Multiphonon vibrational states in 106,108Pd

The nuclei 106Pd and 108Pd have been Coulomb excited using beams of 16O, 58Ni and 208Pb. The data determined 25 and 31 E2 matrix elements in 106Pd and 108Pd, respectively. The experimental data are qualitatively in agreement with the spherical-harmonic quadrupole vibrational model, but quantitatively there are large discrepancies, even for the states generally accepted to be the two-phonon vibrational states. A sum-rule method was applied to the data, the result of which implies that the vibrational quadrupole strengths of the 0+ and 4+ two-phonon states are fragmented and shared between several E2 matrix elements. The E2 matrix elements imply appreciable quadrupole triaxiality with a γ-centroid of about 20°. A systematic comparison is made with neighbouring Pd-isotopes and with 114Cd.