ESR ofPb3+Centers in ThO2

Abstract

Spectra of ${\mathrm{Pb}}^{3+}$ in sites having cubic and axial symmetry have been identified in Th${\mathrm{O}}_2$ crystals grown from a Pb${\mathrm{F}}_2$-based solvent which have been irradiated with electrons or $\gamma$ rays. That part of the spectra arising from the 20%-abundant ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ isotope ($I=\frac{1}{2}$) is characterized by a large zero-field hyperfine splitting. The ${\mathrm{Pb}}^{3+}$ center with $〈111〉$ axial symmetry shows a superhyperfine interaction with a ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nucleus. Part of this spectrum has previously been attributed to the $F$ center in Th${\mathrm{O}}_2$. After exposure to ionizing radiation, no centers were detected in Th${\mathrm{O}}_2$ crystals grown from a ${\mathrm{Li}}_2$O· 2W${\mathrm{O}}_3$-based flux. Neutron irradiation of all crystals produced another center with $〈100〉$ axial symmetry.