International audienceThe level structures of 70 Co and 70 Ni, populated from the β decay of 70 Fe, have been investigated using β -delayed γ -ray spectroscopy following in-flight fission of a 238 U beam. The experimental results are compared to Monte-Carlo Shell-Model calculations including the pf+g9/2+d5/2 orbitals. The strong population of a (1+) state at 274 keV in 70 Co is at variance with the expected excitation energy of ∼1 MeV from near spherical single-particle estimates. This observation indicates a dominance of prolate-deformed intruder configurations in the low-lying levels, which coexist with the normal near spherical states. It is shown that the β decay of the neutron-rich A=70 isobars from the new island of inversion to the Z=28 closed-shell regime progresses in accordance with a newly reported type of shell evolution, the so-called Type II, which involves many particle-hole excitations across energy gaps

The low-lying structures of the midshell νg 9/2 Ni isotopes 72 Ni and 74 Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72 Co and 74 Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first (8 + ) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g 9/2 shell. We find that the disappearance of the seniority isomerism in the (8 + 1 ) states can be explained by a lowering of the seniority-four (6 + ) levels as predicted years ago. For 74 Ni, the internal de-excitation pattern of the newly observed (6 + 2 ) state supports a restoration of the normal seniority ordering up to spin J = 4. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78 Ni.

The level structure of the neutron-rich 77 Cu nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of 77 Ni are produced by inflight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in 77 Cu are identified for the first time by correlating γ rays with the β decay of 77 Ni, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the singleparticle structure near 78 Ni and suggests a single-particle nature for both the 5=2 − 1 and 3=2 The evolution of the shell structure is one of the key motivations to study atomic nuclei with large neutron excess. The goal is to understand effects due to this excess of neutrons that are responsible for deviations from the conventional harmonic oscillator description with a strong attractive spin-orbit coupling, which characterizes the shell structure and properties of nuclei near the line of β stability. Such deviations are related to the monopole components of the effective nucleon-nucleon interaction and their strong effects on the single-particle energies (SPEs). The spindependent central component influences the energies of all single-particle orbitals, while the tensor interaction alters the spin-orbit splitting when specific orbits are filled by neutrons or protons [1][2][3][4][5][6][7][8].For the chain of Ni (Z ¼ 28) isotopes between N ¼ 40 and N ¼ 50, theoretical models predict significant changes in the proton SPEs as the ν1g 9=2 shell is filled by neutrons [3,4,[9][10][11][12]. Here, the tensor force responsible for SPE shifts

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.

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