Isospin symmetry and proton decay: Identification of the 10{sup +} isomer in {sup 54}Ni

The {gamma} decays from an isomeric 10{sup +} state at 6457 keV in the nucleus {sub 28}{sup 54}Ni{sub 26} have been identified using the GSI fragment separator in conjunction with the RISING Ge-detector array. The state is interpreted as the isobaricanalog of the 6527-keV 10{sup +} isomer in {sub 26}{sup 54}Fe{sub 28}. The results are discussed in terms of isospin-dependent shell-modelcalculations. Clear evidence is presented for a discrete l=5 proton decaybranch into the first excited 9/2{sup -} state of the daughter {sup 53}Co. This decay is the first of its kind observed following projectile fragmentation reactions.