SYSTEMATICS OF LOW-LYING DIPOLE EXCITATIONS IN THE DEFORMED EVEN-EVENNUCLEI ER-164,ER-166,ER-168,ER-170

Photon scattering experiments on the deformed even-even nuclei Er-164, Er-166,Er-168,Er-170 provided detailed information on the excitation e nergies, spins, transition probabilities, and K quantum numbers of low -lying dipole excitations. In the case of Er-168, in addition, paritie s of the excited states were determined model independently by measuri ng the linear polarization of the scattered photons using a sectored G e-Compton polarimeter with a BGO-anti-Compton shield. The results for M1 excitations are compared with the systematics of low-lying orbital M1 strength (''scissors mode'') in the neighboring rare-earth nuclei. The observed enhanced El transitions are discussed in terms of collect ive excitations (K=0 octupole bandheads or candidates for 2(+)x3(-) tw o-phonon excitations) and in the framework of recent microscopic quasi particle phonon model calculations.