Precise mass values of nuclides are of great importance for the basicunderstanding of nuclear structure and decay. The neutron-rich area ofnuclides up to the neutron dripline has the largest discovery potentialfor new nuclear properties. The neutron dripline is reached only for lightelements up to oxygen.The combination of the fragment separator FRS with the storage ring ESR atGSI is a unique facility in the world for research with exotic nuclei. Thesoftware package TOFSIM was developed to simulate the crucial parametersettings for the performance of these complex facilities in the presentexperiments.
In this study we used neutron-rich nuclides produced viafragmentation of 456 A MeV 70Zn projectiles and via fission of 238Uprojectiles at different energies (380-415 A MeV). The FRS separatedin-flight the selected exotic nuclei and injected them into the ESR. Bareor H-like ions were investigated in the ESR operated in the isochronousmode as a high-resolution time-of-flight mass spectrometer.The present experimental studies are the first isochronous massmeasurements that covered a larger area of n-rich short-lived nuclides.More than 500 peaks corresponding to 280 neutron-rich isotopes from oxygento promethium were carefully analyzed. For 41 nuclides experimental massvalues were obtained for the first time and for 20 isotopes the currentvalues in the literature were improved. A mass resolving power of 200 000was achieved for ions with the best isochronicity and the accuracy rangedfrom 140-400 keV. The nuclide with the shortest known half-life (17 ms)was 13B, however, we can investigate nuclides with much lower half-lives,since the method allows to go down to a few hundreds microseconds.
Comparisons of our experimental results with different masspredictions reveal large deviations particularly for the most neutron-richnuclides. A remarkable result is that the pure microscopic theoreticalmodel is better (sigma(rms)=575 keV) in this new territory than themicroscopic-macroscopic FRDM description (sigma(rms)=667 keV). In therecentlymeasured neutron-deficient new mass surface the FRDM prediction was stilla factor of two superior due to the method of parameter adjustment toexperimental data. This reflects the advantage of microscopic theories forunknown mass areas far from the valley of beta stability. Much room forimprovements is also observed from the comparison with the Atomic MassEvaluation (AME). The AME has for our previously measured newneutron-deficient mass surface a sigma(rms)-deviation of 148 keV and hasnow forthe 41 new masses in this work 651 keV. For individual nuclides thedeviation is even larger, e.g. for 109Nb and 114Tc isotopes thedifferences from AME reach 1.5 MeV.
The region of the new masses from this experiment can also contributeto the knowledge of nucleosynthesis for r-process nuclei and help todetermine the corresponding path.
In summary this pioneering experiment performed at the FRS-ESRfacility at GSI contributes a lot to improve the knowledge on neutron-richnuclides. In future experiments our goals are to increase the resolutionand accuracy of the method and to reach more exotic nuclei, particularlyalong the predicted r-process path for Z>30.
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