Optically stimulated luminescence from quartz measured using the linear modulation technique

The optically stimulated luminescence (OSL) from heated natural quartz has been investigated using the linear modulation technique (LMT), in which the excitation light intensity is increased linearly during stimulation. In contrast to conventional stimulation, which usually produces a monotonically decreasing signal, linearly increasing the stimulation power gives peaks in the signal as a function of time. In cases where the OSL signal contains more than one component, the linear increase in power of the stimulation light may result in a curve containing overlapping peaks, where the most easily stimulated component occurs at a shorter time. This allows the separation of the overlapping OSL components, which are assumed to originate from different traps. The LM-OSL curve from quartz shows an initial peak followed by a broad one. Deconvolution using curve fitting has shown that the composite OSL curve from quartz can be approximated well by using a linear combination of first-order peaks. In addition to the three known components, i.e. fast, medium and slow components from continuous-wave-OSL studies, an additional slow component is also identified for the first time. The dose responses and thermal stabilities of the various components are also studied.     

Use of the LM-OSL technique for the detection of partial bleaching in quartz

We present a study of the sensitivity to light (ease-of-bleaching) of the trapped charge in sedimentary quartz grains using an optically stimulated luminescence (OSL) technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as linear modulation OSL (LM-OSL). In controlled laboratory conditions, this technique has been employed to study the ease-of-bleaching of the trapped charge in quartz by comparing the OSL curves of quartz aliquots which have been either: (1) fully bleached, followed by a laboratory dose of β-irradiation, or (2) partially bleached, followed by the laboratory β-dose. The ratio of the OSL signals due to the β-dose from the partly and fully bleached aliquots is illustrated to be a potential indicator of the degree of optical resetting of the OSL signal in dating material. The key parameter governing the ease-of-bleaching is the photoionization cross-section of the trap involved. The concept is also demonstrated in a model study from which very good agreement with the experimental observations has been found. Potential applications of the technique to dating are discussed.     

How to visualize quartz OSL signal components

The optically stimulated luminescence (OSL) signal of natural quartz measured under constant stimulation power (CW-OSL) is a featureless decay curve where underlying components cannot be identified easily. Visual interpretation of quartz OSL signals is easier if signals are shown in spectrum-like form. This can be achieved either through ramped stimulation, or by transforming a measured CW-OSL curve into a pseudo OSL curve. We show that both give similar results, but that transformation should be preferred as CW-OSL provides best signal-to-noise ratios. We present different transformation methods to obtain pseudo OSL curves and discuss the advantages and disadvantages of each method. In addition, we show that different pseudo OSL transformations can be used to emphasize specific aspects of the OSL signal. We conclude that transformation of CW-OSL to pseudo hyperbolically modulated OSL provides the most useful visualization of the quartz fast OSL component. Pseudo parabolic modulated OSL is the transformation of choice for showing the slow component(s). The pseudo OSL curves can be used for rapid visual inspection of the relative contribution of the OSL components prior to further analysis.     

Thermal assistance in TA – OSL signals of feldspar and polymineral samples; comparison with the case of pure quartz

Thermally assisted optically stimulated luminescence (TA – OSL) is studied for the cases of polymineral, rich in K-feldspar sample as well as one pure sample of K-feldspar. For both cases, the shape of the TA – OSL signal indicates an initial, fast decaying part which is followed by a flat, very slowly decaying part with intensity much larger than the ordinary background noise signal. Thermal assistance characteristics indicate that for the case of pure K-feldspar, the signal originates from a unique very deep trap. The experimental features of the TA – OSL signal in the case of polymineral sample resemble much the corresponding TA – OSL features of pure quartz, in terms of both glow curve shape, especially at high stimulation temperatures, as well as signal intensity and its dependence on the stimulation temperature. Nevertheless, TA – OSL stimulation at low, ambient temperatures provide strong hints towards the contribution of K – feldspars to this signal. Proper selection of the stimulation temperature could possibly discriminate the TA – OSL signal originating from quartz and feldspars.     

Study of sensitivity change of OSL signals from quartz and feldspars as a function of preheat temperature

Optically stimulated luminescence (OSL) signals from feldspar and quartz samples were studied using infrared (860 nm) and green light (420–575 nm) stimulation. A serious problem connected with the regeneration technique used for dating is associated with a change of OSL sensitivity to radiation in the course of the measurement process. A typical effect seen is a large increase of the apparent strength of our beta source when calibrated against a gamma source. If the regeneration procedure is used, it is shown that the sensitivity increases up to 50% during the measurement process and as a result, the equivalent dose (ED) would be underestimated. A study of sensitivity changes in feldspars and quartz was carried out with emphasis on the effect of preheat and annealing on the OSL signal. Measurement results obtained are presented, and possible elimination of errors in dating caused by sensitivity changes is discussed.     

Optically stimulated luminescence from BeO ceramics: An LM-OSL study

Optically Stimulated Luminescence (OSL) signals of BeO ceramics were investigated using continuous wave (CW) OSL and Linearly Modulated (LM) OSL. It was found that both curves can be approximated using a linear combination of two first-order components. Experiments on the measurement temperature dependence have shown that these two components have nearly the same thermal quenching energies around 0.57 eV. Dependences of the OSL signal on preheat temperature and radiation dose were also examined. Thermal annealing experiments have shown that OSL signals originate from traps which are unstable near 340 °C, thus proving the suitability of the signals for dosimetric purposes. Dose response was found to be linear and a minimum detectable dose of ～10 μGy was found.     

Behavioural studies of stimulated luminescence from feldspars

Since the work of Mejdahl in the mid-1980s feldspars have been used widely for palaeodosimetry. They have proved particularly popular because of the technical ease with which an optically stimulated luminescence signal can be obtained, and the potential for their use in dating over a wider time range than quartz. However, while the use of infrared stimulated lminescence has been a great success, the realisation of the potential for dating over a wide time range has been hampered by a series of behavioural problems, in particular concerned with the stability of the luminescence signal. This paper surveys the current state of knowledge of a range of aspects of the stimulated luminescence behaviour of feldspar. Particular attention is paid to the thermal and optical properties of the thermoluminescence (TL) and optically stimulated luminescence (OSL) signals from feldspars since these are of greatest relevance to their use as palaeodosimeters. Results obtained from well-characterised museum specimens demonstrate the variety of responses that are seen from different types of feldspar, particularly in their TL behaviour. Less variation is seen in their OSL response. In contrast, the response of potassium-rich feldspars separated from Quaternary sediments tends to be very consistent when many grains are analysed simultaneously, suggesting that averaging of the signal simplifies the behaviour of the system.     

Recuperation of infrared stimulated luminescence of feldspars

After bleaching the optically stimulated luminescence (OSL) signal to a low residual level, the signal has been found to increase during subsequent storage or preheating. This effect is well known in quartz as “recuperation of OSL after bleaching” (Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after bleaching. Quat. Sci. Rev. 7. 387–393.). A better understanding of recuperation in feldspars could help the dating specialist, because this process might be different from the recuperation observed in quartz. This paper highlights a few examples of a larger study, which cannot be shown here in complete detail. We carried out a recuperation study of infrared-stimulated luminescence (IRSL) of different feldspars from a mineral collection, mainly alkali feldspars and one albite. The samples were irradiated with doses of 200, 1000 and 2500 Gy in a ⁶⁰Co gamma cell. Subsequently, the samples were stored in the dark at room temperature (3 weeks up to 6 months, depending on the applied dose), so that the very intense irradiation-induced phosphorescence can decrease for many orders of magnitude. The emitted OSL was measured through detection filters also used in dating (Schott UG 11, Hoya U 340 for detection of near UV-emissions and Schott BG 39 for detection in the visible range). Recuperation times up to 100 days were used. The recuperated-OSL emissions were measured either with the optical filters mentioned above or with a modified experimental set-up using a variable interference filter with a continuous detection range from 400 to 700 nm. In some feldspars very intense recuperation signals (up to 100% and more of the initial signal) were observed when optical stimulation was performed with IR and broadband detection using the BG 39 or when detection was carried out in the near ultraviolet region. The IRSL emissions at 410 and 560 nm, measured with the interference filter, showed no recuperation despite a clearly detectable first shine-down.     

Characteristics of LM-OSL from several different types of quartz

We present Linear-Modulation OSL (LM-OSL) curves from several different types of quartz, including sedimentary quartz, bulk rock crystal and synthetic quartz. The LM-OSL method consists of linearly increasing the intensity of the stimulation light while continuously monitoring the OSL emission from the sample. With this technique, one obtains peaks of luminescence intensity versus time (Bulur, 1996) in which the position of the peak is inversely related to the photoionization cross-section of the trap. The shape of the LM-OSL curve is directly related to the shape of the conventional OSL decay curve in which the stimulating light source is maintained at constant intensity (so-called Continuous-Wave OSL, or CW-OSL). In the latter, the OSL from the traps with the highest photoionization cross-section decays most rapidly with stimulation time, while OSL from those traps with smaller photoionization cross-sections is characterized by longer decay times. In this paper, data are presented comparing the CW-OSL and LM-OSL curves for several different quartz types with particular focus on the changes in the shapes of each of these curves with sample type pre-heat temperature, and measurement temperature. Several different LM-OSL components are found for each sample, each with different photoionization cross-sections, but a universal behavior for quartz is not observed; each sample is different. From the dependence upon pre-heat temperature, features associated with shallow traps can be identified. Traps which empty very rapidly can be observed more easily in the LM-OSL curves than in the conventional CW-OSL curves, and the temperature dependence of the photoionization cross-section for the various components can be clearly discerned.     

TL and OSL dosimetric properties of Opal gemstone for gamma radiation dosimetry

In this work, the response of the natural material Opal was studied in relation to its thermoluminescence (TL) and optically stimulated luminescence (OSL), after exposure to the gamma radiation of a ⁶⁰Co source. The structure of the powdered Opal was verified using the X-ray diffraction, scanning electronic microscopy and energy-dispersive X-ray spectroscopy techniques. The material, in its stone form, was turned into powder and mixed to Teflon (also in powder) in three different concentrations, and then pellets were manufactured. The aim of this work was to evaluate the response of these pellets in high-doses of gamma radiation beams, and to observe their possible application as dosimeters, using the TL and OSL techniques. The dosimetric properties of the samples were analyzed by means of different tests, as: TL emission curves and OSL signal decay curves, reproducibility of TL and OSL response, minimum detectable dose, TL and OSL dose–response curves (5 Gy–10 kGy), and fading. The results obtained in this work, for the TL and OSL phenomena, demonstrated that the pellets of Opal + Teflon present an adequate performance e possibility of use as dosimeters in beams of high-dose gamma radiation.     

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.     

A computer program for the deconvolution of the thermoluminescence glow curves by employing the interactive trap model

An efficient computer program has been developed to deconvolute thermoluminescence (TL) glow peaks and optically stimulated (OSL) curves by employing a method of the interactive trap model (ITM). The program is designed to be used easily on the MS Windows-based computer with a graphical user interface. This program could be used to analyze the TL glow curves by using the traditional first-order kinetics (1OK), second-order kinetics (2OK), general order kinetics (GOK), mixed order kinetics (MOK) and the general approximation (GA) method as well as ITM. The program was tested with the generated data and the experimental results of deconvoluted TL glow curves of LiF TLD-100 by assigning five interacting traps and one recombination center. A complete version with full functionalities of this program can be downloaded from the web site http://physica.gnu.ac.kr/TLanal.     

Investigation of a fibre-coupled beryllium oxide (BeO) ceramic luminescence dosimetry system

The purpose of this study is to investigate the potential use of a beryllium oxide (BeO) ceramic as a radioluminescence (RL) and optically stimulated luminescence (OSL) probe material for fibre-coupled luminescence dosimetry. A portable dosimetry system, named RL/OSL BeO FOD was developed, consisting of a 1 mm diameter, 1 mm long BeO ceramic cylinder coupled to a silica/silica optical fibre. The reader measures the RL signal and also uses a 450 nm laser diode to stimulate the BeO ceramic. A second background optical fibre is used to remove the stem effect. The RL/OSL BeO FOD was characterised in a solid water phantom, using a 6 MV x-ray beam. The RL was found to be reproducible and have a linear response to doses ranging from 30 cGy–15 Gy and dose rates from 100 cGy/min – 600 cGy/min. The OSL response was linear to doses of 10 Gy, becoming supralinear at higher doses. Measured percentage depth curves using the RL/OSL BeO FOD agreed with those measured using an IC15 ion chamber to within 5%, beyond the build up region. It was also found that the RL from BeO ceramic is unaffected by the delivered dose to the probe and hence, it remains constant for a given dose-rate. The insensitivity of the RL to accumulated dose makes BeO ceramic potentially capable of accurate dose-rate measurements without any corrections for the accumulated dose. This study demonstrates the feasibility of BeO ceramic as a versatile fibre-coupled luminescence dosimeter probe.     

OSL from BeO ceramics: new observations from an old material

In BeO ceramics, exposed to ionizing radiation, an intense OSL signal was observed. The properties of the signal and its behavior under various experimental conditions were investigated. It was found that the OSL signal is a composite signal and exhibits strong thermal quenching. The quenching energy was estimated as 0.5 eV. The excitation spectrum of the OSL signal was obtained as a broad peak in the region 420–550 nm with maximum around 435 nm. The possible correlation between the OSL signal and the peaks in the TL glow curve was also examined. It was interesting to observe that the highly light sensitive TL peak near 220°C does not contribute to the OSL signal. The OSL signal was found to originate from a trap near 340°C.

To check the possibility of using the material for radiation dosimetry the dose response and thermal stability of the OSL signal were also investigated. The dose response was found to be quite linear up to 10 Gy. The thermal activation energy of the OSL trap was determined as 1.7 eV using isothermal annealing and heating rate methods thus proving the suitability for dosimetry.     

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.     

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.     

Further investigations of tunneling recombination processes in random distributions of defects

The shape of infrared stimulated luminescence signals (IRSL) from feldspars has been the subject of numerous studies in the field of luminescence dating. Specifically linearly modulated IRSL signals (LM-IRSL) are commonly assumed to consist of several first order components corresponding to distinct optical stimulation cross sections. This paper models the shape of LM-IRSL signals using a recently proposed kinetic model, which describes localized electronic recombination in donor–acceptor pairs of luminescent materials. 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. The model has been used previously successfully to describe both thermally and optically stimulated luminescence (TL, OSL). This paper shows that it is possible to obtain approximate solutions for the distribution of donors in the ground state as a function of two variables, time and the distance between donors and acceptors. Approximate expressions are derived for several possible modes of optical and thermal stimulation, namely TL, OSL, linearly modulated OSL (LM-OSL), LM-IRSL and isothermal TL (ITL). Numerical integration of these expressions over the distance variable yields the distribution of remaining donors at any time t during these experimental situations. Examples are given for the derived distributions of donors in each experimental case, and similarities and differences are pointed out. The paper also demonstrates how LM-IRSL signals in feldspars can be analyzed using the model, and what physical information can be extracted from such experimental data. The equations developed in this paper are tested by fitting successfully a series of experimental LM-IRSL data for Na- and K-feldspar samples available in the literature. Finally, it is shown that the equations derived in this paper are a direct generalization of an equation previously derived for the case of ground state tunneling.     

Quartz fast component opticallystimulated luminescence: Towards routine extraction for dating applications

Using an uncontaminated fast component is a key for improving the reliability of quartz OSL dating for many deposits. So far no approach to extract the fast component of quartz OSL has routinely been adopted for dating practice. Key challenges for extracting fast components are (1) the difficulty of finding a unique solution in curve-fitting deconvolution of OSL decay curves and (2) the relatively poor dating precision when using experimental fast component extraction. Here, a simple mathematic solution for fast component extraction is presented that is not relying on curve-fitting deconvolution and can easily be adopted into routine dating practices. By using specifically selected data points from smoothed OSL decay curves, the precision of equivalent doses calculated using the extracted fast component can be improved over equivalent doses calculated using bulk OSL. The fast component extraction is tested on a group of age-constrained samples containing both insufficiently bleached and sufficiently bleached deposits. Fast component OSL ages are as accurate as bulk OSL ages for the sufficiently bleached deposits, but more accurate for samples where bulk OSL is affected by insufficient bleaching. We also demonstrate how using a curve smoothing procedure can improve dating precision in case of both sufficiently and insufficiently bleached deposits.     

A comparison of single and multiple aliquot TT-OSL data sets for sand-sized quartz from the Arabian Peninsula

The quartz OSL signal from dune sands from Saudi Arabia and Oman start to saturate at doses of about 100 Gy. In order to try to date dune sands with greater expected doses, a previously published, single-aliquot, regenerative-dose protocol (SAR) for thermally-transferred optically stimulated luminescence (TT-OSL) was tested. Dose recovery tests, recycling and recuperation ratios showed robust functioning and dose response curves demonstrated the potential to extend the dose range to beyond 600 Gy. Multiple aliquot additive dose (MAAD) TT-OSL protocols were used to test for sensitivity changes in the SAR TT-OSL protocol up to doses of 1200 Gy. A strong dose dependent deviation of the SAR TT-OSL relative to the MAAD TT-OSL dose response is observed. Comparison of the TT-OSL and OSL sensitivity data obtained from the MAAD and SAR data sets shows a lack of proportionality between TT-OSL and OSL for the SAR data which will result in a problem when SAR dose response curves are constructed using many regeneration points with doses above 300 Gy.     

TL, OSL, and phototransferred TL in beta-irradiated anion-defective Al2O3

The behavior of TL, OSL and PTTL under thermal and/or optical stimulation in beta-irradiated -Al₂O₃ was investigated. The noticeable change of the shape of the main TL peak after thermal or optical stimulation clearly shows that this peak is related to at least two traps. The OSL curves recorded after preheating up to a given increased end temperatures are described by two exponential decays curves. The first one is associated to the decay of the traps responsible for the main TL peak. The second exponential is related with the phototransfer from the deep to the main traps. A simple model of the OSL process, based on simultaneous refilling of shallow traps and their reverse filling due to phototransfer from deep traps, under illumination with light is proposed. The model describes well the obtained experimental data.