Annealing of Radiation Defects in X-Irradiated LiBaF 3
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Annealing of Radiation Defects in X-Irradiated LiBaF3 Peteris Kulis, Uldis Rogulis, Maris Springis, Ivars Tale, Aris Veispals, Andis Groza and Valters Ziraps Institute of Solid State Physics, University of Latvia, 8 Kengaraga, LV-1063, Riga, Latvia ABSTRACT Results of application of the glow rate technique GRT for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in LiBaF3 crystals, pure and containing hetero-valence oxygen centers are presented. It is shown that depending on impurity composition in crystal two alternative mechanisms are involved in annealing of color centers. It is proposed that either the anion vacancy governed migration of F- centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F- type centers are responsible for the recombination of radiation defects above RT. INTRODUCTION According to [1], intensive, non-saturating with dose absorption bands arise at 270, 317 nm and 420 nm after X-irradiation of an undoped LiBaF3. Experimental data argue for complex nature of these absorption bands. The ratio of bands intensities varies from sample to sample and considerably changes in course of optical or thermostimulated bleaching [1–3]. Optical stimulation in UV - blue spectral region leads to the strong decay of the 420 nm band, whereas the 270 nm band is more stable under stimulation [3]. By comparing LiBaF3 with KMgF3 and LiF crystals the absorption bands at 270, 420, 632, 386 and 480 nm have been attributed to F, F2, F2+, F3 and F3+ centers [3]. The following data obtained by the selective light stimulated bleaching indicate for presence of color centers having absorption in all three main bands at 270, 317 and 420 nm as well [2]. Illumination by monochromatic light corresponding to any of these bands results in their simultaneous bleaching together with creation of new absorption bands at 632 nm and 700 - 1100 nm [2,3]. Subsequent stimulation by 632 nm leads to a partial simultaneous restoration of the initial absorption spectrum [2]. The anisotropy of absorption induced by bleaching with polarized light indicates that the 420 nm band is due to centers of axial symmetry with transition dipole moments oriented along the direction. This result together with the derivative-shaped magnetic circular dichroism (MCD) band centered at 300 nm, allows us to propose that both the 317 and 270 nm bands are related to the σ-transition of the same axial center [1,2]. Taking into account the predicted location of F- type absorption bands in accordance with the Mollwo-Ivey relation, it is concluded that FA-centers are the most feasible candidates for the defects having absorption at 420, 317 and 270 nm [3]. Thermostimulation starting from RT results in simultaneous complete bleaching of the 270, 317 and 420 nm absorption bands at ~550K [4]. Taking into account that different kinds of color
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