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.     

Real time dose rate measurements with fiber optic probes based on the RL and OSL of beryllium oxide

This work covers the examination of fiber optical probes based on the radioluminescence and real time optically stimulated luminescence of beryllium oxide. Experiments are carried out to determine the fundamental dosimetric and temporal properties of the system and evaluate its suitability for dose rate measurements in brachytherapy and other applications using non-pulsed radiation fields. For this purpose the responses of the radioluminescence and optically stimulated luminescence signal have been investigated in the dose rate range of 20 mGy/h to 3.6 Gy/h and for doses of 1 mGy up to 6 Gy. Furthermore, a new, efficient analysis procedure, the double phase reference summing, is introduced, leading to a real time optically stimulated luminescence signal. This method allows a complete compensation of the stem effect during the measurement. In contrast to previous works, the stimulation of the 1 mm cylindrical beryllium oxide detectors is performed with a symmetric function during irradiation. The investigated dose rates range from 0.3 to 3.6 Gy/h. The real time optically stimulated luminescence signal of beryllium oxide shows a dependency on both the dose rate and the applied dose. To overcome the problem of dose dependency, further experiments using higher stimulation intensities have to follow.     

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.     

Scintillation and optical stimulated luminescence of Ce-doped CaF2

Scintillation and optical stimulated luminescence of Ce 0.1–20% doped CaF₂ crystals prepared by Tokuyama Corp. were investigated. In X-ray induced scintillation spectra, luminescence due to Ce³⁺ 5d–4f transition appeared around 320 nm with typically 40 ns decay time. By ²⁴¹Am 5.5 MeV α-ray irradiation, 0.1% doped one showed the highest scintillation light yield and the light yield monotonically decreased with Ce concentrations. Optically stimulated luminescence after X-ray irradiation was observed around 320 nm under 550 or 830 nm stimulation in all samples. As a result, intensities of optically stimulated luminescence were proportional to Ce concentrations. Consequently, scintillation and optically stimulated luminescence resulted to have a complementary relation in Ce-doped CaF₂ system.     

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.     

Optimization of CW-OSL parameters for improved dose detection threshold in Al2O3:C

Continuous wave optically stimulated luminescence (CW-OSL) is relatively a simple technique that offers good signal to noise ratio (SNR) and involves simple instrumentation. This study reports the influence and optimization of CW-OSL parameters on minimum detectable dose (MDD) using α-Al₂O₃:C phosphor. It is found that at a given stimulation intensity MDD in CW-OSL mode depends on signal integration time. At lower integration times MDD is inferior. It exhibits an improvement for intermediate values, shows a plateau region and deteriorates as integration time increases further. MDD is found to be ∼127 μGy at 4 mW/cm² stimulation intensity for integration time of 0.1 s, which improves to ∼10.5 μGy for 60 s. At stimulation intensity of 72 mW/cm², MDD is 37 μGy for integration time of 60 s and improves significantly to 7 μGy for 1 s.     

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.     

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.     

The estimation of basic experimental parameters in the fine-grained quartz multiple-aliquot regenerative-dose OSL dating of Chinese loess

The sensitivity-corrected multiple-aliquot regenerative-dose (MAR) protocol provides a reliable approach for fine-grained quartz optically stimulated luminescence (OSL) dating. For reliable estimation of the equivalent dose (D_e), we investigated certain basic experimental parameters in the fine-grained quartz MAR OSL dating of Chinese loess. (1) For suitable bleaching of the natural OSL signal of the regenerative-dose aliquots, the effect of bleaching duration using sunlight, SOL2 and blue LEDs on D_e was studied, and it is found that the appropriate method is a short-duration SOL2 (e.g. 5 min) or blue LEDs (e.g. 60 s) bleaching. (2) To select the appropriate test dose, the relationship between the test dose and D_e was investigated based on three samples having D_e values of approximately 11, 31 and 137 Gy respectively. It is suggested that the test dose for sensitivity correction may be limited to less than approximately 10–20 Gy. (3) Three commonly used fitting modes for quartz OSL growth curve were compared at three regenerative-dose scales. The results indicate that the mode of two saturating exponential functions plus a constant is appropriate and universal. (4) The comparison of D_e values derived using OSL approach with those obtained using the recuperated OSL (ReOSL) protocol shows that the reliable D_e estimation in the fine-grained quartz MAR OSL dating of Chinese loess may be limited to less than approximately 300 Gy. (5) The comparison of growth curves for 18 samples from the Weinan, Xifeng and Jingyuan sites shows that it is feasible to construct a standardized growth curve (SGC) for fine-grained quartz OSL signal in the Chinese Loess Plateau (CLP).     

Yellow stimulated luminescence from potassium feldspar: Observations on its suitability for dating

Yellow stimulated luminescence (Y-OSL) is the light detected from potassium-rich feldspars at 410 nm under stimulation by a yellow light source emitting 590 nm. The investigation of this study aimed at understanding basic luminescence physics of Y-OSL in order to assess the suitability of the technique for dating. The Y-OSL signal properties tested were signal intensity, thermal assistance, thermal stability, sensitivity to daylight and the suitability of a single aliquot regenerative (SAR) protocol to be employed for equivalent dose (D_e) estimation. D_e measurements were conducted on samples of Holocene, last glacial and Tertiary age. The tests were undertaken on sedimentary feldspar separates extracted from aeolian, fluvial and coastal deposits.Results from experiments show that the signal intensity increases by measuring Y-OSL at elevated temperature suggesting thermal assistance characteristics similar to infrared stimulated luminescence (IRSL). The yellow stimulated signal remains unaffected by preheat temperatures up to ～200 °C suggesting higher thermal stability than the IRSL signal. The Y-OSL signal is less light sensitive than the IRSL signal and D_e residuals obtained from modern samples are up to 7 Gy indicating suitability of the technique for ‘older’ and well-bleached sediments. The dose recovery tests successfully recovered the given dose if the specific light sensitivity of Y-OSL is taken into account. For every sample Y-OSL D_e values obtained by a single aliquot regenerative dose protocol (SAR) are higher than those obtained by an IRSL SAR approach. From these results we infer high thermal stability and a minimal anomalous fading of the Y-OSL signal. We conclude that Y-OSL has a high potential to date Quaternary sediments that were sufficiently bleached in nature.     

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.     

Investigations into the reliability of SAR-OSL equivalent doses obtained for quartz samples displaying dose response curves with more than one component

Fast component dominated quartz single aliquot regenerative dose optically stimulated luminescence (SAR-OSL) dose response curves that display continuing growth at high doses are increasingly reported in literature. This behaviour would result in higher equivalent doses being obtained. Here we document the characteristics of OSL signals from fine (4–11 μm) and coarse (63–90 μm) quartz extracted from Romanian loess that display such behaviour. For very high doses (>1 kGy up to 5–15 kGy) the data could be closely fitted to a double saturating exponential regression model. Nonetheless, the saturation charcteristics of these fine and coarse quartz grains are very different, with average saturation chracteristic doses of D₀₁ ≈ 175 Gy and D₀₂ ≈ 1800 Gy in the case of the fine material, while in the case of the coarse material values of D₀₁ ≈ 55 Gy and D₀₂ ≈ 600 Gy have been obtained. Our results imply a hitherto unexplained mechanism in OSL production at high doses and question the reliability of obtaining SAR-OSL equivalent doses in the high dose region when a second function is needed to describe the dose response.     

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.     

Fiber-coupled radioluminescence dosimetry with saturated Al2O3:C crystals: Characterization in 6 and 18 MV photon beams

Radioluminescence (RL) and optically stimulated luminescence (OSL) from carbon-doped aluminum oxide crystals can be used for medical dosimetry in external beam radiotherapy and remotely afterloaded brachytherapy. The RL/OSL signals are guided from the treatment room to the readout instrumentation using optical fiber cables, and in vivo dosimetry can be carried out in real time while the dosimeter probes are in the patient. The present study proposes a new improved readout protocol based solely on the RL signal from Al₂O₃:C. The key elements in the protocol are that Al₂O₃:C is pre-dosed with ∼20 Gy before each measurement session, and that the crystals are not perturbed by optical stimulation. Using 6 and 18 MV linear accelerator photon beams, the new RL protocol was found to have a linear dose-response from 7 mGy to 14 Gy, and dosimetry in this range could therefore be performed using a single calibration factor (∼6 × 10⁶ counts per Gy for a 2 mg crystal). The reproducibility of the RL dosimetry was 0.3% (one relative standard deviation) for doses larger than 0.1 Gy. The apparent RL sensitivity was found to change with accumulated dose ((−0.45 ± 0.03)% per 100 Gy), crystal temperature ((−0.21 ± 0.01)%/ °C), and dose-delivery rate ((−0.22 ± 0.01)% per 100 MU/min). A temporal gating technique was used for separation of RL and stem signals (i.e. Cerenkov light and fluorescence induced in the optical fiber cable during irradiation). The new readout protocol was a substantial improvement compared with the combined RL/OSL protocol, that required relatively long readout times and where the optical stimulation greatly affected the RL sensitivity. The only significant caveat was the apparent change in RL-response with accelerator dose-delivery rate.     

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.     

The potential of luminescence signals from electronic components for accident dosimetry

This study investigated the optically stimulated luminescence of a large number of electronic components extracted from both old and new generation mobile phones and chip modules of phone cards. Most resistors and all chip modules studied present a linear dose response (R > 0.99) in the dose range investigated (200 mGy up to 6 Gy, respectively 10 Gy), while capacitors, inductors and integrated circuits generally have a non-linear growth (exponential or cubic). For our experimental setup, an average specific luminescence of ∼20,000 cts in 2 s/Gy (n = 10) and ∼6000 cts in 2 s/Gy (n = 14) was obtained for two types of chip modules with a relatively high degree of homogeneity (relative standard deviation of 23% and 31%) and a minimum detectable dose of 7 mGy for immediate measurement. The investigated signals show small sensitivity changes (generally <10%) after repeated cycles of irradiation and readout. Preliminary fading measurements are presented. It can be concluded that most mobile phones and phone card components have a significant potential as retrospective luminescence dosimeters.     

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.     

Natural and laboratory TT-OSL dose response curves: Testing the lifetime of the TT-OSL signal in nature

This study compares natural and laboratory generated thermally transferred optically stimulated luminescence (TT-OSL) dose response curves (DRCs) for fine-grain quartz extracts from the Luochuan loess section in central China. Both DRCs saturate at high doses relative to the quartz OSL signal; the natural TT-OSL DRC saturates at about 2200 Gy and laboratory DRCs saturate at about 2700 Gy. However, the natural and laboratory TT-OSL DRCs deviate from one another at circa 150 Gy resulting in TT-OSL equivalent dose underestimation relative to palaeodoses expected from dose rates and independent age control. The lifetime of the TT-OSL signal at 10 °C, calculated from values of trap parameters E and s, is compared against the value for lifetime of the TT-OSL signal in nature at average burial temperature as determined from the age underestimation caused by deviation of the natural and laboratory generated DRCs. These two independent assessments of TT-OSL signal lifetime at Luochuan give similar values, suggesting that laboratory measurements of thermal stability reflect natural burial lifetimes and can potentially be used to correct TT-OSL ages for the difference between natural and laboratory dose response curves.     

Optically stimulated luminescence in Ag doped Li2B4O7 single crystal and its sensitivity to neutron detection and dosimetry in OSL mode

Optically stimulated luminescence (OSL) measurements have been carried out on single crystals of Ag doped Li₂B₄O₇ (LTB:Ag) after exposure to various nuclear radiations. The time integrated OSL intensity is found to be linear in the range from 0.1 Gy to 500 Gy. Fading of the OSL signal was found to be around 36% in 48 h. The presence of ⁶Li and ¹⁰B has been gainfully utilized to measure doses of thermal neutrons. Further, the large difference between the wavelength of the stimulation source (∼460 nm) and emission from the LTB:Ag at 270 nm has enhanced the signal-to-noise ratio in a simple OSL set-up with suitable filters. The high sensitivity of the LTB:Ag to thermal neutrons will be useful in variety of applications including personal dosimetry in mixed-fields and imaging devices for neutron radiography.     

Radiation dose response correlation between thermoluminescence and optically stimulated luminescence in quartz

The fast, linearly modulated optically stimulated luminescence (LM-OSL) component in quartz is the main dosimetric signal used for the dating applications of this material. Since the blue light stimulation (470 nm, 40 mW cm ${}^{?2}$) time needed to obtain the fast LM-OSL component is less than 50 s the electron trapping levels responsible for it are still highly populated. In this way an active radiation history is created which could play an important role in the dosimetric characteristics of the fast OSL signal. In the present work the dose response behavior of the fast OSL signal is investigated in quartz samples with an annealed radiation history and quartz samples possessing an artificial radiation history. A computerized curve de-convolution analysis of the LM-OSL curves for 50 s stimulation time showed that it consists of three individual OSL components. The faster component C₁ with peak maximum time around 5 s has a linear dose response in virgin samples, which turns to a slight superlinearity as a function of the artificial radiation history. On the other hand the component C₂ with peak maximum time at 12 s is slightly superlinear which turns into strong superlinearity as a function of artificial radiation history. Finally, component C₃ with peak maximum time at about 45 s is strongly superlinear for both virgin samples and as a function of artificial radiation history. The implications to practical application are discussed.