Anomalous regeneration of OSL in sodium chloride – Experiment and modeling

Recently, a new optical phenomenon was observed in sodium chloride (NaCl). Performing series of optically stimulated luminescence (OSL) readouts it was found that subsequent OSL decay, after some delay, starts from significantly higher intensity level than the final intensity of the previous readout. This unusual behavior was called the ‘regeneration effect’ (Biernacka and Mandowski, 2013). Regeneration occurs in spite of the significant fading of OSL – well known in this material. Nevertheless, these two contradictory processes take place in two different time scales. Previously, it was proved also, that the regeneration phenomenon cannot be explained on the basis of the classical simple trap model (STM). This paper analyses optimal measurement conditions leading to the highest regeneration effect. It was found that the increase of OSL intensity could be as high as 190%. A simple phenomenological model explaining the mechanism of regeneration is proposed. The model involves simultaneous localized and delocalized (i.e. band-like) transitions. Characteristic lifetimes of regeneration and fading processes are of the order of 560 s and 22000 s, respectively.     

The BeOmax system – Dosimetry using OSL of BeO for several applications

The BeOmax-system uses the OSL of BeO for dose measurement. The OSL-Material is Thermalox 995™ from Materion Ceramics^® in quadratic or cylindrical form. For an easy handling complete dosimeters with encapsulated quadratic detectors and identification code for automatic evaluation are also available. Stimulation of detectors is performed with a blue LED (455 nm), OSL signal (UV-region) is detected with a Hamamatsu photo sensor module (PSM) from the opposite detector side. Several filters to avoid stimulation light reaching the PSM are necessary. An electronic and special software offers an easy evaluation of the PSM-signals and to specify dose. The dose characteristic is linear from 10 μGy up to 10 Gy. Then supralinearity connected with a change of the shape of the decay curve starts. The saturation begins with some hundred grays. The variation coefficient of dose measurements is below 2% for dose higher than 0.1 mGy. The energy dependence shows an underestimation of low energy photons. The BeOmax-System can be used in medicine, for industrial applications and for scientific research. Two examples will be shown.     

Empirical insights into multi-grain averaging effects from ‘pseudo’ single-grain OSL measurements

In this study we assess the signatures of multi-grain averaging effects for a series of sedimentary samples taken from the archaeological site of Hotel California, Atapuerca, Spain. We focus on the special case of equivalent dose (D_e) measurements made on single-grain discs that contain more than one quartz grain in each of the individual grain-hole positions with the aims of (i) providing insight into the nature and extent of averaging effects in very small multi-grain aliquots of sedimentary quartz, and (ii) assessing the suitability of ‘pseudo’ single-grain D_e measurements for this particular dating application. Pseudo single-grain OSL measurements made on standard discs loaded with 90–100 μm grains (equivalent to ～30 grains per hole) yield significantly different D_e distribution characteristics and finite mixture model (FMM) burial dose estimates compared with single-grain OSL measurements. Grains with aberrant luminescence behaviours, which are routinely rejected during single-grain analysis, exert strong averaging effects on the pseudo single-grain and multi-grain aliquot D_e distributions. Grain-hole averaging effects arising from pseudo single-grain measurements also give rise to ‘phantom’ dose components and are apt to provide bias assessments of quartz signal characteristics and grain type classifications. Though this is a site-specific study, it serves as a cautionary note for interpretations of other pseudo single-grain OSL and D_e datasets – particularly those obtained from measurements of discs containing several tens of grains per hole and those derived from complex depositional environments. The use of custom single-grain discs drilled with smaller sized grain holes is recommended as a means of limiting grain-hole averaging effects when dealing with very fine (<180 μm) sediments.     

Quantification of cross-irradiation and cross-illumination using a Risø TL/OSL DA-15 reader

The automated Risø TL/OSL DA-15 reader is widely used for luminescence dating applications. The nature of the device is such that the irradiation and illumination of a sample during the OSL procedures may affect discs in adjacent positions. It is critical to confirm that these factors do not introduce significant systematic errors. The effects of such cross-talk (cross-irradiation and cross-bleaching) are examined and quantified, both for specific and general cases. Depending upon the protocol used, cross-irradiation and cross-bleaching could accumulate to have an effect on the D_e estimate, and for the latter this could be significant (i.e. 10%).     

Improvement in dose threshold of α-Al2O3:C using thermally assisted OSL

α-Al₂O₃:C phosphor was studied for improvement of its dose threshold using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The dose threshold of phosphor depends on the standard deviation of the background signal which affects the signal-to-noise ratio of the instrument. In case of OSL measurement, the background signal is due to the scattering of the stimulation light intensity from the planchet. On reducing the stimulation light intensity, the scattering component and thus the standard deviation of the background signal reduces considerably. The reduction in stimulation light intensity increases the readout time due to the dependence of decay constant of OSL signal on the former. The decay constant depends on the photo-ionization cross-section of the OSL active traps in the phosphor participating in OSL phenomenon and thus on the readout temperature due to the temperature dependence of photo-ionization cross-section. In order to achieve the same decay constant for two sets of measurements for α-Al₂O₃:C i.e. to take the OSL measurement in the same time at lower stimulation light intensity as that taken for higher light intensity, the temperature of measurement for α-Al₂O₃:C for the former was raised. Moreover, the increase of readout temperature does not affect the standard deviation in the background signal of OSL readout. The optimized elevated temperature for α-Al₂O₃:C was found to be 85 °C as the main dosimetric peak starts giving signal due to depletion of its filled traps, at temperatures higher than 90 °C. As a result of lowering the stimulation intensity at higher temperature (85 °C), the standard deviation in the background signal and thus the overall dose threshold of α-Al₂O₃:C was found to improve by 1.8 times.     

Characterization of deep energy level defects in α-Al2O3:C using thermally assisted OSL

Commercially available α-Al₂O₃:C powder was studied for deep energy level defects by a newly suggested method using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The method involves simultaneous application of continuous wave optically stimulated luminescence (CW-OSL) as well as thermal stimulation up to 400 °C, using a linear heating rate of 4 K/s. By using this method, two well-defined peaks at 121 °C and 232 °C were observed. These TA-OSL peaks have been correlated to two different types of deeper defects which can be bleached at 650 °C and 900 °C respectively on thermal treatment. These deeper defects, having larger thermal trap depth and relatively lower photoionization cross-section at room temperature for stimulation with blue LED (470 nm), are stable up to 500 °C, so they can store absorbed dose information even if the sample is inadvertently exposed to light or temperature. As only a fraction of signal is bleached during TA-OSL readout, multiple readouts could be performed on an exposed sample using this technique. The dose vs TA-OSL response from deep traps of α-Al₂O₃:C was found to be linear up to 10 kGy, thus extending its application for high dose dosimetry. The value of thermally assisted energy (E_A) associated with these traps in α-Al₂O₃:C has been determined to be 0.268 eV and 0.485 eV respectively and the corresponding values of photoionization cross-section at room temperature (25 °C), for optical stimulation with blue light (470 nm), are 5.82 × 10^?20 and 3.70 × 10^?22 cm², respectively. The process of thermally assisted OSL has been formulated analytically as well as theoretically for describing the temperature dependence of optical cross-section and evaluation of thermally assisted energy associated with deep traps.     

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.     

Nanostructured layers of anion-defective gamma–alumina – New perspective TL and OSL materials for skin dosimetry. Preliminary results

Thin- layer material based on nanostructured Al₂O₃ of the surface density 5 mg/cm² was obtained. The material is characterized by high OSL and TL yields comparable with those for TLD-500 which is one of the leaders among the TL and OSL detectors. The dose response, fading and dependence of TL yield on heating rate was studied. It is established that high luminescence yield of the samples under study correlates with the content of anion vacancies and γ-phase of Al₂O₃. The data for time-resolved luminescent spectroscopy are presented, which evidence for possible correlation between high TL and OSL activity and the F-type centers. It is noted that the material needs to be modified for successful use in dosimetry. In addition further studies to decrease the contribution of unstable (at 300 К) components to OSL and TL yields are required.     

Spectroscopy of the gamma and X ray leakage radiation from the built-in sources of a Risø TL/OSL reader

In this work we evaluate the intensity and the spectral shape of the leakage radiation from the built-in beta and alpha sources of a Risø TL/OSL reader. LiF (TLD-100), fluorite-based pellets and Al₂O₃:C detectors were used in order to determine the dose rate delivered to the dosimeters when the sources are closed. The leakage spectra under both alpha and beta sources were registered with a CdTe semiconductor detector. The spectrum measured under the beta source shows the X-ray beam generated by the interaction of the beta particles with the lead used to shield the source. Besides, the 59.4 keV gamma ray from ²⁴¹Am was registered under the alpha source. Dose rates from 50 to 100 μGy/h were obtained for the dosimeter positions in the turntable under the beta and the alpha sources with the luminescent dosimeters.     

Sources of uncertainties in OSL dating of archaeological mortars: The case study of the Roman amphitheatre “Palais-Gallien” in Bordeaux

Archaeological mortars are more convenient and much more representative for the chronology of buildings than brick or wood constructions that can be re-used from older buildings. Before dating unknown samples of mortars, further investigation of OSL from mortars is required and the most efficient methodology needs to be established. In this study we compared the ages obtained by OSL dating of quartz extracted from mortars of the Roman amphitheatre Palais-Gallien in Bordeaux with independent age information.Resetting of the OSL signal occurred during the preparation of mortar when grains of sand (quartz) were extracted and mixed with lime and water. The mortar was subsequently hidden from light by embedding within the structure which is the event to be dated.Various factors contribute to uncertainties in the age determination. The frequency of measured equivalent doses reveals a large scattering. Optical bleaching of certain grains can be partial due to the short duration of the exposure to light. We worked with the single grain technique in order to find and select the grains that were sufficiently exposed to daylight. To determine the average equivalent dose, we tried three different approaches: a calculation of an arithmetic mean and one following either the central age model or the 3-parameter minimum age model, the latter turned out to be the only relevant way to evaluate the experimental data. The proportion of grains included in the calculation of the average equivalent dose represents 2.7–4.7 % of the overall analysed grains. The results obtained for the three out of four samples are approaching the expected age, however, the minimum doses and the corresponding ages are significantly over-estimated in case of two samples.The studied material is very coarse, which causes heterogeneity of irradiation at the single grain scale, and contributes to a dispersion of equivalent doses. Different analytical methods (scanning electron microscopy with energy dispersive X-ray spectroscopy cartography, Beta-radiography imaging, inductively coupled plasma mass spectrometry) were employed to demonstrate the presence of this phenomenon.Despite the extremely large proportion of high equivalent doses in equivalent dose distributions, there is an apparent presence of well-bleached grains at the beginning of equivalent dose distributions. The study shows the potential of dating mortars by single grain OSL.     

The TL and room temperature OSL properties of the glow peak at 110 °C in natural milky quartz: A case study

The LM–OSL signal of quartz, while measured at room temperature, is dominated by an intermediate, broad and intense OSL component, so that its contribution and general characteristics are derived very accurately. Through a series of dose–response, bleaching and thermal decay at room temperature experiments, in conjunction with curve fitting studies, a component resolved analysis is carried out studying the correlation between this specific component, termed as LM–OSL component C₂ and the 110 °C TL glow peak in quartz. The dose–response of these two luminescence components behaves exactly similar being linear at low doses and saturating at almost 100 Gy. Both signals decay exponentially under illumination, providing identical optical detrapping cross-section values. Residual of both luminescence signals after thermal decay at room temperature follows an exponential law, yielding similar mean half-lives. All previous luminescence features provide strong evidence for the electron trap being the same for both the 110 °C TL trap and the LM–OSL component C₂. The results of the present work are very promising and clearly support the possibility of extrapolating the TL pre-dose methodology to the OSL pre-dose effect using only the LM–OSL component C₂.     

In-Vacuo thermal processing of α-Al2O3 single crystals in boron ambience and its implication on TL & OSL response

Single crystals of α-Al₂O₃ (10×10 mm², 0.4 mm thick) were annealed in vacuum at about 1500 °C in the ambience of boron. The OA studies on these samples showed bands at 203, 232 and 258 nm signifying that such a treatment leads to the formation of F and F⁺ centers in significant concentrations, these bands, however, were not found in the Al₂O₃ crystals processed in the similar manner in the absence of boron. The Al₂O₃:B samples were irradiated to different absorbed doses of ⁹⁰Sr/⁹⁰Y β-source and the continuous wave OSL (CW-OSL) was recorded on the samples using 470 nm blue light stimulation. These samples have shown a linear TL and CW-OSL response in the dose range of 20 mGy to 15 Gy. The minimum detectable dose, corresponding to 3σ limit of the variation of the output of the unirradiated dosimeters, was found to be 100 μGy. Irradiated samples stored in dark at room temperature for a period of two months show negligible fading. The TL and OSL sensitivities of the samples were found to be strongly dependent on process temperature and time. The TL response is marked by the absence of low temperature peak (<100 °C), unlike the case of α-Al₂O₃:C, implying that the boron doping does not lead to formation of shallow traps. The Al₂O₃:B samples show faster photoionisation cross-section as compared to α-Al₂O₃:C. This approach of processing of single crystal Al₂O₃ in the boron ambience thus represents a potential way of introducing dosimetrically pertinent defects in Al₂O₃ single crystals.