Deformation increase of high-spin core-excited isomers in the astatine nuclei

Quadrupole moments of six high-spin isomers in the At isotopes have been measured with the level-mixing-spectroscopy method: ${}_{}{}^{208}{}_{}{}^{}\mathrm{At}$(${16}^{\mathrm{-}}$), ${}_{}{}^{209}{}_{}{}^{}\mathrm{At}$(29/$2^{+}$), ${}_{}{}^{210}{}_{}{}^{}\mathrm{At}$(${19}^{+}$,${15}^{\mathrm{-}}$), ${}_{}{}^{211}{}_{}{}^{}\mathrm{At}$(39/$2^{\mathrm{-}}$,29/$2^{+}$). The results show that level mixing spectroscopy is a promising technique to determine quadrupole moments of isomers that are difficult to measure by other in-beam hyperfine interaction methods. A large increase of the quadrupole moment is observed if neutrons are excited across or removed from the N=126 shell closure. This behavior is explained in terms of an enhanced core softness for fewer core neutrons; the aligned valence particles, moving in equatorial orbits, then easily polarize the core towards oblate deformation.