Multipole collectivity in 168Er from inelastic scattering.

Abstract

The structure of the low-lying states of $^{168}\mathrm{Er}$ has been investigated by use of inelastic scattering. Differential cross sections for the elastic and inelastic scattering of 36 MeV \ensuremath{\alpha} particles and 16 MeV deuterons from $^{168}\mathrm{Er}$ have been measured. Isoscalar transition strengths for l=2, 3, and 4 excitation have been extracted by a coupled-channels distorted wave calculation involving collective model form factors. A number of known levels corresponding to the ground state band, \ensuremath{\gamma} band, and octupole bands were seen. Results for the ${2}^{+}$, ${4}^{+}$, ${6}^{+}$, and ${8}^{+}$ states in the ground state band and ${2}^{+}$, ${4}^{+}$, and ${6}^{+}$ states in the \ensuremath{\gamma} band imply the importance of the hexadecapole (${\ensuremath{\beta}}_{4}$) and the hexacontatetrapole (${\ensuremath{\beta}}_{6}$) shape components. The octupole states at 1431, 1633, and 1913 keV are excited with isoscalar strengths equal to 0.046, 0.058, and 0.023 ${e}^{2}$${\mathrm{b}}^{3}$, respectively, which are comparable with the corresponding B(E3) strengths known from Coulomb excitation of the 1431 and 1633 keV states. The octupole states at 2269, 2324, and 2486 keV with isoscalar E3 transition strengths 0.055, 0.022, and 0.018 ${e}^{2}$${\mathrm{b}}^{3}$, respectively, are newly identified. For quadrupole excitation the results agree in several ways with predictions of the interacting boson approximation model and the geometrical models and for octupole states there is a good agreement with theoretical results of Neergaaumlrd and Vogel.