Identification of strong E1 and M1 groundstate transitions in deformed rare earth nuclei

Abstract

Systematic nuclear resonance fluorescence (NRF) experiments have been performed at the bremsstrahlung facility of the Stuttgart Dynamitron to investigate the distribution of magnetic and electric dipole strengths in deformed nuclei. Precise excitation energies, transition strengths, spins and decay branching ratios were deduced for numerous low lying dipole excitations in deformed rare earth nuclei. Measurements of the linear polarization of resonantly scattered photons using simultaneously two Compton polarimeters enabled model independent parity assignments. For the first time positive parities could be established for groups of states in the neighbouring deformed nuclei 150Nd, 160Gd, 162Dy. Most of these states are concentrated near 3 MeV and should be attributed to orbital M1 excitations (“Scissors Mode”). The deformation dependence of the orbital M1 strength has been studied in the Nd isotopic chain. Completing previous polarization measurements on 142,150Nd the transitional nucleus 146Nd has been investigated. The surprising novel result of the present systematic studies, however, was the first observation of enhanced electric dipole excitations in the same deformed nuclei at excitation energies of 2.414, 2.471, and 2.520 MeV, respectively. The transition energies and the enhanced B(E1)↑ strengths of 3–5·10−3e2fm2 support the interpretation in terms of the predicted new type of collective electric dipole excitations in deformed nuclei due to reflection asymmetric shapes like octupole deformations and/or cluster configurations. Furthermore, all three states systematically exhibit decay branching ratios Rexp = B(1− → 21+)/B(1− → 01+), which hint at K-mixing. First results for the neutron-odd, deformed nucleus 163Dy are presented.