TL Processes Research Articles

Thermally modulated OSL related to the fast component of the OSL signal in quartz

The methods of optically stimulated luminescence (OSL) measurements used until recently applied optical stimulation at a constant temperature, with a constant stimulation energy and a constant or linearly increased flux of stimulation photons. During such stimulation the optical cross-section of traps is invariable and it is hard to separate the signals of different origins. It was shown recently that the selective optical emptying of traps is much easier when the optical cross-section is changed during the stimulation. Due to the dependence of the optical cross-section on temperature advantageous changes in this trap parameter can be induced by increasing the sample temperature during optical stimulation. Such a method can be called thermally modulated OSL (TM-OSL). When the experimental conditions are properly selected the TM-OSL curve has the form of a peak. The shape and position of this peak are uniquely determined by the trap parameters and, therefore, curve analysis can be used to estimate the optical depth of the trap (photoionization threshold) and the strength of electron-lattice coupling. These parameters define traps unambiguously and allow a direct correlation of the traps active in the OSL and TL processes to be made. The position of the TM-OSL peak on the temperature axis depends greatly on the stimulation energy, the heating rate and the photon flux used for optical stimulation, so these experimental factors can be used to separate the individual OSL components. This work focuses on the selective detection of the fast component of the quartz OSL signal. The traps responsible for the fast OSL component can be depopulated by light with the wavelength of 620 nm, which is far longer the usual 470 or 530 nm used in OSL measurements of quartz. The results of such experiments, when supported by computer simulations of the thermally modulated OSL process, allow the value of 3 eV to be estimated for the optical depth of traps responsible for the fast OSL component in quartz.

Features of OSL and TL properties of anion-defective corundum crystals exposed to thermo-optical treatment

The results of systematic investigation of the mechanisms and role of phototransfer in TL and OSL processes in α-Al2O3−δ crystals under controlled filling of the main and deep traps depleted at 450 and 720 K, respectively, are presented.Optical depletion spectra of the main and deep traps were measured and compared. It was found that green light stimulation only depletes the main trap, while blue light depletes both the main and deep traps. Optical depletion of the deep trap results in phototransfer of charges that can be realized by 2 mechanisms: either through the main trap to recombination centers or directly to the latter. However, the dominant contribution to the OSL signal is provided by phototransfer through the main trap. It is also found that UV emission at 3.7 eV dominates over F-emission at 3.0 eV in the phototransferred TL spectrum. It is assumed that this UV emission is not related to the F+ centers.

Analysis of TL and OSL kinetics of lithium aluminate

Lithium aluminate (LiAlO2) polycrystalline material showing high OSL sensitivity and linear dose response was prepared at IFJ Krakow. The kinetic parameters of OSL and TL processes were evaluated using various experimental techniques: LM-OSL deconvolution, TL glow-curve deconvolution, variable heating rate and isothermal decay. The OSL signal was found to consist of four components, one of them exhibiting a very slow decay. The TL glow-curve possesses two apparent peaks at approximately 85 °C and at 165 °C, which both seem to follow first order kinetics. These peaks seem to have a composite structure and as many as six peaks were found in the glow-curve.

Properties of thermoluminescence glow curves from tunneling recombination processes in random distributions of defects

Localized electronic recombination processes in donor–acceptor pairs of luminescent materials have been recently modeled using a new kinetic model based on tunneling. 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. An approximate semi-analytical version of the model has been shown to simulate successfully thermally and optically stimulated luminescence (TL and OSL), linearly modulated OSL (LM-OSL) and isothermal TL processes. This paper presents a detailed analysis of the geometrical properties of the TL glow curves obtained within three different published versions of the model. The dependence of the shape of the TL glow curves on the kinetic parameters of the model is examined by allowing simultaneous random variations of the parameters, within wide ranges of physically reasonable values covering several orders of magnitude. It is found that the TL glow curves can be characterized according to their shape factors μg, as commonly done in TL theory of delocalized transitions. The values of the shape factor are found to depend rather weakly on the activation energy E and the frequency factor s, but they have a strong dependence on the parameter ρ′ which characterizes the concentration of acceptors in the model. It is also shown by simulation that both the variable heating rate and initial rise methods are applicable in this type of model and can yield the correct value of the activation energy E. However, the initial rise method of analysis for the semianalytical version of the model fails to yield the correct E value, since it underestimates the low temperature part of the TL glow curves. Two analytical expressions are given for the TL intensity, which can be used on an empirical basis for computerized glow curve deconvolution analysis (CGCD).

Genetic synthesizing of band schemes for thermoluminescence in dosimetric crystals

A method has been developed for the evolutional construction of band schemes forr TL processes on the basis of two integrated genetic algorithms executed in a single cycle. The first algorithm selects the structure of the model, while the second algorithm determines its optimal parameter values. Bit “switches” are used to modify the generalized set of kinetic equations for description of TL in solids which allows synthesizing and sorting various multi-trap systems with different internal competitions and additional mechanisms. The efficiency of the proposed approach is verified by a test search of a variety of preset model structures and the approximation of the corresponding “experimental” glow curves. It is shown that the devised genetic constructor provides reliable information about the physical content of the sought-for models and ensures a highly accurate quantitative description of the analyzed data.

Thermoactivated spectroscopy in dosimetric [formula omitted]-Al 2O 3

Thermally stimulated luminescence and exoelectron emission (TL and TEE) near the main dosimetric (440 K) “chromium” (550 K) peaks were studied in anion-deficient α - Al 2 O 3 - δ crystals by methods of modulation thermoactivated spectroscopy with optical and X-ray excitation. Temperature dependences of kinetic parameters of TL and TEE processes were calculated from the obtained results. An anomalous behaviour and overestimated values of the activation energy in the region of main peak were revealed. Possible causes of the observed deviations were discussed. A model of the defect responsible for the TL and TEE peaks near 440 K was proposed.

Microdosimetry, track structure and the response of thermoluminescence detectors

Several properties of the response of thermoluminescence detectors after doses of ionising radiation can be explained by analysing the structure of charged particle tracks. Information on the topology of interaction points around the path of a charged particle can be represented through the radial distribution of average dose around the particle's path (Track Structure) or through quantities such as lineal energy, y , or specific energy, z , applied in Microdosimetry. A principal assumption in all microdosimetric and track structure models is that, on a per average dose basis, the local efficiency of the TL processes taking place in any small volume located in the non-uniformly distributed dose distribution around the particle's path is similar to that after irradiation with a uniformly distributed dose of γ -rays. Since saturation of γ -ray response at high doses then translates to saturation of this response over regions of high dose close to the path of the heavy charged particle, track structure theory and microdosimetric models are able to explain the decrease of TL efficiency after irradiation by heavy charged particles, and the anomalously low photon energy response of LiF:Mg,Cu,P after doses of X-rays. The enhanced photon energy response and enhanced response after heavy charged particle irradiation of LiF:Mg,Ti are connected with the supralinearity of this detector at higher doses of gamma-rays: if this response is sublinear (prior to saturation) the measured photon energy response is lower, and if it is supralinear—it is higher than that expected from the calculation of the interaction cross sections alone. The explanation of the correlation between dose-, energy- and LET-responses of TLDs is perhaps the most important contribution of microdosimetry to solid state dosimetry.

An approximation of phosphorescence decay kinetics of ideal phosphors by a general order kinetics model

The phosphorescence of ideal phosphors has been investigated using a general order kinetics model. The regions where the model describes the phosphorescence processes exactly and approximately have been identified. In order to characterize these processes, the mean value of the kinetics order has been defined for the part of the curve of immediate interest. It has been shown that the value of the order of kinetics for ideal phosphors in the transition region cannot have a unique physical interpretation. The general order kinetics model enables theoretical explanation of some important physical characteristics of TL processes for the ideal phosphors. For the theoretical curves, obtained by the band model, the parameters of phosphorescence curves have been determined using the general order kinetics model. The results differ from the exact values by 3%. We conclude that the general order kinetics model enables a better understanding of the physical processes, and an approximate determination of the phosphorescence decay parameters. This points out the theoretical and practical value of the model.

Thermal Quenching of TL in Alpha-Al2O3 Dosimetric Crystals

Thermal quenching of TL processes in dosimetric anion-defective a-Al2O3 single crystals was studied by investigating the temperature dependence of radioluminescence and photoluminescence in the range of 77-550 K. It is established that the process of quenching is most effective in the temperature range of the dosimetric TL peak. The comparison of experimental results with the data of a computer analysis of the TL glow curves is given. The kinetic equations modified for a joint study of the thermal quenching and TL processes were used. According to the results of the comparison it is concluded that there exists connection of thermal quenching and of experimentally observed peculiarities of TL.

The Analysis of Thermoluminescent Glow Peaks in CaF<sub>2</sub>:Tm (TLD-300)

Computerised glow curve fitting, the variable heating rate method, Tm-Tstop procedure and the dose variation method are used to determine the number of peaks, the order of kinetics, the activation energy E and the attempt-to-escape frequency s associated with the glow peaks in single crystals of CaF2:Tm (TLD-300). Dose variation and the Tm-Tstop procedure indicate that there are only first-order kinetic TL processes in the temperature range from 300 K to 600 K. Glow curve analysis indicates that the glow curve in this temperature range can be best described as a superposition of six peaks. Trapping parameters found with glow curve fitting are found to be constant for glow curves measured at heating rates varying from 0.24 K.s-1 to 3.0 K.s-1. Glow curve fitting and the variable heating rate method yield trapping parameters for main peak 3 which agree reasonably well (E = 1.15 ± 0.02 eV and 1.22 ± 0.04 eV respectively). For all other peaks there is no, or doubtful, agreement. The discrepancy may be caused by the presence of several emission bands at different wavelengths. The possibility of using computerised glow curve analysis in obtaining more information on the radiation quality in mixed radiation fields is discussed.

The role of iron in the TSC and TL for gamma-irradiated MgO

Abstract Simultaneous measurements of optical absorption, TL, and TSC, for two MgO samples containing 180 and 650ppm Fe, y-irradiated at 14 and 100°C, show the importance of iron in the TL and TSC processes, and indicate that the TSC peaks observed above about 90°C are due to electrons. As in earlier work the red TL emission is observed to be due to Fe2+. With increasing iron content, V-type centres associated with Fe2+ play an important role.

Mechanism of Non-Linear TL Response Characteristics of Thermoluminescence Phosphors - Competing Quasi-Continuous TL Trap Distribution Model

Abstract A new TL model, namely a competing quasi-continuous trap distribution model, has been proposed for explaining the physical mechanism of TL processes. It takes into account the capture reaction mechanism of TL traps (activation energy) and the effect of the trap distribution on the accompanied competition processes. The model is quite general in nature and it has been examined for a number of TL phosphors with diverse trap distribution characteristics e.g. CaSO4:Dy, CaF2:Dy, Li2B4O7:Mn and Li2B4O7:Cu.

Thermoluminescence and optical absorption studies rubidium bromide doped with barium

AbstractThermoluminescence (TL) and optical absorption studies are made on RbBr:Ba2+. An absorption band due to Z1‐centres appears at 1.58 eV on F‐light bleaching subsequent to γ‐irradiation. A TL glow peak attributable to Z1‐centres is identified. TL emission under the main glow peak shows a broad emission at 2.65 eV with a shoulder at 1.95 eV. This 1.95 eV emission is attributed to VM2‐centres which give rise to an absorption band around 4.1 eV. A tentative energy band scheme is suggested explaining the various TL processes.

Optical absorption, thermoluminescence (TL) glow and emission studies on rubidium chloride

Single crystals of suprapure RbCl after irradiation and subsequent F-light bleaching show an absorption band on the low energy side of F-band and enhancement of F-excited states, suggesting the formation of F A(K) centres (in the “Suprapure” labelled material about 100 ppm of potassium is present). TL of such irradiated samples show four glow peaks. Upon F-light bleaching a new glow peak appears on the low temperature side. The glow peaks corresponding to F A(K) centres have been identified. The spectral distribution of TL emission under different glow peaks has been studied. It is found that all the glow peaks have broad emission bands around 1.5, 2.5 and 2.8 eV. However, the high temperature peak shows additional emission around 2.1 and 3.9 eV. An attempt is made to interpret these emission bands in terms of recombination processes involving F and V type centres in these irradiated crystals. A tentative energy level diagram indicating various TL processes is also suggested.

Dynamics and thermodynamics of the liquid state (T < Tg) of amorphous polymers

Abstract The liquid state behavior of the following atactic polymers has been reviewed: the alkyl and cycloalkyl methacrylates; polyisobutylene; polybutadiene, its random copolymers with styrene and acrylonitrile, and S-B diblocks; polystyrene; and the poly-α-olefins C3, C5, and C6. Just as Tg manifests itself through time-dependent (relaxational) and thermodynamic (transition) effects, so also does a T Tg or Tβ relaxation. With increasing syndiotactic content, PMMA's reveal strengthening Tβ and Tl processes. Relaxation maps plotting log frequency-T−1 are presented for PBD and PIB, showing both Tg and Tl. The latter exhibits non-Arrhenius behavior with an activation enthalpy about half that at T...