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.
The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.
The nuclear structure of 67 Co has been investigated through 67 Fe β-decay. The 67 Fe isotopes were produced at the LISOL facility in proton-induced fission of 238 U and selected using resonant laser ionization combined with mass separation. The application of a new correlation technique unambiguously revealed a 496(33) ms isomeric state in 67 Co at an unexpected low energy of 492 keV. A 67 Co level scheme has been deduced. Proposed spin and parities suggest a spherical (7/2 − ) 67 Co ground state and a deformed first excited (1/2 − ) state at 492 keV, interpreted as a proton 1p − 2h prolate intruder state.
In-gas-cell laser spectroscopy study of the 57,59,63,65 Cu isotopes has been performed for the first time using the 244.164 nm optical transition from the atomic ground state of copper. The nuclear magnetic dipole moments for 57,59,65 Cu relative to that of 63 Cu have been extracted. The new value for 57 Cu of µ( 57 Cu) = +2.582(7)µN is in strong disagreement with the previous literature value but in good agreement with recent theoretical and systematic predictions.PACS numbers: 21.10. Ky, 27.40.+z, 27.50.+e, 42.62.Fi With more than 3000 nuclei known so far, the present nuclear chart offers a vast landscape to study mesoscopic systems. Many of these nuclei cannot be described by ab initio calculations and theory uses models based on a fundamental or phenomenological approach in order to describe observables of isotopes yet to discover. The confrontation of experimental data with the theoretical predictions does not only allow for fine tuning of theory but also for discovering new aspects of the interactions at work in the atomic nucleus. This is especially the case when studying isotopes with extreme proton-toneutron ratios. In nuclear structure, the identification of the magic numbers 2, 8, 20, 28, 50, 82, 126 [1] is the foundation for the shell model of the nucleus. While these magic numbers are well established in nuclei close to the valley of β-stability, their universality is strongly questioned [2].Of special interest is the magic number 28 as it is the smallest magic number issued from the spin-orbit interaction added to the nuclear potential. Both the N = 28 isotones [3,4] and the nickel (Z = 28) isotopes [5,6] are under intensive investigation to probe their magic character. With N = Z = 28, 56 Ni is expected to be doubly magic. While it displays a high 2 + 1 excited state in comparison to the other nickel isotopes [5] and a sudden change in the two-neutron and two-proton separation energies [7], both characteristic of a doubly magic nucleus, the evolution of the transition strength B(E2) and the behavior of the nuclei in the vicinity point towards particle excitations across the shell gaps and a breaking of this magic core [8,9,10].The nuclear magnetic dipole moment is a very sensitive tool to study the nuclear structure in the vicinity of magic nuclei.
The neutron-rich isotopes 65,67 Fe and 65 Co have been produced at the LISOL facility, Louvain-La-Neuve, in the proton-induced fission of 238 U. Beams of these isotopes have been extracted with high selectivity by means of resonant laser ionization combined with mass separation. Yrast and near-yrast levels of 65 Co have also been populated in the 64 Ni+ 238 U reaction at Argonne National Laboratory. The level structure of 65 Co could be investigated by combining all the information from both the 65 Fe and 65 Co β decay and the deep-inelastic reaction. The 65 Fe, 65 Co, and 67 Fe decay schemes and the 65 Co yrast structure are fully established. The 65,67 Co level structures can be interpreted as resulting from the coexistence of core-coupled states with levels based on a low-energy proton-intruder configuration.
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