Speaker
Description
The recent in-beam γ-ray spectroscopy of $^{78}$Ni has raised questions about the collectivity of the nuclear states in the vicinity of the double shell closure [1]. The tentative identification of intruder states at low excitation energy seems, in fact, to confirm a shape-coexistence scenario already predicted by LSSM calculations [3]. More generally, the robustness of the N = 50 shell closure as Z = 28 is approached [4, 5] and the onset of deformation around N = 50 remain open problems [1–3]. Electromagnetic transition matrix elements are among the most sensitive probes of the nuclear wave function, and measuring them for transitions in this exotic region allows the theoretical description of the evolution of intruder configurations to be constrained.
To complete the picture of the structure of these excited states, we performed an in-beam γ-ray spectroscopy experiment using a fusion-fission reaction to populate neutron-rich fission fragments in medium-spin states [6], coupled with a lifetime measurement using the RDDS method. The experiment was carried out at the INFN Legnaro National Laboratories [7] and represented the first measurement with the newly developed $^{238}$U beam, which impinged on a $^9$Be target mounted on a plunger device with a Ni degrader. The fission products were identified with the PRISMA spectrometer and their de-excitation γ rays were detected with AGATA. Several transitions with unknown lifetimes were observed in isotopes around and beyond N = 50, and preliminary results of the analysis will be presented