Speaker
Description
Nuclear fission provides broad access to neutron-rich nuclei in the $\mathrm{A}\approx 100$ region and enables detailed $\gamma$-ray spectroscopy across the $\mathrm{N}\approx 60$ shape-transition landscape. This region is characterized by a sharp onset of deformation at $\mathrm{N}=60$ near $\mathrm{Z}\approx 38-40$, an abrupt loss of collectivity when moving to lower $\mathrm{Z}$, and an increasing role of triaxial degrees of freedom toward higher $\mathrm{Z}$. These features provide stringent constraints on microscopic and configuration-mixing descriptions of shape evolution.
Selected recent results are presented using two complementary experimental strategies. Event-by-event isotopic identification in inverse-kinematics fission at GANIL with the VAMOS++ spectrometer coupled to the AGATA $\gamma$-ray tracking array provides clean, nucleus-resolved spectroscopy. Within this framework, prompt $\gamma$-ray spectroscopy along the neutron-rich Kr chain has been used to map the evolution of collectivity across $\mathrm{N}\approx 60$ and to delineate the low-$\mathrm{Z}$ limit of this deformation region.
In parallel, high-fold $\gamma-\gamma$ coincidence spectroscopy with large $\gamma$ arrays (e.g., FIPPS and Gammasphere) enables complex level schemes to be disentangled and weak branches or isomer-linked structures to be accessed. Br and Nb case studies are used to illustrate how combining isotopic identification and high-fold coincidence approaches yields a more complete picture of shape evolution and coexistence, and constrains the interplay between axial and triaxial structures on the high-$\mathrm{Z}$ side of the transition.
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