Introduction
Optically stimulated luminescence (OSL) or thermoluminescence (TL) refer to phenomena whereby a material previously subjected to irradiation releases a luminescent signal in response to the application of an appropriate optical or thermal stimulus [1-4]. TL is a well-established technology that is widely used in various dosimetric applications. These include personal, workplace, and environmental dosimetry; medical dosimetry; imaging of ionizing radiation; archaeological and geologic dating; and assessment of radiation accidents [1, 5]. Currently, thermoluminescent dosimeters (TLDs) are predominantly used to assess individual radiation exposure [6-8]. LiF:Mg, Cu, P (GR-200A) is the most prominent example of this approach [9-13].
The basic principles of TL and OSL methods are the same except that the former uses heat to read the signal, whereas the latter uses light stimulation to read the signal [14]. The distinctive stimulation patterns associated with the TL and OSL methods provide the latter with certain advantages [2, 15]. A major advantage of the OSL method over the TL method is the ability to read signals at room temperature without heating the detector. The dosimeters can be used in combination with plastic adhesives at room temperature. In the OSL mode, the intensity of the excitation light and the excitation time can be controlled to read out only a portion of the signal at a time [14]. This is in contrast to the TL mode, in which all signals are read simultaneously. It is possible to perform multiple readout sessions and achieve a rapid readout by reading only a portion of the signal simultaneously. In the InLight reader, each detector was stimulated for a duration of 1 s. To provide multiple readings, a percentage of the OSL signal was discharged with weak stimulation, with a value of only 0.07%. In contrast, a value of 0.25% was obtained with strong stimulation [14]. Recently, several Individual Monitoring Service (IMS) laboratories have transitioned from using TLD to OSL dosimeters [16].
In view of the benefits offered by OSL technology, numerous research teams have addressed the development of new OSL materials; however, only two commercially available OSL dosimeters based on Al2O3:C and BeO have been reported. In contrast, the number of TL dosimeters is significantly higher [1]. Interestingly, α-Al2O3:C is an ideal OSL material with high stability and sensitivity. Many methods have been adopted to prepare α-Al2O3:C OSL dosimeters; however, only α-Al2O3:C OSL dosimeters prepared using crystal growth technology have the highest OSL sensitivity [17-22]. The growth of carbon-doped alumina crystals is susceptible to fluctuations in the growth conditions, which can result in considerable variations in the properties of α-Al2O3:C detectors between samples and batches [14]. To enhance the uniformity of OSL dosimeters, commercially available Luxel+ and InLight OSL dosimeters were manufactured by blending α-Al2O3:C powders with polyester and depositing them on a transparent polyester film [14]. This resulted in the initial dose of the linear range of the dose response of the product being worse than that of the material itself. It has been reported that BeO exhibits a pronounced exponential decline in sensitivity upon initial use [23], accompanied by signal fading following radiation exposure [24].
Many countries are developing novel and enhanced OSL materials. Many materials with OSL properties have been reported [25-29]. However, they have not been widely adopted in commercially available dosimeters because of different inherent limitations. One of them is high fading. In particular, the high fading exhibited by OSL materials used in personal dosimetry and environmental monitoring applications is unacceptable because it inevitably leads to substantial dosimetry inaccuracies. The minimal dose of detectable Li2B4O7:Ag is as low as 15 μGy; however, its CW-OSL signal decreased by approximately 30% over a 24-h period after irradiation [30]. Following one day of radiation exposure, the OSL signals of CaSO4:Dy exhibited a decay of approximately 50%, whereas those of CaSO4:Eu exhibited a decay of approximately 10% [31, 32]. The OSL signals from Mg4SiO4:Tb and LiAl2O4:Tb exhibited a decline in intensity between 30% and 40%, spanning approximately 20 h [33]. In a study on NaMgF3 doped with Ce, Tm, and Tb, the OSL response decreased by 20%, 13%, and 35%, respectively, after irradiation for 1 d, 60 h, and 5 d [34, 35]. The minimum detectable doses (MDDs) for MgO:Li,Ce, and Sm are found to be as low as 0.2 μGy. However, a 15% fading in the OSL signals was observed within the initial hour [36].
LiMgPO4 is one of the numerous new OSL materials with excellent dosimetric properties that have been investigated by research teams worldwide [26, 37-50]; however, its main drawback is high fading. A recent study summarized the OSL fading behavior of LiMgPO4. Various conditions, including changes in the dopant species, impurity concentrations, preparation processes, and prereadouts to eliminate the effects of low-temperature peaks, have been used to minimize the fading of LiMgPO4 phosphors. It has been observed that fading can be reduced by preheating or optical treatment. However, the application of preheating or optical treatment resulted in a significant reduction in OSL intensity, with a reduction of 60% to 70% [51]. Similarly, the fading of LiMgPO4:Tb, Sm, B was reduced from 30% without pretreatment to approximately 7% by preheating at 160 °C for 30 s after 30 days of irradiation [52].
One day after irradiation, the fading of LiMgPO4 doped with B alone was reduced from 15% to 30% for LiMgPO4:Tb, B [53]. The low-temperature peak of LiMgPO4:B is still very high, and the fading is still unable to meet the requirements of personal dose monitoring. By varying the preparation temperature, the fading of the LiMgPO4:Tb, Sm, and B phosphors was significantly reduced. The residual OSL signal was as high as 92.3% after 24 h of irradiation [47].
The fading of the OSL signals in LiMgPO4:Er after irradiation has been reported to be reduced by improving the preparation process [41]. It was found that the fading of LiMgPO4:Er samples heated to 900 °C for 10 h was less than 5%, which was observed within 30 d after irradiation when the integration time was greater than or equal to 20 s. However, the fading was greater than 5% when the integration time was less than 20 s.
To gain insight into the features of energy transfer in doped LiMgPO4, LiMgPO4:RE (RE–Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm) was synthesized [39]. Both LiMgPO4:Er and LiMgPO4:Gd exhibit broad emission bands at 360 nm in their TL spectra, and the intensity of the latter is higher than that of the former [39]. However, the OSL characteristics of LiMgPO4:Gd were not studied.
Significant efforts have been made to develop new OSL materials with properties comparable to those of Al2O3:C, including a simple synthetic process, easily available raw materials, low cost, and suitability for mass production. The main aim of this study was to improve the time stability of the OSL signal of LiMgPO4 to achieve fast and multiple OSL readouts. Five samples of LiMgPO4:Gd phosphor powder were prepared using five different preparation methods.
The effects of the preparation process on the OSL decay curves, TL glow curves, OSL sensitivity, MDD, and fading characteristics of LiMgPO4:Gd phosphors after 24 h of irradiation were investigated. The impact of annealing on the OSL sensitivity, relative residual OSL signals measured after 24 h of irradiation, and MDD of LiMgPO4:Gd phosphors heated to 900 °C for 15 h were also investigated. The fundamental characteristics of LiMgPO4:Gd annealed at 350 °C for one hour were examined.
Materials and methods
Material
A simple solid-state reaction method was used to synthesize LiMgPO4:Gd materials. The following reagents were weighed according to a stoichiometric ratio of Li2CO3 (lithium carbonate, 99.99%), 3MgCO3·Mg(OH)2·3H2O (magnesium carbonate, 99.9%), and NH4(H2)PO4 (ammonium dihydrogen phosphate, 99.99%) thoroughly mixed. An appropriate quantity (0.25 mol%) [39] of Gd2O3 was added to the mixture, which was then mixed thoroughly.
Five distinct processes were employed to prepare the LiMgPO4:Gd phosphors. The synthesis of the LiMgPO4:Gd phosphor was divided into three stages. The two initial stages of the five preparatory processes for LiMgPO4:Gd were identical, with only the third stage differing. In the first phase of the procedure, the mixture was placed in an alumina crucible and pushed into the center of the tube furnace. It was then heated from room temperature to 300 °C for 30 min, after which it was maintained at 300 °C for 2 h. The crucible was then removed from the furnace and rapidly cooled to room temperature. The samples were manually pulverized and sieved. In the second step, the material obtained in the first step was subjected to a temperature increase from 300 to 650 °C for 30 min, after which it was maintained at 650 °C for 2 h. The third step of the first three preparations involved heating the material obtained in the second step from 650 to 900 °C for 30 min. This was followed by additional periods of 10, 15, or 20 h at 900 °C, respectively. In the third step of the preparation process for 4 and 5, the material obtained in the second step was heated from 650 to 950 °C and 1000 °C, respectively, for 30 min, and then maintained at 950 °C and 1000 °C for 2 h, respectively.
For comparison, an α-Al2O3:C detector (TLD-500K) with a 5 mm diameter and 0.9 mm thickness was used.
Characterizations
The powder structure was characterized by powder X-ray diffraction (XRD; Bruker D8 Advance). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed to confirm the presence of Gd in the structure using a Hitachi SU8600.
The Risø TL/OSL reader TL/OSL-DA-20 was used to measure the OSL decay curves and TL glow curves of the samples, as well as the β-source irradiation. Many studies have been published on this TL/OSL reader; therefore, it will not be described in detail here [41, 52]. A continuous-wave OSL (CW-OSL) method was used for OSL measurements. The stimulus power was 90% and the duration was 88 s. TL glow curves were measured from room temperature to 450 °C at a heating rate of 5 °C s-1.
To test the effect of the preparation process on the OSL decay curves, TL glow curves, OSL sensitivity, MDD, and fading characteristics of the LiMgPO4:Gd phosphors, the five types of samples for the five preparation processes were fully bleached using the TL readout method, which involves pretreatment at a ramp rate of 5 °C per second, followed by a readout temperature of 450 °C; then, their OSL background signals were measured. A 90Sr/90Y β source was used to irradiate the samples for 2 s at a nominal dose rate of 50 μGy s-1 to measure the OSL decay curve, TL glow curve, and fading signal 24 h after irradiation.
To ascertain the effect of annealing temperature on the OSL sensitivity, MDD, and fading of LiMgPO4:Gd 24 h after irradiation, LiMgPO4:Gd powders synthesized at 900 °C for 15 h were subjected to annealing treatment at temperatures between 350 °C and 500 °C at intervals of 50 °C for an hour. The background signal was quantified immediately following the annealing of the samples, which were subjected to irradiation at ambient temperature for 2 s at a nominal dose rate of 50 μGy s-1 using a 90Sr/90Y β source integrated into the Risø reader. Subsequently, OSL signals were recorded. The samples were fully bleached using the TL readout method, which entailed a pretreatment with a temperature ramp rate of 5 °C, followed by a readout to 450 °C, and then irradiated at ambient temperature using a 90Sr/90Y β source integrated into the Risø reader at a nominal dose rate of 50 μGy s-1 for 2 s, Subsequently, the OSL signal was recorded 24 h after irradiation.
To experimentally assess the long-term fading of LiMgPO4:Gd, a sample of LiMgPO4:Gd synthesized at 900 °C for 15 h was used. Before irradiation, the samples were annealed at 350 °C for one hour. To investigate the kinetic order of LiMgPO4:Gd, the samples were completely bleached. This involved pretreatment at a rate of 5 °C per second, followed by a readout temperature of 450 °C, irradiation with a dose of 0.1 Gy using a β-in-situ irradiator, and subsequent reading of the TL glow curves at 450 °C. The heating rate of the TL readings was set to 5 °C, and this was repeated six times at irradiation doses of 0.2, 0.5, 1, 2, 5, and 10 Gy.
To experimentally assess the long-term fading of LiMgPO4:Gd, the sample was irradiated with a dose of 50 μGy from a 60Co gamma source. The OSL signals were read at 0 days (approximately 40 min) and 1, 2, 5, 10, 20, and 30 days after irradiation.
To study the reusability of LiMgPO4:Gd, five samples of LiMgPO4:Gd, each weighing 10 mg, were extracted and placed at five distinct positions within the TL/OSL reader. The samples were fully bleached using the TL readout method, which entailed a pretreatment with a temperature ramp rate of 5 °C per second, followed by a readout to 450 °C, and then irradiated at ambient temperature using a (90Sr)/90Y β source integrated into the Risø reader at a nominal dose rate of 50 μGy s-1 for 2 s before reading out the OSL signal; ten such cycles were performed.
Results
X-ray diffraction (XRD)
Figure 1 shows the XRD patterns of LiMgPO4:Gd samples produced through five distinct preparation processes in conjunction with Inorganic Crystal Structure Database (ICSD) Card No. 201138. The measured diffraction patterns agreed with the reference pattern with respect to the positions of the majority of reflections. However, the differences in sensitivity may also be attributable to variations in sample preparation.
Elemental Analysis
Figure 2 depicts the map sum spectra of LiMgPO4:Gd samples produced using five distinct preparation processes. Figure 2 illustrates the presence of gadolinium in the structure.
Effect of preparation processes
The effects of the five preparation processes on LiMgPO4:Gd phosphors are shown in Fig. 3. The OSL decay curves, normalized by the maximum of the LiMgPO4:Gd phosphors produced using the five preparation processes, are shown in Fig. 3a. The LiMgPO4:Gd phosphor heated to 900 °C for 15 h exhibited the slowest decay. In contrast, the LiMgPO4:Gd phosphor heated to 1000 °C for 2 h exhibited the fastest decay. The remaining three curves exhibit relatively similar decay profiles.
The normalized OSL signals of the five LiMgPO4:Gd phosphors prepared by the five processes, compared to TLD-500k for various integration times, are shown in Fig. 3b. The OSL signals were averaged over five readings for each data point. All standard deviations were less than 10%. Note that the OSL signals of the LiMgPO4:Gd phosphors from the five preparation processes are lower than those of the TLD-500k phosphors for the different integration times. The highest OSL sensitivity was obtained for the LiMgPO4:Gd phosphor heated to 900 °C for 15 h.
The relative residual OSL signals of the five LiMgPO4:Gd phosphors prepared using the five preparation methods with different integration times measured after 24 h of irradiation compared to those measured immediately after irradiation are shown in Fig. 3c. The OSL signals were averaged over five readings for each data point. For the sample heated to 1000 °C for 2 h, the relative residual OSL signals measured after 24 h of irradiation were approximately 0.701–0.88, while the relative residual OSL signals measured after 24 h of irradiation for the other four samples were stable for the different integration times.
The OSL MDDs of the five LiMgPO4:Gd phosphors prepared using the five processes are presented in Fig. 3d. The preparation process has a significant impact on the MDD. Relatively low MDDs were observed in the samples heated to 950 °C for 15 h and different integration times.
Typical TL glow curves and TL integrated signals of the five LiMgPO4:Gd phosphors prepared using the five processes are presented in Figs. 3e and 3f. For clarity, Fig. 3e shows the TL signal between 50 °C and 300 °C because of the minimum TL signal between 300 °C and 450 °C. The LiMgPO4:Gd phosphor heated to 900 °C for 15 h exhibited the highest TL sensitivity. In contrast, the LiMgPO4:Gd phosphor heated to 1000 °C for 2 h exhibited the lowest TL sensitivity.
Considering its high sensitivity, low fading, and MMD, the LiMgPO4:Gd phosphor heated to 900 °C for 15 h is concluded to be optimal. The following study is conducted only on the LiMgPO4:Gd phosphor.
Effect of annealing on LiMgPO4:Gd
The effect of annealing on LiMgPO4:Gd phosphors is shown in Fig. 4. Figure 4a shows the OSL signals of four distinct types of LiMgPO4:Gd phosphors, each subjected to annealing processes at temperatures of 350, 400, 450, and 500 °C for 1 h. The single unannealed LiMgPO4:Gd phosphor was normalized to TLD-500k at various integration times. Each data point in the figure corresponds to the mean value of the OSL signals for the five samples. The standard deviations of these values were less than 10%. The OSL sensitivity of LiMgPO4:Gd can be enhanced by annealing. The LiMgPO4:Gd phosphor annealed at 350 °C for 1 h exhibited the highest OSL sensitivity. It is observed that as the annealing temperature exceeds 350 °C, the OSL sensitivity decreases by the rise in annealing temperature. The initial and total OSL signals were found to be essentially identical to those of TLD-500k at an annealing temperature of 350 °C. The initial OSL signal of the LiMgPO4:Gd phosphor annealed for 1 h at 350 °C relative to that of TLD-500k was approximately 0.98.
Figure 4b depicts the relative residual OSL signals after 24 h of the irradiation to the signals immediately after irradiation for four distinct types of LiMgPO4:Gd phosphors annealed at 350, 400, 450, and 500 °C for 1 h, and a single unannealed LiMgPO4:Gd phosphor. The highest residual OSL signal was observed for the unannealed phosphor. For annealed phosphors, the OSL signal can be considered stable when the integration time is less than 10 s.
Figure 4c depicts the OSL MDD for four distinct types of LiMgPO4:Gd phosphors annealed at 350, 400, 450, and 500 °C for 1 h, and a single unannealed LiMgPO4:Gd phosphor. The OSL MDD of LiMgPO4:Gd was significantly reduced by annealing. The OSL MDD of LiMgPO4:Gd phosphors annealed for one hour at 350 °C was as low as 2.2 μGy when the integration time was one second, and 16.2 μGy when the integration time was 0.1 s.
Considering the high sensitivity, low fading, and MDD, annealing the LiMgPO4:Gd phosphor at 350 °C for 1 h was optimal. The following study is conducted only on the LiMgPO4:Gd phosphor.
Basic properties of LiMgPO4:Gd
TL dose response and kinetic order
The effects of dose, heating rate, and OSL readings on the TL of the LiMgPO4:Gd phosphor and OSL decay curve are shown in Fig. 5. In Fig. 5a, the glow curves for the LiMgPO4:Gd phosphors were recorded under irradiation conditions of 0.1–10 Gy; the phosphors were initially maintained at room temperature and subsequently heated to 450 °C at a constant rate of 5 °C. For clarity, Fig. 5a shows the TL signal between 50 °C and 250 °C, because of the minimum TL signal between 450 °C and 450 °C. The peak temperature of LiMgPO4:Gd was found to be consistently positioned at 131.4 ± 1.1 °C for varying irradiation doses. The peak shift method [5] indicates that the peak of the TL glow curve in the LiMgPO4:Gd phosphor represents a first-order phenomenon because the temperature at which the peak occurs remains constant. The dose responses of the LiMgPO4:Gd phosphors are shown in Fig. 4b. Over the investigated dose range of 0.1–10 Gy, the LiMgPO4:Gd phosphors exhibited an excellent linear response (Fig. 5b).
TL kinetics parameters
Figure 5c shows five glow curves for the LiMgPO4:Gd phosphors that were evaluated and recorded at ramping rates of 0.2 to 5 °C. The samples were irradiated with 0.1 Gy and subjected to an increase in temperature from room temperature to 450 °C. For clarity, the TL signal between 50 and 250 °C is shown in Fig. 4c, because of the minimum TL signal between 250 °C and 450 °C. As the heating rate increased from 0.5 °C to 5 °C, the peak temperature gradually increased and the peak and total integral TL signals decreased.
The variable heating rate (VHR) method [5] was used to obtain the kinetic parameters of the peaks in the glow curve. This method requires only peak temperature information. Therefore, it can be applied to peaks with well-defined peak temperatures. However, this method can only be applied to peaks with first-order kinetics. The kinetic parameters of LiMgPO4:Gd can be obtained using the VHR method because the peak of the TL glow curve in this sample appears to be a first-order phenomenon.
The theory of TL kinetics proposes that in first-order kinetics, the following relationship exists between the peak temperature (Tm) and heating rate (β) [54]:
When plotting the natural logarithm of
TL after OSL
Figure 5e shows a comparison between the measured TL glow curve immediately following radiation exposure and the TL glow curve following the OSL readout. The peak height of the TL glow curve of the LiMgPO4:Gd sample measured immediately was approximately 14 times greater than that of the TL glow curve measured after the use of OSL. The total area of the TL peak after the OSL measurements was 8.5% of the area recorded immediately after irradiation over a temperature range of 50-250 °C. It can be observed that the OSL signal present in LiMgPO4:Gd is derived from the principal TL glow peak. Furthermore, the majority of traps that lead to this signal can be eliminated using OSL readings.
OSL decay curve
The decay curves per unit mass for LiMgPO4:Gd and TLD-500k are compared in Fig. 5f. As the measurement time increased, LiMgPO4:Gd began to decrease faster and then TLD-500k decreased faster. Therefore, the sensitivity of LiMgPO4:Gd relative to that of TLD-500k decreased and then increased as the measurement time increased. During this investigation, the elimination of approximately 2.2% and 2.9% of the OSL signals, respectively, was observed in the case of LiMgPO4:Gd and TLD-500k following each readout at a readout time of 0.1 s.
OSL long-term fading
Table 1 presents the relative residual OSL signals of the LiMgPO4:Gd phosphors as functions of time after irradiation and integration time. For a maximum integration time of 5 s, the OSL signal was stable, with no fading 30 days after radiation exposure. The data points represent the mean values of the OSL signals obtained from five aliquots.
| Integration time (s) | 0 day | 1 day | 2 days | 5 days | 10 days | 20 days | 30 days |
|---|---|---|---|---|---|---|---|
| 0.1 | 1.000 | 1.000 | 1.007 | 0.994 | 0.985 | 0.972 | 0.979 |
| 0.2 | 1.000 | 1.004 | 1.013 | 1.009 | 1.007 | 0.981 | 1.011 |
| 0.5 | 1.000 | 1.004 | 1.013 | 1.018 | 1.022 | 1.024 | 1.037 |
| 1 | 1.000 | 1.000 | 1.006 | 1.012 | 1.021 | 1.036 | 1.036 |
| 2 | 1.000 | 0.992 | 0.996 | 1.000 | 1.008 | 1.028 | 1.018 |
| 5 | 1.000 | 0.978 | 0.977 | 0.973 | 0.973 | 0.998 | 0.972 |
| 10 | 1.000 | 0.962 | 0.955 | 0.941 | 0.932 | 0.965 | 0.921 |
| 20 | 1.000 | 0.940 | 0.926 | 0.904 | 0.883 | 0.930 | 0.867 |
| 50 | 1.000 | 0.915 | 0.896 | 0.868 | 0.839 | 0.899 | 0.822 |
| 88 | 1.000 | 0.911 | 0.892 | 0.863 | 0.834 | 0.894 | 0.817 |
Reusability
Table 2 presents the repeatability of the normalized OSL signals for the LiMgPO4:Gd phosphors with different integration times. The coefficient of variation for the repeated cycles of the LiMgPO4:Gd samples at different integration times was less than 0.8%, demonstrating the excellent reproducibility of the powders.
| Integration time (s) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Mean | COV |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.1 | 1.000 | 0.996 | 0.993 | 0.988 | 0.996 | 0.987 | 0.983 | 0.980 | 0.978 | 0.980 | 0.988 | 0.79% |
| 0.2 | 1.000 | 0.997 | 0.994 | 0.991 | 0.996 | 0.986 | 0.985 | 0.982 | 0.982 | 0.979 | 0.989 | 0.75% |
| 0.5 | 1.000 | 0.994 | 0.992 | 0.991 | 0.993 | 0.985 | 0.984 | 0.981 | 0.980 | 0.979 | 0.988 | 0.72% |
| 1 | 1.000 | 0.996 | 0.993 | 0.993 | 0.995 | 0.988 | 0.987 | 0.984 | 0.983 | 0.982 | 0.990 | 0.62% |
| 2 | 1.000 | 0.997 | 0.995 | 0.994 | 0.996 | 0.990 | 0.990 | 0.987 | 0.985 | 0.985 | 0.992 | 0.52% |
| 5 | 1.000 | 0.997 | 0.995 | 0.994 | 0.996 | 0.992 | 0.991 | 0.988 | 0.987 | 0.988 | 0.993 | 0.44% |
| 10 | 1.000 | 0.997 | 0.996 | 0.995 | 0.997 | 0.994 | 0.992 | 0.990 | 0.989 | 0.989 | 0.994 | 0.38% |
| 20 | 1.000 | 0.998 | 0.997 | 0.996 | 0.997 | 0.994 | 0.992 | 0.991 | 0.991 | 0.990 | 0.995 | 0.35% |
| 50 | 1.000 | 0.998 | 0.997 | 0.996 | 0.997 | 0.995 | 0.992 | 0.991 | 0.991 | 0.991 | 0.995 | 0.34% |
| 88 | 1.000 | 0.998 | 0.997 | 0.996 | 0.998 | 0.996 | 0.992 | 0.991 | 0.991 | 0.991 | 0.995 | 0.34% |
Discussion
Sensitivity
The sensitivity of OSL materials to radiation is one of their most important properties. Low doses must be measured in personal, workplace, and environmental dosimetry, and OSL materials must have high sensitivity. The sensitivity and MDD of integrated passive detectors for personal, workplace, environmental photon, and beta radiation monitoring are not specified in the IEC 62387 standard. Instead, they only correspond to the dose linearity floor of the entire dosimetry system, which is mandatory at 0.1 mSv. However, to assess the sensitivity of a novel material, it is common practice to compare the response signal of such a material with that of a well-known material used commercially in dosimetry systems [1], measured under identical conditions.
The two principal advantages of OSL dosimetry over TL dosimetry are the speed of readout and the capacity for multiple readouts. The former enhances the processing capacity of many dosimeters, whereas the latter safeguards against data loss and ensures the preservation of evidence. To satisfy the demand for rapid and multiple readouts, it is necessary to read only a minimal quantity of the OSL signal at any given time. To provide multiple readings, the InLight OSL Reader removes a minimal amount of the OSL signal per reading, with a weak stimulus resulting in a 0.07% reduction and a strong stimulus resulting in a 0.25% reduction [14].
In this study, LiMgPO4:Gd eliminated approximately 2.2% of the OSL signal at each readout, with a readout time of 0.1 s. This method can also be used for multiple readouts. For LiMgPO4:Gd in this study, to eliminate the OSL signal by 0.07% per readout, the stimulus had to be reduced. Relative to TLD-500k, the initial OSL signal, i.e., the sensitivity of the LiMgPO4:Gd phosphor annealed for 1 h at 350 °C with a readout time of 0.1 s, was approximately 0.98, which is sufficient for low-dose measurements in personal, workplace, and environmental dosimetry.
Fading
The change in the OSL response of an irradiated material over time is referred to as OSL fading. A variety of new OSL materials have been investigated; however, most exhibit severe fading and are not routinely used for dosimetry. OSL signal fading is more difficult to solve than TL signal fading, which is a combination of OSL signals from different traps with different fading characteristics. OSL signal fading cannot be solved by splitting the spectrum into different peaks and removing unstable peaks as in TL signal fading. The problem of OSL signal fading can also be solved by preheating in the same manner as the problem of TL signal fading. However, this approach appears to contradict the premise that the OSL method is suitable for timely processing of a large number of dosimeters. Development of an OSL material that stabilizes the signal over a long period after irradiation is the best approach to solve the problem of OSL signal fading.
The OSL signal fading can be significantly improved by modifying the preparation process. In the case of MgB4O7:Ce,Li phosphor, the observed fading of OSL signals following the two preparation processes differed significantly. One study reported that the OSL signals of MgB4O7:Ce,Li phosphors exhibited fading of approximately 10–15% over the course of 72 h post-irradiation [55], while another observed that the OSL signals of MgB4O7:Ce,Li phosphors synthesized through distinct routes exhibited fading of less than 1% over a period of 40 d [56]. In the case of LiMgPO4:Tb,B, the fading of powder samples was reported to be approximately 20-25% after a period of three weeks following irradiation, although the crystalline samples exhibited a greater degree of fading, exceeding 50% within a period of 24 h following irradiation [42]. For LiMgPO4:Er, the loss of OSL signal intensity after irradiation for 24 h was approximately 40-85% for samples heated to 1000 °C for a period of 2 h, whereas the loss of signal after irradiation for the same duration was below 5% for samples heated to 900 °C for a period of 10 h, when the integration time was above 0.5 s [41]. For LiMgPO4:Gd in this study, the relative residual OSL signals, measured after 24 h of irradiation, ranged from 0.701 to 0.880 for the sample heated for 2 h at 1000 °C. In contrast, the relative residual OSL signals measured after 24 h of irradiation for the other four samples were stable at different integration times. The dependence of the fading on the preparation process was less pronounced for LiMgPO4:Gd than for LiMgPO4:Er.
A reduced number of studies have addressed the relationship between OSL fading and integration time [41]. Signal fading in the LiMgPO4:Er samples was significantly dependent on the integration time. The signal loss in LiMgPO4:Er over a period of 30 d of radiation exposure was reported to be less than 5% when the integration time was greater than 20 s. In contrast, the signal loss on day 30 was approximately 12% when the integration time was 0.1 s [41]. No fading of the OSL signal of LiMgPO4:Gd in this study was observed within 30 days of irradiation when the integration times were less than five seconds, on the 30th day, a total OSL signal fading of approximately 18% was observed. To prevent fading, the integration time (i.e., readout time) of LiMgPO4:Er must be at least 20 s [41]. However, a readout time of 20 s is considerably longer than the typical requirement for a fast OSL readout and is not optimal for multiple readouts. The integration time (i.e., readout time) of LiMgPO4:Gd in this study was as low as 0.1 s, which is sufficient for fast and multiple OSL readouts.
Superiority of LiMgPO4:Gd
The properties and economics of α-Al2O3:C (TLD-500K) and LiMgPO4:Gd are compared in Table 3. Based on the results of this study (Figs. 3b, 4a, and 5f), LiMgPO4:Gd presents no significant advantages over the commercial dosimeter material α-Al2O3:C (TLD-500K). With a comparable sensitivity to OSL and an effective atomic number (Table 3) and fading to satisfy fast and multiple measurements, LiMgPO4:Gd has the advantages of easy preparation and low cost.
| α-Al2O3:C (TLD-500K) | LiMgPO4:Gd | |
|---|---|---|
| Initial OSL sensitivity | 1.000 | 0.980 |
| Total OSL sensitivity | 1.000 | 0.991 |
| Effective atomic number | 11.28 | 11.44 |
| Manufacturing | Difficulty | Easy |
| Cost | High | Low |
LiMgPO4:Gd is significantly easier to prepare than α-Al2O3:C. One disadvantage of α-Al2O3:C is its high melting point (2050 °C), which presents a challenge for its synthesis, and the harsh growth conditions caused by carbon doping, which can limit its optimization scope. The optimal conditions for the preparation of LiMgPO4:Gd are heating to 900 °C for 15 h.
The growth of carbon-doped alumina crystals is susceptible to fluctuations in the growth conditions, which result in considerable variations in the properties of different parts of the same crystal, as well as between different crystals. To improve the homogeneity of OSL dosimeters, commercially available Luxel+ and InLight OSL dosimeters were produced by blending α-Al2O3:C powders with polyester and depositing them on a transparent polyester film. This results in an initial dose that is situated within the linear range of the dose-response curve, exhibiting a considerably inferior response to that of the material itself, which is partly due to the fact that the OSL sensitivity of α-Al2O3:C powders is much lower than that of α-Al2O3:C crystals. The use of LiMgPO4:Gd for the preparation of dosimeters, such as those of Inlight OSL dosimeters, may confer certain advantages in terms of sensitivity. LiMgPO4:Gd exhibits properties similar to those of Al2O3:C. It is easy to synthesize, readily available, inexpensive, and suitable for large-scale production.
Conclusions
In this study, five samples of LiMgPO4:Gd phosphor powder were prepared via five different production processes using a solid-state reaction preparation method. X-ray diffraction was used to determine the structures of the five prepared samples. The effects of the preparation process on the OSL decay curves, TL glow curves, OSL sensitivity, MDD, and fading characteristics of the LiMgPO4:Gd phosphors after 24 h of irradiation were investigated. The highest OSL sensitivity was obtained for the LiMgPO4:Gd phosphor heated to 900 °C for 15 h. For the sample heated to 1000 °C for 2 h, the relative residual OSL signals measured after 24 h of irradiation were approximately 0.701-0.88, while the relative residual OSL signals measured after 24 h of irradiation for the other four samples were stable for different integration times. The dependence of fading on the preparation process was found to be less pronounced for LiMgPO4:Gd than for LiMgPO4:Er. Considering its high sensitivity, low fading, and MDD, the LiMgPO4:Gd phosphor heated to 900 °C for 15 h is optimal.
The effect of annealing on OSL sensitivity, relative residual OSL signals measured after 24 h of irradiation, and MDD of LiMgPO4:Gd phosphors heated to 900 °C for 15 h were also investigated. The OSL sensitivity of LiMgPO4:Gd was enhanced by annealing. The LiMgPO4:Gd phosphor annealed at 350 °C for 1 h exhibited the highest OSL sensitivity. For the annealed samples, the OSL signal can be considered stable when the integration time is less than 10 s. The OSL MDD of LiMgPO4:Gd is significantly reduced by annealing. Considering its high sensitivity, low fading, and MDD, annealing at 350 °C for 1 h is optimal.
The basic properties of LiMgPO4:Gd annealed at 350 °C for 1 h were investigated. Over the investigated dose range from 0.1 to 10 Gy, LiMgPO4:Gd phosphors exhibited an excellent linear response. The peak in the TL glow curve of the LiMgPO4:Gd sample represents a first-order phenomenon. E and s were 1.44 eV and 4.82E+17 s-1, respectively. The OSL signal of LiMgPO4:Gd was derived from the principal TL glow peak. At a maximum integration time of 5 s, the OSL signal was stable, with no fading 30 d after radiation exposure.
LiMgPO4:Gd eliminated approximately 2.2% of the OSL signal at each readout at a readout time of0.1 s, which is sufficient for fast and multiple OSL readout. The sensitivity of the LiMgPO4:Gd phosphor annealed for 1 h at 350 °C with a reading time of 0.1 s, was found to be approximately 98% of that observed for TLD-500k, which is sufficient for low-dose measurements in personal, workplace, and environmental dosimetry.
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