Single crystals of Th${\mathrm{O}}_2$ grown in our laboratory have been subjected to ionizing radiations of 2-MeV electrons, ${\mathrm{Co}}^{60}$ $\gamma$ rays, and 150-keV x-rays. The electron-spin-resonance spectrum of these irradiated single crystals indicates that an electron is trapped near an ${\mathrm{O}}^{--}$ vacancy in the crystal lattice of Th${\mathrm{O}}_2$. The ESR spectrum is the same regardless of which radiation was used to bombard the samples. Optical and ESR low-temperature studies have demonstrated that no paramagnetic ion impurities are present in the samples prior to irradiation, and similar studies performed on irradiated samples indicate that the observed signal is not the result of valence changes produced by irradiation of ions in the crystal. The observed spectrum can be interpreted as due to a defect having local tetrahedral symmetry which is very slightly distorted axially along the body diagonal in the over-all cubic symmetry of the Th${\mathrm{O}}_2$ lattice. Room-temperature $g$ values are $g_{\parallel }=1.9739\mathrm{}$ and $g_{\perp }=1.9644\mathrm{}$. The linewidth at half-maxima ranges from approximately 32 G at room temperature to less than 1.5 G near liquid-helium temperature. All evidence so far examined leads us to conclude that we have observed the Th${\mathrm{O}}_2$ $F$ center.

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