Spektroskopie der Oktupolkorrelationen von 231Pa und 229Pa mit adaptierten Koinzidenzteilchendetektorsystemen

The shape of a nucleus can be defined in a classical view by a
sphere of constant density of positive space-charge in a body-fixed
frame of reference. Experimental observations can be interpreted as
deviations from spherical symmetry of the nucleus. Static shapes of
nuclei in the periodic table near the valley of stability
predominantly are rotationally symmetric and mostly quadrupole
deformed (American football or lens). For nuclei in time-dependent
excited states asymmetric terms are known, like octupole
deformation (pear shape). The group of actinides lies in a
transition region with possible "static octupole deformation". For
a fixed proton number (Z = 91) the impact of changing octupole
correlations with neutron number on the configuration of 31Pa and
229Pa was analysed in the present work. A detector optimized for
the reaction 231Pa(22 MeV p,t)229Pa at the Munich Universities
tandem accelerator laboratory gave experimental clues of the 229Pa
ground-state energy. A detailed study of231Pa by Coulomb-excitation
with 148 MeV 32S und 255, 260, 261 MeV 58Ni projectiles can answer
open questions on octupole correlations and determines a multitude
of transition matrix elements as well as their global parameters in
an enlarged level scheme. A segmented coincidence detector for
backscattered particles with read-out electronics and software for
high resolution, combined with Compton-suppressed and
background-reduced-gamma spectroscopy following Coulomb-excitation
was developed for the NORDBALL gamma-spectrometer of the Niels Bohr
Institute Tandem Accelerator Laboratory, Risø (Denmark). An
optional extension of the detector system, built by the University
of Warsaw (Poland), can be integrated. For some experiments a
Si-detector of the College of Industrial Technology Amagasaki
(Japan) was used. The 1/2[530] ground-state band 231Pa could be
followed up to spin 39/2-, the 1/2[400]+1/2[660] parity-partner
band up to 23/2+ and the 3/2[651] side band up to 37/2+. The
simulation of Coulomb-excitation with the code GOSIA helped to fit
the parameters of a model system consisting of three bands with 47
levels and interconnecting 672 matrix elements of E1, E2, E3, E4
and M1 transitions to experimental data within chi^2/n=1. As far as
possible the calculation remained model-independent and led to
values for model parameters of multipole moments and deformation.
The determination of signs of matrix elements is discussed.
Different methods of error analysis were used. The results were
related to the experimental known properties of neighbouring
nuclei. A comparison of the results with calculations by the
Charles University, Prague (Czech Republic) in the
one-quasiparticle-plus-phonon model with Coriolis-coupling is
given.