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.     

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.     

Thermoluminescence of pure and doped ThO2

Thermoluminescence (TL) measurements in the temperature range 295–675 K indicate the existence of at least five trapping centres for single crystals of nominally pure “reduced” ThO₂ and three trapping centres for “oxidized” ThO₂. Deliberate doping with Ca²⁺, Y³⁺ and Ta⁵⁺ impurities decreases TL emission. For reduced ThO₂, accidentally incorporated rare-earth impurities, Pr³⁺, Tb³⁺, Er³⁺ act as the electron-hole recombination sites. In oxidized crystals the impurity Fe³⁺ is thought to be involved in the recombination process. In the temperature range 80–295 K an additional eight trapping centres exist for both oxidized and reduced ThO₂. For the reduced crystals the emission is probably associated with Fe in the +2 state, and in the +3 state for oxidized crystals. The differences in the TL glow curve intensities are partially explained by differences in the temperature dependence of the luminescence efficiency of the different recombination centres.     

ODEPR of indium colour centres in the X- irradiated storage phosphor KBr:In

Abstract

The results of measurements of the magnetic circular dichroism of the optical absorption (MCDA) and optically detected electron paramagnetic resonance (ODEPR) of X-irradiated KBr:In crystals are presented. The MCDA bands and ODEPR parameters of In°(1) centers and In²⁺ centres have been measured. The mechanism of the energy storage in KBr:In crystals is found not to be simply the formation of correlated F centre-In²⁺ centre pairs as was assumed previously. Considerable similarities to the storage phosphor BaFBr:Eu²⁺ were found for the photostimulated emission and read-out properties.     

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.     

Lumineszenz und Photochemie von Schwefelzentren in KCl und KBr

Interstitial hydrogen atoms are produced in alkali halides doped withSH ^–-molecules by UV-irradiation at low temperature. Tempering of the crystals leads to the formation of other sulphur centres which are investigated by optical luminescence and by electron spin resonance. The kinetics of these centres is discussed and models for several molecules are developped.     

An EPR study of Er3+ impurities in RbTiOPO4 single crystals

An electron paramagnetic resonance (EPR) study of Er³⁺ ions in single crystals of RbTiOPO₄ (RTP) is presented. The EPR spectra show the existence of six different Er³⁺ centres. The g-matrix has been determined for these centres from the analysis of the angular dependences of the spectrum in three planes of the crystal. The study supports that erbium can enter the Rb⁺ and Ti⁴⁺ low-symmetry sites of RTP. This conclusion differs from those for KTP:Er³⁺ and RTP codoped with Nb and Er. The different occupancies found for Er in these various crystals is suggested to be due to the differences in Er concentration.     

Anomaly in electron channeling due to imperfections produced thermally in a potassium chloride crystal

Lattice imperfections were produced in KCl crystals by a thermal process. The heat treated specimen was irradiated with 50 KeV electrons and the corresponding range of electrons in the crystal has been estimated. It was found that the electron range in the crystal depends upon the temperature of heat treatment, being a decreasing function of this temperature up to about 625 K, above which it becomes an increasing function. This anomaly has been explained in terms of dissolution of vacancies through dislocation loops.     

The yield of color center formation in KBr doped with KI

Measurements have been made of F and H centre production rates in KBr doped with KI, when electron irradiated at 5°K. Significant concentrations of I⁻ reduce the production rates of both centres. In the mixed crystals complex H bands are formed and the thermal annealing behaviour of these bands has been measured.     

Luminescence study of alumina nanopowders prepared by various methods

Mixed nanopowders of transition alumina prepared by combustion synthesis and phase pure ultra-porous α-alumina by oxidation method were investigated using low temperature time-resolved cathodoluminescence and photoluminescence spectroscopy under VUV-XUV excitation. In all samples along with the 7.6 eV emission of self-trapped excitons of α-alumina, luminescence bands due to F, F⁺ centres with maxima at 3 and 3.8 eV and other UV–visible luminescence bands of intrinsic and extrinsic origin with varying intensity depending on sample preparation method and thermal treatment were studied. In alumina nanopowders the excitonic excitation peak at ∼9.1 eV near fundamental absorption edge is shifted to the higher energies by 0.15 eV in comparison with the same feature in single crystals. The nanostructure of alumina is responsible for this shift.     

Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh²⁺ centre was identified as a [RhCl₆]^4? complex with a single charge compensating Na⁺ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh²⁺ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh²⁺ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh²⁺ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl₆]^4? complex with two next nearest neighbour Na⁺ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.     

Thermal and non-radiative transitions of electrons at imperfections in ionic crystals

The theory of non-radiative and thermal transitions of electrons at imperfections in ionic crystals is elaborated. The results obtained are used to confirm Mott's and Gurney's theories on the detachment process in two steps of electrons from F centres during the absorption of light in the F band and to explain theoretically the temperature dependence of quantum efficiency. Good agreement with experiment was obtained. The possibility of the luminescence of alkali halide crystals containing F centres at sufficiently low temperatures was also determined.     

Red emission of PbWO4 crystals

Luminescence characteristics of a large number of undoped and doped PbWO₄ crystals, grown by the Czochralski or Bridgman method, as-grown or annealed in the nitrogen atmosphere or in air, were studied in the 4.2–300 K temperature range. Two types of red emission centres were found. The centres with the emission band, peaking at 4.2 K at 1.57 eV, were observed in most of the crystals studied. The centres with the emission band, peaking at 4.2 K at 1.48 eV, were observed only in the PbWO₄ : Mo⁶⁺, Y³⁺ crystal. It is suggested that incompletely compensated lead vacancies are responsible for the appearance of the red emission.     

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.     

Lithium fluoride colour centres-based imaging detectors for proton beam characterization at high doses

Proton beams of 7 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films thermally evaporated on glass substrates in the dose range from 10³ to 4 × 10⁶ Gy, inducing the formation of stable photoluminescent colour centres (mainly F₂ and F₃⁺), emitting in the visible spectral range. Using a conventional fluorescence microscope, the transversal proton beam intensity was mapped by acquiring the photoluminescence image of the irradiated spots. Image analysis allowed measuring the integrated photoluminescence intensity as a function of the irradiation dose: a linear optical response was obtained up to different maximum dose values, after which a quenching was observed, depending on the nature of the samples (crystals or films). The colour centres formation was investigated by optical absorption spectroscopy at room temperature and the Principal Component Analysis was applied to the absorption spectra of irradiated LiF crystals. In samples irradiated at highest doses, it allowed clearly identifying the formation of more complex aggregate defects, which appears strictly related to the observed photoluminescence quenching effect.     

Investigation of luminescence processes in YAG single crystals irradiated by 50 MeV electron beam

Absorption, emission and excitation spectra of 50 MeV electron beam irradiated and as-grown YAG single crystals were studied and compared in the 10–300 K temperature range using time-resolved luminescence spectroscopy under UV/VUV/XUV excitation by synchrotron radiation and cathodoluminescence. The emission spectra consist of intrinsic (excitonic) and defect related non-elementary bands in the VIS/UV range. It is shown that fast electrons create stable F and F⁺ color centers with characteristic emission and absorption bands in the visible/UV range. Induced absorption caused from these defects starts at 4.2 eV. Energy transfer from host to color centers is not an efficient process.     

Absorption background and laser properties of YAP:Nd

YAP:Nd laser crystals suffer to a greater extent than YAG:Nd from colour centres formation. Stable brownish-yellow or green colourations are evoked by non-oxidizing treatment at high (>1500 °C) temperature. The former colouration may be attributed to holes in the oxygen neighbourhood of Y³⁺ at Al sites, the latter to the F⁺ centres. Brownish-red colouration of less stability is induced by optical pumping of Nd³⁺ containing crystals. It may be attributed to holes in oxygen neighbourhood of Nd³⁺.Dependences of the colourations on the preparation methods are described. YAP:Nd,Zr laser rods, oxygen annealing of which was followed by medium (1100–1400 °C) temperature treatment in vacuum or hydrogen showed no anomalous colouration and extremely low losses at Nd³⁺ emission wavelength independent of pumping power. Using such rods, an overall efficiency of 2·5% and a slope efficiency of 3·0% in CW lasers were reached.     

EPR study of free radical in gamma-irradiated bis(cyclopentadienyl)zirconium dichloride single crystal

Bis(cyclopentadienyl)zirconium dichloride (BCZD; zirconocene dichloride) single crystals were exposed to ⁶⁰Co-γ irradiation at room temperature. The irradiated single crystals were investigated between 125 and 470?K by electron paramagnetic resonance spectroscopy. The spectra of the crystals were found to be temperature independent. The paramagnetic center was attributed to the cyclopentadienyl radical. The g values of the radiation damage center observed in BCZD single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained.     

Effect of temperature on processes of radiation-induced generation of primary defects in MgF<Subscript>2</Subscript> crystals

The spectral and kinetic parameters of the absorption and emission relaxation initiated by a single nanosecond-range electron pulse in MgF₂ crystals are studied in the temperature range 20–550 K. It is found that the relationship between the two channels of dissipation of the electronic excitation energy (generation of F centers and short-lived excitons) does not depend on the temperature of the irradiated crystal in the temperature range 20–120 K and changes in favor of the F centers at T≥150 K. The temperature quenching of the 3.2-eV exciton emission at T≥60 K is detected. The ionic structure of the short-lived excitonic state is discussed.     

Optical properties of FeCl2 and FeBr2 around the néel temperature

The ³T₂(³F) crystal field transition for FeCl₂ and FeBr₂ crystals is studied in the temperature range around the magnetic transition. The temperature dependence of the intensity of the lines at 2.395 eV in FeCl₂ and at 2.31 and 2.95 eV in FeBr₂ is typical of a “hot” exciton-magnon mechanism. Specific magnetic rotation measurements at 80 and 4.2 K are also reported. Values of 10⁴ deg/cm are found in correspondence to the absorption edge of both crystals.