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.

A very exotic process of -delayed fission of 180 Tl is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the fissionfragment mass distribution of the post--decay daughter nucleus 180 Hg (N=Z ¼ 1:25) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two 90 Zr fragments, with magic N ¼ 50 and semimagic Z ¼ 40. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for -delayed fission of 180 Tl is 3:6ð7Þ Â 10 À3 %, approximately 2 orders of magnitude larger than in an earlier study. DOI: 10.1103/PhysRevLett.105.252502 PACS numbers: 24.75.+i, 23.40.Às, 27.70.+q Nuclear fission, discovered more than 70 years ago [1], represents one of the most dramatic examples of a nuclear metamorphosis, whereby the nucleus splits into two fragments releasing a large amount of energy. Initially, the fission process was described within the liquid-drop model [2,3], in which shape-dependent surface and Coulomb energy terms define the potential-energy landscape through which fission occurs. However, this macroscopic approach naturally leads to symmetric fragments and cannot explain observed asymmetric mass splits of actinides. Only by including a microscopic treatment based on shell effects can asymmetric fission be described [4]. Importantly, only in fission below or slightly above the barrier, so-called low-energy fission, can the interplay between the macroscopic liquid-drop contribution and the microscopic single-particle shell corrections be most fully explored.Until recently, such low-energy fission studies were limited to nuclei from around thorium (Th) to fermium (Fm) using spontaneous fission, fission induced by thermal neutrons or -delayed fission. These studies showed the dominance of asymmetric fission over symmetric fission for most isotopes of these elements [5][6][7] and suggested that structure effects due to, specifically, the spherical shell structure of doubly magic 132 Sn dominate the mass split. A decade ago, a new technique, developed at GSI [8]-Coulomb-excited fission of radioactive beamsallowed for a more extensive experimental survey of lowenergy fission in other regions of the nuclidic chart. These studies demonstrated the transition from mostly asymmetric fission in the actinides towards symmetric fission as the dominant mode in the light thorium to astatine region. This is also consistent with earlier studies by Itkis et al. [9], in which fission of stable targets in the mass 185-210 region was induced by bombardment with protons and 3;4 He beams. Itkis et al. found mostly symmetric mass distributions in the region around 208 Pb, with about four systems in the mass A $ 200 region having a slight reduction of PRL 105, 252502 (2010)

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...

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