Color Centers and Point Defects in Irradiated Thoria

Abstract

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.

Inferring relative dose-dependent color center populations in proton irradiated thoria single crystals using optical spectroscopy

“…This is consistent with a previously reported observation of color change following proton irradiation in commercial arc-fused thoria single crystals. 46 The deep blue color in the irradiated crystals indicates the formation of color centers associated with point defects, where trapped electrons and holes contribute to emergence of new states in the electronic band structure.…”

“…Energetic protons have been shown previously to produce color centers in the thoria single crystals. 46 The PL spectra reveal a broad asymmetric peak at ∼1.67 eV and another lower intensity peak at ∼1.92 eV. Distinct optical absorption peaks are seen at ∼1.8 eV and ∼2.0 eV in the optical absorption spectra of the irradiated thoria crystals.…”

“…43,44 The presence of F-centers in single crystal thoria subject to ionizing radiation was first reported by Neeley and co-workers using ESR spectroscopy in 1967. 45 Subsequent studies on single crystal thoria in the 1970s investigated the effects of irradiation and annealing conditions on the optical absorption, 46,47 photoluminescence 48 and thermoluminescence 49,50 behavior of thoria. Regions of the sample exposed to ion irradiation changed color to an intense blue due to the formation of distinct optical absorption peaks.…”

Charged defects during alpha-irradiation of actinide oxides as revealed by Raman and luminescence spectroscopy

“…This process does not have any effect on the normal lattice vibrations, thereby explaining the absence of Raman signals. On the other hand, the electrons are also involved in the formation of colored defects associated with the blue color in the irradiated ThO 2 [24][25]. A unique mechanism could be held responsible for the irradiation defects in UO 2 and PuO 2 .…”

Thermal Energy Transport in Oxide Nuclear Fuel

“…This has resulted in the AO2 family being the subject of more exotic crucible methods, such as the cold crucible technique (ThO2 and UO2 [212]), closed capsule zone melting (UO2 [213]), and using a floating zone furnace (UO2 [214,215]). An alternative route to avoiding crucible destruction is to have a much more rapid melt growth, such as in arc melting (UO2 [214,216,217,218] and ThO2 [219,220,221,222,223]), a solar furnace (UO2 [224] and ThO2) [225]), or the Verneuil method (UO2 [226]). Although many of these techniques yield large (>1 cm 3 ) products, they are never single crystal for the AO2 family.…”