There is strong circumstantial evidence that the shape of atomic nuclei with particular values of Z and N prefers to assume octupole deformation, in which the nucleus is distorted into a pear shape that loses the reflection symmetry of a quadrupole-deformed (rugby ball) shape prevalent in nuclei. Recently, useable intensities of accelerated beams of heavy, radioactive ions have become available at the REX-ISOLDE facility at CERN. This has allowed electric octupole transition strengths, a direct measure of octupole correlations, to be determined for short-lived isotopes of radon and radium expected to be unstable to pear-like distortions. The data are used to discriminate differing theoretical approaches to the description of the octupole phenomena, and also help restrict the choice of candidates for studies of atomic electric-dipole moments, that provide stringent tests of extensions to the Standard Model.

The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low-ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20. Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8-11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich 30;32 Mg nuclei [12]. Coulomb excitation experiments have shown that 30 Mg has a rather small BðE2Þ value for the 0 þ gs ! 2 þ 1 transition [13,14] placing this nucleus outside the island of inversion. The excited deform...

Abstract. The Miniball germanium detector array has been operational at the REX (Radioactive ion beam EXperiment) post accelerator at the Isotope Separator On-Line facility ISOLDE at CERN since 2001. During the last decade, a series of successful Coulomb excitation and transfer reaction studies have been performed with this array, utilizing the unique and high-quality radioactive ion beams which are available at ISOLDE. In this article, an overview is given of the technical details of the full Miniball setup, including a description of the γ-ray and particle detectors, beam monitoring devices and methods to deal with beam contamination. The specific timing properties of the REX-ISOLDE facility are highlighted to indicate the sensitivity that can be achieved with the full Miniball setup. The article is finalized with a summary of some physics highlights at REX-ISOLDE and the utilization of the Miniball germanium detectors at other facilities.

Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Seidlitz, M., Muecher, D., Reiter, P., Bildstein, V., Blazhev, A., Bree, N., ... Wiens, A. (2011). Coulomb excitation of , 181-186. https://doi.org/10.1016/j.physletb.2011.05.009 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 11-04-2019Physics Letters B 700 (2011) © 2011 Elsevier B.V. All rights reserved. MotivationShell structure is one of the most important frameworks for understanding nuclear structure and the properties of atomic nuclei. Recent experimental and theoretical findings indicate that magic numbers are subject to the proton-to-neutron ratio and new magic numbers are revealed when going to more exotic nuclei. Such a new magic number was proposed at N = 16 for some nuclei be- ground state for these nuclei [6]. Later shell model calculations by Warburton et al. [7] showed that the 1 f 7/2 orbital becomes lower in energy, reducing the sd shell gap and an anomalous inverted level structure was proposed, which is based on 2-particle 2-hole (2p2h) neutron cross shell configurations in the ground state. ExperimentThe Coulomb excitation experiment was performed at the REX-ISOLDE facility at CERN [37,38] The scattered beam and recoiling target nuclei were detected by a CD-shaped 500 μm thick double sided silicon strip detector (DSSSD), consisting of four identical quadrants [40]. Each quadrant comprised 16 annular strips at the front side and 24 radial strips at the back side for identification and reconstruction of the trajectories of the scattered nuclei. The detector covered forward angles between 16.4 • and 53.3 • in the laboratory system. De-excitation γ -rays following Coulomb excitation of projectile and target nuclei were detected by the MINIBALL γ -spectrometer, consisting of eight triple cluster detectors in close geometry, each containing three 6-fold segmented HPGe crystals [41]. The photopeak efficiency of the array at 1.3 MeV was 8% after cluster addback. The high segmentation of the setup ensured a proper Doppler correction for in-flight γ -ray emission at v/c ∼ 8% by combining the angular information of the γ -ray with the direction and velocity of the scattered beam particle that was detected in coincidence.Two additional particle detectors were used downstream after the scattering chamber to monitor the position of the beam and M. Seidlitz et al. / Physics Letters B ...

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