Excitation mechanism of Bi3+ luminescence centres in CaS

A systematic approach to investigate the excitation mechanism of Bi³⁺ luminescence centres in CaS has been made from simultaneous measurements of photoluminescence (PL) and photoconductivity (PC), thermoluminescence (TL) and thermally stimulated conductivity (TSC). Changes in PL and PC have been studied in the temperature range 295–505 K. An attempt has been made to explain the correlation between these phenomena with a simple trapped-carrier-free-carrier recombination model. It is found that the charge transfer process dominates in all these phenomena. Conductivity is primarily due to electron traffic through the conduction band, whereas their subsequent recombination through the excited state (³P₁) accounts for the characteristic emission of Bi³⁺.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

Thermoluminescence of anion-deficient aluminum oxide single crystals after high-dose irradiation by nanosecond electron pulses

The effect of deep traps filled by a pulsed electron beam on the dosimetric thermoluminescence (TL) peak at 450 K in anion-deficient aluminum oxide single crystals has been investigated. After filling deep traps, the dosimetric TL peak becomes nonelementary and is characterized by a complex dependence of the TL intensity on the crystal annealing temperature with alternating fall and rise portions. The influence of the occupancy of deep centers of different nature and different energy depths on the structure of dosimetric TL peak is analyzed. The suggestion that basically electron traps are depleted in the temperature ranges of 600–750 and 900–1000 K while holes are depleted at T = 780–900 and above 1000 K is substantiated. The possibility of using TL deep traps for high-dose dosimetry of pulsed electron beams is demonstrated.     

Shallow traps in pure KTaO 3 crystals

We made Thermally Stimulated Conductivity (TSC), Thermoluminescence (TL) and Electron Spin Resonance (ESR) measurements on single crystals of potassium tantalate in the temperature range 4.2-290 v K. We revealed two sorts of O m shallow hole centers which are responsible for Photoconductivity (PC) and Photoluminescence (PL) enhancement. Both O m centers were identified by their ESR spectra. We show that these centers serve as radiative electron-hole recombination centers. The measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18-30 v K and 65-70 v K. Both TSC and TL are attributed to the thermal ionization of the same shallow donor centers related with oxygen vacancies. Experimental data were treated in a simple one-trap/one-recombination center model, which takes into account the presence of "thermally disconnected" deep electron traps.     

The contribution of hole traps to thermoluminescence of the dosimetric peak in anion-defect α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> single crystals

The thermoluminescent properties of anion-defect alumina single crystals with different FWHMs of the main (dosimetric) peak at 400–500 K are studied. New experimental evidence in favor of the hole nature of traps associated with the high-temperature part of this peak are presented. The introduction of hole trap centers into analysis provided theoretical justification for the experimentally observed dependences of the thermoluminescence (TL) intensity, the temperature position of the main peak, and its FWHM on the occupancy of deep traps. The hole nature of traps of the high-temperature part of the main TL peak is confirmed by the results of examination of specific TL features of shallow trap centers, which govern TL at 350 K, and the temperature variation of the main TL peak spectrum.     

掺Gd3+,Y3+对PbWO_4低温热释光的影响

通过热释光方法研究了PbWO₄(PWO),PWO:Y³⁺,PWO:Gd³⁺多晶粉末及PWO,PWO:Y单晶的低温(<300K)热释光现象.多晶粉末中,掺杂Y³⁺或Gd³⁺都会大大降低甚至消除200K附近的热释光峰,同时产生新的热释光峰,分别位于125和150K(掺Y掺Gd).这表明掺三价离子除了起到电荷补偿作用以减少Pb³⁺,O^-浓度外,还可以产生新的陷阱能级.对于PWO:Y单晶,掺杂Y³⁺可以消除253K的热释光峰,即消除较深(～0.89eV)的陷阱,但PWO单晶中较浅的陷阱(～0.42eV)对应130K热释光峰仍然存在,对此进行讨论,它最可能源于氧空位缺陷.根据Pb³⁺,Gd³⁺,Y³⁺的电子库仑势不同,在PWO晶体中替代Pb²⁺后形成的电子陷阱深度有别(E_Pb>E_Gd>E_Y),从而解释了相应的热释光峰值温度的不同 关键词： 4')" href="#">PbWO₄ Y和Gd掺杂 热释光 陷阱     

The effects of deep trap population on the thermoluminescence of Al2O3:C

The influence of deep traps on the 450 K thermoluminescence (TL) peak of Al₂O₃:C is studied. Depending upon the sample and on the degree of deep trap filling, features such as the TL width, area and height can vary considerably. These effects are interpreted to be due to: (a) sensitivity changes introduced by competition mechanisms involving deep electron and hole traps, and (b) the multiple component nature of the 450 K TL peak. The influence of the deep traps on the TL was studied using different excitation sources (beta irradiation or UV illumination), and step annealing procedures. Optical absorption measurements were used to monitor the concentration of F- and F⁺-centers. The data lead to the suggestion that the competing deep traps which become unstable at 800–875 K are hole traps, and that the competing deep traps which become unstable at 1100–1200 K are electron traps. Both the dose response of the TL signal and the TL sensitivity are shown to be influenced by sensitization and desensitization processes caused by the filling of deep electron and hole traps, respectively. Changes in the TL peak at low doses were also shown to be connected to the degree of filling of deep traps, emphasizing the influence of deep trap concentration and dose history of each sample in determining the TL properties of the material. Implications of these results for the optically stimulated luminescence properties are also discussed.     

Investigations of thermoluminescence properties of multicrystalline LiF: Mg,Cu, Si phosphor prepared by edge defined film fed growth technique

Thermoluminescence (TL) properties of LiF: Mg, Cu, Si phosphor prepared in multicrystalline form using edge defined film fed growth (EFG) technique has been investigated. The effect of preparation route on TL properties and thermal stability has been studied. To improve the TL dosimetry properties, phosphor is subjected to different annealing temperatures ranging from 250 °C to 450 °C. The shape of the glow curve structure and peak temperature remains similar at different annealing temperatures, however peak intensities vary. The consistency in the glow curve structure with annealing temperature elucidate that TL trapping states are stable in nature. Thermal annealing at 300 °C for 10 min gives maximum TL intensity with main dosimetry peak at 209 °C. The TL intensity of the main dosimetry peak is increased by a factor of five as compared to as-grown crystal. The thermal stability of LiF: Mg, Cu, Si is found to be better than LiF: Mg, Cu, P. Trapping parameters are calculated to have an insight study of defect states. A simple glow curve structure, tissue equivalency, thermal stability, low residual signal, linear response and reusability makes LiF: Mg, Cu, Si a suitable phosphor for radiation therapy, radio diagnostics and personnel dosimetry applications.     

A study of thermally stimulated currents in thin films of a good ionic conductor (βPbF2) with blocking electrodes

Thin film samples of βPbF₂ are studied by stimulated currents, the TSC peak of lower temperature, independent of the polarization voltage is attributed to the space charge depolarization. On the basis of simplifying hypothesis an exact analytical solution of this problem is also proposed; the comparison with the experimental results allows to get a value of the activation energy of mobile species and to estimate the thickness of the space charge. Electron transfers at the electrodes are assumed to explain the second peak.     

Afterglow mechanism and thermoluminescence of red-emitting CaS:Eu,Pr phosphor with incorporated Li ion upon visible light irradiation

This paper reports on the afterglow mechanism and thermoluminescence (TL) of a red-emitting CaS:Eu²⁺,Pr³⁺ phosphor with incorporated Li⁺ ion upon irradiation by visible light (D₆₅ lamp). In the TL glow curve of the CaS:Eu²⁺,Pr³⁺ phosphor, a TL peak was observed near 120 °C. The luminescence center of the CaS:Eu²⁺,Pr³⁺ phosphor was the Eu²⁺ ion and the trap depth of the CaS:Eu²⁺,Pr³⁺ phosphor with the cation vacancy (Trap 1) which formed by incorporation of the Pr³⁺ ion was 0.202 eV. A cation vacancy (Trap 2) was formed by incorporation of the Li⁺ ion in the CaS:Eu²⁺,Pr³⁺ phosphor. In the TL glow curve of the CaS:Eu²⁺,Pr³⁺ phosphor with incorporated Li⁺ ion, two TL peaks were observed near 120 and 200 °C. The TL luminance of the CaS:Eu²⁺,Pr³⁺ phosphor with incorporated Li⁺ ion increased with an increase in the initial Li/Ca atomic ratio. The two TL peaks moved to the high-temperature side with an increase in heating rate. The cation vacancy (Trap 2) calculated from the Hoogenstraaten method was 0.118 eV. The afterglow time of the CaS:Eu²⁺,Pr³⁺ phosphor with incorporated Li⁺ ion was prolonged by generation of a shallow trap.     

Nature of the anomalies of the thermolumenescence properties of dosimetric α-Al2O3 crystals

The results of investigations into the anomalies of the thermoluminescence properties of dosimetric corundum crystals are presented. The decisive role of deep-lying traps in the quenching of luminescence in anion-defect Al₂O₃ monocrystals is shown. The existence of deep-lying traps is demonstrated by the method of direct observations of thermoluminescence (TL) peaks associated with them. Experimental evidence for the influence of the degree of occupation of deep-lying traps on the main features of the TL dosimetric peak at 450 K is given. The results obtained are interpreted for a model of the interactive system of traps, which differs radically from the models described in the literature by a consideration of the temperature dependence of the probability of trapping of charge carriers on deep-lying traps. We believe that the heat quenching of luminescence is due to the thermal ionization of excited F-center states. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 3, pp. 55–65, March, 2000.     

Thermoluminescence and thermally stimulated currents in PbMoO4 crystals

Thermally stimulated depolarization (TSD) currents have been studied in the range from 4 K to room temperature by utilizing direct contacts as well as blocking contacts. Charge carrier injection and subsequent trapping seem to be responsible for internal electric fields. A glow peak near 186 K arises from a dipolar relaxation. The results of additional optical excitation during polarization provide some more information on charge transport involved in relaxation processes. The correlation to the results of photoconductivity and annealing procedure leads to the assumption of oxygen vacancies as the dominant defect centres. The study of thermoluminescence (TL) after X-irradiation gives evidence on defect states in question. In addition, from the results quasi-free charge carriers are assumed to be thermally generated above 100 K.     

On the role of hole trapping centers in the interactive mechanism of the trap interaction in anion-defect alumina single crystals

The effect of deep hole traps on the intensity and shape of the dosimetric peak of thermoluminescence (TL) has been studied at 450 K in anion-defect alumina single crystals. It has been shown that filling of deep hole centers leads to a decrease in the sensitivity to radiation of crystals with a small half-width of the TL peak and has no effect on the sensitivity of crystals with a broadened peak. It has been assumed that traps responsible for the TL dosimetric peak broadening are of hole nature, which can be caused by the presence of Ti3+ ions in the corundum lattice. The results obtained have been interpreted within the modified model of the interactive system of traps.     

Effect of Fe,Co, Ni and Cd CO‒activation on luminescence emission and decay of CaS:Ce phosphors

Cathodoluminescence (CL), photoluminescence (PL), thermoluminescence (TL) spectra, and decay after laser beam excitation of cerium-activated CaS phosphors doped with Fe, Co, Ni or Cd have been investigated. These studies shed some light on the nature of luminescence centres and the association of various groups of traps with the dopants. Effect of concentiation variation of these four elements on emission and decay characteristics of CaS:Ce phosphor has been discussed. In contrast with the earlier publication [W. Lehmann and F. M. Ryan, J. Electrochem. Soc. 119, 275 (1972)] we have found that Ni is an effective killer, Cd ineffective and Co has no clearly observable effect. Fe has also been found to be an effective killer for CaS:Ce phosphors. Ni and Fe appear to be more effective quenchers for shorter wavelength emission band characteristic of Ce in CaS.     

EPR,thermoluminescence, and thermally stimulated conductivity in BaWO4 crystals

In BaWO₄ crystals electrons and holes trapped at WO₄ complexes are identified by electron paramagnetic resonance (EPR) after X-irradiation at T = 80 K. The thermal decay of the intrinsic hole centres at about 100 K is accompanied by a simultaneous decrease of electron traps (WO₄)^3- and glow maxima of thermoluminescence (TL) and of thermally stimulated conductivity (TSC). This connection is explained by a thermally activated hopping of the $\text{(WO}{}_4\text{)}\text{3-}\text{2}$ hole centres followed by radiative recombination with electron traps (WO₄)^3-. A qualitative kinetic calculation based upon EPR data and the shift between TL and TSC glow peak confirms this model.     

Trapping parameters and kinetics in CaS: Ce,Cl luminophor

By means of thermally stimulated luminescence (TL) technique the trapping parameters and order of the kinetics in TL process in the CaS: Ce, Cl luminophor are investigated. In the temperature range 300–450 K, two glow-peaks are observed showing the presence of two group of traps in this luminophor whose trap-depths are 0.66 ± 0.16 eV and 1.13 ± 0.11 eV respectively as determined by three different methods. The order of the kinetics in TL processes in this luminophor comes out to be first order for the first group and second-order for the second group of traps.     

Photoenhancement of the photoconductivity (PC) of electron irradiated semiconducting diamond

The spectrum of the non-enhanced PC of electron irradiated semiconducting diamond extends from the UV towards the visible and near infrared. It's long wavelength tail was found in the present work to exhibit a well defined threshold shifted with temperature from about 1.5eV at 76 K to about 1.25 eV at 500 K. Pre-illumination in the “UV-band” produced an enhanced PC band with a temperature independent threshold at 1.08 0.03 eV. This photoenhanced band was found to be closely related to a thermally-simulated current peak (TSC) at 500 K with an activation energy of 0.50 eV excited by the pre-illumination in the UV-band. The prhotenhanced band was bleached out thermally with the exhaustion of the TSC peak below 600 K. Some of the characteristics of the photoenhanced band including the linear dependence of the square root of the PC on photon energy may suggest that internal photoemission of holes plays a role in the formation of this band.     

Phototransferred Thermoluminescence of Dosimetric α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Crystals Irradiated by a Pulsed Electron Beam

The thermoluminescence (TL) of deep traps of anion-defective alumina monocrystals irradiated by a high-dose (more than 1 kGy) pulsed electron beam (130 keV) is studied. The deep traps in the studied material are classified according to the TL temperature range. It is demonstrated that the phototransferred thermoluminescence (PTTL) in the temperature range of the main TL peak is induced by optical charge migration from deep traps that are emptied at 400–470 and 470–600°C. An anomalous PTTL enhancement in crystals subjected to stepped annealing in the 350–400°C interval is observed. It is demonstrated that this effect may be caused by competing processes of charge transfer that involve deep traps corresponding to the TL peak at 390°C. The applicability of PTTL in the dosimetry of high-dose (1–50 kGy) pulsed electron beams is established.     

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.     

Luminescence properties of CaS:Ce, Sm nanophosphors

CaS:Ce, Sm nanophosphors were synthesized via solid state diffusion method. X-Ray diffraction confirmed the cubic crystalline phase of CaS:Ce, Sm nanoparticles. The particle size calculated using Debye-Scherrer formula was found to be 52 nm. The morphological investigations of the nanoparticles were made using TEM and found to have nearly spherical morphology with diameter 45-50 nm, which is in close agreement with the XRD result. The PL emission characteristics of CaS:Ce, Sm as a function of cerium and samarium concentrations have been studied and CaS:Ce_0.6Sm_0.4 system has maximum emission intensity, hence it was opted for further studies. The CaS:Ce_0.6Sm_0.4 system showed independent emission of Sm and Ce when excited at 330 and 450 nm, respectively. To study the energy transfer between cerium and samarium, the CaS:Ce_0.6Sm_0.4 was excited at wavelengths other than the excitation wavelengths of Ce (450 nm) and Sm (330 nm). The existence of Ce emission (at an excitation of 390 nm) even in the absence of Ce excitation band and Sm emission at an excitation of 405 nm, which is the excitation band of Ce, indicates the energy transfer at these two wavelengths. Thermoluminescence characteristics of ⁶⁰Co irradiated CaS:Ce_0.6Sm_0.4 have been investigated for different doses of 0.14-125 Gy. All the glow curves show a single peak at 475 K. With increasing dose, the intensity of this peak increases and a shoulder is formed on the lower temperature side at 415 K at 21 Gy of exposure. CaS:Ce_0.6Sm_0.4 shows almost linear dose dependence up to 125 Gy.