Lifetimes of nuclear excited states are a very important observable in nuclear physics. They deliver information on the structure of excited states, and, in combination with branching ratios and multipole mixing ratios, are necessary to deduce transition matrix elements. These are directly linked to the wave functions of the system and their prediction poses an important test for nuclear structure models. Isomers are a special kind of excited state. Their unusually long lifetime is an embodiment of their unique structure.
A study of isomers and their decay often yields a better understanding of evolution of the microscopic structure within a mass region.
The measurement of lifetimes in exotic nuclei poses a great experimental challenge. This challenge is met by the recent developments in application of inverse Coulomb excitation and Plunger measurements with radioactive ion beams. Another method which is applicable for the measurement of lifetimes down to 5 ps is the delayed coincidence method with very fast LaBr3(Ce) detectors. This technique was
successfully applied in combination with a fission fragment separator for the first time in this work to measure lifetimes in very neutron-rich nuclei.
Experiments on neutron-rich nuclei with neutron number N = 59 and N = 60, performed within the scope of this thesis, are discussed and the results are presented. A new microsecond isomer with a 76.5 keV E1 decay transition was unambiguously assigned to the N = 60 nucleus 97Rb. Data on a new decay branch of the K=9/2+[404] deformed isomer in 97Sr are presented. Lifetime measurements of rotational states in 99Y and states with previously unknown structure in 100Nb were performed successfully. Lifetime measurements in the beta background reproduced literature values. These experiments, employing the delayed coincidence technique, convincingly demonstrate the feasibility of precision gamma gamma fast timing measurements of excited states in exotic nuclei, which involve low peak-to-background ratio and an extended beam spot.
The data of a large isomer spectroscopy campaign at Lohengrin at the Institute Laue-Langevin, Grenoble, are presented. These measurements yielded more precise microsecond isomer lifetimes in light fission fragments. Furthermore, it was possible to deduce isomer population ratios for many of the observed isomers. These results are important data for comparison with model predictions of mean spin population after fission, and essential information for understanding the very complex fission process.
Finally, the experimental results are discussed and compared to nuclear model calculations. For 97Rb a spin assignment of (1/2, 3/2, 5/2)- can be made for the state with the 76.5 keV transition. Comparison to calculations, made by Simpson and Daugas, shows hints at a possible oblate character of the low-lying isomeric state. The new decay branch in 97Sr allows a spin assignment
of 5/2+ to the state at 522 keV. Calculations within the IBFM indicate a simple one-quasi-particle structure of this state. The lifetime measurements in 99Y give further evidence for the regular rotational structure of the ground-state band in this nucleus.
A calculation applying the quasi particle rotor model, assuming axially symmetric deformation, yields good agreement for the ground-state band. The lifetime measurements in the decay cascade of the microsecond isomer in 100Nb clearly indicate non-collective nature of
these excitations
