[en] GODDESS is a coupling of the charged-particle detection system ORRUBA to the gamma-ray detector array Gammasphere. This coupling has been developed in order to facilitate the high-resolution measurement of direct reactions in normal and inverse kinematics with stable and radioactive beams. GODDESS has been commissioned using a beam of ¹³⁴Xe at 10 MeV/A, in a campaign of stable beam measurements. The measurement demonstrates the capabilities of GODDESS under radioactive beam conditions, and provides the first data on the single-neutron states in ¹³⁵Xe, including previously unobserved states based on the orbitals above the N=82 shell closure.

[en] We report on the first measurement of the absolute transition strength B(E2;0₁⁺→2₁⁺) in the self-conjugate nucleus ⁶⁸Se. It is found that the 0₁⁺→2₁⁺ transition displays a strength similar to that for the triaxial ⁶⁴Ge nucleus, in sharp contrast to the much stronger collectivity observed for the oblate ⁷²Kr nucleus. Shape evolution along the N=Z line from zinc to strontium is analyzed through beyond-mean-field calculations using the D1S force. Over this narrow mass region the nuclear structure shows clear-cut transition from prolate to oblate and back to prolate, with ⁶⁸Se standing as pivotal between triaxial and oblate deformations.

[en] The structure of nuclei provides insight into astrophysical reaction rates that are difficult to measure directly. These studies are often performed with transfer reactions and β-decay measurements. These experiments benefit from particle-γ coincidence measurements which provide information beyond that of particle detection alone. The Hybrid Array of Gamma Ray Detectors (HAGRiD) of LaBr₃(Ce) scintillators has been designed with this purpose in mind. The design of the array permits it to be coupled with particle detector systems, such as the Oak Ridge Rutgers University Barrel Array (ORRUBA) of silicon detectors and the Versatile Array of Neutron Detectors at Low Energy (VANDLE). It is also designed to operate with the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) advanced target system. HAGRiD’s design avoids compromising the charged-particle angular resolution due to compact geometries which are often used to increase the γ efficiency in other systems. First experiments with HAGRiD coupled to VANDLE as well as ORRUBA and JENSA are discussed.

[en] Electron capture on neutron-rich medium-mass nuclei is a key process where the electrons that impede the collapse of the core of massive stars are captured, thereby producing very neutron-rich nuclei. As the core collapses, the supernova is then initiated. For the electron capture to proceed, however, the allowed Gamow-Teller (GT) transition must be unblocked either by thermal excitations or by mixing of proton configurations from a higher-lying shell into the ground-state configuration of the nucleus. The present paper presents an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University, in which we study the configuration mixing in the neutron-rich⁷⁶Zn isotope. The experiment utilised single-proton and single-neutron knockout with detection of reaction-residue γ rays and measurement of the parallel momentum of the residue. Through this we investigate the proton components of the ⁷⁶Zn ground state, with a particular interest in π-g_9/2, which may unblock the GT electron capture even at low temperatures and thereby open a new pathway for the initiation of the collapse of the pre-supernova stellar core.

[en] The neutron-rich N=28 nucleus ⁴⁴S was studied using the two-proton knockout reaction from ⁴⁶Ar at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4⁺ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ''intruder'' configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in ⁴⁴S, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

[en] The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas ⁶⁸Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus ⁶⁶Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in ^62,64,66Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for ^64,66Fe than expected and allow tests of new calculations.

[en] The transition rates for the 2₁⁺ states in ^62,64,66Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N=40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N=40 as governed by the number of valence protons with respect to Z≅30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

[en] The mean lifetime of the 2₁⁺ state in the near drip-line nuclide ²⁰C was measured for the first time using the Recoil Distance Method with intermediate energy radioactive beams via a knockout reaction. The measured value of τ_21⁺ = 9.8±2.8(stat)_-1.1^+0.5(syst) ps gives a reduced transition probability of B(E2;2₁⁺→0_g.s.⁺) 7.5_1.7^+3.0(stat) e²fm⁴. This value is in good agreement with shell model calculations using a p--sd shell model space and isospin-dependent (N--Z) effective charges.

[en] Probing shell structure at a large neutron excess has been of particular interest in recent times. Neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 offer an exotic testing ground for shell evolution. The development of the N = 40gap between neutron fp and lg_9/2 shells gives rise to highly interesting variations of collectivity for nuclei in this region. While ⁶⁸Ni shows doubly magic properties in level energies and transition strengths, this was not observed in neighbouring nuclei. Especially neutron-rich Fe isotopes proved particularly resistant to calculational approaches using the canonical valence space (fpg) resulting in important deviations of the predicted collectivity. Only an inclusion of the d_5/2-orbital could solve the problem [1]. Hitherto no transition strengths for ⁶⁶Fe have been reported. We determined B(E2,2⁺₁→0⁺₁) values from lifetimes measured with the recoil distance Doppler-shift method using the Cologne plunger for radioactive beams at National Superconducting Cyclotron Laboratory at Michigan State University. Excited states were populated by projectile Coulomb excitation for ^62,64,66Fe. The data show a rise in collectivity for Fe isotopes towards N = 40. Results [2] are interpreted by means of a modified version of the Valence Proton Symmetry [3] and compared to shell model calculations using a new effective interaction recently developed for the fpgd valence space [4].

[en] The neutron-rich N=50 isotones ⁸²Ge and ⁸⁴Se were investigated using intermediate-energy Coulomb excitation on a ¹⁹⁷Au target and inelastic scattering on ⁹Be. As typical for intermediate-energy Coulomb excitation with projectile energies exceeding 70 MeV/nucleon, only the first 2⁺ states were excited in ⁸²Ge and ⁸⁴Se. However, in the inelastic scattering on a ⁹Be target, a strong population of the first 4⁺ state was observed for ⁸⁴Se, while there is no indication of a similarly strong excitation of the corresponding state in the neighboring even-even isotone ⁸²Ge. The results are discussed in the framework of systematics and shell-model calculations using three different effective interactions.