Progress in mass measurements of stored exotic nuclei at relativistic energies

Exotic nuclei produced via projectile fragmentation were separated in fligh t by the fragment separator FRS and injected into the storage ring ESR at ( 300-400) MeV/u. Ultra-cold fragment beams with half-lives larger than 10 s were investigated by Schottky Mass Spectroscopy (SMS) applying electron coo ling. A new efficient data acquisition system recorded the time correlation s allowing corrections for possible drifts of the revolution frequencies. I n addition, it allows to observe the nuclear decay of the fragments circula ting in the ESR. Representative results of previously unknown masses of abo ut 150 neutron-deficient isotopes in the range of 52 less than or equal to Z less than or equal to 85 are discussed. Results for lead isotopes are com pared with different mass models. Exotic nuclei with half-lives shorter tha n the time required for electron cooling have been investigated by time-of- flight measurements with the ESR being operated in the isochronous mode. Th is novel experimental technique gives access to all nuclei with half-lives down to the microsecond range and has been successfully applied in first ex periments with light fragments produced by Cr-52 and Kr-84 projectiles. A m ass resolving power of better than m/Deltam = 0.9 . 10(5) (FWHM) was achiev ed with isochronous mass spectrometry (IMS).