Luminescence of aggregate centers in CsBr:Eu2+ single crystals

We have used the Bridgman method to grow CsBr:Eu²⁺ single crystals, adding an activator to the mix in the form of Eu₂O₃ in amounts of 0.0125, 0.0250, and 0.0500 mole %. At T = 300 K, we studied the absorption spectra, the photoluminescence (PL) spectra, and the photostimulated luminescence (PSL) spectra of the grown crystals. We have established that the structure of the photoluminescence and photostimulated luminescence centers in crystals grown from the CsBr:Eu₂O₃ mix includes isolated dipole centers Eu²⁺-V_Cs, emitting in bands with maxima at 432 nm and 455 nm respectively, and in crystals grown at activator concentrations of 0.025 and 0.050 mole % they also include aggregate centers (AC) based on CsEuBr₃ nanocrystals with emission bands at 515 m and 523 nm. We have shown that the maximum concentration of aggregate centers of the CsEuBr₃ nanocrystal type in CsBr:Eu²⁺ crystals is achieved for an activator content in the mix within the range 0.01–0.05 mole %. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 359–362, May–June, 2006.     

Effects of Na and K co-doping on growth and scintillation properties of Eu:SrI2 crystals

We grew Na and K co-doped Eu:SrI₂ [Na,Eu:SrI₂ and K,Eu:SrI₂] crystals by a modified micro-pulling-down method to reveal the co-doping effects on the crystal growth and scintillation properties. The non-codoped, Na0.5%, Na1.0%, K0.5% and K1.0%,Eu:SrI₂ crystals indicated high transparency while the milky parts were generated in the Na5.0% and K5.0%,Eu:SrI₂ crystals. The light yields of Na,Eu:SrI₂ and K,Eu:SrI₂ crystals under γ-ray irradiation were decreased by the Na and K co-doping. On the other hand, there was a small change within 940–1020 ns in the decay times by the Na and K co-doping. In the light yield proportionality under γ-ray irradiation, the non-proportionality in the low energy region was improved by Na and K co-doping.     

Mechanoluminescence and thermoluminescence of BaFCl:Sm 2+ and BaFBr:Sm 2+ crystals

The alkaline-earth fluorohalide crystals MFX, where M=Ca, Sr, Ba, Pb and X=Cl, Br, I, form an important class of materials crystallizing in the PbFCl-type tetragonal structure which is also called the matlockite structure. These compounds have long been of interest because of the various defect species which can be detected by spin resonance and associated techniques. The crystals were prepared by slow cooling of the melt of a stoichiometric mixture of BaF ₂ and the corresponding chloride or bromide under 0.2 bar of ultrapure argon (5N5), often slightly fluorinated. We have studied the mechanoluminescence (ML) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals. It is seen that after the impact of a moving piston, initially the ML intensity increases with time, attains a maximum value and then it decreases with time up to a particular minimum value, and then it increases again, attaining a peak value and finally disappears. The first peak lies in the deformation region and the second peak lies in the post-deformation region. The ML intensity of the BaFCl:Sm ²⁺ crystal is much higher than the ML intensity of the BaFBr:Sm ²⁺ crystal. For different impact velocities, the ML intensity increases with velocity; and the total ML intensity attains a saturation value for higher impact velocities. The total ML intensity increases with the increase in the applied load. It is suggested that the moving dislocation produced during deformation of crystals captures holes from hole-trapped centers (like H centers), and the subsequent radiative recombination of the dislocation holes with electron gives rise to ML. Thermoluminescence (TL) of BaFBr:Sm ²⁺ and BaFCl:Sm ²⁺ crystals was studied after exposure to ultraviolet rays with the help of a TLD reader. The peak of TL for the BaFBr:Sm ²⁺ crystal is found at ～247°C and for BaFCl:Sm ²⁺ crystals at 283°C. The TL intensity initially increases with increase in the UV radiation and then it attains saturation for higher values of UV exposure. The absorption spectrum was recorded with the help of a UV–visible spectrophotometer (Shimadzu). The band found at 275 nm was attributed to H centers.     

Gd3+, Eu2+ in SrFCl and BaFCl single crystals: EPR results

Electron Paramagnetic Resonance results are reported on Gd³⁺, Eu²⁺ incorporated into BaFCl and SrFCl crystals.     

Scintillation properties of BeO single crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 49, No. 2, pp. 286–291, August, 1988.     

Low-temperature thermoluminescence study of GaSe:Mn layered single crystals

Mangan doped GaSe single crystals have been studied by thermoluminescence measurements performed with various heating rates between 0.4 and 1.0 K/s in the temperature range of 10?300 K. Thermoluminescence spectra exhibited four distinguishable peaks having maximum temperatures at 47, 102, 139 and 191 K revealing the existence of trapping levels in the crystals. Curve fitting and initial rise methods were applied to observed peaks to determine the activation energies of four trapping levels. Capture cross-sections of each level were also evaluated using the obtained energy values. Moreover, heating rate dependencies of the obtained peaks were investigated. It was shown that increase in the heating rate resulted in the decrease in thermoluminescence intensity and shift of the peak maximum temperatures to higher values. Discrete, single trap behaviour was established for acceptor level related with the peak at 191 K by analysing the sequentially obtained peaks with different stopping temperatures between 15 and 65 K.     

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals

Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship T_M1nτ^?1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu²⁺ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.     

Scintillation properties of pure and Ce3+-doped SrF2 crystals

In this paper results of scintillation properties measurements of pure and Ce³⁺-doped strontium fluoride crystals are presented. We measure light output, scintillation decay time profile and temperature stability of light output. X-ray excited luminescence outputs corrected for spectral response of monochromator and photomultiplier for pure SrF₂ and SrF₂-0.3 mol% Ce³⁺ are approximately 95% and 115% of NaI–Tl emission output, respectively. A photopeak with a 10% full width at half maximum is observed at approximately 84% the light output of a NaI–Tl crystal after correction for spectral response of photomultiplier, when sample 10 × 10 mm of pure SrF₂ crystal is excited with 662 keV photons. Corrected light output of SrF₂-0.3 mol% Ce³⁺ under 662 keV photon excitation is found at approximately 64% the light output of the NaI–Tl crystal.     

Optical absorption and fluorescence investigation of the precipitated phases of Eu2+ in KBr single crystals

Europium precipitation in monocrystalline KBR has been studied using optical absorption and photoluminescence techniques. The annealing of quenched samples at temperatures below 100°C produces the growth of an emission band peaking at 433 nm which appears to be related with the presence of Suzuki-type precipitates in the crystals. The aging at higher temperatures, however, produces the appearence in the emission spectrum of two other bands peaking at 427 and 459 nm. The data presented in this paper strongly suggest that the former band is associated with the stable dihalide phase EuBr₂ in the host KBr, while the latter is due to a metastable precipitate whose structure appears to be quite similar to that of europium bromide. Some of the characteristics of the second phase precipitates were obtained by measuring the crystal field splitting (10 Dq) of the 4f⁶5d configuration of the Eu²⁺ ions in each one of them.     

Radiative energy transfer from Pb2+ to Eu2+ ions in NaCl,Kcl, and KBr single crystals

In the present work, the emission and excitation spectra of NaCl, KCl, and KBr doubly doped with europium and lead ions were investigated. In all cases, excitation with light lying in the A-band of the Pb²⁺ ions produces in addition to the ³P₁ → ³S_o Pb²⁺ emission, the 4f⁶ 5d (^t2g) → 4f⁷ europium emission. This fact indicates that energy transfer occurs from Pb²⁺ to Eu²⁺ ions. From the data obtained, it was determined that the energy transfer process is of a radiative nature and that it is more efficient in KCl than in either of NaCl or KBr. A possible explanation for this fact is given.     

Fluorescence mechanisms of mixed crystals Sr1-xBaxF2:Eu2+

Abstract

The spectroscopy of Sr_1-xBa_xF₂:Eu²⁺ mixed crystals is reported and explained by using an impurity-trapped exciton model. The broadening of the absorption and emission structures are interpreted as a Fano effect.     

Characteristic features of thermoluminescence kinetics in dosimetric aluminum oxide crystals

We have studied the thermoluminescence curves in the 420 nm band (F centers) and the 330 nm band (F⁺ centers) within the temperature range of the dosimetric peak (T_max = 450 K) in anion-deficient aluminum oxide crystals. Assuming general-order kinetics, we have analyzed the thermoluminescence decay curves on the rising and falling sides of the dosimetric peak, in samples with different degrees of deep trap filling. We have established differences in the order of the kinetics within different temperature ranges of the dosimetric peak. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 187–190, March–April, 2006.     

Effect of hydrostatic and chemical pressure on BaF2 and BaF2: Eu2+ crystals

The effect of hydrostatic pressure on a BaF₂ crystal was studied within the shell model in the pair-wise potential approximation. The structural phase transition from the cubic to orthorhombic phase was simulated. The behavior of the unit-cell parameters of the α-and β-BaF₂ phases under hydrostatic pressure (from 0 to 12 GPa) was investigated. The fundamental vibration frequencies of BaF₂ under hydrostatic pressure (0–3.5 GPa) were calculated for both phases. The effect of chemical pressure on the BaF₂ crystal was studied by simulating Ba_1?x Me_xF₂ mixed crystals (Me=Ca, Sr). It was shown that at impurity concentrations up to 15–20 at. % the lattice constant varies in the same way as it does when hydrostatic pressure increases to P _c , which corresponds to a phase transition to the orthorhombic phase. The effect of chemical and hydrostatic pressure on BaF₂: Eu²⁺ doped crystals was also studied. The dependence of the absorption and luminescence zero-phonon line shift on the Eu²⁺-ligand distance was calculated.