Time-resolved Optically Stimulated Luminescence Research Articles

A systematic multi-technique comparison of luminescence characteristics of two reference quartz samples

Further developments in luminescence dating, dosimetry and temperature-sensing require a deep understanding of luminescence processes and their driving parameters. Natural quartz is one of the most widely used minerals for these purposes. Still, poor reproducibility of results often hampers comparability and credibility from findings in the literature. We identified the lack of suitable natural reference samples as a pivotal problem impeding significant progression. Ideally, basic investigations involve several laboratories working on well-characterised reference quartz samples with different characteristics. Investigations should include multiple complementing methods to analyse luminescence properties and mineralogical and geochemical composition.Here, we present such a multi-technique luminescence comparison of two natural quartz samples. Next to the recently introduced Fontainebleau (FB) reference quartz, we propose another reference sample derived from the ‘Silver Sands of Morar’ (lab code ‘MR’; Scotland, UK). Our experiments confirm that both quartz samples behave fundamentally different in terms of signal composition and sensitivity. The comparative characterisation of both samples targeted electron traps via thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques and luminescence centres via radioluminescence and time-resolved OSL spectrometry. In summary, we conclude that all observed differences are likely the results of divergent defect concentrations rather than variances in defects' composition (nature). The measurement data of our study are accessible open-access for inspection by others.

Time-resolved optically stimulated luminescence of quartz in the nanosecond time domain

Time-resolved optically stimulated luminescence (TR-OSL) is one of the few methods to characterise the type of recombination centre involved in luminescence production and its response to thermal, optical and irradiation treatments. TR-OSL experiments for natural quartz yielded lifetimes mainly in the range 30–45 μs, often supplemented by shorter or longer values in the μs range. For distinct types of bedrock quartz and K-feldspar much shorter lifetimes in the ns range have been reported. Here we further explore the characteristics of this short lifetime component in quartz and look for links to specific components in the quartz OSL signal with emphasis on the slow component. Our experiments were carried out on a newly developed measurement system available as an attachment to Freiberg Instruments lexsyg research readers that allows recoding TR-OSL signals with a minimum dwell time of 2.5 ns. We demonstrate that all of the nine investigated natural quartz samples give rise to a short lifetime component in the range 30–200 ns, which does not originate from measurement artefacts or feldspar contaminants. For most samples, two further lifetimes of 0.7–3.2 μs and 39–160 μs accompany this component as indicated by multi-exponential curve fitting. Our results show that the short lifetime component does not change substantially with annealing up to 500 °C, whereas it appears to rise slightly with irradiation up to a dose of 200–800 Gy. The process behind the decrease of the short lifetime component down to ∼30 ns with measurement temperature up to 110 °C could not be certainly identified. With the measurement data compiled so far, we presently cannot establish a link between the short lifetime component and the OSL slow component.

AN INTRODUCTION TO TIME-RESOLVED OPTICALLY STIMULATED LUMINESCENCE

This short and concise monograph is an excellent introduction for anyone contemplating using optically stimulated luminescence (OSL) in time-resolved mode. Time-resolved OSL refers to the separation in time of the stimulation and emission phases of OSL leading to a clearer elucidation of the numerous processes that can participate in the phenomenon. The book is very well written and easy to read and provides an easy-to-comprehend description of OSL in general and time-resolved processes in particular for new students and researchers in the field. Unlike time-resolved photoluminescence phenomena, OSL involves the participation of localised and delocalised optical transitions and charge transport through the lattice. The delay between the onset of stimulation and the start of the emission is dictated by multiple transition rates and charge-carrier lifetimes. Untangling these can be difficult but the problem can be approached in a logical and rigorous way by considering the different rate processes that give rise to the effects and the various mathematical relationships that describe them. Dr Chithambo is to be congratulated on doing this in a way that is both easy to read and easy to absorb.

TL and OSL properties of Mn2+-doped MgGa2O4 phosphor

The oxide MgGa2O4 spinel ceramics doped with Mn2+ ions was synthesized by a solid-state reaction at 1200 °C in air. The activator concentration was equal 0.05 mol% of MnO. Phase purity of the synthesized samples was analyzed by X-ray diffraction technique. This spinel ceramics show efficient green emission in the range from 470 to 550 nm with a maximum at about 505 nm under UV or X-ray excitations, which is due to Mn2+ ions. MgGa2O4: Mn2+ exhibits intense thermoluminescence (TL) and optically stimulated luminescence (OSL) after influence of ionizing radiation. Are complex nature of the TL glow curves is associated with a significant number of structural defects that are responsible for the formation of shallow and deep electron traps. In this work, time-resolved OSL characteristics of the samples exposed to beta particles are reported for the first time. A light from green LED was used for optical stimulation. Obtained TL and OSL results suggest MgGa2O4:Mn2+ as perspective material for further research and possible application in radiation dosimetry.

Development and Performance Testing of a Time-resolved OSL Measurement System

Development and Performance Testing of a Time-resolved OSL Measurement System

Time-resolved OSL studies of YAlO3:Mn2+ crystals

Time-Resolved Optically Stimulated Luminescence (TR-OSL) from single crystalline YAlO3:Mn2+ samples was investigated using a green light emitting diode (λ ∼ 525 nm) as stimulation light source. The TR-OSL decay curve of the material can be described with a single exponential decay function with a lifetime about 80 ms that does not depend on irradiation dose in the range from 50 mGy to 1 kGy. This OSL decay is superposed on a photoluminescence signal with a much shorter (3.5 ms) decay lifetime. The Mn2+ photoluminescence decay with a lifetime of 3.5 ms can be easily eliminated by corresponding time resolution using pulsed OSL readout. Dose response and thermal stability of the OSL signal are consistent with the previous thermoluminescent (TL) studies of the material.

Extending the area of investigation of fine versus coarse quartz optical ages from the Lower Danube to the Carpathian Basin

Despite the general satisfactory performance of quartz in the single aliquot regeneration protocol (SAR), previous optically stimulated luminescence (OSL) dating studies of key loess sections in Romania, Lower Danube region, revealed a disturbing disagreement among the ages obtained on fine (4–11 μm) grains and coarse (63–90 μm) grains respectively. The current study aims at expanding these investigations, both by extending the area of study from the Lower Danube Basin to the Carpathian Basin and by applying time-resolved optically stimulated luminescence (TR-OSL) on quartz, in order to gain further insights into the above mentioned behaviour. The samples from the Orlovat loess paleosol section (Vojvodina, Serbia) showed a similar behaviour to that previously reported on Romanian loess. A marked difference between the dose saturation characteristics of fine and coarse quartz OSL signals is observed for both continuous wave (CW-OSL) and pulsed OSL (POSL), where the dose response (up to 1000 Gy) is well described by a sum of two saturating exponential functions. TR-OSL measurements show one single, characteristic quartz lifetime for both natural as well as regenerative signals in the entire dose range investigated. A general disagreement between the ages obtained on the two grain sizes for samples with equivalent doses higher than about 100 Gy is reported as in the case of Romanian loess, inferring that the age discrepancy between the two grain sizes might be more widespread than previously thought.

More on the TR-OSL signal from BeO ceramics

Time Resolved Optically Stimulated Luminescence (TR-OSL) from BeO ceramics was investigated using blue (445 nm) and near-IR light (852 nm) for stimulation. Stimulation spectrum of the TR-OSL signal – as measured in the interval 700 to 420 nm- was observed to increase monotonically with the decreasing stimulation wavelength. In addition to the “fast” and “slow” components observed with blue light stimulation, IR stimulated TR-OSL spectra of irradiated BeO ceramics were observed to have two components with average lifetimes around ∼2.5 μs and ∼17 μs. Emission spectra of the both IR stimulated TR-OSL components were observed to have a broad emission band peaking around 330 nm. Thermal stability of the IR stimulated TR-OSL signal was studied by making preheating experiments in the range from 100 °C to 190 °C. It was observed that the IR stimulated OSL signal is stable up to ∼150 °C and decay afterwards. Radiation dose response of the IR stimulated luminescence signal was obtained in the range from 5 to 500 Gy. Both blue and IR stimulated TR-OSL signals grew up to 100 Gy and exhibited saturation for higher doses. Additionally, measurement temperature dependence of the components was also investigated and for the ∼2 μs component thermal assistance with activation energy around 0.16 eV was observed. It seems that the fast component of the blue stimulated TR-OSL component can be correlated to the ∼2 μs IR stimulated TR-OSL component.

Time-resolved OSL of natural zircon: A preliminary study

Time-resolved OSL (TR-OSL) from natural zircon (ZrSiO4) minerals was investigated using 445 nm blue laser light for stimulation. Analyses of the TR-OSL spectra have showed that the decay is composed of two exponential components with lifetimes varying around ∼17 μs and around ∼110 μs respectively. The behaviour of these signal components, was examined under various sample treatments and experimental conditions. Preheating experiments showed that the OSL signal is stable up to temperatures ∼250 °C then becomes unstable. The dose response of the TR-OSL signal from zircon was determined in the range from 1 Gy to 1 kGy and observed to be increasing linearly. Practically, no effect of radiation dose on the lifetimes of signal components was observed. In addition, the effect of measurement temperature on the TR-OSL decay lifetimes was also investigated. Thermal quenching energies of the “fast” and the “slow” components were found to be very close to each other i.e. 0.18 and 0.24 eV respectively.

Time-resolved OSL studies on BeO ceramics

Time-Resolved Optically Stimulated Luminescence (TR-OSL) from BeO ceramics was investigated using a blue laser (445 nm) as stimulation light source. It was observed that, at relatively low dose levels (up to ∼25 Gy) the TR-OSL decay curve can be approximated with a single exponential decay function with a lifetime of ∼26 μs at room temperature. Beyond 25 Gy a new decay component with a lifetime of a ∼2 μs was observed in addition to the ∼26 μs component. Thermal stability, radiation dose response, optical bleaching, measurement temperature dependence of the components of the TR-OSL signal were investigated in detail. As result of these studies, a new OSL component which becomes unstable after 150 °C was observed. OSL decay rate of this component was found to be higher than the one which becomes unstable after 300 °C. In order to obtain information about the temperature dependence of the luminescence efficiency, luminescence emission lifetime was determined in the temperature range from 30 to 130 °C with 10 °C steps. Using the temperature dependence of the lifetime, thermal quenching energy was determined to be around 0.56 eV for the 26 μs component. For the ∼2 μs component an enhancement in the component intensity was observed pointing to a thermally assisted process with activation energy of 0.15 eV.

Thermal dependence of time-resolved blue light stimulated luminescence in α-Al2O3:C

This paper presents time-resolved optically stimulated luminescence (TR-OSL) experiments in the important dosimetric material Al2O3:C. During these experiments short pulses (0.5s) of light from blue LEDs (470nm) are followed by relaxation periods (2.5s) of the charge carriers at different stimulation temperatures. During the pulse excitation period the integrated TR-OSL signal increases with the stimulation temperature between 50 and 150°C, while between 160 and 240°C the signal intensity decreases. This behavior is interpreted to arise from competing effects of thermal assistance (activation energy, Eth=0.067±0.002eV) and thermal quenching (activation energy W=(1.032+0.005)eV). Changes in the shape of the TR-OSL curves were analyzed at different stimulation temperatures using analytical expressions available in the literature. The TR-OSL signals contain a slower temperature-dependent phosphorescence signal, the “delayed-OSL” described previously for this material. The temperature dependent luminescence lifetimes obtained from analysis of the optical stimulation period are identical to those obtained from the corresponding relaxation period. However, the values of these luminescence lifetimes are systematically higher than previously reported values from time-resolved photoluminescence (TR-PL) studies carried out in this important dosimetric material. These results are discussed within the context of a recently published kinetic model.

Time-resolved measurements of optically stimulated luminescence of Al2O3:C and Al2O3:C,Mg

This study investigates the components of the optically stimulated luminescence (OSL) signal from enhanced aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg), a new material developed for optical data storage and nuclear track detection. The objective is to understand the OSL properties of Al2O3:C,Mg in comparison to Al2O3:C (the most commonly used OSL detector) and to explore the potential of Al2O3:C,Mg for applications in luminescence dosimetry. The study includes continuous-wave OSL (CW-OSL) and time-resolved OSL (TR-OSL) measurements under pulsed laser stimulation. The results show that the OSL emission from Al2O3:C,Mg is similar to Al2O3:C, but the UV emission at 330 nm attributed to F+ centers is more intense in Al2O3:C,Mg than in Al2O3:C. Moreover, we observed a delayed UV emission with a luminescence lifetime of ∼3 ms which can be explained by the influence of shallow traps related to thermoluminescence (TL) peaks below room temperature.

Influence of shallow traps on time-resolved optically stimulated luminescence measurements of Al2O3:C,Mg

The objective of this paper is to investigate the influence of shallow traps on the signals from Al2O3:C,Mg obtained using time-resolved optically stimulated luminescence (TR-OSL) measurements through experiments and numerical simulations. TR-OSL measurements of Al2O3:C,Mg were carried out and the resulting optically stimulated luminescence (OSL) curves were investigated as a function of the temperature. The numerical simulations were carried out using the rate-equations for a simplified model of Al2O3:C,Mg containing two types of luminescence centers with different luminescence lifetimes and three types of electron traps (a shallow trap, a main dosimetric trap, and a thermally disconnected deep trap). Both experimental results and simulations show that the OSL signals during and between the stimulation pulses are affected by the presence of shallow traps. However, with an appropriate choice of timing parameters, the influence of shallow traps can be reduced by calculating the difference between the signals during and between stimulation pulses. Therefore, TR-OSL can be useful in dosimetry using materials having a large concentration of shallow traps and OSL components with short luminescence lifetimes, for example Al2O3:C,Mg and BeO. Our results also show that the presence of shallow traps has to be taken into account when using the TR-OSL for discrimination between luminescence centers with different luminescence lifetimes, or separation between the OSL from different materials based on their characteristic luminescence lifetimes. The experimental results also show evidence of thermal assistance in the OSL process of Al2O3:C,Mg.

Dose and dose rate dependence of time-resolved OSL from Korean paleosol quartz

Abstract Using a newly developed small X-ray irradiator equipped with a mini X-ray generator (Varian VF-50J) and TR-OSL measurement system attached to an MCS (ORTEC MCS-PCI), the influence of dose and dose rate of X-ray radiation on TR-OSL was investigated in paleosol quartz from the Chungdang-dong paleolithic site, Korea. All lifetimes were calculated by a curve-fitting method using a single stretched-exponential function with β = 0.86. Physical characteristics, natural lifetime, thermal quenching activation energy ΔE, and thermal assistance activation energy Ea were firstly determined. From the results of the investigation of dependence on dose and dose rate of X-ray radiation using fully bleached samples, the lifetime decreased significantly to about 20 μs; when the X-ray dose was increased to 300 Gy at a dose rate of 0.5 Gy/s. Also, despite different radiation dose rates, at a dose of 50 Gy both lifetime and related intensity were independent of dose rate change.

Towards a non-fading signal in feldspar: Insight into charge transport and tunnelling from time-resolved optically stimulated luminescence

Feldspars are an attractive alternative to quartz for extending the dose range, and for dating volcanic terrains such as on Mars and Iceland. Unfortunately, charge stored in the feldspar lattice undergoes anomalous fading leading to an underestimation in the dose estimates. In this paper we use the time-resolved optically stimulated luminescence (TR-OSL) technique to investigate the processes that give rise to the signal following infrared (IR), green and blue stimulation, with an objective to understand tunnelling and charge transport during thermo-optical excitations. We show that the TR-OSL shape is governed by the energy of excitation and the subsequent charge recombination route through the excited state of the trap, the band tail states or the conduction band. The role of band tail states in charge recombination is specifically examined using the signal shown to decay over several ms; we identify two dominant recombination routes, viz., phonon (0.05–0.06 eV) assisted diffusion, and quantum mechanical tunnelling, depending on the energy state of the detrapped electron. As would be expected, diffusion in the band tails is identical for both resonant and non-resonant excitations, where in the latter case the band tail state occupancy likely arises from thermalisation of conduction band electrons. The important outcome of this study is a comprehensive physical model based on a single dosimetric trap that successfully explains wide-ranging luminescence phenomena in feldspars, in particular, the luminescence efficiency and thermal partitioning of charge in different energy states and the subsequent recombination routes. The model predicts three different systematic approaches to preferentially sampling the most stable signal. We finally present evidence for a non-fading signal using one of these methods based on pulsed IR stimulation.

Charge recombination processes in minerals studied using optically stimulated luminescence and time-resolved exo-electrons

A time-resolved optically stimulated exo-electron (TR-OSE) measurement system has been developed using a Photon Timer attached to a gas-flow semi-proportional pancake electron detector within a Risø TL/OSL reader. The decay rate of the exo-electron emission after the stimulation pulse depends on the probability of (1) escape of electrons into the detector gas from the conduction band by overcoming the work function of the material and (2) thermalization of electrons in the conduction band, and subsequent re-trapping/recombination. Thus, we expect the exo-electron signal to reflect the instantaneous electron concentration in the conduction band. In this study, TR-OSE and time-resolved optically stimulated luminescence (TR-OSL) were measured for the first time using quartz, K-feldspar and NaCl by stimulating the samples using pulsed blue LEDs at different temperatures between 50 and 250 °C after beta irradiation and preheating to 280 °C. The majority of TR-OSE signals from all the samples decayed much faster than TR-OSL signals irrespective of the stimulation temperatures. This suggests that the lifetime of OSL in these dosimeters arises mainly from the relaxation of an excited state of the recombination centre, rather than from residence time of an electron in the conduction band.

Optically stimulated phosphorescence in quartz over the millisecond to second time scale: insights into the role of shallow traps in delaying luminescent recombination

Time-resolved optically stimulated luminescence (TR-OSL) curves from quartz are usually measured over a few hundred microseconds because this time range best illustrates the main component in quartz which lies in the range 30–45 µs. In this study we present the decay form of quartz TR-OSL and optically stimulated phosphorescence (OSP) covering over 8 orders of magnitude from 50 ns to ∼8 s. A detailed characterization of the previously unstudied slowly decaying signals (millisecond–second time scales) is undertaken to understand the origin of these components and the role of re-trapping following optical stimulation. We present preheat and stimulation temperature dependence for both the TR-OSL and OSP curves in these time ranges and use the latter data to determine the E and s values for the participating shallow traps. We observe an abnormal decay behaviour seen as a sudden increase in the decay rate (a ‘kink’) conspicuous at about 2–3 s in the OSP curves measured at 75 and 100 °C. We satisfactorily reproduce this behaviour with a numerically solved kinetic model consisting of four energy levels. The physical interpretation of the kinetic rate equations is discussed in terms of a three trap–one centre model or a one trap–two centres model involving localized charge transfer.

Towards multi-exponential analysis in optically stimulated luminescence

Optically stimulated luminescence (OSL) data from quartz can follow different mathematical forms depending on the stimulation mode. These data can be described in terms of different multi-exponential models and can be numerically fitted using several well-known methods. Here we make a comparative analysis of the performance and stability of two models, the decay and peak form, and we consider different transformation methods for obtaining the peak form. For the numerical computations we use a nonlinear least squares (NLS) method and a method based on a first-kind Fredholm integral equation (FIE). Our analysis uses artificial data with three components (seven parameters including the background) and ten different levels of background, both the signal and the background contain Poisson distributed noise. Parameters derived using both models are acceptable (statistically consistent and on an average within ∼1% of the expected value) and no obvious preference is observed for any particular model, although there may be a suggestion that peak-form data show a smaller mean bias. This conclusion seems to be independent of the type of peak transformations investigated here. Furthermore, it is found that transformation of OSL decay data to a peak form gives better results than direct measurement of peak-form data by, for example, varying the stimulation light intensity. The comparison of the two numerical methods suggests that the NLS method performs somewhat better than the FIE method; however, the latter has the advantage that it does not require the user's judgement on the number of components in the data. Testing of the NLS procedure on a measured quartz time-resolved OSL signal transformed into peak form yielded reliable parameter estimates even when the signal intensity was deliberately reduced by a factor of 16.

Optimising the separation of quartz and feldspar optically stimulated luminescence using pulsed excitation

Abstract In luminescence dating, the two most commonly used natural minerals, quartz and feldspar, are exposed to different dose rates in the natural environment, and so record different doses. The luminescence signals also have different stabilities. For accurate dosimetry, the signals from these two minerals must be separated, either by physical separation of the mineral grains, or by instrumental separation of the luminescence signals. The luminescence signals from quartz and feldspar have different luminescence lifetimes under pulsed optical stimulation. This difference in lifetime can be used to discriminate between the two signals from a mixed quartz-feldspar sample. The purpose of this study is to identify optimum measurement conditions for the best separation of quartz OSL from that of feldspar in a mixed sample using pulsed stimulation and time-resolved OSL. We integrate the signal from 5 μs after the LEDs are switched off until just before the LEDs are switched on again, with the pulse on -time equal to the pulse off -time of 50 μs. By using only the initial interval of the pulsed OSL decay curve (equivalent to 0.2 s of CW-OSL using blue light at 50 mW cm −2 ) we find that the quartz to feldspar pulsed OSL intensity ratio is at a maximum. By using these parameters with an additional infrared (IR) stimulation at 175 °C before measurement (to further reduce the feldspar signal intensity), we obtain a factor of 25 enhancement in signal separation compared to that from a conventional prior-IR CW measurement. This ratio can be further improved if the counting window in the pulse off -time is restricted to detect between 20 and 50 μs instead of the entire off -period.

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

Abstract 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.