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Scintillation Detectors and Materials Scintillation Detectors and Materials
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
Although there are many types of color centers, perhaps one of the best understood is the F-center, so called from the German Farbzentrum, translated “color center”. An F-center is formed by the displacement of a negative ion in the crystal structure, forming an anionic vacancy, which can trap an electron. The defect is formed by the coupling of a positive acting vacancy and the negative electron. This vacancy-electron combination produces energy levels in the band gap of the crystal, which can reabsorb luminescent photons, and through the Stokes shift, re-emit them at longer wavelengths, or lose the excited electron by phonon interactions. The net effect is to reduce the light yield. F-centers are native defects in alkali halide crystals, but can also be formed by x-ray, electron, or gamma-ray irradiation. In some cases, F-centers can be removed or reduced by annealing the crystal. There are many variants of these F-centers, all based on additional defects coupled to an F-center [Kittel 1956]. Included in the list of F-center variants are F′-centers, R-centers, and M-centers [Singh 2012], although the F-center is the most common. An F′-center is an anion vacancy with two trapped electrons [Kittel 1956], an M-center is a formation of two localized F-centers, and an R-center is a formation of three localized F-centers (see Fig. 13.9).
Optical Properties of Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
Subsequently, it was found that F-centres can also be produced by heating a crystal in the vapor of an alkali metal: this gives a clue to the nature of these defects. The excess alkali metal atoms diffuse into the crystal and settle on cation sites; at the same time, an equivalent number of anion-site vacancies are created and ionisation gives an alkali-metal cation with an electron trapped at the anion vacancy (Figure 8.7). In fact, it does not even matter which alkali metal is used; if NaCl is heated with potassium, the color of the F-centre does not change because it is characteristic of the electron trapped at the anion vacancy in the host halide. Work with ESR has confirmed that F-centres are indeed unpaired electrons trapped at vacant lattice (anion) sites. (a) The F-centre, an electron trapped on an anion vacancy. (b) H-centre.
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Published in Evgeny Y. Tsymbal, Igor Žutić, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
Yun-Yi Pai, Anthony Tylan-Tyler, Patrick Irvin, Jeremy Levy
Oxygen vacancies are intrinsic, pervasive point defects for SrTiO3. Oxygen vacancies and vacancy clusters modify the band structure and create deep, localized in-gap states [59, 63], at, or in the vicinity of, defect sites. Each oxygen vacancy, in principle, donates two electrons. Depending on the configuration of vacancy clustering, these electrons can remain localized on the vacancy site, or fill the d orbitals of nearby Ti atoms. The defect sites with unpaired electrons are referred to as F-centers or color centers. Both F-centers and Ti 3d orbital can possess a local moment [88, 89].
Radiation effects on luminescent and structural properties of YPO4: Pr3+ nanophosphors
Published in Radiation Effects and Defects in Solids, 2018
Ivica Vujčić, Tamara Gavrilović, Milica Sekulić, Slobodan Mašić, Bojana Milićević, Miroslav D. Dramićanin, Vesna Đorđević
The phenomenon of radiation-induced color change is called ‘activation of color centers’. The details are quite complex and involve an alteration of the orbital distribution of an atom’s valence (outermost) electrons, causing the atom to absorb photons of a different frequency (color) prior and after irradiation (31). Depending on the type of the defects in the crystal, the color centers may be electrons located in anion vacancies (F center), or holes located in cation vacancies (V center), or interstitial anion atoms (H center) or ions (I center) (11). In this case, an F-center (German Farbe, ‘color’) is formed, as described in (12). Radiation-induced vacancy acts like a positively charged particle and attracts and traps an electron. The trapped electron can absorb only certain colors of light. However, these color changes do not significantly affect the properties of powders, and the color can be restored simply by heating the powders to the temperature of the annealing (32). The observed reflectance was converted to the CIE chromaticity coordinates which are listed in Table 2 and plotted in Figure 8.
TL–ESR correlation studies in LiMgPO4:Tb,B phosphor
Published in Radiation Effects and Defects in Solids, 2018
S. N. Menon, T. K. Gundu Rao, D. K. Koul, S. Watanabe
BO32− ion, PO22− radical, O− ion, and F+ center have been identified in the irradiated LMP. The BO32− ion, F+ center, and the PO22− radical are found to correlate with the TL peak at 260°C. O− ion with an isotropic g value is related to the TL peak at 175°C. Only two defect centers, the PO22− radical and an F+ center, are found to be forming in undoped LiMgPO4. The precursor of F+ center, the F center, relates to the low-temperature-dominant TL peak at 110°C, while the PO22− radical is associated with the 260°C TL peak. A TL mechanism involving BO32− is proposed.
Radiolytic Production of Fluorine Gas from MSR Relevant Fluoride Salts
Published in Nuclear Science and Engineering, 2023
Lance Davis, Ralph Hania, Dennis Boomstra, Dillon Rossouw, Florence Charpin-Jacobs, Jan Uhlir, Martin Maracek, Helmut Beckers, Sebastian Riedel
Finally, it appears that the observed production decrease and recovery following the movement of the SAGA facility out of and back into the radiation field are more significant for the heavier salts (FLiBe-UF4 and ThF4) compared to LiF. This may be related to higher accumulated radiation damage (due to higher gamma absorption) in the heavier salts in the time period up to the first facility removal operation. For the halide sublattice of the heavier salts, the number of primary defects (F-center and H-center), extended defects (metallic colloids and halogen bubbles), and associated vacancy voids within the salt crystal at the time of the facility movement would be higher than that of the lighter salts. Since vacancy void growth is proportional to dose absorbed,7 the vacancy voids are expected to be larger in the heavier salt. The combined effect of higher number density and larger dimensions of extended defects (specifically metallic colloids) and larger vacancy voids increases the significance of the recombination back reaction, which may explain the observed pressure decrease of the heavier salt samples upon removal from the gamma field. In addition, the F2 production curves for the heavier FLiBe-UF4 (Fig. 11) and ThF4 (Fig. 12) salts indicate a marked F2 increase around the period during which the SAGA facility was removed from the gamma field. In Refs. 41 and 42, it is shown that the recombination reaction between metallic colloids and halogen gas filled vacancy voids is significantly energetic, leading to a very rapid temperature and pressure increase within the vacancy void. This may provide a complementary/partial explanation for the observed F2 increase (spike) preceding the F2 production decrease. It is noteworthy to mention that a similar observation was made in ORNL work21 following an interruption of (60Co) gamma irradiation of MSRE fuel salt.