The optical absorbance of commercially pure thoria single crystals was measured after oxidizing and reducing treatments, irradiation by 18‐MeV protons, 48‐MeV 16O ions, and 60Co gamma photons, and thermal annealing at temperatures up to 950° C. For unirradiated specimens, absorption bands previously observed at 2.80, 3.03, and 4.03 eV were confirmed, and additional bands were identified at 1.67, 2.04, 2.45, 3.61, and 4.52 eV. Reduction and irradiation enhanced the 3.03 and 3.61 eV bands but diminished the 2.80, 4.03, and 4.52 eV bands. The centers responsible for the two groups of bands are thus presumed to be electron‐type and hole‐type, respectively. Impurity atoms, also, are probably involved in the centers. Ion irradiation produced an intense blue color in the exposed volume of the specimens, and additional absorption bands appeared at 1.49, 1.80, 2.03, and 2.32 eV. The behavior of these bands on annealing and on subsequent low‐exposure gamma irradiation indicated that the corresponding centers are combinations of impurity atoms and oxygen interstitials.

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