First stage f centre formation in pure and doped KCl crystals by room temperaure X-irradiation

The electrical conductivity and optical absorption of potassium chloride crystals, pure and doped with divalent cation impurities, have been measured before and after X-irradiation at room temperature. The concentration of free positive ion vacancies at room temperature has been calculated from conductivity for each crystal before irradiation and is found to be much less than the first stage F centre concentration. This shows that both free and associated positive ion vacancies are the latent source of F centres in the first stage colouration. Pb^{+ +} ions trap electrons producing Pb^{+ +} and Pb⁰ centres and making free the associated cation vacancies. Such centres are not produced in Ca-doped crystals where impurity-vacancy complexes trap F centres producing Z centres. The higher ratio of F centres to positive ion vacancies in Pb-doped crystals indicates that free cation vacancies are more effective in producing F centres. However, the concentration of divalent cations is found to decrease in both the crystals after irradiation, the decrease being more in Ca-doped crystals.The author is indebted to Prof. H. N. Bose for helpful discussions. Thanks are also due to Dr. M. L. Mukherjee for providing the crystals.     

The variation of electron range in pure and Mg-doped crystals of NaCl due to thermal treatment

Pure and Mg-doped NaCl crystals were subjected to heat treatment at different elevated temperatures and subsequently quenched to room temperature. The specimens were irradiated with 50 keV electrons. F centres were produced uniformly in the crystals. From the depth of coloration, the electron range was estimated by observing the number of F centres/cm² at different depths below the surface of the crystal. The electron range is found to diminish with the rise of temperature of the heat treatment up to a certain temperature above which the range indicates an increasing trend. The phenomenon is found to occur both in pure and heat-treated impurity doped samples.     

Crystalline properties and colour centres in alkali halide thin films

Abstract

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.     

On the relation between F and C centres in irradiated sodium chloride crystals

Results of experiments on the F?C conversion by reirradiation of NaCl crystals containing C centres and subsequent F-bleaching show that the C centre is a complex containing a maximum of four electrons.     

Photostimulated thermoluminescence of x-irradiated NaCl

The thermoluminescence and photostimulated thermoluminescence of X-ray coloured NaCl crystals has been studied, together with the thermal annealing of F, F′ and M centres. Three glow peaks centred at 315, 341 and 348°K are obtained in the temperature range 300–400°K. The first peak (315°K) is ascribed to electron trapping by Cu²⁺ centres formed by X-irradiation. The other peaks (314 and 348°K) are related to the thermal annealing of M and F centres, respectively.     

Thermally stimulated luminescence origin in LiF crystals irradiated in a reactor at different temperatures

Thermally stimulated luminescence as well as optical absorption and emission spectra have been studied in LiF crystals irradiated in a reactor at different temperatures. It was shown that aggregate colour centres give rise to thermally stimulated luminescence peaks registered below 450°C. Peak at 470°C is observed only in crystals that have been irradiated at standard temperature of the reactor experimental channels. The peak is caused by interaction of dislocations and F centres.     

Triboluminescence and the mechanism of discharge of gases between the newly-created surfaces of crystals during their fracture

The present paper reports on the mechanism of discharge of the gases between the newly-created surfaces of sugar, tartaric acid and six other crystals during their fracture. It has been shown that the TBL intensity obtained by crushing the crystals inside ths liquid, anhydrous diethylether, diminishes more considerably than when they are crushed in air but it depends on the mode of mechanical crushing in the same manner as the TBL intensity obtained by crushing the crystals in air does. The TBL intensity obtained by crushing the crystal inside the liquid is higher for those crystals which show higher TBL when crushed in air. It has been concluded that some part of TBL is due to the excitation of the TBL centres and the remaining part to the discharge of the gases between the newly-created surfaces, to the exo-emission of the electrons from the TBL centres at the instant of fracture of the crystal.Thanks are due to Dr. J. N.Das, Professor and Head of the Physics Department, Government Science College, Raipur (M. P.) India for his guidance and help. The author also wishes to thank the Director-General, C. S. I. R. New Delhi, for the award of a Post-Doctoral Fellowship during the tenure of the work.     

Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E _a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Emission spectra for ArNe binary crystals

Emission spectra for ArNe binary crystals have been measured in the range of 6–18 eV at the Ar concentration of 0.01 – 100 mol%. Luminescence was excited by slow electrons at T = 5°K. The spectrum of excitons bound on the atomic and molecular type centres is observed. The energies of radiative transition in a crystal appear to be very close to those in a gas phase because of the relaxation processes at which the resonance exchange interaction decreases considerably. The bands of atomic centres display a fine structure which reflects the effect of the lattice crystal field of Ne. For the ArNe crystals it has been shown that excitons are not thermalized completely and there is the independent relaxation of the lowest states of various Wanier series to local excitons. A scheme for the relaxation process in argon involving the formation of a molecular centre Ar₂¹ is given.     

Characteristics of the fast electron emission produced during the cleavage of crystals

The present paper reports the fast electron emission produced during the cleavage of alkali halide crystals and models the dynamics of the process. The mechano-emission arises as a result of the ionization of surface traps at the expense of the energy which is released in the annihilation of the defects which are formed during cleavage. The slow electrons which appear upon the ionization of surface traps are subsequently accelerated in the field of negatively charged segment of the freshly cleaved surface. Considering the basic mechanism of fast electron emission, expressions are derived which are able to explain satisfactorily the temporal, thermal, charge, surface, coloration, water adsorption and other characteristics of the fast electron emission produced during the cleavage of crystals. The decay time of the charges on the newly created surfaces, and the velocity of cracks can be determined from the measurements of fast electron emission produced during the cleavage of crystals. It is shown that two types of diffusing centres are responsible for the charge relaxation and thereby for the emission of fast electrons produced during the cleavage of alkali halide crystals.     

Thermoluminescence and optical studies on X-irradiated terbium-doped potassium bromide crystals

This paper reports the thermoluminescence (TL) and other optical studies made on terbium-doped KBr crystals X-irradiated at room temperature. Photoluminescence studies confirm the presence of terbium ions in the KBr matrix in their trivalent form. Formation of Z₁ centres on F-bleaching of X-irradiated crystals is observed. The characteristic emission due to Tb³⁺ ions in the spectral distribution under optically stimulated emission and thermoluminescence emission confirm the participation of the Tb ions in the recombination process. TL process has been identified to be due to the thermal mobilization of electrons produced during the colouration process.     

Irradiation damage in carbon-doped silicon irradiated at low temperatures by 2 MeV electrons

Abstract

Irradiation of silicon by 2 MeV electrons at 130°K leads to the removal of carbon from substitutional sites and the formation of centres with axial symmetry having vibrational modes at 921 and 930 cm^?1 for ¹²C; large isotope shifts are found in crystals containing ¹³C and ¹⁴C. This centre is considered to involve interstitial carbon atoms but not oxygen impurities. On annealing such irradiated crystals to room temperature the concentration of these centres is reduced and a new transient centre involving carbon has been detected. Further annealing leads to the formation of the well known 11.56μm absorption band in pulled crystals and it is shown that this may be correlated with another band at 1115.5 cm^?1. Again large isotope effects are found in crystals containing ¹³C and ¹⁴C and this centre is ascribed to a [O-C] complex involving an interstitial carbon atom.     

Synthesis, Characterization, Luminescence and Defect Centres in CaYAl3O7:Eu3+ Red Phosphor

CaYAl(3)O(7):Eu(3+) phosphor was prepared at furnace temperatures as low as 550°C by a solution combustion method. The formation of crystalline CaYAl(3)O(7):Eu(3+) was confirmed by powder X-Ray diffraction pattern. The prepared phosphor was characterized by SEM, FT-IR and photoluminescence techniques. Photoluminescence measurements indicated that emission spectrum is dominated by the red peak located at 618 nm due to the (5)D(0)-(7)F(2) electric dipole transition of Eu(3+) ions. Electron Spin Resonance (ESR) studies were carried out to identify the centres responsible for the thermoluminescence (TL) peaks. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0126 is identified as an O(-) ion while centre II with an isotropic g-factor 2.0060 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F(+) centre appears to correlate with the observed high temperature TL peak in CaYAl(3)O(7):Eu(3+) phosphor.     

Luminescence of [Be] centre in MgO:Be

Doping of MgO single crystals with Be results in the formation of numerous Be-containing paramagnetic centres, easily detectable by EPR, and creates an intensive luminescence band at 6.2 eV, observable at T<200 K, and gives rise to new thermoluminescence peaks at 147 and 190 K. A paramagnetic centre with a rhombic symmetry that decays at 160 K was identified as a [Be]⁺ (i.e. O⁻–Be²⁺) centre—a hole trapped by O²⁻ lattice ion near a Be²⁺ perturbing defect. The luminescence excitation and isochronal annealing studies led to the conclusion that the 6.2 eV luminescence arises at the radiative decay of electron excitations near Be²⁺. These excitations can be created at the recombination of electrons with the holes localised in the [Be]⁺ centres, at the recombination of holes with the electrons trapped in the Be¹⁺ centre or at a direct excitation of oxygen near the Be²⁺ ion.     

Eigenst?rstellen in elektronenbestrahltem Zinkoxid

The ESR of zinc oxide single crystals irradiated with 6 × 10¹⁸ electrons/cm² consists of five spectra resulting from interstitial oxygen ions and from holes trapped at zinc vacancies. Irradiation with visible light during the ESR measurements allows to determine the energy levels of the centres. Further the influence of the paramagnetic defects on the electric conductivity is discussed.     

高能电子和正电子在晶体沟道中的辐射

介绍了高能电子和正电子在晶体中的沟道辐射,对超相对论电子和正电子在周期弯曲晶体中的相干辐射进行了分析,并提出了初步的实验设想. The channeling radiation of high energy electrons and positrons in crystals was introduced. According to the new idea proposed by A.V.Korol, the coherent radiation of ultra relativistic electrons and positrons channeled in periodically bent crystals was analyzed. The characteristics of the radiation were obtained by using classic electromagnetic theory and a tentative experimental plan for testing was suggested.     

Über Eigenstörstellen in Zinkoxid

Irradiation of zinc oxide single crystals with fast electrons produces both interstitial ions and vacancies. ESR investigations supply knowledge about the local structure of these intrinsic centres. The characteristic photosensitivity of the ESR spectra has been studied in detail. The electron configurations are discussed under the conditions of the covalent ZnO bond. A band model is proposed to explain the observed photosensitivities.     

Eigenstörstellen in elektronenbestrahltem Zinkoxid

The ESR of zinc oxide single crystals irradiated with 6 × 10¹⁸ electrons/cm² consists of five spectra resulting from interstitial oxygen ions and from holes trapped at zinc vacancies. Irradiation with visible light during the ESR measurements allows to determine the energy levels of the centres. Further the influence of the paramagnetic defects on the electric conductivity is discussed.     

Investigations on the photographic elementary process in AgCl single crystal foils

The behaviour of the latent image produced by actinic radiation (λ = 365 nm and λ = 407 nm) in AgCl monocrystal foils highly doped with Cd and grown and annealed under various conditions was studied by extinction measurements in the near infrared. The photographic elementary process in these highly doped crystals cannot be described satisfactorily by the classical Gurney Mott model. Therefore another model was used based on the creation of anion vacancies and molecular chlorine complexes. The radiation-induced electrons occupy these anion vacancies, and quasi- metallic centres are formed. By this model the behaviour of the light-induced latent image can also be described as the nuclear particle track formation in the Cd doped AgCl crystals.