Paramagnetic Resonance ofGd3+155in a ThO2Single Crystal: Study of the Hyperfine Structure (Allowed and Forbidden Lines)

The hyperfine structure of ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}_{}^{3+}{}_{}{}^{}$ ions in ${\mathrm{Th}}^{4+}$ substitutional cubic sites of a single crystal of Th${\mathrm{O}}_2$ (fluorite structure) has been studied by means of electron-paramagnetic-resonance techniques. The observation of "forbidden" transitions traditionally labeled by $\Delta m=1\mathrm{}$ or 2 is reported. Theoretical calculations using first-order perturbation theory for the hyperfine structure and nuclear Zeeman terms after a numerical diagonalization of the remaining (electronic Zeeman plus crystal-field terms) spin Hamiltonian are in good agreement with experimental results.