Optically Stimulated Luminescence Process Research Articles

Surface mount multilayer ceramic capacitors as optically stimulated luminescent dosimeters

The choice of dosimetric materials for precise retrospective dose assessment in case of any emergencies is very challenging. Since few decades, electronic components like ceramic resistors, inductors, IC chips, surface mount resistors and inductors have been proven to be suitable candidates for dose estimation using thermoluminesce (TL)/optically stimulated luminescence (OSL) techniques. In this work, OSL technique is applied to study dosimetric properties of surface mount multilayer ceramic capacitors (SM MLCC) available in commonly used instruments like printer, computer mouse, USB flash drive and personal gadgets like mobile phone etc. To understand the OSL process involved, the elemental composition and crystallinity of the SM MLCCs were analyzed using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) techniques. It is found that the main constituent is barium titanate with aluminium silicate as major impurity along with some trace elements and is responsible for the origin of the radiation induced OSL signal. Some important dosimetric characteristics like OSL sensitivity, repeatability, OSL dose response, and OSL signal stability upon storage were studied. The stimulation time was optimized as 50 s and used for all OSL measurements. The dose response was found to be linear from 500 mGy to 15 Gy and simply increased (up to 40 Gy). Fading analysis showed that the OSL signal decreases by ∼ 25 % in 1 day, ∼ 50 % in 4 days, and remained stable after that at least up to 8 days. The dose was recovered using single aliquot regenerative-dose (SAR) technique with very good accuracy. These results of dosimetric studies and validation study suggest that surface mount multilayer ceramic capacitors have potential use as retrospective dosimeters.

OSL CHARACTERISTICS: THEORY AND EXPERIMENTS.

Fundamental effects such as the participation of crystal lattice vibrations should be taken into account while considering photo-ionisation in solids at measurement temperatures characteristic for dosimetric applications. The impact of these effects on the optically stimulated luminescence (OSL) can be recognised for example by changes in the optical cross-section of the trap with the stimulation energy and the temperature. The OSL signal observed during the stimulation with variable energy, during the linear heating of a sample or during the stimulation with modulation of the shape of a spectral band can provide important information about the properties of the trap. Moreover, such methods when applied with carefully selected experimental parameters enable the effective separation of the OSL signal originating from different traps. The consequences of such an approach to optical stimulation will be presented using the modelling of the OSL process and compared with experimental results.

Properties of Charge Carrier Traps in Lu2O3:Tb,Hf Ceramic Storage Phosphors Observed by High-Pressure Spectroscopy and Photoconductivity

In this work, the kinetic study of charge carrier trapping processes, photocurrent excitation measurements, and high-pressure spectroscopy investigation of an optically stimulated luminescence (OSL) process in Lu2O3:Tb,Hf ceramics are presented. The OSL process was experimentally and theoretically treated to explain the OSL mechanism and kinetics. The obtained results allowed to propose an energetic structure of the system and a theoretical model that explains the charge exchange between Hf4+ and Tb3+ (upon charging the system). It was shown that the OSL took place via the conduction band and the depths of Hf traps were determined for the thermal (1.36 eV) and optical (2.95 eV) release and the significant difference between these two values was fully justified.

Thermoluminescence, photoluminescence and optically stimulated luminescence characteristics of CaSO4:Eu phosphor: Experimental and density functional theory (DFT) investigations

The CaSO4:Eu phosphor in nanocrystalline form was obtained by chemical method. The sample was annealed at various temperatures and quenched. The structural, electronic and optical properties are studied using various experimental techniques. As synthesized CaSO4:Eu particles have nanorod shapes with diameter of ~15 nm and length of ~250 nm. After annealing (at around 900 °C) a significant increase in their size (~2–4 μm) with phase transformation from hexagonal to orthorhombic was observed. Thermoluminescence (TL) and optically stimulated luminescence (OSL) intensities were found to increase with temperature up to 900 °C and decrease thereafter for 1 Gy of test dose of β-rays from 90Sr-90Yr source. However, the maximum OSL sensitivity was found to be more than that of CaSO4:Eu microcrystalline phosphor (prepared by acid recrystallization method) contrary to the usually found in the literature but much less than that of commercially available α-Al2O3:C phosphor. The activation energy for thermally assisted OSL process was found to be 0.0572 ± 0.0028 eV. The dose ranges of TL and OSL response was found from 0.04 Gy to 100 Gy and 0.02 Gy–100 Gy, respectively. The experimental results are also correlated with computational calculations based on density functional theory (DFT). The crystal structures and electronic structures of both hexagonal and orthorhombic CaSO4 and CaSO4:Eu materials show that they are direct band gap (5.67–5.86 eV) insulators, with Ca2+ substitution by Eu2+ found to introduce donor states in the band gap near Fermi level and the valence band edge of CaSO4 on doping with Eu2+ impurity ions.

Ceramic resistors as optically stimulated luminescent retrospective dosimeters

Application of optically stimulated luminescence (OSL) as a retrospective dosimetric technique is studied using ceramic resistors available in commonly used instruments like personal computers, printers, household electronic appliances etc. In order to understand the OSL process the composition of the ceramic resistors was analysed using XRD, FTIR and ED-XRF techniques and it is found to have hydrated calcium aluminium silicate as the major constituent along with trace impurities for the origin of radiation induced OSL. The important retrospective dosimetric characteristics like its OSL sensitivity, reproducibility, OSL dose response and stability of the signal upon storage after exposure have been studied. The dose response was found to be linear from 100 mGy to 5 Gy. It is also observed that the OSL signal decreases by 30% in the first 3 days and thereafter there is negligible fading up to 30 days. The correlation between TL and OSL signals in the material was also studied. Using Single Aliquot Regenerative (SAR) technique, the dose was recovered with very good accuracy. These dosimetric studies showed that carbon film ceramic resistors have potential use as emergency dosimeter.

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

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

Effect of Al3+ co-doping on the luminescence properties of Cu doped Na2SiF6

Studies were carried out to assess the correlation between thermoluminescence (TL) and optically stimulated luminescence (OSL) of this phosphor. It was observed that the OSL and TL glow curve consists of a wide distribution of traps having different photo-ionization cross-sections, trap depths and frequency factors. In case of Al doped sample, some of the traps up to 200°C are assumed to act as a source traps for the observance of OSL due to thermal transfer of charge carriers into the deep traps beyond 480°C. This suggests that Al impurities play an important role in the thermal transfer OSL process. As most of the work on this phenomenon is done on natural materials (mainly quartz) in which aluminum is a natural impurity, this study will explain the role of Al in this phenomenon.

Optically stimulated luminescence (OSL) from Ag-doped Li2B4O7 crystals

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li2B4O7) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag+ ions substituting for Li+ ions. They also have Ag+ ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag+ ions that replace Li+ ions and electrons are trapped at the interstitial Ag+ ions, i.e., the radiation forms Ag2+ (4d9) ions and Ag0 (4d105s1) atoms. These Ag2+ and Ag0 centers have characteristic EPR spectra. The Ag0 centers also have a broad optical absorption band peaking near 370nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400nm light produces an intense OSL response when emission is monitored near 270nm. Electrons optically released from the Ag0 centers recombine with holes trapped at Ag2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370nm peak of the absorption band of the Ag0 electron traps). Oxygen vacancies are also present in the Ag-doped Li2B4O7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described.

Thermally modulated optically stimulated luminescence (TM–OSL) as a tool of trap parameter analysis

The methods of optically stimulated luminescence (OSL) measurements used until recently, used optical stimulation with a constant energy and a constant or linearly increased flux of stimulation photons. During such a stimulation the ratio of probabilities of the optical release of electrons from different traps is constant and it is hard to separate the signals of different origins. It was shown recently that advantageous changes of the probability ratio during the OSL experiments, and more information about traps can be obtained by optical stimulation with the increasing stimulation energy. This method, however, needs a strong tuneable light source that supplies a stable flux of photons and because of that it cannot find a wide application. Inducing the appropriate changes of the probabilities of the optical release of electrons from traps by increasing the sample temperature during the optical stimulation with a constant stimulation band do not face such obstacles. Such a stimulation can be realised by means of the standard OSL readers after a slight modification and offers the possibility for direct estimation of optical trap depth. The simulations of the OSL process during linear heating show that the experimental parameters such as the heating rate, the stimulation light intensity and the stimulation energy strongly affect the shape of the OSL curve and can be the very useful tools for the OSL process regulation. By this kind of stimulation one can reach very deep traps that are not detectable by thermoluminescence measurements below 500°C. The resolution of the OSL signal originating from different traps is remarkable.

Identification of defects responsible for optically stimulated luminescence (OSL) from copper-diffused LiAlO2 crystals

A large optically stimulated luminescence (CW-OSL) response has been observed from bulk lithium aluminate (LiAlO2) crystals that have been copper-diffused. After introducing the copper, an optical absorption band peaking at 277nm is present and is assigned to Cu+ (3d10) ions at Li+ sites. Photoluminescence (PL and PLE) from copper-diffused samples shows an emission band near 359nm and an excitation band near 273nm. These PL bands are also assigned to the Cu+ ions. After an irradiation at room temperature with x-rays, the copper-diffused crystals give an intense OSL signal when stimulated with light in the 425–500nm region. The OSL emitted light has a peak near 357nm and thus is directly associated with recombination at a Cu ion. Electron paramagnetic resonance (EPR) shows that the x-rays produce two distinct Cu2+ (3d9) centers, both at Li+ sites. One of the trapped-hole centers is a Cu2+ with no nearby defects and the other is a Cu2+ with a neighboring Li+ vacancy. Monitoring the EPR spectra before and after OSL shows that only the isolated Cu2+ ions (i.e., those with no nearby defect) participate in the OSL process. The electron traps participating in the OSL are shown with EPR to be transition-metal ions that are unintentionally present. Specifically, Fe2+ ions replacing Li+ ions, possibly with a nearby lithium vacancy, convert to Fe+ (3d7) ions when they trap an electron during irradiation. These electrons are optically released by the OSL exciting light and migrate to the holes trapped as isolated Cu2+ ions, where electron–hole recombination gives the characteristic Cu+ emission. By combining OSL, PL, and EPR, we establish the general OSL mechanism in copper-diffused LiAlO2 crystals.

Estimating the parameters of traps in quartz by the variable energy of stimulation optically stimulated luminescence (VES-OSL) method

Optical-cross section that is a trap parameter estimated from the measurements of optically stimulated luminescence (OSL) is not a uniquely determined physical quantity. It depends not only on temperature and the energy of stimulation light but also, in the simplest case, on the optical depth of trap, the frequency of vibration mode and on the Huang-Rhys factor, i.e. the average number of phonons involved in the process of optical excitation of electrons from trap to conduction band. Conventional OSL measurement techniques do not enable to determine directly these parameters but they could be estimated by applying the variable energy of stimulation optically stimulated luminescence (VES-OSL) method. Recently it was put in to practice and the first VES-OSL curves were presented. In this study the outcomes of VES-OSL experiments are presented together with the first attempt of direct estimating the optical depth of traps active in OSL process in quartz.

State of art: Optically stimulated luminescence dosimetry – Frontiers of future research

Since the commercial adoption of the optically stimulated luminescence (OSL) technique in dosimetry, almost 20 years ago, we have seen major advances in the deployment of OSL dosimeters in different areas, including personal, medical, and space dosimetry. The objective of this paper is to provide a critical overlook at the OSL technique from three different points of view: strengths, challenges and opportunities. We discuss factors that made the OSL technique successful: its simplicity, accuracy, wide dynamic range of measured dose, ease for automation, re-read capability, ability to perform imaging, and the availability of diverse instruments and materials. We look into problems that were overcome and others that remain in several areas of new applications into which OSL has expanded in the past 10 years, such as medical, space, neutron and accident dosimetry. Finally, we discuss unexplored possibilities, new driving forces, and open questions. We hope the broad overview presented here will encourage more discussion and stimulate the research that will advance our fundamental understanding of the OSL process.

The dependence of quartz OSL on stimulation energy

Quartz is the mineral most commonly used for sediment dating. In dating practice, the optically stimulated luminescence (OSL) of quartz is measured mainly using the stimulation light whose wavelength is 470 ± 30 nm. The parameters of traps active in the OSL process are also determined for this stimulation band. The zeroing of the OSL is the fundamental condition of applicability of the luminescence dating for specific sediment and takes place in sunlight whose spectrum differs significantly from the band 470 ± 30 nm. In order to be able to know the course of OSL process in nature, a wider knowledge of the dependency of the trap parameters on the stimulation band is needed. Here the results are presented for the OSL measurements carried out with different wavelengths of stimulation light. For each stimulation band the components of the OSL signal are determined by the fitting procedure, and in this way the wavelength dependence of an individual component is found. The experiment has been repeated for two temperatures of OSL detection – the room temperature, which corresponds to natural conditions, and for 125 °C, which is the temperature usually applied for OSL measurement in dating. Four OSL components are presented in both experimental series. The values of their optical cross-section changes along with stimulation energy and temperature, as it is predicted by the model of OSL process including crystal lattice vibrations.

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-rate dependence of optically stimulated luminescence signal

Optically stimulated luminescence (OSL) is the luminescence emitted from a semiconductor during its exposure to light. The OSL intensity is a function of the total dose absorbed by the sample. The dose-rate dependence of the OSL signal of the semiconductor CaS doped Ce and Sm was studied by numerical simulation and experiments. Based on a one-trap/one-center model, the whole OSL process was represented by a series of differential equations. The dose-rate properties of the materials were acquired theoretically by solving the equations. Good coherence was achieved between numerical simulation and experiments, both of which showed that the OSL signal was independent of dose rate. This result validates that when using OSL as a dosimetry technique, the dose-rate effect can be neglected.

TL, OSL and ESR studies on beryllium oxide

Abstract Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the thermoluminescence (TL) and optically stimulated luminescence (OSL) processes in BeO phosphor. Two defect centres were identified in irradiated BeO phosphor by ESR measurements, which were carried out at room temperature and these were assigned to an O− ion and Al2+ centre. The O− ion (hole centre) correlates with the main 190 °C TL peak. The Al2+ centre (electron centre), which acts as a recombination centre, also correlates to the 190 °C TL peak. A third centre, observed during thermal annealing studies, is assigned to an O− ion and is related to the high temperature TL at 317 °C. This centre also appears to be responsible for the observed OSL process in BeO phosphor.

Modelling the thermal bleaching of OSL signal in the case of a competition between recombination centres

The thermal bleaching of the optically stimulated luminescence (OSL) has been investigated by computer simulations for a model including three traps and two luminescence centres. The deepest trap is active only during the OSL process. Two other traps are active only during the thermal bleaching. The thermal bleaching effects on the OSL intensity as well as on the OSL curve shape are presented for the wide range of trap and luminescence centre parameters and for the different settings of optical detection window. The conventional OSL curve analysis consisting in decomposition of the OSL curve into first order components is applied to the simulation results and the optical cross section spectra obtained as a result of this analysis are compared with the model assumptions. The simulations show that OSL signal can decrease to undetectable level even when the traps related to this signal are not emptied during thermal bleaching. The residual level of the OSL signal after bleaching process, however, depends strongly on centre parameters and concentrations. The modifications of optical detection spectral window lead to significant changes of bleaching effects. The thermal bleaching influences also the optical cross section spectra obtained as a result of the OSL curve decomposition.

Optically stimulated luminescence and thermoluminescence of terbium-activated silicates and aluminates

This paper presents a preliminary characterization of the optically stimulated luminescence (OSL) and thermoluminescence (TL) properties of terbium-doped samples as potential new OSL dosimeters. The samples include lithium aluminate (LiAlO 2:Tb, Li 2Al 2O 4:Tb), magnesium silicate (Mg 2SiO 4:Tb, Mg 2SiO 4:Tb,Co) and calcium silicate (CaSiO 3:Tb). The OSL and TL emission spectra show the characteristic lines of Tb 3 + . The OSL intensity of the most sensitive samples is one order of magnitude lower than Al 2O 3:C. Nevertheless, the sensitivity and dose response are suitable for dosimetry applications, with the lower limit of detection estimated to be in the range from ∼ 30 – 50 to ∼ 200 μ Gy , depending on the sample. The OSL response is linear over the range of doses investigated, from 2 mGy to 5 Gy, except for CaSiO 3:Tb samples. OSL stimulation spectra show that blue light (470 nm) is six times more efficient than green light (532 nm) to stimulate the OSL emission. Investigations on the fading properties also show that the signal fades between ∼ 30 % and 40% in a period of 20 h, but after that the signal remains relatively stable in some samples. The fading seems to be related to the contribution of shallow traps to the OSL process.

Investigation of neutron converters for production of optically stimulated luminescence (OSL) neutron dosimeters using Al 2O 3:C

This paper presents the optically stimulated luminescence (OSL) properties of neutron dosimeters in powder and in the form of pellets prepared with a mixture of Al 2O 3:C and neutron converters. The neutron converters investigated were high density polyethylene (HDPE), lithium fluoride (LiF), lithium fluoride 95% enriched with 6Li ( 6LiF), lithium carbonate 95% enriched with 6Li ( 6Li 2CO 3), boric acid enriched with 99% of 10B ( H 3 10 BO 3 ) and gadolinium oxide (Gd 2O 3). The proportion of Al 2O 3:C and neutron converter in the mixture was varied to optimize the total OSL signal and neutron sensitivity. The neutron sensitivity and dose-response were determined for the OSL dosimeters using a bare 252Cf source and compared to the response of Harshaw TLD-600 and TLD-700 dosimeters ( 6LiF:Mg,Ti and 7LiF:Mg,Ti). The results demonstrate the possibility of developing an OSL dosimeter made of Al 2O 3:C powder and neutron converter with a neutron sensitivity (defined as the ratio between the 60Co equivalent gamma dose and the reference neutron absorbed dose) and neutron–gamma discrimination comparable to the TLD-600/TLD-700 combination. It was shown that the shape of the OSL decay curves varied with the type of the neutron converter, demonstrating the influence of the energy deposition mechanism and ionization density on the OSL process in Al 2O 3:C.

Investigation on relation between TL and OSL of Li2B4O7:Cu, In

Polycrystal Li2B4O7 (LBO) doped with Cu and In was prepared and then sintered at different temperatures. X-ray diffraction (XRD) was applied to get the parameters of the LBO structure, thermoluminescence (TL) and optically stimulated luminescence (OSL) were measured, and a second-order exponential decay model was fitted to the OSL decay curves. The results indicate that the original number of OSL traps that have captured electroncs is linearly related with the sum of TL decay during the OSL process. Mean decay constant of OSL is related to the sintered temperature. The possible reason is that the sintered temperature affects the crystal sizes of the polycrystal, and consequently affects the stimulating lights’s intensity and the photoionization cross-section of the electrons, which have been captured by the traps.