Nuclear masses and lifetimes are the basic properties to understand nuclear existence and structure resulting from the strong interaction. Both informations are also important for the understanding of nuclear astrophysics and fundamental interaction. Most experimental information was in the past obtained for nuclides close to the valley of stability, i.e., experimental data for exotic nuclei are still rare and important. In the present experiment masses and lifetimes of stored exotic nuclei in Tl -- U region were measured with the FRS-ESR facility.
The exotic nuclei were produced by fragmentation of 670 MeV/u 238U projectiles in a 4g/cm2 Be target placed at the entrance of the fragment separator FRS. The produced exotic nuclei were separated in flight with the FRS and injected into the storage-cooler ring ESR for high precision mass and half-life measurement. The injected hot ions were cooled by electron cooling. The relative velocity spread of the stored ions was reduced down to the level of 5 10-7 for low intensity beams. The cooled beam was stored in ESR for about 5 minutes for each injection cycle and the revolution frequency of the stored ions was precisely measured. The unknown masses were directly determined from the revolution frequency using known masses in the same spectrum for calibration. In addition to the atomic mass, the half-life of stored ions has been determined with the time-resolved Schottky frequency analysis.
The neutron-rich region from Tl to U was investigated in this work. The ultimate sensitivity, down to single particles, and the high resolving power of the time-resolved Schottky mass analysis (SMS) were the basis for the discovery of the five isotopes: 236Ac, 224At, 222Po, 221Po and 213Tl and the six new isomers: 234mAc, 234nAc, 228mAc, 228mFr, 214mBi and 213mBi. The isotope identification was unambiguous going along simultaneously with the first mass determination for the new isotopes and isomers. Experimental masses of 30 known isotopes in this heavy neutron-rich region were measured for the first time. Furthermore, the half-lives of 12 nuclides including 235Ac were determined experimentally for the first time. The total accuracy of the mass measurement in the present experiment was 30 keV mainly determined by the systematical errors.
Our experimental results have been compared to theoretical models. In this comparison microscopic and macroscopic theories have been applied. The new experimental mass data contribute to a better knowledge of the most neutron-rich nuclides in the Pt -- U region.
Verknüpfung zu Publikationen oder weiteren Datensätzen
