Population inversion of nuclear states by a Penning trap mass spectrometer

According to the energy-mass equivalence, E = mc(2), 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 Cu-68. The resulting unique form of ion ensembles opens up a large variety of new nuclear-physics experiments. Mass excesses of -65567.0(1.6) keV for the 1(+) ground state and -64850.3(1.5) keV for the 6(-) isomeric state have been determined.