Systematic study of the fragmentation of low-lying dipole strength in odd-<i>A</i>rare earth nuclei investigated in nuclear resonance fluorescence experiments

Abstract

Nuclear resonance fluorescence experiments were performed on the rare earth nuclei $^{155}\mathrm{Gd}$ and $^{159}\mathrm{Tb}$ to study the fragmentation of the M1 scissors mode in odd deformed nuclei and to establish a kind of systematics. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.1 MeV) and high resolution Ge-\ensuremath{\gamma} spectrometers detailed information was obtained on excitation energies, decay widths, transition probabilities, and branching ratios. The results are compared to those observed recently for the neighboring odd nuclei $^{161,163}\mathrm{Dy}$ and $^{157}\mathrm{Gd}$. Whereas in the odd Dy isotopes the dipole strength is rather concentrated, both Gd isotopes show a strong fragmentation of the strength into about 25 ${(}^{155}$Gd) and 90 transitions ${(}^{157}$Gd) in the energy range 2--4 MeV. The nucleus $^{159}\mathrm{Tb}$ linking the odd Dy and Gd isotopes exhibits an intermediate strength fragmentation. In general the observed total strength in the odd nuclei is reduced by a factor of 2--3 as compared to their neighboring even-even isotopes. The different fragmentation behavior of the dipole strengths in the odd Dy and Gd isotopes is unexplained up to now. \textcopyright{} 1996 The American Physical Society.