arXiv subjects

A. Covello

Publications and source records attributed to A. Covello.

At least 19 recordsLinked to original sources

arXiv2019

Early signal of emerging nuclear collectivity in neutron-rich $^{129}$Sb

Radioactive $^{129}$Sb, which can be treated as a proton plus semi-magic $^{128}$Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, $B(E2)$, for the $2^+$ $\times$ $πg_{7/2}$ multiplet members and candidate $πd_{5/2}$ state were measured. The results indicate that the total electric quadrupole strength of $^{129}$Sb is a factor of 1.39(11) larger than the $^{128}$Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wavefunctions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

nucl-ex↗

arXiv2015

Shell-model study of quadrupole collectivity in light tin isotopes

A realistic shell-model study is performed for neutron-deficient tin isotopes up to mass A=108. All shell-model ingredients, namely two-body matrix elements, single-particle energies, and effective charges for electric quadrupole transition operators, have been calculated by way of the many-body perturbation theory, starting from a low-momentum interaction derived from the high-precision CD-Bonn free nucleon-nucleon potential. The focus has been put on the enhanced quadrupole collectivity of these nuclei, which is testified by the observed large B(E2;0+ -> 2+)s. Our results evidence the crucial role played by the Z=50 cross-shell excitations that need to be taken into account explicitly to obtain a satisfactory theoretical description of light tin isotopes. We find also that a relevant contribution comes from the calculated neutron effective charges, whose magnitudes exceed the standard empirical values. An original double-step procedure has been introduced to reduce effectively the model space in order to overcome the computational problem.

nucl-th↗

arXiv2014

Two-neutron transfer in Sn isotopes beyond the N = 82 shell closure

We have performed microscopic distorted-wave Born approximation (DWBA) calculations of differential cross sections for the two reactions 136Sn(p,t)134Sn and 134Sn(t,p)136Sn, which are within reach of near-future experiments with radioactive ion beams. We have described the initial and final nuclear states in terms of the shell model, employing a realistic low-momentum two-body effective interaction derived from the CD-Bonn nucleon-nucleon potential that has already proved quite successful in describing the available low-energy energy spectrum of 134Sn. We discuss the main features of the predicted cross sections for the population of the low-lying yrast states in the two nuclei considered.

nucl-th↗

arXiv2014

From Kuo-Brown to today's realistic shell-model calculations

This paper is an homage to the seminal work of Gerry Brown and Tom Kuo, where shell model calculations were performed for 18O and 18F using an effective interaction derived from the Hamada-Johnston nucleon-nucleon potential. That work has been the first successful attempt to provide a description of nuclear structure properties starting from the free nucleon-nucleon potential. We shall compare the approach employed in the 1966 paper with the derivation of a modern realistic shell-model interaction for sd-shell nuclei, evidencing the progress that has been achieved during the last decades.

nucl-th↗

arXiv2014

Realistic shell-model calculations and exotic nuclei

This paper presents a short overview of the shell-model approach with realistic effective interactions to the study of exotic nuclei. We first give a sketch of the current state of the art of the theoretical framework of this approach, focusing on the main ingredients and most relevant recent advances. Then, we present some selected results for neutron-rich nuclei in various mass regions, namely oxygen isotopes, $N=40$ isotones, and nuclei around $^{132}$Sn, to show the merit as well as the limits of these calculations.

nucl-th↗

arXiv2014

Realistic shell-model calculations for isotopic chains "north-east'' of 48Ca in the (N,Z) plane

We perform realistic shell-model calculations for nuclei with valence nucleons outside 48Ca, employing two different model spaces. The matrix elements of the effective two-body interaction and electromagnetic multipole operators have been calculated within the framework of the many-body perturbation theory, starting from a low-momentum potential derived from the high-precision CD-Bonn free nucleon-nucleon potential. The role played by the neutron orbital 1d5/2 has been investigated by comparing experimental data on yrast quadrupole excitations of isotopic chains north-east of 48Ca with the results of calculations including or not including this single-particle state in the model space.

nucl-th↗

arXiv2013

Behavior of odd-even mass staggering around 132Sn

We have performed shell-model calculations of binding energies of nuclei around $^{132}$Sn. The main aim of our study has been to find out if the behavior of odd-even staggering across N=82 is explainable in terms of the shell model. In our calculations, we have employed realistic low-momentum two-body effective interactions derived from the CD-Bonn nucleon-nucleon potential that have already proved quite successful in describing the spectroscopic properties of nuclei in the $^{132}$Sn region. Comparison shows that our results fully explains the trend of the experimental staggering.

nucl-th↗

arXiv2013

Evolution of single-particle states beyond $^{132}$Sn

We have performed shell-model calculations for the two one valence-neutron isotones $^{135}$Te and $^{137}$Xe and the two one valence-proton isotopes $^{135,137}$Sb. The main aim of our study has been to investigate the evolution of single-particle states with increasing nucleon number. To this end, we have focused attention on the spectroscopic factors and the effective single-particle energies. In our calculations, we have employed a realistic low-momentum two-body effective interaction derived from the CD-Bonn nucleon-nucleon potential that has already proved quite successful in describing the spectroscopic properties of nuclei in the $^{132}$Sn region. Comparison shows that our results reproduce very well the available experimental data. This gives confidence in the evolution of the single-particle states 4 figures predicted by the present study.

nucl-th↗

arXiv2013

Shell-model study of single-neutron strength fragmentation in 137Xe

We have performed shell-model calculations for the nucleus $^{137}$Xe, which was recently studied experimentally using the $^{136}$Xe($d,p$) reaction in inverse kinematics. The main aim of our study has been to investigate the single-neutron properties of the observed states, focusing attention on the spectroscopic factors. We have employed a realistic low-momentum two-body effective interaction derived from the CD-Bonn nucleon-nucleon potential that has already proved quite successful in describing the spectroscopic properties of nuclei in the $^{132}$Sn region. Comparison shows that our calculations reproduce very well the experimental excitation energies and yield spectroscopic factors that come close to those extracted from the data.

nucl-th↗

arXiv2012

Pairing and realistic shell-model interactions

This paper starts with a brief historical overview of pairing in nuclei, which fulfills the purpose of properly framing the main subject. This concerns the pairing properties of a realistic shell-model effective interaction which has proved very successful in describing nuclei around doubly magic 132Sn. We focus attention on the two nuclei 134Te and 134Sn with two valence protons and neutrons, respectively. Our study brings out the key role of one particle-one hole excitations in producing a significant difference between proton and neutron pairing in this region.

nucl-th↗

arXiv2012

Effective shell-model hamiltonians from realistic nucleon-nucleon potentials within a perturbative approach

This paper discusses the derivation of an effective shell-model hamiltonian starting from a realistic nucleon-nucleon potential by way of perturbation theory. More precisely, we present the state of the art of this approach when the starting point is the perturbative expansion of the Q-box vertex function. Questions arising from diagrammatics, intermediate-states and order-by-order convergences, and their dependence on the chosen nucleon-nucleon potential, are discussed in detail, and the results of numerical applications for the p-shell model space starting from chiral next-to-next-to-next-to-leading order potentials are shown. Moreover, an alternative graphical method to derive the effective hamiltonian, based on the Z-box vertex function recently introduced by Suzuki et al., is applied to the case of a non-degenerate (0+2) hbaromega model space. Finally, our shell-model results are compared with the exact ones obtained from no-core shell-model calculations.

nucl-th↗

arXiv2012

g9/2 nuclei and neutron-proton interaction

We have performed shell-model calculations for nuclei below 100Sn, focusing attention on the two N=Z nuclei 96Cd and 92Pd, the latter having been recently the subject of great experimental and theoretical interest. We have considered nuclei for which the 0g9/2 orbit plays a dominant role and employed a realistic low-momentum two-body effective interaction derived from the CD-Bonn nucleon-nucleon potential. This implies that no phenomenological input enters our effective Hamiltonian. The calculated results for 92Pd are in very good agreement with the available experimental data, which gives confidence in our predictions for 96Cd. An analysis of the wave functions of both 96Cd and 92Pd is performed to investigate the role of the isoscalar spin-aligned coupling.

nucl-th↗

arXiv2011

Fully microscopic shell-model calculations with realistic effective hamiltonians

The advent of nucleon-nucleon potentials derived from chiral perturbation theory, as well as the so-called V-low-k approach to the renormalization of the strong short-range repulsion contained in the potentials, have brought renewed interest in realistic shell-model calculations. Here we focus on calculations where a fully microscopic approach is adopted. No phenomenological input is needed in these calculations, because single-particle energies, matrix elements of the two-body interaction, and matrix elements of the electromagnetic multipole operators are derived theoretically. This has been done within the framework of the time-dependent degenerate linked-diagram perturbation theory. We present results for some nuclei in different mass regions. These evidence the ability of realistic effective hamiltonians to provide an accurate description of nuclear structure properties.

nucl-th↗

arXiv2010

Shell-model study of exotic Sn isotopes with a realistic effective interaction

We report on a shell-model study of Sn isotopes beyond N=82 employing a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential renormalized through use of the V-low-k approach. At present, the most exotic Sn isotope for which some experimental information exists is 134Sn with an N/Z ratio of 1.68. It is the aim of our study to compare the results of our calculations with the available experimental data and to make predictions for the neighboring heavier isotopes which may be within reach of the next generation of radioactive ion beam facilities. The very good agreement between theory and experiment obtained for 134Sn gives confidence in the predictive power of our realistic shell-model calculations.

nucl-th↗

arXiv2010

Realistic shell-model calculations for neutron-rich calcium isotopes

We study the neutron-rich calcium isotopes performing shell-model calculations with a realistic effective interaction. This is derived from the CD-Bonn nucleon-nucleon potential renormalized by way of the V-low-k approach, considering 48Ca as an inert core and including the neutron 0g9/2 orbital. We compare our results with experiment and with the results of a previous study where 40Ca was assumed as a closed core and the standard 1p0f model space was employed. The calculated spectroscopic properties are in both cases in very good agreement with the available experimental data and enable a discussion on the role of the g9/2 single-particle state in the heavy-mass Ca isotopes.

nucl-th↗

arXiv2010

Shell-model calculations for neutron-rich carbon isotopes with a chiral nucleon-nucleon potential

We have studied neutron-rich carbon isotopes in terms of the shell model employing a realistic effective hamiltonian derived from the chiral N3LOW nucleon-nucleon potential. The single-particle energies and effective two-body interaction have been both determined within the framework of the time-dependent degenerate linked-diagram perturbation theory. The calculated results are in very good agreement with the available experimental data, providing a sound description of this isotopic chain toward the neutron dripline. The correct location of the dripline is reproduced.

nucl-th↗

arXiv2010

Realistic shell-model calculations: current status and open problems

The main steps involved in realistic shell-model calculations employing two-body low-momentum interactions are briefly reviewed. The practical value of this approach is exemplified by the results of recent calculations and some remaining open questions and directions for future research are discussed.

nucl-th↗

arXiv2010

Shell-model interpretation of high-spin states in 134-I

New experimental information has been recently obtained on the odd-odd nucleus 134-I. We interpret the five observed excited states up to the energy of ~3 MeV on the basis of a realistic shell-model calculation, and make spin-parity assignments accordingly. A very good agreement is found between the experimental and calculated energies.

nucl-th↗