Electron-Spin-Resonance Spectrum of Divalent Europium in Thorium Dioxide

Either 2-MeV electron or ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ $\gamma$ irradiation was employed to reduce trivalent Eu in Th${\mathrm{O}}_2$ to the divalent state. The ESR spectrum of the ${\mathrm{Eu}}^{2+}$ ions could only be observed at temperatures below 77°K and posessed trigonal symmetry about the four $〈111〉$ axes. The axial crystal field was so strong that only the $|M_J=\frac{1}{2}〉$ to $|M_J=-\frac{1}{2}〉$ transition was observed with a highly anisotropic $g$ value ($g_{\mathrm{II}}=1.9678\mathrm{}\pm 0.0005$ and $g_{\perp }=7.867\mathrm{}\pm 0.002$). Hyperfine structure was resolved but the angular variation was complex because of a nuclear quadrupole interaction that was of the same order of magnitude as the hyperfine interaction. Spin-Hamiltonian constants which describe the ESR spectrum were determined with the use of a computer. Electron irradiation at 77°K was not successful in producing ${\mathrm{Eu}}^{2+}$ in isomorphous Ce${\mathrm{O}}_2$. In addition, crystals of Th${\mathrm{O}}_2$ and Ce${\mathrm{O}}_2$ doped with trivalent thulium were also electron-irradiated at 77°K, but no change in the Tm valence was detected.