Energy dependent response of Al 2 O 3 and its potential application in personal monitoring.

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1 Energy dependent response of Al 2 O 3 and its potential application in personal monitoring. Nelson, V. K. a, b a Sydney University, Faculty of Health Sciences, Discipline of Medical Imaging and Radiation Sciences, Cumberland Campus, New South Wales, Australia. b Macarthur & Liverpool Cancer Therapy Centers, Campbell town, New South Wales, Australia. Abstract. Thermoluminescent (TL) dosimetry, especially using LiF, is widely used for both personal monitoring and patient dose measurement. However, the evaluation of dose may be hampered by the energy dependence of the TL materials, which provides the highest contribution to the total uncertainty in TL dosimetry. For accurate assessment of dose, information on the spectrum of radiation and knowledge of TL response at commonly encountered photon energies is essential. The aim of this study was to measure the energy dependent response of Al 2 O 3 and LiF (TLD500 and TLD100), and explore the possibility of employing a system of dual TL detectors in personal monitors for estimation of x-ray energy to improve dose estimation. Ten TL chips of each type of above-mentioned materials were irradiated to x-ray beams of varying tube potential from 75 kvp to 300 kvp and Co60 gamma radiation. The responses were normalized to 1 mgy dose of Co60 radiation. Energy -response factors for both materials were calculated and the ratio of energy-response factors of TLD500 & TLD100 were plotted against the effective energy to investigate the sensitivity of this method to varying x-ray energy. The ratio of energy-response factors of TLD500 and TLD 100 varies sharply between 30 kev and 100keV. The method described above can be used to estimate the effective energy of radiation a TL monitor has been exposed by including TLD500 TL detector with the LiF TL detectors, commonly employed in personal monitoring. This will improve the dose estimation by applying appropriate energy correction to the response of LiF based personal monitors, which are known to exhibit energy dependency in the kilovoltage region. KEYWORDS: Thermoluminescent Monitoring. Dosimeters (TLDs); X-Ray Energy; Personal 1. Introduction Thermoluminescent dosimeters (TLDs) have proved to be useful in several gamma dose intercomparison projects [1]. Radiation protection and safety standards require that routine dose monitoring services be established for persons working with ionising radiation for risk benefit analysis. A large amount of occupational dose emanates from diagnostic radiology and such an extensive application demands accurate personnel radiation monitoring [1]. Thermoluminescent (TL) dosimetry methodology is employed for this purpose, particularly LiF: Mg, Ti, TL dosimeters, because of their near tissue equivalence and low fading rate [2]. However, the evaluation of dose may be compromised due to the energy dependence of the TL materials, and non-unity biological effect of the low energy x and gamma radiation. The TL properties of quite a number of different materials have been investigated; however, there are still only a limited number of materials with properties attractive for TL dosimetry. Lithium fluoride, LiF: Mg, Ti (TLD100) has been the most popular dosimeter for medical applications due to its near tissue equivalency and reliability [3]. One disadvantage of using LiF (TLD100) for personnel dosimetry is its energy dependent TL response [3], [4], [5]. The sources of uncertainty in TL dosimetry include energy dependence, directional dependence, and non-linearity of response, fading, exposure to light, and type of radiation [6]. However, energy and direction dependent responses provide the highest contribution to the total uncertainty in TL dosimetry [1]. This implies that the energy dependence of the response constitutes a systemic source of uncertainty to the measured dose. To minimize this uncertainty due to energy dependence, it would be ideal to perform calibration of dosimeters with the same radiation quality as 1

2 the control and field dosimeters. This is not always practical to achieve as the quality of radiation is often not known and the quality of calibration radiation and field radiation may not be the same. The calibration qualities in use are derived from narrow-spectrum and heavily filtered x-rays (ISO 1990), and are therefore different from the wider-spectrum and less filtered x-rays found in clinical situations [7]. Therefore, it is possible that the mismatch between calibration and diagnostic x-ray qualities does compromise the accuracy of personal doses in diagnostic x-ray facilities. Consequently, accurate determination of the x-ray energy would be of practical interest for cases such as personnel dosimetry, to reduce uncertainty in dose estimation. Radiation energy estimation employing two thermoluminescent materials in tandem has been suggested in previous studies [8]. However it is not clear that this technique has been proven to be valid at lower energies [8]. As a large amount of occupational dose emanates from low energy x-rays used in diagnostic radiology, a method is required which can estimate x-ray energies in the diagnostic radiology range with good accuracy. The properties of Al 2 O 3 studied by a number of workers [9], [10] indicate that this TL material would be quite useful in personal dosimetry for workers in diagnostic radiology and nuclear plant where both the doses and the radiation energy are in the lower range. In this work, Al 2 O 3 : C (TLD500) has been used in addition to LiF:Mg,Ti (TLD100), in an attempt to extend the range of the method of employing two TL dosimeters to estimate radiation energy and examine its accuracy at lower effective energies commonly encountered in diagnostic radiology facilities. 2. Materials and Methods 2.1 Dosimeter preparation Twenty dosimeters of each type, TLD100 and TLD500 (Harshaw) were annealed in an oven (SEM TLD Oven) using the annealing cycles described in Table 1. The recommended annealing temperature of C for TLD500 was not available on the TLD oven, therefore both type of dosimeters were annealed using similar cycles. After this annealing, the TLDs were cooled to room temperature before being subjected to three TL read-out cycles, Table 2. A Harshaw 5500 TLD system manufactured by Harshaw-Bicron Ltd was used to read the dosimeters in nitrogen gas environment. Ten dosimeters of each TLD type were selected on the basis of their similarities in sensitivity, reproducibility and dose response performance. Irradiation was performed with a 6 MV x-ray beam from Siemens Primus Linac and TLDs were irradiated in solid water (RMI) at 5 cm depth to a dose of 20mGy. The standard deviation for batch reproducibility of each selected TLD was matched within ±5 %. The maximum coefficient of variation of readings of each TLD batch was less than 6 %. Table 1: Annealing cycles for both TLD materials. Ramp Rate ( 0 C/Hr) Anneal Temp ( 0 C) Anneal Time (Hrs) Anneal Temp C Anneal Time (Hrs) Table 2: Reading cycles of both TLD materials. TLD Type Heating Rate ( 0 C/Hr) Pre-Heat Temp ( 0 C) Read Temp 0 C Read Time (sec) TLD TLD

3 2.2 Radiation Sources The source of orthovoltage x-ray source was an x-ray therapy machine (Pantak Therapax 300). The Pantak Therapax DXT generated eight x-ray qualities of half-value layer (HVL) ranging from 2.2 mm Al to 3.8 mm Cu ( nominal kvp). The HVL measurements were within ±0.05mm. The characteristics of x-rays used are summarized in Table 3. The source of megavoltage radiation was a Theratron Co-60 unit (Atomic Energy Canada Ltd) at the secondary standard dosimetry laboratory of the Australian Nuclear Science and Technology Organization (ANSTO). This unit is also used for calibration of dosimetry equipment. Table3: Characteristics of X-Ray Beams used in this investigation. Beam Quality 2.3 Dosimeter irradiation Tube Potential (kv) Added Filtration HVL (mm) Q mm Al 2.2 Al Q mm Al 3.2 Al Q mm Al 4.0 Al Q mmCu+2.5mm Al 7.2 Al Q mm Cu mm Al 1.70 Cu Q mm Sn+0.5 mm Cu Cu mm Al Q mm Sn+0.25 mm Cu mm Al 3.80 Cu The TLDs were irradiated on a polystyrene tray (section 2.2.4) covered with thin plastic sheet to prevent movement and soiling of dosimeters during transport and irradiation. Six TLDs for each TLD type were exposed to doses, varying from mgy from the orthovoltage therapy unit. While irradiation from lower energy beams (HVL < 5 mm) were carried out with TLDs on the surface, a 2 mm build up was used for rest of the beams. The depth of irradiation for Co-60 was 5 cm. One TLD of each type was not irradiated and therefore used as a control. The irradiations were performed simultaneously with all types of TLD within 24 hours post annealing. After each irradiation, the TLDs were kept for another 24 hours before read-out. After each TLD read-out, the dosimeters were annealed, ready for the next irradiation. The experiment was repeated three times and the average read-out values taken for each beam energy. 2.4 Energy-Response Factor The energy-response factor of a given radiation beam quality Q relative to Co-60 gamma radiation is given by F Q Co ( Dw / D ) LiF TLD Co = [11] ( Dw / D )Q 3. Results LiF TLD Table 4 shows the responses of both types of TLD materials to the orthovoltage x-ray beams and the Co-06 gamma radiation. The effective energy (E eff) is the energy of a monoenergetic radiation beam with same HVL. Table 5 shows the energy response factors of both types of materials irradiated in kilovoltage photon beams and normalized to C0-60 response. The Energy-response factor for Al 2 O 3 varies from approximately 1 for 300 kv x-rays up to 2.6 times for 100 kv x-rays and for LiF from 1.13 for 300 kv x-rays to 1.76 for 70 kv x-rays. Both of these dosimeter types generally over respond when irradiated with kilovoltage x-rays as a significant amount of interaction is due to photoelectric effect. The difference in over response of Al 2 O 3 and LiF is due to the difference in their effective 3

4 atomic number and the fact that photoelectric mass cross section is proportional approximately to the cube of the atomic number (Z 3 ) [11]. As the kilovoltage x-ray energy increases, the probability of Compton interaction increases and the energy-response factor decreases since the Compton interaction probability does not vary significantly with variation in atomic number. Table 4 and Fig. 1 show the variation of response factor ratio of Al 2 O 3 and LiF, which is largest at approximately 30keV. This ratio of response factors Al 2 O 3 and LiF varies sharply with radiation quality between 30 kev and 100 kev, where most of the diagnostic radiology work is performed. Table 4: TL response per mgy of radiation dose from both types of materials. kv HVL Eeff KeV TLD100 SD TLD500 SD mm Al mm Al mm Al mm Al mm Cu mm Cu mm Cu Cobalt Table 5: Energy response factors of Al 2 O 3 and LiF and ratio of energy response factors. KVp HVL TLD100 % SD TLD500 % SD Ratio Norm. to Co-60 % SD mm Al mm Al mm Al mm Al mm Cu mm Cu mm Cu Cobalt Figure 1: Variation of the energy-response factor ratio of Al 2 O 3 and LiF with x-ray energy. 3.0 Energy-Response Factor Ratio Al 2 O 3 /LiF (Normalised to Co-60) Effective Energy (kev) 4

5 4. Discussion The sources of uncertainty in thermoluminescence dosimetry include energy dependence, directional dependence, non-linearity of response, fading, exposure to light and type of radiation [6]. However the energy and the directional responses provide the highest contribution to the total uncertainty in thermoluminescence dosimetry [1]. This phenomenon was also noted in the personnel dosimetry using film [12]. When dealing with complex spectral sources of megavoltage radiation, the measured personal dose is rarely effected by the energy dependence of TL response [13]. However, in case of kilovoltage produced by diagnostic x-ray machines and synchrotons, the energy dependent TL respose of personal monitors cannot be neglected. The aim of this research was to find suitable TL materials that can be used in a personnel monitor for estimation of the x-ray energy a radiation worker has been exposed to and consequently improve the dose estimation. In addition to random errors an energy dependent systemic error of the order of 5%, if the energy varies from 20 kev to Cobalt-60 and much higher if the irradiation field contains mainly low energy x-rays, can be introduced in the final dose estimation [14]. In personnel dosimetry the most popular method of energy discrimination has been the fitting of metallic filters within the personal monitoring badge. The potential source of errors in dose assessment using TL dosimeters and metallic filters is thus two fold. Firstly, the error in energy discrimination using filters (±30%) [15], secondly the error due to change in response of TL dosimeters from the low energy scattered radiation originating from the filters themselves [16]. Fig. 2 compares the TL response of TLD100 measured by Muhogora et al [1] to ISO4037 specified x-ray beams, and field x- ray beams measured in preset study with same effective energy as those specified by ISO4037. This comparison gives an idea of the magnitude of potential error (up to 50%) that can be introduced by the change in x-ray energy brought about by the scattered radiation from filters fitted in a personnel monitor. Figure 2: Relative energy response of TLD100 to ISO4037 beams: Muhogora et al and present study (Q1= 31 kev; Q2= 48 kev; Q3, =65 kev; Q4, =83 kev; Q5=100keV; Q6= 120 kev; Q7, =662 kev). 1.8 Present Study Muhogora et al (2002) 1.6 Thermoluminescnt Response Normalised to Q7 Beam Q1 Q2 Q3 Q4 Q5 Q6 Q7 Effective Photon Energy (kev) The use of thermoluminescence materials with large difference thermoluminescence sensitivity and energy response for estimating the energy of incident beam was proposed by [3]. This work presents the measured energy response of highly sensitive thermoluminescence material TLD500 and TLD100 to determine the effective energy of an incident beam. The results show that by using the ratio of 5

6 thermoluminescence response of two thermoluminescence materials with different thermoluminescence sensitivity and energy dependence the energy of incident x-ray beam can be determined. The results also show that the ratios of energy-response factors of TLD500 and TLD100 vary very sharply with incident photon energy below 80 kev. When a combination of these materials is used to estimate the energy in adition to dose from kilovoltage x-ray beams, this method has an accuracy of approximately ±10%. With the recommended uncertainty (at 95% confidence level) for personal monitoring being 21% for annual dose and 45% for dose levels at the lower end of the dose range to be monitored i.e. 170 microgray for monthly doses [6], this accuracy compares favourably with the current methods employed in personnel dosimetry. A number of national and private personnel radiation monitoring agencies use personnel monitoring badges with TLD cards designed to hold two TL dosimeters. The proposed method can be implemented by incorporating additional spaces in this card to hold one each of TLD100 and TLD 500 dosimeters for energy estimation. 5. Conclusion Accurate dose estimation, essential for safe use of radiation, requires knowledge of the effective energy of the incident x-ray beams and the energy sensitivity of the chosen detector that does not have a uniform energy response. A number of methods have been described in the literature to ascertain the energy of incident radiation. All of these have their advantages and limitations. In personnel dosimetry the most popular method of energy discrimination is achieved by fitting filters within the thermoluminescent dosimeter badge. The potential source of error in dose assessment in this method is the low energy scattered radiation from the filters, which may alter the response of the thermoluminescent dosimeter. In this study the thermoluminescent response of Al 2 O 3 (TLD500) and LiF (TLD100) thermoluminescent materials to various x-ray energies has been studied and a method has been described to use these as an indicator of incident x-ray energy. This method uses the sensitivity ratios of two thermoluminescent materials as an indicator of incident photon energy. In situations where the energy of the incident radiation is not known, this method can be used to estimate the quality of radiation a worker has been exposed to. The 10% accuracy of energy estimation described in this work compares favourably with the 30% accuracy of current methods employed in either personnel dosimetry or dosimetry intercomparison. Acknowledgements I wish to thank Mr. Justin Davies of the Secondary Standard Laboratory oat ANSTO for his help in carrying out Cobalt-60 irradiations. REFERENCES [1] MUHOGORA, W. E., et al., Energy response of LiF: Mg, Ti, Ti dosimeters to ISO 4037 and typical diagnostic x-ray beams in Tanzania, J. Radiol. Prot., 22 (2002)175. [2] HOROWITZ, Y. (Ed), Thermoluminescence and Thermoluminescent dosimetry. Vol III. CRC Press, Boca Raton (1984)133. [3] KRON, T., Thermoluminescence Dosimetry and Its Applications in Medicine Part 2: History and Applications. Aus. Phy. & Engg. Sci. in Med. 18 (1995)1. [4] EDWARDS, C. R., et al., The response of a MOSFET, p-type semi-conductor and LiF TLD to quasi-monoenergetic x-rays. Phys. Med. Biol. 42(1997) [5] DAVIS, S. D. et al., The response of LiF thermoluminescent dosimeters to photon beams in the energy range 30 kv to 60 C0 gamma rays, Radiation Protection Dosimetry, 106 (2003) 33. [6] CHRISTENSEN, C. P. and GRIFFITH, R. V., Required accuracy threshold in individual monitoring, Radiation Protection Dosimetry 54 (1994) 279. [7] CURRY, T. S., DOWDEY, I. E. and MURRAY, R. C., Christensen's Physics of Diagnostic Radiology, Lea and Febiger, London (1990). [8] PUITE, K.J., A Thermoluminescence System for the Intercomparison of Absorbed Dose and Radiation Quality of X-rays with a HVL of 0.1 to 3.0 mm Cu, Phys. Med. Biol. (1976)

7 [9] AKSELROD, M.S., KORTOV, V. S. and GORELOVA, E. A., Preparation and Properties of Alpha-Al 2 O 3 : C, Radiation Protection Dosimetry 47 (1993) 159. [10] MCKEEVER, S. W. S., et al., Characteristics of Al2O3 for thermally and optically stimulated luminescence dosimetry, Radiation Protection Dosimetry, 84(1999) 163. [11] MOBIT, P., AGYINGI, E. and SANDISON, G., Comparison of the energy-response factor of LiF and Al2O3 in radiotherapy beams, Radiation Protection Dosimetry, (2006). [12] Crosby, E H. Comparison of Film Badges and Thermoluminescent Dosimeters, Health Physics, 23 (1972) 371. [13] ANGELONE, M. et al., Energy dependence of TLD-300 response from 6 kev to 1250 kev, Radiation Protection Dosimetry, 100(2002) 381. [14] OBERHOFER, M. and SCHARMANN, A. (Eds.) Applied Thermoluminescence Dosimetry, Adam Higler, Bristol, (1981). [15] KALMYKOV, L.Z., Materials for thermoluminescent dose detector and photon radiation energy detectors intended for intercomparison procedures of radiation therapy units, Medical Physics 21 (1994) [16] PRADHAN, A. S. and BAKSHI, A. K., Calibration of TLD badges for photons of energy 6 MeV and dosimetric intricacies in high energy gamma ray fields encountered in nuclear power plants, Radiation Protection Dosimetry, 98 3 (2002)

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