γ spectroscopy of the Y96 isotope: Searching for the onset of shape coexistence before N=60 [gamma spectroscopy of the Y-96 isotope: Searching for the onset of shape coexistence before N=60]

Medium and high spin states of the Y-96 nucleus, located in the shape-coexistence region near Z = 40 and N = 60, were populated in thermal-neutron-induced fission of U-233 and U-235 targets, diluted in a scintillator. gamma rays were measured with the FIssion Product Prompt gamma-ray Spectrometer (FIPPS) high-purity germanium (HPGe) detector array, using double and triple gamma-ray coincidence techniques and taking advantage of the efficient fission tag provided by the scintillating target material. A complex level scheme, extending up to 5.2 MeV and including excitations above the 8(+) beta-decaying isomer, was investigated, and firm spin and parity assignments were given to a number of states, on the basis of angular correlation analysis and considerations on the gamma-decay patterns. While the structures built on the 0(-) ground state and the 8(+) isomer show irregular patterns typical for spherical shapes, the (6(+)) isomeric state at 1655 keV [with half-life of 181(9) ns], and the rotational band built on it [with spin-parity values between (6(+)) and (9(+))], can be explained by Hartree-Fock-Bogoliubov calculations, if an oblate deformation is assumed. This is the first observation of a deformed structure in an N = 57 isotone, lying three neutrons away from the N = 60 line. An important finding is also the 115-keV transition which connects the (6(+)) 181(9)-ns isomer to the beta-decaying 8(+) spherical isomer, allowing us to firmly place the latter at 1541 keV excitation energy. This may be relevant for calculations of electron and antineutrino spectra from fission of actinides, for which Y-96 is a prominent product.