| According to the energy-mass equivalence, E=mc2, excited long-lived nuclear states, so-called isomers, are distinguished from the ground state by their additional mass. The ultra-high accuracy provided by Penning traps now allows to weigh nuclear excitation energies. With the on-line Penning trap mass spectrometer ISOLTRAP, we have developed a mass-separation procedure of sufficient resolving power to isolate particles of a selected isomeric state from an ensemble delivered from the radioactive beam facility ISOLDE at CERN, first demonstrated for $\chemCu}$. The resulting unique form of ion ensembles opens up a large variety of new nuclear-physics experiments. Mass excesses of -65567.0(1.6) $\un{keV}$ for the 1+ ground state and -64850.3(1.5) $\un{keV}$ for the 6- isomeric state have been determined. |