Systematics of low-lying dipole strengths in odd and even Dy and Gd isotopes

Photon scattering experiments on the odd, deformed nuclei ${}_{}{}^{161,163}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ provided detailed information on the excitation energies and transition probabilities of low-lying dipole excitations. In the case of the even-even nuclei ${}_{}{}^{162,164}{}_{}{}^{}\mathrm{Dy}$ in addition spins and parities of the excited states could be determined model independently by measuring the angular distributions and the linear polarization of the scattered photons using a Compton polarimeter. The results are compared with the systematics obtained for the neighboring even-even isotopes ${}_{}{}^{160}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{156,158,160}{}_{}{}^{}\mathrm{Gd}$ in previous photon scattering experiments. Whereas in the odd Dy isotopes a concentration of dipole strength is observed, which fits nicely into the systematics of the orbital M1 mode, the dipole strength in ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ is completely fragmented into about 90 transitions.