S. V. Nikiforov

An interactive process in the mechanism of the thermally stimulated luminescence of anion-defective α-Al2O3 crystals

The role played by deep traps in the appearance of specific features of the thermally stimu- lated luminescence (TL) of anion-defective a-Al2O3 single crystals is reported. The existence of deep traps has been proved by direct observations of the TL peaks associated with these traps. From the ex- perimental evidence it is adduced that the degree of occupancy of the deep traps aAects the basic fea- tures of the main TL peak at 450 K. A model of an interactive trap system is proposed. The main diAerence of this model from those described in the literature is that it allows for the temperature dependence of the probability of the charge carriers to be captured at the deep traps. The proposed model was used to calculate the basic TL parameters of the main peak (the TL output and the shape of the glow curve) of the crystals in question as a function of the heating rate and the occupancy of the deep traps. The calculated values are similar to those observed experimentally. # 1998 Published by Elsevier Science Ltd. All rights reserved

Specific features of luminescence properties of nanostructured aluminum oxide

Aluminum oxide nanopowders are prepared by the gas phase method and characterized according to the particle sizes and the phase composition. Samples of the nanostructured ceramic material are produced by pressing and annealing in air. The photoluminescence and cathodoluminescence spectra of the Al2O3 nanostructured ceramic material and α-Al2O3 anion-defect single crystals are investigated under comparable conditions. The luminescence bands of centers formed by oxygen vacancies are revealed in the spectra of two types of samples. The nanostructured ceramic material is characterized by the appearance of a new luminescence band at 3.4 eV and a decrease in the luminescence decay time. The inference is made that the characteristic features of the luminescence of the nanostructured ceramic material can be associated with the presence of non-equilibrium phases and the specific features of relaxation processes.

Thermal and optical ionization of F-centers in the luminescence mechanism of anion-defective corundum crystals

A model describing the luminescence mechanism of anion-defective single crystals of aluminum oxide was developed including thermal ionization of excited states of F-centers. The model was validated by experimental studies on the effect of deep traps on specific features of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electric conductivity of test crystals.

The role of deep traps in luminescence of anion-defective α-Al2O3: C crystals

This paper presents the results of a study of deep traps responsible for thermally stimulated luminescence in single crystals of the anion-defective corundum α-Al2O3: C observed at temperatures of about 703, 778, 830, and 903 K. The effect of the filling of these traps by carriers of both signs on the sensitivity to radiation of the main thermoluminescence peak at 450 K, optically stimulated luminescence, and phototransferred thermoluminescence has been investigated. The results obtained are treated in terms of a model involving interactive competition of traps of different depth.

Mechanism of F-center luminescence in anion-defective aluminum oxide single crystals

New experimental data illustrating the effect of deep traps on the luminescence properties of anion-defective α-Al2O3 single crystals are presented. It was established that deep traps have electronic nature and their filling occurs through photoionization of F centers and is accompanied by F → F+-center conversion. Model concepts were developed that describe the luminescence mechanism in anion-defective aluminum oxide single crystals with inclusion of thermal ionization of the excited F-center states. The validity of the model was supported by experimental data obtained in a study of thermoluminescence, thermally stimulated exoelectron emission, and thermally stimulated electrical conductivity.

Interactive process in the mechanism of thermostimulated luminescence in anion-defect α-Al2O3 crystals

A study of the role of deep traps in the specific features of the thermoluminescence (TL) of anion-defect α-Al2O3 single crystals is reported. The existence of deep traps is proven by direct observation of the associated TL peaks. Experimental support for the effect of deep-trap filling on the main characteristics of the main TL peak at 450 K is presented. A model involving trap interaction is proposed, which differs radically from the others described in the literature by taking into account the temperature dependence of the carrier capture probability by deep traps. This model was used to calculate the dependences on heating rate and deep-trap filling of the main parameters of the main TL peak for the crystals under study (TL yield, glow-curve shape, and sensitivity to the stored light sum), which were found to be close to those observed experimentally.

Mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output for anion-defect α-Al2O3 crystals

The mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output was investigated for anion-defect α-Al2O3 single crystals. The experiments revealed a new specific feature: the nonlinearity of the dose dependence of the thermoluminescence output depends on the heating rate of crystals during recording of thermoluminescence. It was established that this effect and other thermoluminescence features studied earlier for anion-defect α-Al2O3 crystals are adequately described in the framework of a unified mechanism associated with the interactive interaction of dosimetric and deep traps. Recommendations are given for a decrease in the range of the superlinear portion in the dose dependence of the thermoluminescence output for the crystals under investigation.

Specific features in the kinetics of thermally stimulated luminescence of α-Al2O3 crystals containing defects

A study of specific features in the kinetics of thermoluminescence (TL) of defective α-Al2O3 crystals is reported. The TL properties are experimentally shown to be related to the presence of oxygen vacancies of thermochemical or radiation origin. It is established that the differences in the TL kinetics in α-Al2O3 samples grown under different conditions can be accounted for by the hypothesis that the energy spectra of the trapping levels have different widths. As is demonstrated with a titanium impurity, this broadening of the energy spectrum can originate from the presence of traces of native impurities in the α-Al2O3 lattice.

Luminescent and Dosimetric Properties of Nanostructured Ceramics Based on Aluminum Oxide

The main features of thermoluminescence (TL) of nanostructured ceramics based on anion-defective aluminum oxide have been investigated. The kinetic parameters of the TL dosimetric peak at 475 K have been determined. The possibility of using nanostructured ceramics for beta-radiation dosimetry of high doses (up to 1 kGy) with thermoluminescence of deep traps has been justified. It has been found that the light sum of the dosimetric peak decreases with an increase in the heating rate due to the temperature quenching of the luminescence. The obtained results have confirmed that the mechanism of TL quenching in anion-defective aluminum oxide is associated with the temperature dependence of the probability of the capture in deep traps, which can be caused by thermal ionization of excited states of F-centers.

Dosimetry of mixed gamma-neutron fields using TLD-500K detectors based on anion-defective corundum

The outlooks for detection of neutrons using thermoluminescent (TL) photon radiation detectors type TLD-500K based on anion-defective corundum have been analyzed. The detection method uses transformation of neutron radiation into gamma radiation arising when neutrons are captured by cadmium nuclei. The comparative analysis of the data obtained with TL detectors and results of radiometric measurements suggests applicability of this method. A sensor with an optimal embodiment providing a relatively high sensitivity has been developed.