Spektroskopie im superdeformierten Minimum von 240 Pu

The present work gives an overview on the experimental results of the spectroscopy in the second minimum of the double-humped fission barrier of ^240Pu and the derived conclusions on the structure of collective excitation modes in deformed nuclei. From the combined analysis of γ-decay and conversion electron measurements and an experiment on the transmission resonances in the prompt fission probability a detailed level scheme for the second minimum of an actinide nucleus could be established for the first time. Excitation energies, spins, parities and lifetimes of collective excitations above the groundstate band of the isomer up to β-vibrational multiphonon excitations could be established. Contrary to the experiments in the high-spin region of superdeformed nuclei, which were promoted extraordinarily during the last years, the K-purity of the isomer states populated with low spins allows for a clear separation of vibrational and rotational excitations. Therefore low-lying collective vibrational bands could be identified unambiguously as K^π i=0^-, 1^- and 2^- -octupole bands, as well as the lowest-lying β-vibrational band. After determining the energy of the β-vibrational phonon it became possible to identify the resonance groups in the prompt fission probability as the third and fourth β-vibrational phonon. The complete spectroscopy of the K^ π =0^+ states in the transmission resonance experiment made it possible to determine the excitation energy of the fission isomeric ground state from measured level densities with a recently developed method. Very interesting was furthermore the observation of the predominant population of negative parity states (98 %) in the second minimum, most likely attributed to a filtering action of the inner and outer fission barrier. Finally the measured rotational bands give hints on the behaviour of the moments of inertia of superdeformed nuclei with increasing excitation energy. The measurements could help to better understand the multiphonon states in the first minimum, where much stronger mixing with quasiparticle states occur.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Ludwig-Maximilians-Universität

Year

2003

Author dc:creator

• Gassmann, David

Contributors dc:contributor

• Habs, Dietrich

Repository record source_url

https://edoc.ub.uni-muenchen.de/789/

OAI identifier oai:identifier

oai:edoc.ub.uni-muenchen.de:789