Within the framework of the interacting boson model (IBM-2) we analyze $F$-vector $\gamma$ transitions in $\gamma$-soft nuclei for which the O(6) dynamical symmetry of the IBM provides an analytical description. For certain parameter combinations, the IBM-2 Hamiltonian is invariant with respect to the $d$-parity operation which changes the sign of each $d$ boson. This symmetry of the Hamiltonian is responsible for the existence of the $d$-parity quantum number which leads to selection rules for electromagnetic transitions. The breaking of the $d$-parity symmetry can be studied in mixed $E2/M1\mathrm{}$ transitions, and especially the $E2/M1\mathrm{}$ mixing ratio of the $1_1^{+}\to 2_1^{+}$ $\gamma$ transition turns out to be a sensitive measure of IBM-2 parameters for $\gamma$-soft nuclei.

• Received 4 March 1998

DOI:https://doi.org/10.1103/PhysRevC.58.191