Background: Neutron-rich nuclei with protons in the fp shell show an onset of collectivity around N = 40. Spectroscopic information is required to understand the underlying mechanism and to determine the relevant terms of the nucleon-nucleon interaction that are responsible for the evolution of the shell structure in this mass region. Methods: We report on the lifetime measurement of the first 2 + and 4 + states in 70,72,74 Zn and the first 6 + state in 72 Zn using the recoil distance Doppler shift method. The experiment was carried out at the INFN Laboratory of Legnaro with the AGATA demonstrator, first phase of the Advanced Gamma Tracking Array of highly segmented, high-purity germanium detectors coupled to the PRISMA magnetic spectrometer. The excited states of the nuclei of interest were populated in the deep inelastic scattering of a 76 Ge beam impinging on a 238 U target. Results: The maximum of collectivity along the chain of Zn isotopes is observed for 72 Zn at N = 42. An unexpectedly long lifetime of 20 +1.8 −5.2 ps was measured for the 4 + state in 74 Zn. Conclusions: Our results lead to small values of the B(E2; 4 + 1 → 2 + 1 )/B(E2; 2 + 1 → 0 + 1 ) ratio for 72,74 Zn, suggesting a significant noncollective contribution to these excitations. These experimental results are not reproduced by state-of-the-art microscopic models and call for lifetime measurements beyond the first 2 + state in heavy zinc and nickel isotopes.

Excited states in the nucleus133 Sn, with one neutron outside the doubly-magic 132 Sn core, were populated following one-neutron knockout from a 134 Sn beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the γ rays emitted in the decay of the known neutron single-particle states in 133 Sn additional γ strength in the energy range 3.5-5.5 MeV was observed for the first time. Since the neutron-separation energy of 133 Sn is low, Sn=2.402(4) MeV, this observation provides direct evidence for the radiative decay of neutronunbound states in this nucleus. The ability of electromagnetic decay to compete successfully with neutron emission at energies as high as 3 MeV above threshold is attributed to a mismatch between the wave functions of the initial and final states in the latter case. These findings suggest that in the region south-east of 132 Sn nuclear structure effects may play a significant role in the neutron vs. γ competition in the decay of unbound states. As a consequence, the common neglect of such effects in the evaluation of the neutron-emission probabilities in calculations of global β-decay properties for astrophysical simulations may have to be reconsidered.

We report on the observation of a new isomeric state in 68 Ni. We suggest that the newly observed state at 168(1) keV above the first 2 + state is a π (2p-2h) 0 + state across the major Z = 28 shell gap. Comparison with theoretical calculations indicates a pure proton intruder configuration and the deduced low-lying structure of this key nucleus suggests a possible shape coexistence scenario involving a highly deformed state. The atomic nucleus is a complex quantum system consisting of two kinds of strongly interacting fermions. A direct consequence of this fermionic nature, the Pauli principle, is the shell model of the nucleus, one property of which being the existence of magic gaps. Shell structures are present in a number of systems such as atoms, metal clusters, and quantum dots and wires, for instance, and are strongly linked to the symmetries of the mean field. How the shell gaps evolve in nuclei that are further and further away from stability is one of the key questions to which the radioactive beam facilities that are currently under construction hope to bring answers. Already today, the structure of moderately exotic nuclei such as 68 Ni allows one to pave the way toward a general answer to the problem of shell evolution. Unusual configurations which are expected to dominate in the ground-state structure of very exotic nuclei can be identified as excited structures in systems not very far away from stability. The strong contribution of the spin-orbit term in the nucleon-nucleon interaction affects in a major way the single-particle levels with the largest angular momentum, pushing it down in energy. This quenches significantly the N = 40 magic gap from the spherical harmonic oscillator. The intrusion of the 1g 9/2 and the 2d 5/2 neutron orbitals brings collectivity and enhances neutron-pair excitations across N = 40 from the fp shell into the 1g 9/2 . Conversely, however, this parity change hinders quadrupole excitation and mimics some properties usually associated to magicity. In 68 Ni, the observation of a first excited 0 + 2 state at low energy [1] and the high excitation energy of the 2 + 1 state [2] are examples of such properties. These competing consequences of shell quenching make 68 Ni a particularly suited case to study the evolution of shell gaps with isospin.Reactions involving single-proton particle-hole excitations, π (1p-1h), are an ideal tool to learn about the residual interaction. Unfortunately they lie at very high excitation energy. One possibility to circumvent this obstacle is to look for π (2p-2h) states which are lowered in energy thanks to pairing correlations and proton-neutron residual interactions. Studying pair excitation across magic gaps means, therefore, studying these residual interactions. Pair excitations are revealed by the presence of excited 0 + states. In 68 Ni, two such states are reported, mainly of neutron character, originating from the scattering of pairs into the 1νg 9/2 . A state corresponding to the excitation of two protons (2p-2h) has been predicted by ...

The reduced transition probability B(E 2; 0 + 1 → 2 + 1) to the first excited 2 + state of the neutron-rich nucleus 136 Te, with two protons and two neutrons outside the doubly magic 132 Sn core, was measured via Coulomb excitation at relativistic energies at the RIKEN Radioactive Isotope Beam Factory. A value of B(E 2) = 0.191(26) e 2 b 2 was extracted from the measured inelastic scattering cross section on an Au target taking into account the contributions from both Coulomb and nuclear excitations. In addition, an upper limit for the transition strength to a 2 + state of mixed-symmetry character in the excitation energy range of 1.5-2.2 MeV was determined and compared to the predictions of various theoretical calculations. Because of the high statistics gathered in the present experiment the error of the deduced B(E 2) value is dominated by the systematic uncertainties involved in the analysis of Coulomb excitation experiments at beam energies around 150 MeV/u. Therefore, the latter are for the first time assessed in detail in the present work.

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