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.