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.

Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A ¼ 42, 46, 50, and 54 "f-shell" nuclei in ( 3 He, t) charge-exchange reactions. In the 42 Ca → 42 Sc reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the 54 Fe → 54 Co reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei.

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The neutron-rich nuclei 94;96 Kr were studied via projectile Coulomb excitation at the REX-ISOLDE facility at CERN. Level energies of the first excited 2 þ states and their absolute E2 transition strengths to the ground state are determined and discussed in the context of the Eð2 þ 1 Þ and BðE2; 2 þ 1 ! 0 þ 1 Þ systematics of the krypton chain. Contrary to previously published results no sudden onset of deformation is observed. This experimental result is supported by a new proton-neutron interacting boson model calculation based on the constrained Hartree-Fock-Bogoliubov approach using the microscopic Gogny-D1M energy density functional. DOI: 10.1103/PhysRevLett.108.062701 PACS numbers: 25.70.De, 27.60.+j, 29.30.Kv, 29.38.Gj Since the availability of high-intensity radioactive ion beams, the extension of the concept of quantum phase transitions to exotic nuclei is of great interest in nuclear physics [1]. Quantum phase transitions occur in atomic nuclei as a function of the number of protons or neutrons and describe changes of the ground-state shapes [2]. The so-called A % 100 mass region of the nuclear chart around 100 Zr is one of the most popular regions for the study of this phenomenon since the zirconium (Z ¼ 40) and strontium (Z ¼ 38) isotopes undergo a shape transition from almost spherical to strongly deformed shapes when going from neutron number N ¼ 58 to N ¼ 60 [3][4][5][6][7]. This