Thermoluminescence Experiments Research Articles

Thermoluminescence glow curves in preheated feldspar samples: An interpretation based on random defect distributions

Thermoluminescence (TL) glow curves from feldspar samples have been studied extensively in luminescence dating and dosimetry applications. However, the mechanism responsible for their unusual shape and behavior in preheated samples is not well understood. This paper presents new experimental TL glow curves for four types of preheated feldspar samples; an orthoclase, a sanidine and two microclines. Both the preheat temperature and the duration of the preheat are varied, before measurement of the remnant TL glow curve. Kinetic analysis of the experimental results is carried out using a recently proposed physical kinetic model which describes localized electronic recombination in a random distribution of donor-acceptor pairs, with nearest-neighbor interaction. The experimental data are analyzed using two different methods within the model. In the first method one follows the development of the distribution of nearest neighbor distances for each of the four stages of the experiment, namely irradiation followed by heating to a temperature and holding the sample at this temperature for certain time, and finally measurement of the TL glow curve. In the second method the TL glow curves are analyzed by using a single adjustable parameter in the model, which characterizes the thermal history of the sample. Good agreement is found between the two methods. However, the second method is much simpler to use in practical situations, and in principle it can be applied for any thermally or optically pretreated feldspar sample.

Superlong and Color-Tunable Red Persistent Luminescence and Photostimulated Luminescence Properties of NaCa2GeO4F:Mn2+,Yb3+ Phosphor.

An excellent persistent luminescence (PersL) phosphor NaCa2GeO4F:Mn2+,Yb3+ has been synthesized by traditional solid-state reaction. By controlling the occupation sites of Mn2+ emitters, the PersL color of this phosphor can be optionally tuned in the red to green-yellow region, and the maximum spectral shift is more than 50 nm. Significantly, the red PersL can be measured for approximately 13 and 3 h (0.32 mcd/m2) and can be observed for more than 20 and 5 h with dark-adapted vision after exposure to ultraviolet irradiation and artificial sunlight, respectively. The crystal/electronic structure and photoluminescence/PersL properties of this phosphor have been investigated in detail. A series of the excitation temperature-dependent thermoluminescence experiments and the initial rising method have been conducted to study the trap properties of this phosphor. It reveals the reasons for the variation of PersL color, excellent red PersL, and degradation of green-yellow PersL. According to the results, the as-prepared NaCa2GeO4F:Mn2+,Yb3+ can be considered as an excellent red PersL phosphor, and it also has potential for application in optical storage.

Oxygen Vacancy in Hafnia as a Blue Luminescence Center and a Trap of Charge Carriers

The electronic properties of HfO2, in particular, luminescence and charge transport, are determined by defects and traps. The origin of luminescence centers and traps of charge carriers in a HfO2 crystal was studied using luminescence spectroscopy, charge transport, and quantum-chemical calculation. The 2.7 eV luminescence band and 5.2 eV absorption/luminescence excitation band are associated with an oxygen vacancy. The thermal activation energy 1.25 eV, estimated from the charge transport and thermoluminescence experiment, is equal to half of the Stokes shift in photoluminescence spectra. Hence, oxygen vacancies are supposed to operate as electron traps in HfO2, and the charge transport is described by phonon-assisted tunneling between traps.

Thermoluminescence curves simulation using genetic algorithm with factorial design

The evolutionary approach is an effective optimization tool for numeric analysis of thermoluminescence (TL) processes to assess the microparameters of kinetic models and to determine its effects on the shape of TL peaks. In this paper, the procedure for tuning of genetic algorithm (GA) is presented. This approach is based on multifactorial experiment and allows choosing intrinsic mechanisms of evolutionary operators which provide the most efficient algorithm performance. The proposed method is tested by considering the “one trap–one recombination center” (OTOR) model as an example and advantages for approximation of experimental TL curves are shown.

On the determination of activation energy and the order of kinetics in thermoluminescence

The study of thermoluminescence (TL) glow-peaks forms the background of important and useful methods for estimating the kinetic parameters such as activation energy and the order of kinetics . These parameters are, in general, determined by using glow curve deconvolution or peak shape methods. In the present paper, we have investigated the suitability of the modified peak shape method and Kirsh method in order to perform simultaneous evaluation of these kinetic parameters and in TL. For this purpose, we have considered numerically computed TL peaks resulting from (i) General Order Kinetics model, (ii) Non-interactive MultiTrap System model (or Modified One Trap One Recombination centre (M-OTOR) model), and (iii) Interactive MultiTrap System model. We have also considered the application of these methods to experimental TL glow peaks of γ-irradiated colourless calcite that exhibits two intense TL glow peaks at 516 and 613 K, respectively. Theoretical results obtained by using different methods corresponding to different models show good agreement with experiments and, hence, point towards the model independence of methods under consideration, except the cases of strong retrapping.

Thermoluminescence of Uranyl Salts Under Mechano-destruction

The thermoluminescence of the salts UO2(NO3)2·6H2O and UO2(ClO4)2·6H2O that have been previously treated mechanically at 77K was studied. A maximum was observed in thermoluminescence curve at 80-90K. The thermoluminescence peak intensity depends on duration of mechanical treatment at 77K. The trap depth of glow centers has been determined using experimental thermoluminescence curves. The calculated activation energy ETL and frequency factor p0 were 0.21eV and 1010 s–1, respectively

2D Thermoluminescence imaging of dielectric surface long term charge memory of plasma surface interaction in DBD discharges

The charge trapping effect due to the exposure of alumina surfaces to plasma has been studied in a volume dielectric barrier discharge (DBD) in Ar and He noble gases. The long lasting charge trapping of alumina dielectric plates, used as barriers in DBDs, is evidenced by an ex situ thermoluminescence (TL) experiment performed with a standard and a custom two-dimensional (2D)-TL apparatus. The spatial density of trapped surface charges is found to be strongly correlated to the plasma morphology, and the surface spatial memory lasted for several minutes to hours after plasma exposure. In the case of Ar, the plasma channel impact signature on the surface shows a higher equivalent radiation dose with respect to the surface plasma wave and the post-discharge species signature. As a consequence, for the development of discharges, inside the dielectric surface the availability of lower energy trapped electrons is larger in the first region of plasma impact. The reported spatial memory increases the likelihood of the occurrence of plasma filaments in the same position in different runs. In He plasmas, the dielectric barrier shows an almost uniform distribution of trapped charges, meaning that there is no preferred region for the development of the discharge. In all cases a slight asymmetry was shown in the direction of the gas flow. This can be interpreted as being due to the long-living species moving in the direction of the gas flow, corresponding with the TL side experiment on the sample exposed to the plasma afterglow. The maximum values and the integral of the 2D-TL images showed a linear relation with the total charge per ac cycle, corresponding with findings for the TL glow curve. In conclusion, 2D-TL images allow the retrieval of information regarding the plasma surface interaction such as the plasma morphology, trap sites and their activation temperature.

On isothermal heating as a method of separating closely collocated thermoluminescence peaks for kinetic analysis

The experimental separation of overlapping peaks is a common practical concern in thermoluminescence experiments. In this regard, we have investigated the use of isothermal heating as a method for separating closely collocated peaks for kinetic analysis. The method has been developed and applied on a sample of synthetic quartz in which the thermoluminescence intensity is not reproducible in successive measurements on the same aliquot. Preparatory experiments performed to establish a suitable combination of temperature and heating time are described in detail. A comparative analysis of results from conventional thermal cleaning with those from isothermal cleaning, done as a means to assess the effectiveness of the latter, has shown isothermal heating to be a reliable method of separating closely collocated glow-peaks. The method is conveniently applied in combination with a new technique for kinetic analysis based on the use of the temperature-dependence of the area under an isothermal decay-curve. Isothermal heating was also applied to separate collocated peaks in europium-doped orthosilicate (Sr2SiO4:Eu2+), a luminophor, and in carbon-doped aluminium oxide (α-Al2O3:C), a dosemeter. The set studied in Sr2SiO4:Eu2+, unlike in the quartz, consists of peaks where the intensity of the lower-temperature one is greater than subsequent ones at higher temperature. On the other hand, the example investigated in α-Al2O3:C is similar except that the dominant peak subsumes the lower intensity secondary peak to the extent that they appear as one.

Kinetic analysis of thermoluminescence glow curves in feldspar: evidence for a continuous distribution of energies

The thermoluminescence (TL) glow curves from feldspars have been the subject of numerous studies, because of their importance in luminescence dating and dosimetry. This paper presents new experimental TL glow curves in a plagioclase feldspar, measured using the T max-T stop technique of glow curve analysis. Kinetic analysis of the experimental results is carried out for a freshly irradiated sample, as well as for a sample which has undergone optical treatment using infrared light for 100 s at 50°C. Application of the initial rise method of analysis indicates that the TL signals from both samples can be characterized by a continuous distribution of energy levels. By subtracting the TL glow curves measured at successive T stop values, a series of TL glow curves is obtained which are analyzed using the empirical general order kinetics. It is found that all TL glow curves obtained by this subtractive procedure can be described accurately by the same general order parameter b ∼1.7. In a second attempt to analyze the same TL glow curves and possibly extract information about the underlying luminescence process, the shape of TL glow curves is analyzed using a recently proposed physical kinetic model which describes localized electronic recombination in donor-acceptor pairs. Within this model, recombination is assumed to take place via the excited state of the donor, and nearest-neighbor recombinations take place within a random distribution of centers. This recent model has been used recently to describe successfully several types of luminescence signals. This paper shows that it is possible to obtain good fits to the experimental data using either one of these two approaches.

Band-gap engineering in Lu3Al5O12:Pr by Sc3+ or In3+ substitution

Pr3+-doped Lu3−xScxAl5O12 and Lu3−xInxAl5O12 (x=0, 0.025, 0.1, 0.25, 0.5, 0.75, 1, and 2) polycrystals are fabricated by the high-temperature solid state reaction method. Although X-ray excited luminescence measurements show that there is no positive contribution of Sc3+ or In3+ substitution on the scintillation efficiency, but the physical aspects of band-gap engineering such as the cooperative process of excitation and thermal ionization of 5d1 excited state are illustrated in this study. We employ a combination of optical diffuse reflectance, photoluminescence, decay kinetic, thermoluminescence experiments to reveal the influence of Sc3+ or In3+ substitution on electronic structure and luminescent properties in Pr3+-doped lutetium aluminate garnet.

Modeling of the thermoluminescence mechanisms in ZrO2

In this paper, simulations of thermoluminescence experiments are carried out using the comprehensive model for thermoluminescence cited in the literature by several authors, for both natural and laboratory irradiated aliquots. In the present work, we propose a possible, alternative model to explain the thermoluminescence (TL) process in zirconium oxide (ZrO2). The model includes four trapping states and one recombination center (4T1C model). The aim of the present study is to present a model that, by using as guess values the parameters obtained by employing the GOK model, fits very well the experimental glow curve. It is a goal point that the physical model does not need to resort to the quasi-equilibrium approximation. The results of these simulations are in general qualitative agreement with the experiments and confirm that the proposed model in this study presents a good explanation of these phenomena. A linear behavior was also observed between the response and dose range from 1×109 to 9×109cm−3.

On the determination of the order of kinetics in thermoluminescence by peak-shape method

The application of the peak-shape method in the kinetic order model of thermoluminescence (TL) is mainly confined to the determination of activation energy. However, the order of kinetics cannot be accurately evaluated by this method because of the weak dependence of the geometrical shape factor μ g on u m (=E/kT m , E=activation energy, T m =peak temperature, k=Boltzman constant) apart from the strong dependence on the order of kinetics b. In the present work, for the first time, a method has been suggested for the determination of the order of kinetics by taking into account the weak dependence of μ g on u m . The suitability of the method has been tested by applying it to both numerically generated and experimental TL peaks of TLD-100 and albite.

Revealing trap depth distributions in persistent phosphors

Persistent luminescence or afterglow is caused by a gradual release of charge carriers from trapping centers. The energy needed to release these charge carriers is determined by the trap depths. Knowledge of these trap depths is therefore crucial in the understanding of the persistent luminescence mechanism. Unfortunately, the trap depths in persistent phosphors are often difficult to evaluate in an accurate and reliable way. The existing analysis methods are mostly based on single experiments, or they ignore the possibility of a continuous distribution of trap depths. We present a procedure to accurately probe the activation energies, even in the presence of a continuous distribution of energy levels. By performing a series of thermoluminescence experiments with varying excitation duration and at varying excitation temperature, and employing the initial rise analysis method, the depth and shape of such a distribution can be estimated. As an example, we investigated the trap system in the violet persistent phosphor CaAl2O4:Eu,Nd, and show that it consists of a Gaussian-shaped distribution of trap depths. The maximal density of traps lies in the region around 0.9 eV, but the distribution extends to 0.7 eV on the shallow side and 1.2 eV on the deep side. The described procedure can be used to obtain a clear view of the trap system in other persistent phosphors as well. This can lead to a better understanding of the nature of these trapping centers, and the role they play in the persistent luminescent mechanism.

Study of the electronic structure and luminescence of Mn-doped SrS phosphors

In this paper, the electronic structure and the luminescence properties of Mn (1 mole%)-doped SrS phosphors synthesized by using the solid state diffusion method are reported. The X-ray diffraction pattern reveals a single-phase rocksalt structure, and the average crystallite size calculated using a Williamson-Hall plot is 72 nm. The near-edge X-ray absorption fine structure (NEXAFS) collected at Mn L3,2-edges confirms the presence of divalent Mn in SrS. A bright green photoluminescence (PL) at 547 nm is observed at an excitation wavelength of 355 nm, which may be attributed to the 4T1-6A1 inner atomic transition of the 3d electrons in the Mn2+ ion under the cubic crystal field of SrS. We have also studied the thermoluminescence (TL) of UV (254 nm)-irradiated Mn-doped SrS phosphors. The TL glow curve has a well-resolved single peak around 360 K, which shifts slightly to lower temperatures with increasing UV exposure time, confirming second-order kinetics. A kinetic analysis of the experimental TL glow curve has been carried out using Chen’s peak shape method to determine the trapping parameters.

Adaptation of the OTOR-model to explain exactly the thermoluminescence processes during thermal excitation

This paper presents a theoretical model to modify the three simultaneous differential equations of the one-trap and one-recombination center model (OTOR). This amendment will link the OTOR model to the general-order model of kinetics (GOK) to describe the thermoluminescence (TL) emission during thermal processes. The results of the new model are compared with the results of the experimental TL glow curves. The authors conclude that the empirical GOK-model is adequate to simulate the experimental glow curves, while the OTOR-model is inadequate to describe experimental glow peaks at different doses. In order to make the OTOR-model more equivalent to the GOK model, the paper introduces an adjustable parameter C, representing the concentration of inactive charge carriers due to non-radiative recombination in the thermally accessible traps of the OTOR model. The work is then extended to study the effect of this parameter on the shape of the TL glow curves, with special emphasis on the behavior of the high temperature side of the glow peaks.

Modified peak shape method for the determination of activation energy in thermoluminescence

We have reconsidered the peak shape method in thermoluminescence by taking into account the main difficulty of the peak shape method, i.e. the dependence of the symmetry factor μg on um (=E/kTm ) , where E is the activation energy, Tm is the peak temperature and k is the Boltzmann constant. In the present paper, the peak shape method is improved by taking into account this dependence. Finally, the applicability of the present findings has been analyzed by considering both synthetic and experimental thermoluminescence curves including glow curves of the most common dosimetric material LiF : Mg, Ti (TLD-100). It is also found that activation energies as calculated by the present method are in good agreement with those calculated by the rigorous method of curve fitting.

A novel fitting method for evaluating the thermal quenching parameters of TL with an application to undoped CVD diamond

AbstractThere are a number of thermoluminescence (TL) phosphors (and CVD diamond among of them), for which the integrated TL response decreases with the increasing of the heating rate. This effect relates to the thermal quenching (TQ) of luminescence caused by the increasing the non‐radiative recombination with the temperature increasing. In this work we present a new fitting method of evaluation the TQ parameters from a series of experimental TL glow curves recorded with variable heating rates. The method is simple, fast and effective. It quite general and can be applied to any series of TL glow curves consisting of several overlapped peaks and does not assume a particular type of TL kinetics. The only limitation of the method is that the end temperature of heating must be enough to ensure full readout of the analyzed TL peaks for all heating rates. The proposed method was applied to a series of TL glow curves measured with the heating rates from 0.1 to 17 K/s for the undoped CVD diamond film. The evaluated TQ parameters were found to be: an activation energy of 0.37 eV and a dimensionless constant of 3.7 × 103.

Influence of the PsbA1/PsbA3, Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone QA in Photosystem II from Thermosynechococcus elongatus as revealed by spectroelectrochemistry

Ca2+ and Cl− ions are essential elements for the oxygen evolution activity of photosystem II (PSII). It has been demonstrated that these ions can be exchanged with Sr2+ and Br−, respectively, and that these ion exchanges modify the kinetics of some electron transfer reactions at the Mn4Ca cluster level (Ishida et al., J. Biol. Chem. 283 (2008) 13330–13340). It has been proposed from thermoluminescence experiments that the kinetic effects arise, at least in part, from a decrease in the free energy level of the Mn4Ca cluster in the S3 state though some changes on the acceptor side were also observed. Therefore, in the present work, by using thin-layer cell spectroelectrochemistry, the effects of the Ca2+/Sr2+ and Cl−/Br− exchanges on the redox potential of the primary quinone electron acceptor QA, Em(QA/QA−), were investigated. Since the previous studies on the Ca2+/Sr2+ and Cl−/Br− exchanges were performed in PsbA3-containing PSII purified from the thermophilic cyanobacterium Thermosynechococcus elongatus, we first investigated the influences of the PsbA1/PsbA3 exchange on Em(QA/QA−). Here we show that i) the Em(QA/QA−) was up-shifted by ca. +38mV in PsbA3-PSII when compared to PsbA1-PSII and ii) the Ca2+/Sr2+ exchange up-shifted the Em(QA/QA−) by ca. +27mV, whereas the Cl−/Br− exchange hardly influenced Em(QA/QA−). On the basis of the results of Em(QA/QA−) together with previous thermoluminescence measurements, the ion-exchange effects on the energetics in PSII are discussed.

Prevalence of first‐order kinetics in thermoluminescence materials: An explanation based on multiple competition processes

AbstractTypical materials used in thermoluminescence (TL) dosimetry exhibit the following common characteristics: (i) the temperature of glow peak maximum of individual glow peaks remains practically constant over a wide dose range, (ii) there are no systematic changes in the glow curve shapes with the irradiation dose, and (iii) higher order kinetics is rarely seen in dosimetric materials, while first‐order kinetics is a common occurrence in experimental TL work. Theoretical explanation of these experimental characteristics is an open topic of TL research. In the present work these three characteristics are studied by using several models of increasing complexity. The simplest model studied is based on the empirical analytical general order (GO) expressions, followed by two commonly used models, the well‐known one trap one recombination center models (OTOR) and the interactive multiple trap system (IMTS). Previous researchers have studied the behavior of these models using arbitrary values of the kinetic parameters in the models, and by varying these parameters within limited physically reasonable ranges. In this paper, a new method of analyzing the results from such models is presented, in which the average behavior of real dosimetric materials is simulated by allowing simultaneous random variations of the kinetic parameters, within several orders of magnitude. The simulation results lead to the conclusion that the presence of many competitive processes during the heating stage of TL, may be correlated to the remarkable stability of the glow curve shapes exhibited by most materials, and to the prevalence of first‐order kinetics. This correlation is demonstrated further by a series of simulations in which the number of competitor traps is increased systematically, by adding up to 12 competitor traps in the IMTS model. As the number of competitor traps increases, the average behavior of the TL glow curves tends progressively toward first‐order kinetics, and this in turn results in very small average variations in the shape of the TL glow peak. The simulation results in this paper provide a convincing demonstration and explanation of the stability of the shape of TL glow curves in dosimetric materials, and for the prevalence of first‐order kinetics in TL.

Reconstruction of thermally quenched glow curves in quartz

The experimentally measured thermoluminescence (TL) glow curves of quartz samples are influenced by the presence of the thermal quenching effect, which involves a variation of the luminescence efficiency as a function of temperature. The real shape of the thermally unquenched TL glow curves is completely unknown. In the present work an attempt is made to reconstruct these unquenched glow curves from the quenched experimental data, and for two different types of quartz samples. The reconstruction is based on the values of the thermal quenching parameter W (activation energy) and C (a dimensionless constant), which are known from recent experimental work on these two samples. A computerized glow-curve deconvolution (CGCD) analysis was performed twice for both the reconstructed and the experimental TL glow curves. Special attention was paid to check for consistency between the results of these two independent CGCD analyses. The investigation showed that the reconstruction attempt was successful, and it is concluded that the analysis of reconstructed TL glow curves can provide improved values of the kinetic parameters E, s for the glow peaks of quartz. This also leads to a better evaluation of the half-lives of electron trapping levels used for dosimetry and luminescence dating. 2012 Published by Elsevier Ltd.