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

Nuclear resonance fluorescence experiments were performed on the rare earth nuclei ¹⁵⁵Gd and ¹⁵⁹Tb 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-γ 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 ^161,163Dy and ¹⁵⁷Gd. 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 (¹⁵⁵Gd) and 90 transitions (¹⁵⁷Gd) in the energy range 2–4 MeV. The nucleus ¹⁵⁹Tb 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. © 1996 The American Physical Society.