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.

The first excited 2 state of the unstable isotope 110 Sn has been studied in safe Coulomb excitation at 2:82 MeV=u using the MINIBALL array at the REX-ISOLDE post accelerator at CERN. This is the first measurement of the reduced transition probability of this state using this method for a neutron deficient Sn isotope. The strength of the approach lies in the excellent peak-to-background ratio that is achieved. The extracted reduced transition probability, BE2 : 0 ! 2 0:220 0:022e 2 b 2 , strengthens the observation of the evolution of the BE2 values of neutron deficient Sn isotopes that was observed recently in intermediate-energy Coulomb excitation of 108 Sn. It implies that the trend of these reduced transition probabilities in the even-even Sn isotopes is not symmetric with respect to the midshell mass number A 116 as 100 Sn is approached. DOI: 10.1103/PhysRevLett.98.172501 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Substantial interest has recently arisen in the shell structure of atomic nuclei with only a few nucleons outside the double shell closure at 100 Sn. As an example, a series of experiments aiming at isotopes in this region has been carried out using fusion-evaporation reactions in the recent past [1]. With the advent of radioactive ion beams these studies are now taken further using sub-barrier and intermediate-energy Coulomb excitation [2,3]. In this Letter we present the only sub-barrier or ''safe'' Coulomb excitation experiment in this region to date. The study of the reduced transition probability -the BE2-of the first excited 2 state in an even-even nucleus gives a direct handle on the collectivity of that state. It can thus be used to measure systematic changes in the strengths of shell gaps. The general motivation for this kind of study goes back to our incomplete knowledge of the mechanisms that govern shell formation and their implications for the structure of nuclei far from stability. It is well known that a strong spinorbit force was introduced into the nuclear shell-model on Fermi's suggestion by Goeppert Mayer [4] and independently by Haxel, Jensen, and Suess [4] to explain the observed shell gaps. However, these papers were substantially predated by the consideration of a nuclear spin-orbit force by Inglis [5] who noted that the relativistic Thomas term which arises as a consequence of the noncommutation of Lorentz transformations should act also in atomic nuclei. This term, given by the vector product of the velocity and acceleration of the bound nucleon, gives rise to nuclear LS coupling, a result which can be derived from the Dirac equation [6]. In this picture, the acceleration is proportional to the derivative of the potential experienced by the bound particle, a notion still used in mean-field approaches today. As a consequence, the splitting of the shell gaps becomes density dependent and may change with the PRL 98,

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the "island of inversion."

We report on the first low-energy Coulomb excitation measurements with radioactive I 6 ÿ beams of odd-odd nuclei 68;70 Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2:83 MeV=nucleon. rays were detected with the MINIBALL spectrometer. The 6 ÿ beam was used to study the multiplet of states (3 ÿ , 4 ÿ , 5 ÿ , 6 ÿ ) arising from the 2p 3=2 1g 9=2 configuration. The 4 ÿ state of the multiplet was populated via Coulomb excitation and the BE2; 6 ÿ ! 4 ÿ value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z 28 shell and N 40 subshell closures. A comparison with large-scale shell-model calculations using the 56 Ni core shows the importance of the proton excitations across the Z 28 shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with N 40. Radioactive beams provide great opportunities for investigating the nuclear structure away from the stable nuclei. One of the regions of the nuclear chart that has attracted a considerable interest in the past years is the one close to 68 Ni [1][2][3][4][5][6][7][8]. Coulomb excitation experiments with radioactive beams of even-even isotopes showed that the coupling of a few extra particles to the 68 Ni core induces large polarization effects [2,3]. These effects were associated with a weakening of the Z 28 and N 40 gaps when neutrons start filling the 1g 9=2 orbital [2]. Beyond N 40, results of -decay measurements in the neutronrich [69][70][71][72][73] Cu isotopes revealed a dramatic and sudden lowering of the 1f 5=2 orbital with the increased occupancy of the 1g 9=2 orbital [4]. Referred to as monopole migration, this energy shift was interpreted as originating from the residual proton-neutron interaction and it is expected to have profound implications on the structure of the doubly magic nucleus 78 Ni [4,5].Shell-model calculations using different effective nucleon-nucleon interactions were used in order to understand the observed properties in the nuclei around 68 Ni and predict the evolution of the shell structure towards 78 Ni [5][6][7][8]. The calculations indicated that the values of the Z 28 and N 40, 50 energy gaps strongly depend on the effective interaction used. A consistent understanding of the evolution of the nuclear structure in these regions requires also experimental information such as excitation energies and transition rates in the odd-A and odd-odd nuclei.