclear structure description in terms of simple models containing the right physics, in particular, BCS plus quasiparticle random-phase approximation and Hartree-Fock mean field plus random-phase approximation, respectively. The associated two-nucleon transfer spectroscopic amplitudes predicted by such model calculations can thus be viewed as essentially "exact." This fact, together with the availability of optical potentials for the different real and virtual channels involved in the reactions considered, namely A +2Sn+p, A+1Sn+d, and ASn+t, allows for the calculation of the associated absolute cross sections without, arguably, free parameters. The numerical predictions of the absolute differential cross sections, obtained making use of the above-mentioned nuclear structure and optical potential inputs, within the framework of second-order distorted-wave Born approximation, taking into account simultaneous, successive, and nonorthogonality contributions, provide, within experimental errors in general, and below 10% uncertainty in particular, an overall account of the experimental findings for all of the measured A+2Sn(p,t)ASn(gs) reactions, for which absolute cross sections have been reported to date
Spreading width of compound states through coincidence spectra of rotational gamma-rays
Abstract The intrinsic width of (multiparticle-multihole) compound states is an elusive quantity, of difficult direct access, as it is masked by damping mechanisms which control the collective response of nuclei. Through microscopic cranked shell model calculations, it is found that the strength function associated with two-dimensional gamma-coincidence spectra arising from rotational transitions between states lying at energies > 1 MeV above the yrast line, exhibits a two-component structure controlled by the rotational (wide component) and compound (narrow component) damping width. This last component is found to be directly related to the width of the multiparticle-multihole autocorrelation function