Home Search Collections Journals About Contact us My IOPscience Shell model spectroscopy far from stability This content has been downloaded from IOPscience. Please scroll down to see the full text. 2017 J. Phys. G: Nucl. Part. Phys. 44 084002 (http://iopscience.iop.org/0954-3899/44/8/084002) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 188.184.3.52 This content was downloaded on 05/09/2017 at 18:21 Please note that terms and conditions apply. You may also be interested in: The nuclear shell model toward the drip lines A Poves, E Caurier, F Nowacki et al. Shape coexistence: the shell model view A Poves Shape coexistence in neutron-rich nuclei A Gade and S N Liddick Shape coexistence in the N = 19 and N = 21 isotones Gerda Neyens Spectroscopy in and around the Island of Inversion Heiko Scheit Nuclear-structure studies of exotic nuclei with MINIBALL P A Butler, J Cederkall and P Reiter A new view of nuclear shells Rituparna Kanungo Exotic nuclei and nuclear forces Takaharu Otsuka Three-body forces and shell structure in calcium isotopes Jason D Holt, Takaharu Otsuka, Achim Schwenk et al. Made open access 7 August 2017 Journal of Physics G: Nuclear and Particle Physics J. Phys. G: Nucl. Part. Phys. 44 (2017) 084002 (8pp) https://doi.org/10.1088/1361-6471/aa7789 * Shell model spectroscopy far from stability A Poves Departamento de Física Teórica and IFT, UAM-CSIC, Universidad Autónoma de Madrid, Spain E-mail: [email protected] Received 24 November 2016, revised 1 June 2017 Accepted for publication 6 June 2017 Published 29 June 2017 Abstract I will discuss the impact of the research on exotic nuclei carried out at ISO- LDE and later on, following its impulse, in many laboratories worldwide, in our present knowledge of the nuclear dynamics. I will put particular emphasis on the occurrence of islands of inversion (IoI) at several magic numbers for the neutrons, far from stability. I will also discuss the appearance of new local shell closures in the calcium and silicon isotopic chains, and qualify their status as new doubly magic nuclei. Keywords: nuclear structure, shell model calculations, neutron rich nuclei, new magic numbers, islands of inversion (Some figures may appear in colour only in the online journal) 1. Introduction Not so long ago, experimental nuclear spectroscopy was dominated by heavy ion collisions and high spin physics. Mean field methods thrived in the theoretical arena, later joined by the interacting boson model. The shell model realm seemed dormant. Nevertheless, studies of very neutron rich radioactive isotopes were pursued at the ISOLDE facility under the CERN umbrella. And one can safely say that this activity paved the way for the present explosion of new results on nuclei far from stability, which have radically modified our understanding of nuclear dynamics. In this contribution, I will deal with the medium–light neutron rich nuclei from oxygen to calcium with special emphasis on the contributions from ISOLDE. 2. The discovery of the N=20 ‘island of inversion’ The story starts with the mass measurements made at CERN by what would later be the Orsay group of the ISOLDE Collaboration [1]. They found an excess of binding in 31Na and 32Na which could not be explained by the available theoretical models. Simultaneously, a group of * This article belongs to the Focus on Exotic Beams at ISOLDE: A Laboratory Portrait special issue. 0954-3899/17/084002+08$33.00 © 2017 IOP Publishing Ltd Printed in the UK 1 Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. J. Phys. G: Nucl. Part. Phys. 44 (2017) 084002 A Poves theorists at Orsay tried to reproduce this effect with projected Hartree–Fock (PHF) calcula- tions using density dependent interactions [2]. They got a normal, spherical, solution for 31Na with a small shoulder in the prolate side of the energy versus deformation curve. Never- theless, they argued that by applying a rotational correction, this shoulder might become a minimum, and explained that the anomaly was due to the gain in energy due to deformation. Even with this proviso, their model was unable to explain the 31Na data. Three decades and huge developments in beyond mean field (BMF) techniques were necessary to produce this minimum in the easier to treat nucleus 32Mg. All in all, their idea was correct even if the calculations were not conclusive. Already in Thibault’s paper [1], the possibility of having a new region of deformation was contemplated. Further work by the same group [3–5] extended the mass measurements to magnesium isotopes finding the same kind of anomaly in 32Mg. In addition, studying the beta decay of 32Na, they populated the first 2+ state in 32Mg at 881keV. Indeed this is a very low excitation energy compared with the corresponding one in 30Mg (1.48 MeV). Much more so, common sense would predict that the 2+ excitation energy should increase at the magic neutron number N=20 and not decrease almost by a factor two. The idea of the existence of an unexpected new region of deformation, precisely where the current models would have predicted enhanced sphericity, was reinforced. New shell model studies of these nuclei were triggered by a paper by Wildenthal and Chung [6] in 1980 with the provocative title ‘Collapse of the conventional shell-model ordering in the very-neutron-rich isotopes of Na and Mg’. In it, the authors, using their newly fitted high quality interaction for the upper part of the sd-shell, concluded that it was impossible to explain the behaviour of the neutron rich N=20 isotones in this valence space. The question that remains unanswered, and which has continued to produce a lot of confusion until now, is the meaning of the sentence ‘Collapse of the conventional shell-model ordering’. For many, this had to be interpreted as if, at the mean field level, some single particle orbits of the pf-shell would lay below the upper orbits of the sd-shell. Work in this direction was carried out by the Glasgow group [7, 8] by including the 0f7/2 orbit in the valence space, essentially degenerated with the 0d3/2. With this prescription they indeed got an increase in the binding energy of the N=20 isotones, but the proposed solution was far from being physically sound, and did not address at all the issue about deformation risen by the PHF calculations on [2]. Our work in [9] interpreted the collapse of the shell model ordering in a different way: they were not the spherical single particle orbits of the two adjacent major harmonic oscillator shells which collapsed far from stability, instead, what happened was that the intruder con- figurations of (neutron) 2p–2h nature, somehow became more bound than the normal, 0p–0h, ones. Therefore, the collapse must be due to the nuclear correlations. This interpretation makes a natural link with the surmise of [2], provided the correlations are of quadrupole– quadrupole type and the intruder configurations become deformed. The problem was how to get this behaviour out of a realistic shell model calculation. The solution was to include the 1p3/2 orbit—the quadrupole partner of the 0f7/2—in the valence space. This calculation produced deformed solutions for 32Mg, 31Na and 30Ne, keeping a substantial single particle gap between the sd and the pf-shells. Shortly after, Warburton, Becker and Brown [10] made extensive shell model calculations in the same model space reaching the same conclusions. And they coined the term ‘island of inversion’ (IoI) which has enjoyed immense success since then (I would have favoured the alternative ‘Island of Deformation’ though). Further work in the same context was carried out in [11, 12]. Since then, there has been a plethora of new experimental results, including mass measurements, radii isotope shifts, magnetic and quadrupole moments, spectroscopic factors 2 J. Phys. G: Nucl. Part. Phys. 44 (2017) 084002 A Poves etc, many of them performed at ISOLDE [13, 14]. The existence of an IoI at N=20 is now well established, even if its boundaries are not fully delineated yet. A very important mile- stone was the measurement during a coulex experiment at RIKEN of the B(E2) connecting the 2+ with the ground state of 32Mg, which confirmed its well deformed nature corresp- onding to β=0.45 [15]. Other coulex experiments followed soon [16]. Indeed, the activity in the theory sector continued, both in the shell model framework [17] and with BMF methods [18]. The latter only got quantitative agreement with experiment after including the triaxial degrees of freedom, breaking time reversal invariance and allowing configuration mixing via the generator coordinate approximation [19]. A more detailed description of the mechanism producing the IoI can be found in [20]. 3. The heaviest potassium and calcium isotopes We have seen how the availability of very heavy sodium isotopes at ISOLDE made it possible to discover the N=20 IoI. But many other elements were abundantly produced as well, and put to good use. This is the case for potassium, studied mainly by the Strasbourg members of the ISOLDE collaboration. 52K was produced for the first time (hence discovered) at ISO- LDE. Its decay produced and discovered 52Ca as well [21], whose spectroscopy was very interesting, with a first 2+ excited state at 2.56MeV, a rather high energy.