Intrinsic superlinear dose dependence of thermoluminescence and optically stimulated luminescence at high excitation dose rates

Superlinear dose dependence of thermoluminescence (TL) and optically stimulated luminescence (OSL) has been reported for many materials. The theoretical explanation has been ascribed to competition of either traps or recombination centers, during the excitation stage or during the read-out phase. There has been an account in the literature on superlinearity of OSL associated with merely one trapping state and one kind of recombination center. This had to do with the process taking place during the read-out stage, namely the optical stimulation. In the present work, we report on a model of one trapping state and one kind of recombination center which results in a superlinear filling of the center. Thus, one can expect a superlinear dose dependence of the area under the resulting TL glow peak as well as the OSL signal. We follow this situation by writing the simultaneous nonlinear rate equations for the one-trap-one-recombination-center (OTOR) model and study the expected results by numerical simulation consisting of solving the equations with sets of the trapping parameters. We also present analytical results based on simplifying assumptions, and compare the analytical and numerical results. The effect is significant at relatively high dose rates. The main implication is that when one tries to evaluate by TL dosimetry a dose applied at a high rate, calibration of the TL dosimeter using much smaller dose rates may result in inaccurate results.     

The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots

Numerical modelling has shown that the form of the quartz OSL shine plateau (hereafter ‘D_e(t)-plot’) is influenced by the effects of phototransferred TL in the ∼110°C region. It is suggested also that the presence of multiple OSL components (as described by Bailey, Smith and Rhodes, 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas., 27, 123–136; Bailey, 1998. The form of the optically stimulated luminescence signal of quartz: implications of dating. Unpublished PhD thesis, University of London) affects the form of the D_e(t)-plot. Laboratory measurements of a fully reset and artificially dosed sample yielded non-flat D_e(t)-plots, the deviation being greater for the larger of the two simulated palaeodoses, in accordance with theoretical predictions. It is suggested that the so-called ‘shine plateau’ test is of limited use in assessing the bleaching history of quartz sediments.     

Superlinear dose response of thermoluminescence (TL) and optically stimulated luminescence (OSL) signals in luminescence materials: An analytical approach

The phenomenon of superlinear dose response of thermoluminescence (TL) and optically stimulated luminescence (OSL) signals has been reported for several important dosimetric materials. We develop new analytical equations for the filling of traps and centers during irradiation and for the read-out stage of annealed luminescence materials, within the context of a two-trap and two-center model. The equations are applicable for both TL and OSL signals in annealed dosimetric materials, and are derived under the assumptions of low irradiation doses and dominant strong retrapping (weak recombination) processes. For low doses all traps and centers display linear dose response, which leads to quadratic dose response of the integrated TL/OSL signals. A new analytical expression is presented for this well-known quadratic dose dependence, in terms of the kinetic parameters in the model. The effect of elevated irradiation temperature on the integrated TL/OSL signals is also considered, and analytical expressions are obtained for this situation as well. A new type of dose-rate effect is reported based on the modeling results, which is due to irradiation during elevated temperatures. The accuracy of the analytical expressions is verified by comparing with the results of numerical simulations.     

The slow component of quartz optically stimulated luminescence

The slow component of quartz OSL displays a number of properties that clearly distinguish it from the main (‘rapidly bleachable’) part of the quartz OSL signal traditionally used for dating. These properties include an extremely high thermal stability, dose saturation level and a charge concentration dependence in both signal form and decay rate. The physical mechanism responsible for the slow component is thought at present to involve a direct donor–acceptor recombination component, possibly associated with competing pathways below, and possibly up to, the conduction band. The thermal stability and high dose saturation characteristics of the slow component suggest much potential for long-range dating exists although at present it is uncertain whether difficulties associated with partial resetting may preclude routine use of the slow component for dating sedimentary deposits. A single-aliquot additive dose method was however used to obtain an estimate of D_e from the slow component for an Egyptian quartz sample that was in broad agreement with previous estimates based on the standard multiple-aliquot additive dose method.The slow component is often small in magnitude compared to the initial portion of the quartz OSL decay. However, this is not always the case and for some samples significant inaccuracies in D_e estimation may occur when deriving ages from the initial ‘rapidly bleaching’ portion of the OSL decay if the effect of the slow component is ignored or taken to be constant.     

A comparison of natural- and laboratory-generated dose response curves for quartz optically stimulated luminescence signals from Chinese Loess

It has previously been observed that laboratory-generated quartz optically stimulated luminescence (OSL) signals from different samples have similar dose response curves (DRCs) after they are normalized using a test dose. It therefore seems likely that growth of the normalized signal due to natural irradiation of quartz may also follow a general dose response curve. The existence of such a curve is investigated by constructing a natural DRC from the test dose-normalized natural OSL signals of seven samples from the Luochuan section of the Chinese Loess Plateau. The same aliquots are then used to build single aliquot regenerative (SAR) DRCs, making it possible to compare the natural and laboratory constructed curves. Two main differences are observed. Firstly, the laboratory-generated DRCs are best fitted with double saturating exponential functions whereas the natural DRC is equally well fitted with a single saturating function. Secondly, in the laboratory-generated DRCs the normalized OSL signal continues to increase at high laboratory doses (>500 Gy), whereas no growth is seen at these doses in the equivalent natural DRC. These differences between natural- and laboratory-generated DRCs are still apparent even if data are manipulated to isolate the fast component, or if a sensitivity corrected multiple aliquot regenerative (SC-MAR) dose procedure is used. This suggests that the observed differences are not due to the influence of different components or inter-regenerative dose cycle sensitivity changes. The divergence between the natural- and laboratory-generated DRC means that the current maximum limit of quartz OSL dating at the Luochuan section is 150 Gy, as D_e estimates above this value are likely to be underestimations.     

Dose-dependent optically stimulated luminescence of synthetic quartz at room temperature

Physical conditions such as annealing temperature, duration of annealing, ionizing radiation, etc., play a significant role in the applications of optically stimulated luminescence (OSL) dating as well as OSL dosimetry. Many efforts are made to understand the effect of these physical parameters on quartz specimens owing to its use in such applications. Such factors induce changes in OSL decay pattern. The definite correlation between color centers and luminescence sensitivity can be established on account of such pre-treatments to the specimen.The purpose of present investigations is to study the effect of ionizing radiation under identical physical conditions on OSL properties measured at room temperature. The shapes of decay curve and dose-response data are considered for this purpose. This study can reveal the changes in color centers in response to the pre-conditions to the specimen. It was found that the OSL decay remains slow and OSL properties change systematically with the rise in beta dose up to a critical dose; however, it changes the pattern when the beta exposure to the specimen was increased higher than the critical dose. This critical dose was found to be different for different temperature of annealing. The shape of decay curve up to the critical dose was also studied by considering the difference of OSL intensities between two successive durations from the observed OSL decay data. The results are explained based on the changes in available shallow traps during OSL measurement at room temperature with changes in pre-conditions to the specimens. The results also have been confirmed with the corresponding changes in ESR signals.     

Dose dependence of thermally transferred optically stimulated luminescence signals in quartz

The thermally transferred optically stimulated luminescence (TT-OSL) responses of chemically purified fine-grained quartz from seven loess-like samples from Korea are presented. In particular, the experimental procedures used to separate the dose-dependent (recuperated OSL, ReOSL) and dose-independent parts of the signal were explored. The OSL signals used to monitor the sensitivity changes that take place during the measurement sequences used to determine the equivalent dose were investigated. A single aliquot procedure was used for the TT-OSL measurements and resulted in linear growth of the ReOSL with dose up to at least 2 kGy. For this suite of samples, a standardised growth curve (SGC) was constructed for the ReOSL, tested with dose recovery experiments, and was used to obtain D_e values for the seven samples.     

Cross-talk during single grain optically stimulated luminescence measurements of quartz and feldspar

Focussed laser systems designed to measure the optically stimulated luminescence (OSL) signals from individual sand sized mineral grains are now widely used. One system that is commonly used can hold up to 100 grains on a 9.8 mm diameter aluminium disc, and can use either a green or infrared (IR) laser for optical stimulation. The grains are held in holes drilled into the disc surface, and the spacing between the edges of adjacent holes is only 300 μm. Focussing of the laser is essential to prevent inadvertent optical stimulation of adjacent grains, but no measurements have previously been made to determine the extent of cross-talk. Measurements on two systems show that the maximum cross-talk for an adjacent position is 0.22%, but typically the value is 0.04% for the green laser and 0.08% for the IR laser. This magnitude of cross-talk is unlikely to have a significant impact on continuous wave OSL measurements where the period of optical stimulation tends to be short, but may be important in linearly modulated OSL measurements where long periods of stimulation are sometimes used to characterise slow components of the OSL signal.     

Modelling the thermal quenching mechanism in quartz based on time-resolved optically stimulated luminescence

This paper presents a new numerical model for thermal quenching in quartz, based on the previously suggested Mott-Seitz mechanism. In the model electrons from a dosimetric trap are raised by optical or thermal stimulation into the conduction band, followed by an electronic transition from the conduction band into an excited state of the recombination center. Subsequently electrons in this excited state undergo either a direct radiative transition into a recombination center, or a competing thermally assisted non-radiative process into the ground state of the recombination center. As the temperature of the sample is increased, more electrons are removed from the excited state via the non-radiative pathway. This reduction in the number of available electrons leads to both a decrease of the intensity of the luminescence signal and to a simultaneous decrease of the luminescence lifetime. Several simulations are carried out of time-resolved optically stimulated luminescence (TR-OSL) experiments, in which the temperature dependence of luminescence lifetimes in quartz is studied as a function of the stimulation temperature. Good quantitative agreement is found between the simulation results and new experimental data obtained using a single-aliquot procedure on a sedimentary quartz sample.     

Investigating the thermally transferred optically stimulated luminescence source trap in fired geological quartz

The pre-dosed thermoluminescence (TL) emission of quartz has been found to be useful in retrospective dosimetry and archaeometry. Though the pre-dosed optically stimulated luminescence (OSL) and TL emissions have been reported to be similar, the former has been found to be un-reliable for the equivalent dose estimation. As this measurement protocol involves thermal heating at around 400 °C, the work reported in this paper investigated the influence of this heating on the OSL using fired specimens from various regions. The results suggested that the discrepancy in the behaviour of two emissions is caused by the presence of the thermally transferred optically stimulated luminescence (TT-OSL) induced by thermal-activation involved in the pre-dose treatment. This transferred signal was observed to be very significant in the case of samples containing a prominent higher-temperature TL peak at ∼375 °C. The characterization of this signal based on (i) the nature of the glow curves, (ii) thermal-annealing of the OSL trap, (iii) observation of the TT-OSL, (iv) bleaching of the source trap and (v) the correlation between TL and OSL seems to suggest that the trap corresponding to this TL peak is the source trap in the TT-OSL emission mechanism.     

Peculiarities of optically stimulated luminescence in halite

Sodium chloride – NaCl is one of materials indicating strong OSL signal after exposure on ionizing radiation. Previous studies of the OSL response in pure sodium chloride showed coexistence of fading and regeneration of the signal using the newly developed variable delay optically stimulated luminescence technique (VD-OSL). This paper presents investigations of some peculiarities of long time scale OSL properties of rock salt (halite) including sensitization phenomena for various bleaching methods. Dose response characteristics were studied in the range from 200 mGy to 1 Gy for series of halite aliquots without signal bleaching and for single aliquot using zeroing by blue and green light.     

Response variation of optically stimulated luminescence dosimeters

This study investigates the variability in response of optically stimulated luminescence dosimeters (OSLDs). Examining the source of sensitivity variations in these dosimeters allows for a more comprehensive understanding of the Landauer nanoDots and their potential for current and future applications. In this work, OSLDs were scanned with a MicroCT scanner to determine potential sources for the variation in relative sensitivity across a selection of Landauer nanoDot dosimeters. Specifically, the correlation between a dosimeters relative sensitivity and the loading density of Al₂O₃:C powder was determined. When extrapolating the sensitive volume's radiodensity from the CT data, it was shown that there is a non-uniform distribution in crystal growth. It was calculated that a 0.05% change in the nominal volume of the chip produces a 1% change in the overall response. Additionally, the ‘true’ volume of an OSLD's sensitive material is, on average, 18% less than that which has been reported in literature, mainly due to the presence of air cavities in the material's structure. This work demonstrated that the amount of sensitive material is approximately linked to the total correction factor.     

Thermo-optical properties of optically stimulated luminescence in feldspars

Optically stimulated luminescence processes in feldspars are subject to competing thermal enhancement and quenching processes: this article describes the thermal enhancement effects for orthoclase, albite and plagioclase feldspars. It is demonstrated that certain lattice vibrational modes can be selectively probed at specific optical excitation energies. The results are described in terms of the Bohr hydrogen model of the OSL donor defects.     

The influence of optical bleaching on lifetimes and luminescence intensity in the slow component of optically stimulated luminescence of natural quartz from Nigeria

The intensity of optically stimulated luminescence may be decreased to a slow or medium component of its decay curve by optical bleaching, that is, by prolonged exposure of the luminescent sample to stimulating light. In this paper, we report on the influence of irradiation and measurement temperature on luminescence lifetimes as well as on the effect of measurement temperature on luminescence intensity in annealed natural quartz from Nigeria. Measurements were carried out in the slow component region using time-resolved optical stimulation at 470 nm on samples annealed at 500 and 600 °C. Luminescence lifetimes were determined from the resultant time-resolved luminescence spectra by analysing the portion of each spectrum after the stimulating light pulse of duration 11 μs. In preparatory tests, the influence of the duration of optical bleaching on lifetimes was investigated. It was found that lifetimes in samples annealed at 500 °C are independent of the duration of optical bleaching, whereas lifetimes in quartz annealed at 600 °C are affected, decreasing towards a constant value with duration of bleaching. Concerning measurements in the slow-component region, lifetimes were found to decrease with irradiation dose for samples annealed at either 500 or 600 °C. The temperature dependence of lifetimes in both sets of quartz is similar with lifetimes constant at about 36 μs between 20 and 120 °C, but decreasing consistently from then on to about 5 μs at 200 °C, the maximum measurement temperature used in experiments. The luminescence intensity was observed to typically go through a peak as the stimulation temperature was increased from 20 to 200 °C, following a brief initial decrease, a change better exemplified in the quartz annealed at 600 °C. The initial decrease in luminescence intensity is attributed to the dominance of optical stimulation over thermal stimulation. On the other hand, the subsequent change of luminescence intensity with temperature is discussed as evidence of thermal assistance to optical stimulation, initially with activation energy of 0.27±0.07 eV and of thermal quenching subsequently with activation energy equal to 0.93±0.23 eV for samples annealed at either 500 or 600 °C. The temperature dependence of lifetimes is explained as showing increased thermal effect on lifetimes with activation energy values within 0.83±0.01 eV. On the other hand, the influence of irradiation on lifetimes is accounted for in terms of an energy band model for quartz consisting of three luminescence centres and one non-radiative recombination centre.     

Phototransferred effects in TLD-500 luminescent detectors and their correlation with thermally and optically stimulated luminescence responses

The results from studies on the correlation between thermally and optically stimulated luminescence (OSL) responses and the kinetics of charge carrier phototransfer in corundum-based TLD-500 detectors are presented. It is shown that the fast component of the OSL decay curve is related to the depletion of the main trap, while the slow component is associated with the phototransfer process. The effect of preliminary heating on the behavior of OSL decay is studied.     

Thermally and optically stimulated luminescence of glass mosaic tesserae

A series of experiments on the luminescence signals from the glassy matrix of mosaic tesserae has been conducted. The results presented show the relationship between the 280–390 °C thermoluminescence (TL) peak and the optically stimulated luminescence (OSL) emitted under illumination by 450 nm light. We have proposed an appropriate laboratory protocol with repeated irradiations and illuminations of the same aliquot of the sample that allowed for establishing a correlation between TSL and OSL emissions, both very likely associated to the same trap. PACS 78.66.Jg; 87.66.Sq; 78.60.Kn