Evaluation of SrAl2O4:Eu, Dy phosphor for potential applications in thermoluminescent dosimetry

Abstract Purpose To evaluate the use of commercial‐grade strontium aluminate phosphorescent powder as a thermoluminescent (TL) dosimeter for clinical radiotherapy beams. Materials and Method Commercially available Eu2+, Dy3+ co‐doped strontium aluminate powder (SrAl2O4:Eu, Dy) was annealed and then irradiated using 20 × 20 cm2 field size, with 6‐MV (PDD10 = 70.7) and 18‐MV (PDD10 = 79.4) photon beams and and 9‐MeV (R50 = 3.6), 15 MeV (R50 = 5.9) and 18‐MeV (R50 = 7.2) electron beams. To calibrate the relationship between the TL readings and the irradiated doses, TL glow curves were acquired for doses up to 600 cGy at all beam energies. For the percentage depth dose (PDD) measurement, the SrAl2O4:Eu, Dy powder was sandwiched by solid water phantoms, with varying thickness of solid water placed above to determine the depth. PDDs were measured at four representative depths and compared against the commissioning depth dose data for each beam energy. Results Linear dose response models of doses up to 200 cGy were created for all beam energies. Superlinearity was observed with doses greater than 200 cGy. The PDD measurement acquired experimentally agrees well with the commissioning data of the medical linear accelerator. Trapping parameters such as order of kinetics, activation energy and frequency factor have been obtained via TL glow curve analysis. Conclusion The linear dose response demonstrates that SrAl2O4:Eu, Dy is a potential TLD dosimeter for both electron beams and photon beams at different beam energies. The PDD measurements further support its potential use in quality assurance and radiation dosimetry.

improved. 3 Strontium aluminate co-doped with Eu 2+ and Dy 3+ ions (SrAl 2 O 4 :Eu, Dy) is one of the most popular phosphors because of its very intense and long lasting afterglow. 4,5 When a persistent phosphor is excited by ionizing radiation at room temperature, part of the absorbed energy converts to immediate luminescence and persistent afterglow (shallow traps and intermediate traps), while the rest is stored and accumulated in deeper traps that require extra stimulation such as heating to release. 2 The immediate luminescence and afterglow can be observed at room temperature. However, to investigate the energy stored in deep traps in materials, stimulation in the form of heat (thermoluminescence) or light (optically-stimulated luminescence) is required to liberate the charged carriers. 6 For a material to qualify as a thermoluminescent dosimeter (TLD), high electron-hole pair creation and trapping efficiency is required. 7 In addition, the time integration of the TL signal obtained during thermal stimulation should be proportional to the delivered dose. Several commercial TLDs have been thriving for decades such as calcium fluoride and lithium fluoride. 8 However, these popular TLDs have their own limitations. For example, the hazards identification of calcium/lithium fluoride compound is marked as skin corrosion/irritation category 2, which forbids it to have direct contact with patients and limit its application form in in vivo dosimetry. 9 The TL dosimeters are relatively economical compared to other dosimeters such as films because of their reusability. Even so, it still costs more than $10/gram for the LiF TLD powder. The exploration of lower toxicity, more cost-efficient and user-friendly TLD materials that have comparable accuracy is still of great interest.
The incorporation of the rare earth ions into the phosphor material leads to dense trapping levels suitable for TL purpose. 7,10 The effective atomic number of strontium aluminate family is quite high compared to the tissue-equivalent TLDs, making it possibly suitable for high radiation dosimetry. 11 In the past few years, the dose response of certain strontium aluminate phosphors for X-ray and Co-60 beams have been calibrated via thermoluminescence. [12][13][14] These studies have found a linear dose response relationship in the kGy range, which is much greater than the doses of concern for in vivo dosimetry in radiation therapy.
The purpose of this study is to investigate the thermoluminescence properties of irradiated SrAl 2 O 4 :Eu, Dy phosphor to explore its use in radiation dosimetry, for both electron and photon beams within the in vivo dosimetry dose range. The SrAl 2 O 4 :Eu, Dy phosphor is widely used as a glow in the dark pigment and can be directly painted onto the skin without side effects. It is a great candidate for TL dosimeter not only because of its nontoxicity and $0.3/gram price but also because of its real-time phosphorescent emission, which can potentially offer real-time fluorescence monitoring and will be studied in subsequent studies. Dose response experiments are performed using clinically commissioned electron and photon beams of different beam energies. Percentage depth dose (PDD) is measured to verify its potential use in quality assurance (QA) routines. To the best of our knowledge, the trapping parameters such as order of kinetics, activation energy, and frequency factor of the SrAl 2 O 4 :Eu, Dy phosphor have been calculated for the first time to get a better understanding of its phosphorescence mechanism.

2.B.1 | Dose calibration curve
For the dose calibration measurements, d max depth was set for each beam with an SSD of 100 cm for convenient dose conversion from monitor units to cGy. The d max value for each beam is shown in Table 1

2.B.2 | PDD measurement
For the PDD measurements, three additional depths along the commissioned PDD curves were explored besides d max for each beam energy ( Table 2). SrAl 2 O 4 :Eu, Dy powder was irradiated at 20 × 20 cm 2 field size with a fixed monitor unit of 200 using the Elekta Synergy. The measurements were then compared against the commissioned PDD curves for the Elekta.

2.C | Read out
The SrAl 2 O 4 :Eu, Dy samples were stored in a dark room for more than 24-hours post-irradiation to allow the non-TL light emission to decay. A Harshaw TLD reader (Model 3500, Waltham, MA) was used to read out the dose response of the samples. A linear temperature ramp from 20°C to 350°C with a high temperature hold for 60 seconds was applied. When the reading is finished, the SrAl 2 O 4 :Eu, Dy powder can be recycled and re-annealed to be used again (Fig. 1).

3.B | PDD measurement
The PDDs at several representative depths (Table 2) were measured for each beam quality. All the experimentally obtained PDDs agree with the commissioning data of the Elekta Synergy, with most of the subgroups having less than 5% standard deviation among the six trials (Fig. 3)  Due to the high gradient in the electron beams, a comparison of R 50 (half-value depth in water) would likely be more prudent. 16 Using linear interpolation of the experimentally measured data, the R 50 of 9 -, 15-, and 18-MeV electron beams are calculated to be 3.62, 5.79, and 7.04 cm, respectively; while those from the commissioning data are 3.6, 5.9, and 7.2 cm.

3.C | Trapping parameters calculation
Since trapping parameters are dominated by the properties of the trapping centers, the determination of the trapping parameters using the glow curves is helpful for understanding the nature of the SrAl 2 O 4 :Eu, Dy phosphor. 7 From measurements, we found that even at different dose levels, with different beam qualities, the glow curves had the same peak shape. This is expected because the photon beams are indirectly ionizing radiation; it is the induced secondary beta particles that excite the sample. 16  The calculation of the trapping parameters starts with calculating the symmetry factor µ g : where δ is the temperature difference between the higher half intensity temperature T 2 and the temperature of the glow maximum T m , δ = T 2 −T m , ω is the full width at half maximum that is calculated from the temperature difference between T 2 and the lower half intensity temperatureT 1 , ω = T 2 −T 1 . 6 All the temperature units are in Kelvin (K). From the glow curve (Fig. 4), δ is calculated to be 60.4 K, while ω is 111.4 K, which gives a µ g of 0.54. A second order peak is characterized by µ g = 0.52. 17 Thus, a µ g of 0.54 is close enough for us to conclude that it is second order kinetics.
Assume that the frequency factor is independent of the temperature, the activation energy for a second-order peak can be calculated based on the measurement of T m and ω: | 195 where k is the Boltzmann constant in eV/K. 17 With a T m of 448.1 K and a ω of 111.4 K, the activation energy equals 0.47 eV.
The frequency factor s is also defined as the "attempt-to-escape frequency," which is calculated from the activation energy: Where β is the heating rate and E is the activation energy. 17  and plastic, which could be molded to conform to a patient's or a phantom's contour. Its price is also 30 times lower than the popular LiF TLD powder, making it more suitable for daily clinical use. Even though it is not tissue-equivalent, the correction of a reading to the dose in tissue can be made as long as the radiation quality is known. 21 The phosphorescence property of the

CONF LICT OF I NTEREST
No conflicts of interest.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.