Near-yrast structure of odd-A, neutron-rich Pr isotopes

Abstract

The neutron-rich praseodymium isotopes ${}^{151}$Pr and ${}^{153}$Pr have been studied by prompt $\ensuremath{\gamma}$-ray spectroscopy using ${}^{248}$Cm and ${}^{252}$Cf spontaneous-fission sources placed inside the EUROGAM-II and Gammasphere germanium arrays, respectively. Rotational bands based on $3/{2}^{\ensuremath{-}}$[541] states, with similar structures, have been assigned to these nuclei. These bands decay by intraband $E2$ transitions. Interband $E1$ transitions, reported in other works, were not observed. Delayed conversion-electron and $\ensuremath{\gamma}$-ray spectroscopy of an $A=151$ nucleus has been performed at the Lohengrin mass spectrometer. A previously reported 35.1-keV isomer of ${}^{151}$Pr has been determined to decay by an $E1$ transition, and its half-life of 50(8) $\ensuremath{\mu}$s has been measured. Calculations performed using a reflection-symmetric quasi-particle-rotor model successfully reproduce the energies of the excited states of these nuclei and their decay patterns. The spin of the isomer has been assigned to be (1/2${}^{+}$, 3/2${}^{+}$) from a comparison with the calculations. The long half-life of this isomer and the lack of intraband $E1$ transitions show an absence of strong octupole correlations in the observed states of ${}^{151,153}$Pr. This is explained in terms of increasing quadrupole deformation reducing the number of Nilsson orbitals close to the Fermi surface available to form octupole collectivity.