V centers in MgAl2O4 spinels

Identification of defect centers responsible for thermoluminescence emission in sol-gel synthesized calcium silicate phosphor

The thermoluminescence (TL) of calcium silicate phosphor (CSO) prepared by the sol-gel method and sintered at 1200 °C were investigated. From Tm-Tstop curve, TL emission spectrum and computer deconvolution using electron traps with discrete and continuous distributions, the glow curves were found to be composed of four TL peaks (117, 190, 250 and 275 °C) with a single emission band centered at 370 nm. Electron paramagnetic resonance (EPR) investigation has been carried out to identify the defect centers formed in the CSO phosphor by γ-irradiation and find the centers related to the TL process in the phosphor. At room temperature, three defect centers were observed. The first center, characterized by the principal g-values of 2.014, 2.011, and 2.0080 was assigned to an O- ion. The second center with g-values 2.015, 2.013, and 2.010 is also attributed to an O- ion and is associated with the TL peak at 280 °C. The third center, with an isotropic g-value of 2.0011 was identified as the F+ center (singly ionized oxygen vacancy) and relates to the TL peak at 280 °C.

Optical Characteristics of MgAl2O4 Single Crystals Irradiated by 220 MeV Xe Ions

In In this study, the optical properties of magnesium-aluminate spinel were examined after being irradiated with 220 MeV Xe ions. The research aimed to simulate the impact of nuclear fuel fission fragments on the material. The following measurements were taken during the experiments: transmission spectra in the IR region (190-7000) nm, optical absorption spectra in the range (1.2-6.5) eV, and Raman spectra were measured along the depth of ion penetration from the surface to 30 µm. A peak with a broad shape at approximately 5.3 eV can be observed in the optical absorption spectrum of irradiated spinel crystals. This band is linked to the electronic color centers of F+ and F. Meanwhile, the band with a maximum at ~(3-4) eV is attributed to hole color centers. Apart from the typical Raman modes of an unirradiated crystal, additional modes, A1g* (720 cm-1), and Eg* (385 cm-1), manifested mainly as an asymmetric shoulder of the main Eg mode, are also observed. In addition, the Raman spectroscopy method showed that the greatest disordering of crystallinity occurs in the near-surface layer up to 4 μm thick. At the same time, Raman scattering spectroscopy is sensitive to structural changes almost up to the simulated value of the modified layer, which is an excellent express method for certifying the structural properties of crystals modified by swift heavy ions.

Distinctive features of diffusion-controlled radiation defect recombination in stoichiometric magnesium aluminate spinel single crystals and transparent polycrystalline ceramics

MgAl₂O₄ spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F⁺) and hole (V) centers in stoichiometric MgAl₂O₄ spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAl₂O₄ single crystals and optical polycrystalline ceramics are compared. It is demonstrated that both transparent ceramics and single crystals, as well as different types of irradiation show qualitatively similar kinetics, but the effective migration energy E_a and pre-exponent D₀ are strongly correlated. Such correlation is discussed in terms of the so-called Meyer-Neldel rule known in chemical kinetics of condensed matter. The results for the irradiated spinel are compared with those for sapphire, MgO and other radiation-resistant materials.

Synthesis, thermoluminescence, defect center and dosimetric characteristics of LiF:Mg,Cu,P,Si phosphor

LiF:Mg,Cu,P,Si (MCPS), a new tissue equivalent phosphor, was synthesized by solid state method. Powder x-ray diffraction and scanning electron microscope were employed to determine the structural features. The dosimetric characteristics, electron spin resonance (ESR) and defect centers of this newly prepared phosphor were investigated. The MCPS phosphor is highly sensitive when compared with LiF:Mg,Ti and LiF:Mg,Cu,P (MCP), with the TL sensitivity being 35 times and 1.3 times higher respectively. The dosimetric peak occurs at 220°C with a well defined glow curve structure similar to MCP. MCPS phosphor shows a linear dose response till 10Gy. The minimum detectable dose has been found to be 8μGy. The thermal stability of the phosphor could be enhanced by 20°C from 240°C to 260°C when compared to MCP. Defect centers formed in the phosphor by gamma irradiation have been studied by ESR to identify the centers associated with the TL process in this phosphor. Thermal annealing experiments reveal the presence of several defect centers. Center I which shows an isotropic g factor of 2.0233 has been found to relate with the TL peaks at 160°C, 220°C and 265°C. Centers II and III are characterized by isotropic g values of 2.0096 and 2.0019 respectively and are attributed to F centers.

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV-visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F(+) centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3-0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Structural and spectroscopic properties of Yb3+-doped MgAl2O4 nanocrystalline spinel

Pechini's sol–gel method was successfully applied to obtain an Yb³⁺-doped MgAl₂O₄ spinel nanopowder. The regularity between the observed structural and spectroscopic measurements was described in detail.