SYSTEMATIC STUDY OF THE FRAGMENTATION OF LOW-LYING DIPOLE STRENGTH INODD-A RARE-EARTH NUCLEI INVESTIGATED IN NUCLEAR-RESONANCE FLUORESCENCE EXPERIMENTS

Nuclear resonance fluorescence experiments were performed on the rare earth nuclei Gd-155 and Tb-159 to study the fragmentation of the M1 sc issors mode in odd deformed nuclei and to establish a kind of systemat ics. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.1 MeV) and high resolution Ge-gamma spectrometers detailed information was obtained on excitation energies, decay widths , transition probabilities, and branching ratios. The results are comp ared to those observed recently for the neighboring odd nuclei Dy-161, Dy-163 and Gd-157. Whereas in the odd Dy isotopes the dipole strength is rather concentrated, both Gd isotopes show a strong fragmentation o f the strength into about 25 (Gd-155) and 90 transitions (Gd-157) in t he energy range 2-4 MeV. The nucleus Tb-159 linking the odd Dy and Gd isotopes exhibits an intermediate strength fragmentation. In general t he observed total strength in the odd nuclei is reduced by a factor of 2-3 as compared to their neighboring even-even isotopes. The differen t fragmentation behavior of the dipole strengths in the odd Dy and Gd isotopes is unexplained up to now.