OSL Curves Research Articles

New correlation between PTTL and POSL components in Al2O3:C

We describe newly found correlations between thermoluminescence (TL), pulsed optically stimulated luminescence (POSL), and phototransfer TL (PTTL) signals in Al2O3:C. A set of Al2O3:C samples were characterized for their TL and OSL curve shapes, as well as TL/POSL integrals and correlated with their PTTL behavior. POSL was used to separate the contribution from fast UV emission (F+ centers) and slow blue emission (F centers) OSL components in Al2O3:C. The results show that samples with high PTTL exhibit TL curve shapes with a peak at higher temperatures and OSL curves that decay slower compared to samples with low PTTL. The ratio of the integral POSL components, the UV/B ratio, was found to inversely correlate with the PTTL. We discuss the possible causes of our observations and outline the limitations of commonly used luminescence models to explain our data. These correlations add yet another piece to the puzzle of the stimulated luminescence mechanisms of Al2O3:C.

Correlation study between OSL, TL and PL emissions of yellow calcite

The present study shows morphological characterization, experimental results, theoretical fittings of TL and OSL curves of yellow calcite and the correlation between these emissions.The OSL of the sample raises with dose and was theoretically decomposed into three components with a lifetime span of 0.5, 10, and 28 s. TL peaks at 90 and 126 °C were observed in UV region and they could be correlated with OSL emission; these peaks were deconvoluted using general order kinetic theory (GOK) they supplied an activation energy (E) values of 0.7 and 1.0 eV and kinetic order (b) of 1.2 and 2.0 respectively. Both peaks presented good response with beta irradiation in the range between 5 and 41 Gy. TL glow curve recorded at VIS-UV region showed 4 peaks, after deconvolution applying GOK, the peaks temperatures were determined at 92.9, 118.9, 155 and 452 °C; with E equal 0.7, 0.8, 1.3, and 1.05 eV, and b equal 1.01, 1.30, 2.00, and 1.70, respectively.In addition, photoluminescence (PL) spectra were carried out with excitation at 350 nm and the calcite exhibited four broad bands at 413, 440, 470, and 486 nm, and a small one at 390 nm, which can be associated with oxygen vacancies and Me–O defects. The PL results was converted to CIE (Commission International deI'Eclairage) and the coordinates color of the phosphors were determined, values x = 0.14959, y = 0.11,775 (blue) for non-irradiated sample, and x = 0.14593, y = 0.05921 (purplish blue) to irradiated one.

Characterization of the thermally transferred optically stimulated luminescence (TT-OSL) of BeO

The objective of this work was to characterize more in depth the newly discovered thermally transferred optically stimulated luminescence (TT-OSL) of BeO. The studies include a more detailed investigation of the pre-heating temperature and time required to induce the TT-OSL signal; the reproducibility, additivity, and stability of the TT-OSL signal at room temperature and under ambient light exposure; the TT-OSL response to high doses (up to 25 Gy); and the possible recuperation of the OSL signal at room temperature. The results indicate that the TT-OSL is maximum for a pre-heating to 260 °C, if a heating rate of 5 °C/s is used. The TT-OSL decreases with readouts (pre-heating plus OSL readout), indicating depletion of the source of trapped charges. Sensitivity changes are observed in both OSL and TT-OSL signals to the same extend. Furthermore, it was demonstrated that the TT-OSL is additive: the TT-OSL is proportional to the total dose delivered to the detector, whereas the OSL signal is proportional to the dose since the last bleaching. This relationship holds up to ∼5 Gy; above this dose, the TT-OSL starts to saturate. The kinetics of the TT-OSL signal with pre-heating temperature and duration is consistent with a simplified model in which charges from an optically insensitive (or “difficult to bleach”) trap are transferred to an optically sensitive trap, both having similar thermal stabilities with a difference in activation energy of the order of 0.1 eV. The TT-OSL signal is stable in the dark up to the maximum duration investigated here (50 days), decaying slowly under exposure to ambient light. Furthermore, we report evidence of recuperation of the OSL signal at room temperature, which may be caused by a similar process as the one responsible for the TT-OSL signal. The results have obvious implications for practical dosimetry using BeO, including the possibility of re-estimating the doses if the detectors are accidently exposed to light, the possibility to check if the detectors have been exposed to light, the potential recuperation of OSL signal at room temperature and its influence on dosimetry if the detectors have been exposed to high doses, or the potential to use the shape of the OSL curve or the ratio between the OSL and TT-OSL signals for linear energy transfer estimation in ion beam dosimetry.

A correlation study on the TL, OSL and ESR signals in commercial BeO dosimeters yielding intense transfer effects

BeO ceramics have been used for quite a long time as radiation dosimeters besides being used in many other areas. BeO dosimeters can be stimulated both thermally and optically, giving rise to TL and OSL signals. BeO has two main dosimetric peaks, 2 and 3, by ignoring peak 1 that corresponds to a shallow trap, while the OSL curve is composed of two components, termed as C1 and C2. Irradiated BeO dosimeters showed ESR spectra at room temperature consisting of three centers I, II and III. These centers were assigned to Al+2 and O- centres; two different typical oxygen centre behaviours were monitored, one showing anisotropic and one isotropic properties. The present study attempted to perform correlation between the TL peaks 2 and 3, the CW-OSL components C1 and C2 and ESR centers I, II and III. The bleaching and decay features of the ESR centers are presented for the first time in the literature. The OSL component C1 is correlated with the TL peak 2; both luminescence entities are associated with the ESR center I, attributed to the Al+2 center. Finally, thermal stability of center II suggests that it can be also tentatively correlated to both TL peak 2 and OSL component C1. Moreover, the present study provides strong experimental arguments towards a correlation between TL peak 3, CW-OSL component C2 and ESR center III, namely oxygen ion (isotropic). Intense transfer was monitored either (a) from deeper traps to both traps responsible for TL peaks 2 and 3, as well as (b) from the trap of TL peak 3 to that of TL peak 2. Nevertheless, reconstruction as well as indirect TA-OSL signal monitoring indicates that the transfer is more intense to the trap corresponding to TL trap 3. For this reason, even after depleting both TL peaks, intense OSL curves were still measured.

Observation of strong thermally transferred optically stimulated luminescence (TT-OSL) in BeO

This work reports the observation of strong thermally transferred optically stimulated luminescence (TT-OSL) signal of BeO OSL ceramic detectors. If the detector is irradiated and the OSL signal is measured for a prolonged time (bleached), subsequent OSL measurements result in zero signal. If the same detector, however, is heated to temperatures in the 225–335 °C range (at 5 °C/s), subsequent OSL measurements at room temperature result in an intense signal, even without additional irradiation. The TT-OSL is maximum for a pre-heating to 275 °C (if heated linearly at 5 °C/s), reaching a value of 56 % of the original OSL signal. Both OSL and TT-OSL are reproducible (∼3 % standard deviation) and linear with dose over the dose range investigated (up to 100 mGy). The shape of the OSL and TT-OSL curves are essentially identical. A possible explanation for this phenomenon is the thermally stimulated transfer of trapped charges from optically inactive traps to optically active traps responsible for the OSL signal. Correlation between the OSL, TT-OSL and the thermoluminescence (TL) peaks suggests the trapping centers associated with the TL peak at ∼310 °C/s (at 5 °C/s) as the source of the trapped charges. These results have implication for the re-estimation of the OSL signal in BeO detectors accidentally exposed to light. They not only provide further evidence that the material is not completely reset simply by bleaching (optical stimulation), but also open a new line of investigation into the underlying TL/OSL mechanisms in BeO.

Deconvolution of isothermal TA – OSL decay curves from sedimentary quartz using combinations of various contemporary models

Thermally assisted optıcally stimulated luminescence (TA – OSL) stands as an experimental technique which encompasses the simultaneous use of optical and thermal stimulation. The technique can effectively stimulate electrons from traps with delocalization temperatures beyond 500 °C, also known as very deep traps (VDTs). For the case of quartz, TA – OSL is usually measured isothermally, at constant elevated temperature, yielding a decaying shape similar to that reported for the cases of conventional OSL. This feature is prevalent for all quartz samples. In the framework of the present study, a deconvolution analysis was attempted on the corresponding isothermal TA – OSL decay curves of 5 different quartz samples collected from the Aegean Anatolia region, Turkey. Deconvolution was performed on isothermal TA – OSL signals measured at a wide range of stimulation temperatures. Combinations of three different, well established, contemporary luminescence models were applied, two involving delocalized recombination via the conduction band, namely general order kinetics and photo-transfer models, along with one involving both tunneling recombination and delocalized recombination pathways. The use of the model of general order kinetics alone gives physically meaningless results, as the photo-ionization cross section of a specific component indicates a behavior which contradicts the luminescence models, yielding inverse Arrhenius law behavior. A combination of the general order kinetics and the photo-transfer models yields results without major physical inconsistencies. However, the thermal quenching parameters obtained according to the deconvolution procedure do not agree with the corresponding prevalent or even representative values for quartz. This latter feature, in conjunction to (a) the shapes of the residual thermoluminescence signal after TA – OSL, (b) the shapes of the TA – OSL with linear heating and (c) the shapes of the isothermal TA – OSL curves normalized over the initial intensity, suggest that the major part of the TA – OSL signal recombines via tunneling. The use of a combination of tunneling plus general order kinetic models has also been applied for the deconvolution analysis. The physically meaningful corresponding results indicate that in the absence of any other specific luminescence model for the isothermal TA – OSL decay curves, the latter combination of these two models provides the optimum case, not only for deconvolving the isothermal TA – OSL in quartz, but also for providing further insight to the physics of the phenomenon.

Band shape modulation optically stimulated luminescence (BSM-OSL) - Simulations and first experiments

Recently it was shown that making use of the dependency of optical cross-section (OCS) of trap on the experimental parameters can help to enhance the ability to separate signals from different traps. When the light source applied for stimulation delivers the spectral band of a finite width, the physical quantity that governs the luminescence process is not the OCS value but rather a kind of weighted average of the OCS value over the range of the stimulation band. Changes of the shape of the stimulation band allow to regulate the shape of OSL curve. The band shape modulation can be easily attained using at least two sources of light. The shape of a band that is the sum of bands from two sources can be controlled by changing the emission intensity of each source. The presented results of simulations and experiments demonstrate what can be achieved by means of such a band shape modulation. The ability to separate the signal from different kinds of traps depends significantly on the selection of the experimental parameters such as the energy of the maxima of the component spectral bands, the difference of these maxima as well as the rate and the character of light intensity changes. When the experimental parameters are properly selected the OSL curve for a single trap has the form of a peak.

Germanium-doped optical fiber for real-time radiation dosimetry

Over the past three decades growing demand for individualized in vivo dosimetry and subsequent dose verification has led to the pursuit of newer, novel and economically feasible materials for dosimeters. These materials are to facilitate features such as real-time sensing and fast readouts. In this paper, purposely composed SiO2:Ge optical fiber is presented as a suitable candidate for dosimetry. The optical fiber is meant to take advantage of the RL/OSL technique, providing both online remote monitoring of dose rate, and fast readouts for absorbed dose. A laboratory-assembled OSL reader has been used to acquire the RL/OSL response to LINAC irradiations (6 MV photons). The notable RL characteristics observed include constant level of luminescence for the same dose rate (providing better consistency compared to TLD-500), and linearity of response in the radiotherapy range (1 Gy/min to 6 Gy/min). The OSL curve was found to conform to an exponential decay characteristic (illumination with low LED source). The Ge doping resulted in an effective atomic number, Zeff, of 13.5 (within the bone equivalent range). The SiO2:Ge optical fiber sensor, with efficient coupling, can be a viable solution for in vivo dosimetry, besides a broad range of applications.

Derivation of a simplified OSL OTOR equation using Wright Omega function and its application

The Wright Omega function has been employed in the simplification of the OSL differential equation, which may, in turn, be employed in the evaluation of OSL curves. The new equation will help in analyzing the zone between the First Order and the Second Order, which was not theoretically possible. The new eqn is employed to deconvolute the OSL curve of quartz using the Hybrid Genetic Algorithm both in CW and LM mode. From the analysis it was observed that the OSL curve of quartz comprises of three decay curves in CW-OSL and four in LM-OSL mode. This difference may be attributed to the preheat given to the CW-OSL, which removes the 110°C peak.

Estimation of OSL trap parameters by optical “cleaning” — a critical study

Abstract The so-called thermal “cleaning” applied in thermoluminescence analysis allows distinguishing TL signal originating from different traps of comparable thermal depths. Here, the detailed study on the suitability of a similar approach — which can be called optical “cleaning” — for the analysis of OSL process has been carried out by means of computer modelling. The optical “cleaning” is realized by short optical stimulation applied directly after irradiation. It turns out that this approach does not help to separate signals related to various traps having similar optical cross-section (OCS) values. For some sets of trap parameters, sufficiently good reconstruction of the trap optical cross-sections can be achieved by the analysis of the conventional OSL curve obtained directly after sample irradiation, while the OSL curve analysis after “cleaning” does not provide any additional data. For other cases the analysis of both curves with and without “cleaning” leads to optical cross-section values different from those assumed in the model, but just in such situations the optical “cleaning” can be helpful. The differences between the OSL components of the conventional curve and the one obtained after “cleaning” can be used as indicators of an incompatibility of the trap parameters obtained from the OSL analysis with their actual values.

Experimental demonstration of the variable energy of stimulation optically stimulated luminescence (VES-OSL) method

A new method of OSL stimulation using the continuous increase of stimulation energy (VES-OSL) has been previously suggested. Earlier computer simulations showed that this type of stimulation results in a peak-shaped OSL curve that closely resembles the TL peak. VES-OSL measurements were performed using a stimulation source that was specially constructed for this purpose, and the equipment is described in detail. The first results of these measurements are presented for quartz samples and the behaviour of the VES-OSL curves obtained is consistent with earlier theoretical expectations.

Titanium effect on the thermoluminescence and optically stimulated luminescence of Ti,Mg:α-Al2O3 transparent ceramics

Abstract Ti,Mg:α-Al2O3 transparent ceramics were successfully fabricated by conventional sintering process under vacuum condition. The SEM micrographs showed that TiO2 effectively inhibited excessive grain growth of Ti,Mg:α-Al2O3 transparent ceramics as sintering proceeds. The fluorescence spectra, thermoluminescence and optically stimulated luminescence of Ti,Mg:α-Al2O3 transparent ceramics were compared to its single crystals. The influence of different content of TiO2 on the thermoluminescence and optically stimulated luminescence of Ti,Mg:α-Al2O3 transparent ceramics were studied. The results showed that the thermoluminescence peaks began to shift to lower temperature with increasing content of TiO2, and the intensity of thermoluminescence peaks increased. The integral luminous areas of OSL curves increased with the increasing content of TiO2.

Influence of spectral width of stimulation band on the shape of OSL curve

In the practice of OSL measurements stimulation light sources with finite spectral band are used. Till now, however, this fact was not taken into account in the analysis of OSL measurement results. Here the simulation of OSL process under the stimulation with light of a defined spectral band is performed and the results of standard OSL analysis of the simulated curves are compared with optical cross-section values assumed in the kinetics model.

Relative response of TL and component-resolved OSL to alpha and beta radiations in annealed sedimentary quartz

Knowledge of the relative luminescence response to alpha and beta radiation is very important in TL and OSL dating. In the present study the relative alpha to beta response is studied in a sedimentary quartz sample, previously fired at 900 degrees C for 1 h, in the dose region between 1 and 128 Gy, for both thermoluminescence (T) and linearly modulated optically stimulated luminescence (LM - OSL). The LM OSL measurements were performed at room temperature and at 125 degrees C. All OSL signals were deconvolved into their individual components. Comparison of OSL curves after alpha and beta irradiation strongly supports that quartz OSL components follow first order kinetics in both cases. In the case of TL, the relative alpha to beta response is found to be very different for each TL glow-peak, but it does not depend strongly on irradiation dose. In the case of LM OSL measurements, it is found that the relative behaviour of the alpha to beta response is different for three distinct regions, namely the fast OSL component, the region of medium OSL component originating from the TL glow-peak at 110 degrees C when stimulation takes place at room temperature and finally the region of slow OSL component. Following stimulation at ambient temperature, the relative alpha to beta response of all components was not observed to depend significantly on dose, with the value of ratio being 0.03 and a tendency to decrease with increasing dose. However, in the case of measurements performed at 125 degrees C, the relative response of the fast components is much enhanced, and for the remaining components it increases with increasing dose. Special care must be taken to examine the relative alpha to beta response of the fast component at 125 degrees C which contrasts the relative response of the TL peak at ca. 325 degrees C. The implications for the dating of annealed quartz are also briefly discussed.

LiMgPO 4:Tb,B OSL phosphor – CW and LM OSL studies

In the adjoining paper, the authors have proposed LiMgPO 4:Tb,B (LMP) OSL phosphor as a potential alternative to α-Al 2O 3:C for dosimetry applications. This calls for developing an understanding on TL and OSL aspects of this highly sensitive LMP phosphor. CW and LM OSL processes were therefore studied experimentally and kinetic analysis was carried out using theoretical models. Continuous wave (CW) OSL curve for LMP phosphor was found not to follow single decaying exponential implying that the CWOSL curve does not follow first order kinetics. Under pre-readout annealing at 125, 200 and 300 °C for 10 s, the nature of decay profile was unaffected and same holds true for optically bleached CWOSL curves. From linearly modulated (LM) OSL studies, it was found that the shape/geometrical factor μ g was ∼0.72 ± 0.03 for wide range of doses (up to 12 Gy studied) and peak position “ t m ” was also independent of dose. The μ g was found to be unaffected with pre-readout annealing at 125, 200 and 300 °C for 10 s and optical bleaching, however it was found that peak position “ t m ” shifted towards higher side in time with increase of optical bleaching. Dose dependence tests were also carried out for LMOSL curves and it was found that peak position “ t m ” was independent of dose, which is typical characteristic of curves following first order kinetics. Hence LM-OSL curve might be mixture of more than one component. Further from CW and LM OSL studies, it was also found that the individual contribution from first, second and third TL peak toward OSL is ∼33%, ∼45% and ∼22%, respectively. Traps beyond 350 °C were found not to contribute towards OSL when stimulated using blue LEDs. In the present paper, the CW and LM OSL processes for LMP phosphor were studied experimentally and their kinetic analysis was carried out.

On the OSL curve shape and preheat treatment of electronic components from portable electronic devices

The shape of the OSL decay curve and the effect of longer time delays between accidental exposure and readout of alumina-rich electronic components from portable electronic devices are investigated. The OSL decay curve follows a hyperbolic decay function, which is interpreted as an approximation of several closely spaced or even a continuous distribution of single trap OSL decay functions. Longer time delays between exposure and readout lead to a rising dose vs. OSL integration time plot, when applying a low preheat. Supplementary TL investigations indicate a distribution in activation energies for the main dosimetric (190 °C) peak and that the lower temperature part could fade with a higher rate than the higher temperature part. Consequences for the applied preheat treatment prior to dose measurement are discussed.

Comparison between blue and green stimulated luminescence of Al2O3:C

Abstract This paper presents a systematic comparison of OSL signals from Al 2 O 3 :C when stimulated with blue and green light. Al 2 O 3 :C detectors were irradiated with various doses and submitted to various bleaching regimes using yellow, green and blue light. Most of the investigations were carried out using Luxel™-type detectors used in the commercial Luxel™ and InLight™ dosimetry systems (Landauer Inc.). Al 2 O 3 :C single crystals and Al 2 O 3 :C powder were also used to complement the investigations. The results show that, although blue stimulation provides faster readout times (OSL curves that decayed faster) and higher initial OSL intensity than green stimulation, blue stimulation introduced complicating factors. These include incomplete bleaching of the dosimetric trap when the Al 2 O 3 :C detectors are bleached with yellow or green light and the OSL is recorded with blue light stimulation, and an increased residual level due to stimulation of charge carriers from deep traps. The results warrant caution when using blue stimulation to measure the OSL signal from Al 2 O 3 :C detectors, particularly if the doses involved are low and the detectors have been previously exposed to high doses.

Analysis of the quartz OSL decay curve by differentiation

A mathematical transformation based on differentiation is introduced that converts a Continuous Wave Optically Stimulated Luminescence (CW-OSL) decay curve composed of a number of exponentials into a spectrum like curve with different peaks. Peak areas of the transformed curve are equal to the time integrated OSL of the exponentials. It is shown that the peaks of the transformed curve are more separated then the original decay curve components. The transformation is applied to computer generated OSL curves and to measured CW-OSL decay curves of three different quartz samples. Advantages and limitations of this differentiation analysis method are discussed.

Computerized curve deconvolution analysis for LM-OSL

The computerized curve deconvolution analysis (CCDA) technique is well known in the case of thermoluminescence (TL). In the present work we investigate the application of CCDA to the linear modulated optically stimulated luminescence curves (LM-OSL). We derive single LM-OSL peak equations which are based on variables which can be extracted directly from the experimental OSL curve, for both first order and general order LM-OSL peaks. The similarities and differences between TL and OSL CCDA analysis are discussed. The resolution of the technique is also examined in the cases of synthetic curves consisting of two or four constituent components. Finally a new experimental procedure is suggested which can be used to separate composite LM-OSL curves into their constituent components

Complex OSL signal and a trap independence assumption

Simulations of optically stimulated luminescence process were performed for the model which includes two electron traps and one recombination centre. The fulfillment of the trap independence assumption was tested. The fitting procedure was applied for the simulation outputs in order to check if they can be decomposed into the first-order components. A wide range of trap parameters was applied and it was shown that the trap independence assumption is valid only for some special cases but the simulated OSL curves fit often with high accuracy a sum of first-order components which differ significantly from these used in the model.