Proton decay of the highly deformed odd-odd nucleus Eu

The proton decay of the highly deformed odd-odd nucleus Eu has been observed by bombarding a Ni target with a beam of 425 MeV Kr ions. Fusion evaporation residues were separated in-flight using the Argonne fragment mass analyzer and implanted into a double-sided silicon strip detector. Eu decays by the emission of a proton with energy E= 1020(15) keV [Q = 1028(15) keV] and half-life t =0.90ms. The transition is assigned to a highly deformed (β ∼ 0.3) J =1 ground-state configuration by comparing the decay rate with a new theoretical formalism for deformed odd-odd nuclei. These results clearly demonstrate the sensitivity of the proton decay rate to the spectator neutron configuration.