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Rapid shape changes are observed for neutron-rich nuclei with A around 100. In particular, a sudden onset of ground-state deformation is observed in the Zr and Sr isotopic chains at N=60: low-lying states in N≤58 nuclei are nearly spherical, while those with N≥60 have a rotational character. Nuclear lifetimes as short as a few ps can be measured using fast-timing techniques with LaBr 3 (Ce)-scintillators, yielding a key ingredient in the systematic study of the shape evolution in this region. We used neutron-induced fission of 241 Pu and 235 U to study lifetimes of excited states in fission fragments in the A∼100 region with the EXILL-FATIMA array located at the PF1B cold neutron beam line at the Institut Laue-Langevin. In particular, we applied the generalized centroid difference method to deduce lifetimes of low-lying states for the nuclei 98 Zr (N=58), 100 Zr and 102 Zr (N≥60). The results are discussed in the context of the presumed phase transition in the Zr chain by comparing the experimental transition strengths with the theoretical calculations using the Interacting Boson Model and the Monte Carlo Shell Model.

In the EXILL campaign a highly efficient array of high purity germanium (HPGe) detectors was operated at the cold neutron beam facility PF1B of the Institut Laue-Langevin (ILL) to carry out nuclear structure studies, via measurements of γ-rays following neutron-induced capture and fission reactions. The setup consisted of a collimation system producing a pencil beam with a thermal capture equivalent flux of about 108 n s−1cm−2 at the target position and negligible neutron halo. The target was surrounded by an array of eight to ten anti-Compton shielded EXOGAM Clover detectors, four to six anti-Compton shielded large coaxial GASP detectors and two standard Clover detectors. For a part of the campaign the array was combined with 16 LaBr3:(Ce) detectors from the FATIMA collaboration. The detectors were arranged in an array of rhombicuboctahedron geometry, providing the possibility to carry out very precise angular correlation and directional-polarization correlation measurements. The triggerless acquisition system allowed a signal collection rate of up to 6 × 105 Hz. The data allowed to set multi-fold coincidences to obtain decay schemes and in combination with the FATIMA array of LaBr3:(Ce) detectors to analyze half-lives of excited levels in the pico- to microsecond range. Precise energy and efficiency calibrations of EXILL were performed using standard calibration sources of 133Ba, 60Co and 152Eu as well as data from the reactions 27Al(n,γ)28Al and 35Cl(n,γ)36Cl in the energy range from 30 keV up to 10 MeV.

Lifetimes of low-lying yrast states in neutron-rich 94,96,98 Sr have been measured by Germanium-gated γ -γ fast timing with LaBr3(Ce) detectors using the EXILL&FATIMA spectrometer at the Institut Laue-Langevin. Sr fission products were generated using cold-neutron-induced fission of 235 U and stopped almost instantaneously within the thick target. The experimental B(E2) values are compared with results of Monte Carlo shell-model calculations made without truncation on the occupation numbers of the orbits spanned by eight proton and eight neutron orbits and show good agreement. Similarly to the Zr isotopes, the abrupt shape transition in the Sr isotopes near neutron number N = 60 is identified as being caused by many-proton excitations to its g 9/2 orbit.

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 ...