Singular observation of shape and spin hindrance in metastable nuclear decay of $^{197}$Au

Abstract: A $I^π$ = (29/2$^{+}$) metastable state with $T_{1/2}$ $>$ 10 s at $E_{x}$ = 2102 keV has been identified in the stable nucleus, $^{197}$Au. Its decay exhibits the unusual coexistence of a strongly hindered $E2$ transition and a competing $E4$ branch with comparable intensity, indicating the simultaneous presence of shape- and spin-related hindrance mechanisms. Such a decay pattern has not previously been identified in a long-lived nuclear state and serves as a sensitive probe of the interplay between shell structure, configuration mixing, and collective degrees of freedom. This result establishes an important benchmark for modern nuclear-structure models, for which a unified description of all observables remains challenging.