Examination of different strengths of octupole correlations in neutron-rich Pr and Pm isotopes

Abstract

Recent prompt γ-ray spectroscopy studies of 149,151Pr have revealed weak octupole correlations. These are manifested by the absence of strong intraband E1 transitions between bands of opposite parities. On the other hand, in their isotones 151,153Pm, which have only two additional protons, the E1 transition rates are several orders of magnitude stronger, indicating enhanced octupole correlations. The quasiparticle phonon model combined with the particle-rotor model, which allows octupole correlations and Coriolis mixing to be taken into account, has been applied to calculate the E1 transition rates between low-lying states of the nuclei 149,151Pr and 151,153Pm. The dependence of the E1 rates on the magnitude of the quadrupole deformation, the magnitude and sign of the hexadecapole deformation parameter, the Coriolis and recoil attenuation parameters, the energies of the vibrational bandheads, and the parameters of the Nilsson Hamiltonian have been thoroughly investigated. From this investigation an overall conclusion can be drawn that, even if the intrinsic quasiparticle structure of the bandheads and the octupole correlations vary as the parameters of the model are changed, the octupole correlations in the odd Pr isotopes are much weaker than those in the odd Pm isotopes. Moreover, the experimental E1 transition rates can be satisfactorily described with the values of the quadrupole and hexadecapole deformation parameters expected in this region. In the case of 151Pr, two possible dominant configurations have been assigned to the 35.1-keV isomeric state, namely, the 3/2[422] and 1/2[420] orbitals. Our calculations show that the first assignment provides an E1 value closer to the experimental observations.