Nuclear structure investigations by means of gamma spectroscopy and phase transitions in atomic nuclei

The phase transition between spherical and deformed nuclei is discussed for Tb and Gd nuclei (N approximately 88), for the sequence of Kr isotopes (N < 50) and for various examples of hsub(9/2) proton excitations (Z < 82). The change from prolate to oblate deformation in the mass region A approximately 190 can clearly be seen from the change of the static quadrupole moments. Excitation spectra in this region also point to nuclear shapes without axial symmetry. As examples for coexistence of excitations with different deformations there are discussed: the even-mass Hg nuclei, different single particle states in odd-mass nuclei and a rotor description on the basis of a variable moment of inertia. High-spin few-particle states are analyzed in order to find evidence for the suprafluid - unpaired phase transition. For many two-particle states in deformed and transitional nuclei a recoupling of one nucleon pair and rotation alignment of the individual angular momenta is observed. Several high-spin states in the mass region A approximately 180, on the other hand, are understood as combinations of certain quasiparticle states with large angular momentum projections on the symmetry axis. High-spin multiplets in the vicinity of the magic nucleus 146Gd can be interpreted on the basis of simple shell model considerations. A manyfold occupation by unpaired nueleons has been derived for some shell model states from the experimental excitation spectra. A comparison between the yrast levels of N=88 nuclei and the model spectrum of a FERMI gas gives evidence that the pairing energy disappears above spin I approximately 16 h/2π. The results of experimental studies as contributed by the authors to this field of investigations are added as appendix. (author)