Identification Of The One-Quadrupole Phonon 2(1,Ms)(+) State Of Hg-204

One-phonon states of vibrational nuclei with mixed proton-neutron symmetry have been observed throughout the nuclear chart besides the mass A approximate to 200 region. Very recently, it has been proposed that the 2(2)(+) state of Po-212 is of isovector nature. This nucleus has two valence protons and two valence neutrons outside the doubly-magic Pb-208 nucleus. The stable isotope Hg-204, featuring two valence-proton and valence-neutron holes, with respect to Pb-208, is the particle-hole mirror of Po-212. In order to compare the properties of low-lying isovector excitations in these particle-hole mirror nuclei, we have studied Hg-204 by using the projectile Coulomb-excitation technique. The measured absolute B(M1;2(2)(+) -> 2(1)(+)) strength of 0.20 (2) mu(2)(N) indicates that the 2(2)(+) level of Hg-204 is at least the main fragment of the 2(1,ms)(+) state. For the first time in this mass region, both lowest-lying, one-quadrupole phonon excitations are established together with the complete set of their decay strengths. This allows for a microscopic description of their structures, achieved in the framework of the Quasi-particle Phonon Model. (C) 2017 The Authors. Published by Elsevier B.V.