New isomer in 96Y marking the onset of deformation at N = 57.

The level scheme of ⁹⁶Y was significantly extended and a new 201 ns isomer was located at 1655 keV excitation energy, with spin-parity assignment of 5^± or 6⁻. The isomer decays to spherical low-spin structure by transitions with large hindrance and is fed by a short cascade which resembles the beginning of a rotational band. This is in analogy with the feeding and decay pattern of the 4⁻ isomer in ⁹⁸Y, here confirmed, by lifetime analysis, as a bandhead of a rotational structure with sizable deformation. It is suggested that the new isomer in ⁹⁶Y arises from a shape change between deformed and spherical configurations, which indicates the appearance of deformation already at N = 57 in the yttrium chain. The experimental findings for ⁹⁶Y are strengthened by theoretical calculations based on the complex Excited Vampir model. [ABSTRACT FROM AUTHOR]