Fine Structure in the Decay of the Highly Deformed Proton EmitterE131u

Fine structure in the ground-state proton radioactive decay of highly deformed ${}^{131}\mathrm{Eu}$ has been identified. In addition to the previously observed ground-state line, measured here with a proton energy of 932(7) keV, a second proton peak with energy 811(7) keV was observed. We interpret this line as proton decay from the ${}^{131}\mathrm{Eu}$ ground state to the first excited $2^{+}$ state of the daughter nucleus ${}^{130}\mathrm{Sm}$. Comparing the measured branching ratio with calculations enables the ground-state configuration of ${}^{131}\mathrm{Eu}$ to be unambiguously assigned to the $3/2^{+}$[411] Nilsson configuration.