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)

In-source resonant ionization laser spectroscopy of the even-A polonium isotopes (192-210,216,218)Po has been performed using the 6p(3)7s (5)S(2) to 6p(3)7p (5)P(2) (λ=843.38  nm) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in (200-210)Po with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.

Atomic nuclei exhibit single-particle and collective degrees of freedom, making them susceptible to variations in size and shape when adding or removing nucleons. The rare cases where dramatic changes in shape occur with the removal of only a single nucleon are key for pinpointing the components of the nuclear interaction driving nuclear deformation. Laser spectroscopy probes the nuclear charge distribution, revealing attometer-scale variations and highlighting sensitivity to the proton (Z) and neutron (N) configurations of the nucleus. The lead isotopes, which possess a closed proton shell (Z = 82), are spherical and steadily shrink with decreasing N. A surprisingly different story was observed for their close neighbours, the mercury isotopes (Z = 80) almost half a century ago 1, 2 : Whilst the even-mass isotopes follow the trend seen for lead, the odd-mass isotopes 181,183,185 Hg exhibit a striking increase in charge radius. This dramatic 'shape staggering' between evenand odd-mass isotopes remains a unique feature of the nuclear chart. Here we present the extension of laser spectroscopy results that reach 177 Hg. An unprecedented combination of state-of-theart techniques including resonance laser ionization, nuclear spectroscopy and mass spectrometry, has established 181 Hg as the shape-staggering endpoint. Accompanying this experimental tour de force, recent computational advances incorporating the largest valence space ever used have been exploited to provide Monte-Carlo Shell Model calculations, in remarkable agreement with the experimental observations. Thus, microscopic insight into the subtle interplay of nuclear interactions that give rise to this phenomenon has been obtained, identifying the shape-driving orbitals. Although shape staggering in the mercury isotopes is a unique and localized feature in the nuclear chart, the underlying mechanism that has now been uncovered nicely describes the duality of single-particle and collective degrees of freedom in atomic nuclei.

At the radioactive ion beam facility REX-ISOLDE, neutron-rich zinc isotopes were investigated using lowenergy Coulomb excitation. These experiments have resulted in B(E2, 2 74,76 Zn and the determination of the energy of the first excited 2 + 1 states in 78,80 Zn. The zinc isotopes were produced by high-energy proton-(A = 74, 76, 80) and neutron-(A = 78) induced fission of 238 U, combined with selective laser ionization and mass separation. The isobaric beam was postaccelerated by the REX linear accelerator and Coulomb excitation was induced on a thin secondary target, which was surrounded by the MINIBALL germanium detector array. In this work, it is shown how the selective laser ionization can be used to deal with the considerable isobaric beam contamination and how a reliable normalization of the experiment can be achieved. The results for zinc isotopes and the N = 50 isotones are compared to collective model predictions and state-of-the-art large-scale shell-model calculations, including a recent empirical residual interaction constructed to describe the present experimental data up to 2004 in this region of the nuclear chart.

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.