Light induced fading associated with the application of OSL to personal dosimetry

Al₂O₃:C is the best material example that presents OSL response and adequate dosimetric behaviour for OSL dosimetry. It was the first commercial material manufactured for use in personal monitoring based on an OSL reader system from Landauer. The purpose of this paper was to report the results of optical fading experiments for the nanoDot commercial OSL detectors (Al₂O₃:C), provided by Landauer Inc. Five groups of different experimental conditions were formed with all detectors, exposing them to fluorescent and semiconductor light sources and to sun light. The loss of OSL signal when the detectors are kept open, was verified, which was already expected, but a loss in the OSL signal even when the detectors are exposed to light and covered with the manufacturer plastic protection are also revealed. The results show also that the use of Mylar filters can delay the OSL fading of the detectors.     

Evaluating the influence of experimental conditions on the photon energy response of Al2O3:C optically stimulated luminescence detectors

This work presents a determination of the photon energy response of Al₂O₃:C optically stimulated luminescence (OSL) detectors (single crystals and Luxel^® detectors) to photons with mean energies from 10 keV to 1225 keV. Irradiations were performed free-in-air using standard X-ray fields (narrow-spectrum series) specified by the International Organization for Standardization. The OSL readouts were conducted under different conditions (e.g., different optical filters; continuous or pulsed stimulation) and the analyses were performed using the initial OSL intensity or total OSL area. The measured photon energy response of the OSL signal was compared to the ratio between the mass–energy absorption coefficient for Al₂O₃:C and air. The results demonstrate that the photon energy response of Al₂O₃:C is not only dependent on the energy deposited, but also on the experimental conditions. This is attributed to the effect of ionization density on the OSL properties of Al₂O₃:C, which affects the shape of the OSL curve and the relative intensity of the OSL emission bands. The results highlight the importance of maintaining similar OSL readout conditions and using similar types of detectors when determining and applying photon energy correction factors.     

Ionization density dependence of the curve shape and ratio of blue to UV emissions of Al2O3:C optically stimulated luminescence detectors exposed to 6-MV photon and therapeutic proton beams

In this work we characterized the dose and linear energy transfer (LET) (ionization density) dependence of commercial Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to clinical photon and proton beams. We characterized the dose-dependence of the OSL signal, OSL curve shape, and the relative intensities of the blue and ultraviolet (UV) OSL emission bands using different readout protocols and beam qualities. We irradiated OSLDs with absorbed doses ranging from 0.1 Gy to 100 Gy in a 6-MV photon beam and from 0.1 Gy to 50 Gy in 140- and 250-MeV proton beams. Readouts were done with both continuous-wave (CWOSL) and pulsed (POSL) stimulation. The linearity of the OSLD dose–response depended on readout protocol and radiation type. Improved linearity was found for OSLDs irradiated with beams of increased LET, and for OSL signals containing only the blue emission band of Al₂O₃:C (which remained linear for doses up to 10 Gy for 140-MeV proton beam irradiations). The OSL curve shape did not vary with dose in the low-dose region (below 5 Gy depending on readout protocol), but beyond this, curves decayed more rapidly with increasing dose. Similarly, the ratio of blue to UV emission band intensities in the OSL signal did not vary with dose up to 5 Gy (depending on readout protocol), beyond which the ratio decreased with increasing dose. Because both the OSL curve shape and the ratio of blue to UV emission intensities have been investigated as potential parameters for measurements of LET, the constancy of these two quantities at doses relevant to radiotherapy is encouraging for the potential development of novel OSL methods to measure LET. Our findings are expected to contribute to the development of (i) improved readout protocols for commercially available Al₂O₃:C OSLDs and (ii) methods to measure radiation quality and LET.     

Characteristics of LiAlO2 – Radioluminescence and optically stimulated luminescence

Radioluminescence (RL) and optically stimulated luminescence (OSL) results of LiAlO₂ were compared with Al₂O₃:C. For blue (470 nm) optical stimulation, RL + OSL signal in LiAlO₂ exhibited a sharp initial increase followed by a decay within the first 20 s of continuous wave (CW)-OSL and a very slow increase thereafter. The RL + OSL signal was about 1.25 times of its RL signal in LiAlO₂ as compared to 2.5 times in Al₂O₃:C. With the continued beta irradiation, the RL signal exhibited a faster growth in LiAlO₂ than that in Al₂O₃:C. Emission spectrum of LiAlO₂ exhibited multiple emission peaks in the range from 320 to 380 nm as against 410 nm of Al₂O₃:C. RL and OSL emission spectra were similar in both LiAlO₂ and Al₂O₃:C. The intense RL in LiAlO₂ (about 300 times of that of its background signal) for a beta ray dose rate of 4.6 mGy/s appears attractive for radiation dosimetry including real time/online dosimetry and dose mapping.     

Development of Ag doped crystalline SiO2 for possible applications in real-time in-vivo OSL dosimetry

A new Ag-doped crystalline SiO₂ based OSL phosphor is developed. This phosphor shows good OSL properties and the sensitivity is comparable to that of the commercial Al₂O₃:C. For the luminescence integrated over initial 3s, BSL (blue stimulated luminescence) and GSL (green stimulated luminescence) sensitivities were found to be 0.54 and 3.1 times the respective BSL and GSL sensitivities of the commercial Al₂O₃:C. The PL emission is found to vary from UV to visible range with the change in the Ag concentration. Thus the material can be used as BSL or GSL phosphor. The simple preparation procedure, fast decay, very good sensitivity will make this phosphor suitable for real-time dosimetric applications, using OSL technique.     

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.     

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.     

Preliminary study on development and characterization of high sensitivity LiAlO2 optically stimulated luminescence material

Preliminary results of an attempt to prepare LiAlO₂ material with high optically stimulated luminescence (OSL) sensitivity are reported. LiAlO₂ was prepared by melting a stoichiometric mixture of Li₂CO₃ and Al₂O₃ powders in a RF heating furnace. CW-OSL signal as recorded on Risoe TL/OSL reader using blue (470 nm) stimulation was found to be up to 16 times of that of Al₂O₃:C. The promising characteristics of LiAlO₂, open up a possibility of an improved material not only for passive dosimetry of mixed fields of neutron and gamma rays but also for online measurements and dose mapping/imaging applications.     

Growth of α-Al2O3:C crystal with highly sensitive optically stimulated luminescence

In this work, an α-Al₂O₃:C crystal was directly grown by the temperature gradient technique (TGT) using Al₂O₃ and graphite powders as the raw materials. The optical, optically stimulated luminescence (OSL) properties and dosimetric characteristics of as-grown crystal were investigated. As-grown α-Al₂O₃:C crystal shows strong absorption band at 205, 230 and 256 nm. Three-dimensional thermoluminescence (TL) emission spectrum of the crystal shows a single emission peak at ∼415 nm. The OSL decay curve can be fitted to two exponentials, the faster component and the slower component. The OSL response of the crystal shows a linear-sublinear-saturation characteristic. As-grown α-Al₂O₃:C crystal shows excellent linearity in the dose range from 5×10⁻⁶ to 50 Gy. For doses higher than the saturation dose (100 Gy), the OSL sensitivity decreases as the dose increases.     

Linear energy transfer dependence of Al2O3:C optically stimulated luminescence detectors exposed to therapeutic proton beams

Currently, there are no radiation detectors that can be used for routine measurements of linear energy transfer (LET) in particle therapy clinics. In this work, we characterized the LET dependence of Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to therapeutic proton beams in order to evaluate their potential for clinical LET measurements. We evaluated OSLDs that were irradiated with an absorbed dose to water of 0.2 Gy in therapeutic proton beams with average energies ranging between approximately 25 MeV and 200 MeV, resulting in LET in water values between 0.45 and 2.29 keV/μm. We examined two properties of the OSL emission signal in terms of LET dependence: the signal intensities of the blue and ultraviolet (UV) emission bands, and the shapes of the OSL curves. We found that the signal intensity of the UV emission band increased consistently with LET within the range investigated, whereas the intensity of the blue emission band remained constant. Our results also demonstrated that the OSL curve shapes were more LET dependent for signals containing both the blue and UV emission bands than for signals containing only one of the bands. Both metrics we examined in this study – the relative UV/blue emission signal intensities and OSL curve shapes – show potential for LET detection in proton therapy.     

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.     

Optically stimulated luminescence from CaSO4:Eu – Preliminary results

A new OSL phosphor CaSO₄:Eu was developed. The phosphor shows good OSL sensitivity which is about 55% of commercially available Al₂O₃:C. The phosphor also shows good TL sensitivity and the dosimetric peak, which appears around 186 °C, has sensitivity nearly 50% of Al₂O₃:C. After OSL readout of the irradiated sample, the TL peak around 250 °C depletes completely, with partial depletion of peak around 186 °C. Since the traps responsible for the high temperature peak are involved for the observed OSL, the sample shows low post-irradiation fading. The OSL decay is similar to Al₂O₃:C. Thus this phosphor due to its good OSL sensitivity, linear dose response, low fading and simple preparation technique could be useful for radiation dosimetry applications.     

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.     

Photon dosimetry methods outside the target volume in radiation therapy: Optically stimulated luminescence (OSL), thermoluminescence (TL) and radiophotoluminescence (RPL) dosimetry

Dosimetry methods outside the target volume are still not well established in radiotherapy. Luminescence detectors due to their small dimensions, very good sensitivity, well known dose and energy response are considered as an interesting approach in verification of doses outside the treated region. The physical processes of thermoluminescence (TL), radiophotoluminescence (RPL) and optically stimulated luminescence (OSL) are very similar and can be described in terms of the energy band model of electron-hole production following irradiation.This work is a review of the main dosimetric characteristics of luminescence detectors which were used in experiments performed by EURADOS Working Group 9 for in-phantom measurements of secondary radiation (scattered and leakage photons). TL LiF:Mg,Ti detectors type MTS-7 (IFJ PAN, Poland), types TLD-100 and TLD-700 (Harshaw), OSL Al₂O₃:C detectors type nanoDot™ (Landauer Inc.) and RPL rod glass elements type GD-352M (Asahi Techno Glass Coorporation) are described. The main characteristics are discussed, together with the readout and calibration procedures which lead to a determination of absorbed dose to water.All dosimeter types used show very good uniformity, batch reproducibility and homogeneity. For improved accuracy, individual sensitivity correction factors should be applied for TL and OSL dosimeters while for RPL dosimeters there is no need for individual sensitivity corrections.The dose response of all dosimeters is linear for a wide range of doses.The energy response of GD-352M type dosimeters (with Sn filter) used for out-of-field measurements is flat for medium and low energy X-rays.The energy dependence for TLDs is low across the range of photon energies used and the energy correction was neglected. A significant over response of Al₂O₃:C OSLDs irradiated in kilovoltage photon beams was taken into account. The energy correction factor f_en was calculated by using the 2006 PENELOPE Monte Carlo code.With suitable calibration, all dosimeter types are appropriate for out-of-field dose measurements as well as for the in-phantom measurements of radiotherapy MV X-rays beams.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Medical dosimetry using a RL/OSL prototype

A portable and robust instrument has been developed for the routine assessment of patient exposure to ionizing radiation during radiotherapy treatments. The design principles of hardware and software are described, along with preliminary measurements that illustrate the operation of the system and its capabilities. In this study the authors used radioluminescence (RL) and Optically Stimulated Luminescence (OSL) from Al₂O₃:C detectors coupled to a PMMA optical fibre to acquire dose in medical dosimetry. The RL/OSL prototype can provide two independent dose estimates from the same in vivo treatment: one integrated dose estimate (OSL) and one real-time dose estimate (RL), which can be compared to one another. The authors first characterized the dose–response to a calibration source (¹³⁷Cs), analysing the OSL and the RL signal to doses from 0.5 to 3 Gy. Later the percentage dose depth from RL is presented for two gamma (6 and 15 MV) and two electron (6 and 12 MeV) medical beams.     

Optical depth of traps in AL2O3:C determined by the variable energy of stimulation OSL (VES-OSL) method

Standard methods of OSL measurements (CW-OSL or LM-OSL) do not allow for the direct determination of optical depth of traps. The variable energy of stimulation optically stimulated luminescence (VES-OSL) method gives such possibility. It consists in optical stimulation with the continuous increase of stimulation light energy and is analogous to the glow curve method in TL measurements. The VES-OSL curve shape and maximum position can be regulated by the stimulation photon flux, the rate of stimulation energy increase and by measurement temperature. This allows for detecting the OSL from very deep traps that give the TL signal overlapping with strong incandescence. The VES-OSL measurements carried out for Al₂O₃:C showed that traps having the optical depth between 2.0 and 2.8 eV are responsible for the OSL signal related to TL peak at about 200 °C. The OSL signal from the much deeper traps from the range 2.8–3.3 eV was also detected. The TL signal related do these traps cannot be detected below 500 °C.     

White,green, and yellow photoluminescence in the (CaO · Al<Subscript>2</Subscript>O<Subscript>3</Subscript> · SiO<Subscript>2</Subscript>): Eu system

It is shown that nitrogen-laser excited (2CaO · m(Al₂O₃) · SiO₂): Eu photoluminescent phosphors obtained by direct solid-state synthesis at 1350°C emit white, green, and yellow lights when Al₂O₃ content m in the system decreases from 0.40 to 0.05. The coordinates of these colors correspond to the coordinates of light warning systems according to International Commission on Illumination (CIE) and are close to the coordinates of primary colors in the EBU and NTSC TV standards.     

OSL, RL and TL characterization of rare-earth ion doped K2YF5: Application in dosimetry

Optically stimulated luminescence (OSL) properties of K₂YF₅ crystals singly doped with different concentrations of Tb³⁺ ions and doubly doped with Tb³⁺ and Ce³⁺ or Tb³⁺ and Dy³⁺ have been investigated for the first time. Radioluminescence spectra and OSL efficiency for stimulation with different wavelengths of light have been analyzed for each compound. Also, dosimetric characteristics of the most efficient composition, namely K₂YF₅:1 at.% Tb³⁺, have been studied. Finally, the OSL signal peculiarities for K₂YF₅:1 at.% Tb³⁺ have been compared to those of a commercial Al₂O₃:C dosimeter.     

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.