A time-correlated photon counting system for measurement of pulsed optically stimulated luminescence

A versatile light-emitting-diode system for measurement of time-resolved luminescence spectra is described. The instrument is an improvement on a previously reported version [1] also based on pulsed light-emitting diodes. The present configuration offers substantially improved performance in measurement of time-resolved optically stimulated luminescence spectra and records the luminescence with better dynamic throughput. Measurements made on sedimentary quartz annealed at 800 °C as an exemplar show that the luminescence obtained after the light pulse as a proportion of the total measured signal falls within the usable dynamic range of 0.1-98% for pulse-widths between 3 and , a range of pulse-widths and so luminescence yield that is sufficient for most contemporary applications of time-resolved optically stimulated luminescence.     

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.     

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.     

Correlation of optically and thermally stimulated luminescence of natural fluorite pellets

Natural fluorite (CaF₂), a dosimetric material of large usage, presents Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). This study examined the behavior of TL and OSL (stimulated with Blue LEDs) signals from the Brazilian natural fluorite pellets with NaCl as binding agent, as well as their correlations, in order to study and optimize the dosimetric process with this material. A series of experiments were conducted, basically with thermal treatments before OSL acquisition, and optical bleaching before TL readout. The role of NaCl in the TL and OSL emission was investigated. It was observed that natural CaF₂ TL signal is still ample to be used in dosimetric applications, as dose re-assessment in personal dosimetry after an OSL measurement. Also it was verified that the fluorite OSL signal is extinguished by a 350 °C heating and that NaCl has no contribution to the stable part of the OSL signal.     

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.     

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.     

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.     

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.     

Thermoluminescence and optically stimulated luminescence characteristics of Al2O3 doped with Tb

In the present work policrystals of α − Al₂O₃ doped with terbium were synthesized using the solvent evaporation method. The samples were prepared using Al(NO₃)₃·9H₂O and Tb(NO₃)₃·5H₂O reagents, with Tb concentrations between 1 and 5 mol% and thermally treated at high temperature above ∼1400 °C. X-ray diffraction measurements showed the α-phase formation of samples. TL glow curve presented an intense peak at ∼190 °C and two other with low intensity at 290 and 350 °C after gamma irradiation. The best doping concentration which presented high luminescence was the sample doped with 3 mol% of Tb. TL spectra and fluorescence measurements showed similar luminescence spectra with lines attribute to Tb³⁺ ions. A linear behavior to gamma dose between 1 and 20 Gy was observed in TL, using 190 °C peak as well as in OSL signal, this last carried out using 532 nm wavelength stimulation.     

Investigation of regeneration effect of blue luminescence in NaCl using variable delay optically stimulated luminescence (VD-OSL)

Optically stimulated luminescence (OSL) of pure analytical sodium chloride (NaCl) was studied. Performing series of CW-OSL (continuous wave OSL) readouts it was found that subsequent CW-OSL decay, after some delay, starts from significantly higher intensity level than the final intensity of the previous readout. To study this ‘regeneration effect’ a new type of measurements was implemented – the variable delay OSL (VD-OSL) method. The idea of VD-OSL is explained – it allows to study OSL kinetics at very long time scale. VD-OSL data confirm coexistence of two effects in NaCl – regeneration and fading of the OSL signal. After partial CW-OSL readout, regeneration predominates in the short time scale of the order of 10³ s. Then, fading becomes dominant. The same measurements were applied to commonly used Al₂O₃:C material as well. Nevertheless, the results do not show such behavior at the studied time scale. Theoretical arguments are presented that the regeneration mechanism cannot be explained on the basis of the simple trap model.     

The response of thermally and optically stimulated luminescence from Al2O3:C to high-energy heavy charged particles

The thermoluminescence (TL) and optically stimulated luminescence (OSL) response of Al₂O₃ dosimeters to high-energy heavy charged particles (HCP) has been studied using the heavy ion medical accelarator at Chiba, Japan. The samples were Al₂O₃ single-crystal chips, of the type usually known as TLD-500, and Luxel^TM dosimeters (Al₂O₃:C powder in plastic) from Landauer Inc. The samples were exposed to ⁴He (150 MeV/u), ¹²C (400 MeV/u), ²⁸Si (490 MeV/u) and ⁵⁶Fe (500 MeV/u) ions, with linear energy transfer values covering the range from 2.26 to 189 keV/μm in water and doses from 1 to 100 mGy (to water). A ⁹⁰Sr/⁹⁰Y beta source, calibrated against a ⁶⁰Co secondary standard, was used for calibration purposes. For OSL, we used both continuous-wave OSL measurements (CW-OSL, using green light stimulation at 525 nm) and pulsed OSL measurements (POSL, using 532 nm stimulation from a Nd:YAG Q-switched laser). The efficiencies (η_HCP,γ) of the different HCPs at producing OSL or TL were observed to depend not only upon the linear energy transfer (LET) of the HCP, but also upon the sample type (single crystal chip or Luxel^TM) and the luminescence method used to define the signal—i.e. TL, CW-OSL initial intensity, CW-OSL total area, or POSL. Observed changes in shape of the decay curve lead to potential methods for extracting LET information of unknown radiation fields. A discussion of the results is given, including the potential use of OSL from Al₂O₃ in the areas of space radiation dosimetry and radiation oncology.     

The response of thermally and optically stimulated luminescence from Al2O3:C to high-energy heavy charged particles

The thermoluminescence (TL) and optically stimulated luminescence (OSL) response of Al₂O₃ dosimeters to high-energy heavy charged particles (HCP) has been studied using the heavy ion medical accelarator at Chiba, Japan. The samples were Al₂O₃ single-crystal chips, of the type usually known as TLD-500, and Luxel^TM dosimeters (Al₂O₃:C powder in plastic) from Landauer Inc. The samples were exposed to ⁴He (150 MeV/u), ¹²C (400 MeV/u), ²⁸Si (490 MeV/u) and ⁵⁶Fe (500 MeV/u) ions, with linear energy transfer values covering the range from 2.26 to 189 keV/μm in water and doses from 1 to 100 mGy (to water). A ⁹⁰Sr/⁹⁰Y beta source, calibrated against a ⁶⁰Co secondary standard, was used for calibration purposes. For OSL, we used both continuous-wave OSL measurements (CW-OSL, using green light stimulation at 525 nm) and pulsed OSL measurements (POSL, using 532 nm stimulation from a Nd:YAG Q-switched laser). The efficiencies (η_HCP,γ) of the different HCPs at producing OSL or TL were observed to depend not only upon the linear energy transfer (LET) of the HCP, but also upon the sample type (single crystal chip or Luxel^TM) and the luminescence method used to define the signal—i.e. TL, CW-OSL initial intensity, CW-OSL total area, or POSL. Observed changes in shape of the decay curve lead to potential methods for extracting LET information of unknown radiation fields. A discussion of the results is given, including the potential use of OSL from Al₂O₃ in the areas of space radiation dosimetry and radiation oncology.     

Al2O3:C optically stimulated luminescence droplets: Characterization and applications in medical beams

Optically stimulated luminescence (OSL) detectors, which are widely used in radiation protection, offer a number of potential advantages for radiotherapy dosimetry. In this study we characterized 1-μl of OSL droplets consisting of a mixture of Al₂O₃:C powder and a photo-curable polymer, in addition to results described in a previous work (Nascimento et al., 2013). The concentration test showed that droplets have a higher spatial resolution than other commonly used Al₂O₃:C-based detectors. Our results from the dose response, reproducibility and dependence with accumulative dose were obtained for droplets with a powder/polymer concentration that showed a high Signal to Noise Ratio (SNR) without compromising the droplet malleability. Additional test results show the response of such droplets in percentage depth dose curves and dose profiles of clinical beams.     

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.     

A study on the real-time radiation dosimetry measurement system based on optically stimulated luminescence

The optically stimulated luminescent (OSL) radiation dosimeter technically surveys a wide dynamic measurement range and a high sensitivity. Optical fiber dosimeters provide capability for remote monitoring of the radiation in the locations which are difficult-to-access and hazardous. In addition, optical fiber dosimeters are immune to electrical and radio-frequency interference. In this paper, a novel remote optical fiber radiation dosimeter is described. The optical fiber dosimeter takes advantage of the charge trapping materials CaS:Ce, Sm that exhibit OSL. The measuring range of the dosimeter is from 0.1 to 100 Gy.     

Comparative study on the effect of annealing treatments on RTL mechanism in natural quartz from different origins

The behaviour of trap centres and luminescence centres has been investigated for fired and unfired natural quartz from bricks and sediments irradiated at 100 Gy and annealed at different temperatures in the range 350-700 °C. The annealing treatment affects thermoluminescence (TL) glow curve as various changes were observed. The higher sensitization occurred for an annealing in the region 550-600 °C. At this annealing temperature, it has been observed the emergence of two peaks arising at 96 and 180 °C. At lower annealing temperatures, these peaks are overlapped by the peaks localized at 90 and 195 °C, respectively. Concerning the fired quartz, the higher sensitization occurred for an annealing in the region 500-550 °C for peak temperature around 200 °C and an unusual desensitization for the peak temperature around 100 °C. The behaviour of the two types of quartz is analyzed regarding to their kinetic parameters and luminescence emission and compared to literature data.     

Optically stimulated luminescence in doped K3Na(SO4)2 phosphors

Optically stimulated luminescence (OSL) in Cu and Eu doped K₃Na(SO₄)₂ is reported for the first time. The Cu-doped sample shows OSL sensitivity which gets enhanced by co-doping with Mg²⁺ ions. The Cu-doped and quenched sample shows better sensitivity which is almost double than that of the slowly cooled sample, whereas the sensitivity of Mg co-doped sample remains nearly same irrespective of the thermal treatment. The Cu-doped sample shows TL peak around 200 °C and moderate OSL sensitivity. Doping of Mg shifts the TL peak to around 160 °C and is correlated with good OSL sensitivity. Eu-doped sample does not show OSL sensitivity. However, relatively good OSL sensitivity is observed in Aluminium co-doped and slowly cooled sample, which is about 15% of the commercial Al₂O₃:C(Landuer Inc.). A near fully optically sensitive TL peak around 155 °C is observed. The dose response is linear and practically no OSL fading is observed in first five days of storage in slowly cooled sample. This study on conventional sulphate-based TL phosphors will be useful in developing OSL phosphors for radiation dosimetry.     

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.