- Email: [email protected]
A personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detectors for photon fields
ARTICLE IN PRESS
Applied Radiation and Isotopes 59 (2003) 87–93
A personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detectors for photon fields Haiyong Junga,*, Kun Jai Leea, Jang-Lyul Kimb a
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea b Health Physics Department, Korea Atomic Energy Research Institute (KAERI), P.O. Box 105, Yuseong, Daejeon 305-600, South Korea
Abstract A new personal thermoluminescence (TL) dosimeter for photon fields using LiF:Mg,Cu,Na,Si TL detector was developed by taking advantage of its dosimetric properties including energy dependencies. Solid pellet type LiF:Mg,Cu,Na,Si detector was developed and fabricated at Korea Atomic Energy Research Institute (KAERI) and has been studied on its dosimetric properties such as TL grow curve, dose response, energy response and reusability. Its dosimetric properties show the feasibility of application of LiF:Mg,Cu,Na,Si TL detector to personal dosimetry fields. A new dosimeter using LiF:Mg,Cu,Na,Si TL detector was designed and tested through irradiation experiments. This multi-element TL dosimeter allows the measurement of a personal dose equivalent Hp ðdÞ in photon fields. Based on the experimental results of the proposed dosimeter, it was demonstrated that a personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detector is appropriate to estimate personal dose equivalent for wide range energy of photon fields. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: LiF:Mg,Cu,Na,Si; Thermoluminescence; Personal dosimeter; Filter system
1. Introduction Thermoluminescence dosimetry is the most widely used technology for evaluating the personal and environmental radiation exposure. Lithium fluo ride (LiF) is a well-known thermoluminescent (TL) dosimeter used in environmental and personal monitoring due to its high sensitivity, stability and tissueequivalency. The TLD material based on LiF that has been studied most extensively is LiF:Mg,Ti, which is widely used in personal dosimetry and available on the market under trade names like TLD-100 and its variations, TLD-600 and TLD-700 which contain different concentrations of lithium isotopes (Vij, 1993). Since the introduction of LiF:Mg,Ti (TLD-100) many new types of TL materials *Corresponding author. Tel.: +82-42-869-3858; fax: +8242-869-3810. E-mail address: [email protected] (H. Jung).
have been developed and used to evaluate the personal dose equivalent in various radiation fields. Research towards developing more sensitive and more effective thermoluminescence materials has been accomplished (Wang et al., 1986; Zha et al., 1993; Horowitz, 1993; Bilski et al., 1997; Bos, 2001). In Korea, Doh et al. (1989) introduced powder type of lithium fluoride doped with magnesium, copper, sodium and silicon, and accomplished the study on its characteristic about a wide range of dopant concentration in the 1980s. After the proposal of LiF doped with four dopants by Doh et al. and Kim et al. (1989) investigated the main dosimetric properties of powder type LiF:Mg,Cu,Na,Si which the response for low energy photons was higher than that of LiF:Mg,Cu,P. These researches were, however, just the introduction and the feasibility study of new TL material. In the 1990s, more concrete researches on the powder type LiF:Mg,Cu,Na,Si TL phosphor have been accomplished by Korea Atomic Energy Research Institute (KAERI) (Nam et al., 1998,
0969-8043/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-8043(03)00120-9
ARTICLE IN PRESS H. Jung et al. / Applied Radiation and Isotopes 59 (2003) 87–93 9000
Relative TL Intensity (a. u.)
1999). Powder type TL phosphor has about twice the sensitivity in comparison with LiF:Mg,Cu,P (Nam et al., 1999). However, powder type TL phosphor has many disadvantages for the practical handling of the material. Therefore, it is needed to develop the suitable shaped solid type TL detector for application of practical dosimetry fields. During the last few years the LiF:Mg,Cu,Na,Si TL material has been studied for practical pellet type TL detector by Nam et al., the dosimetry group of health physics department in KAERI (Nam et al., 2000). But the sensitivity of developed pellet type TL detectors was not exceed 50% of Chinese GR-200A, and had poor reusability that is 10% decrease of the readout values after a reuse of 8 times (Nam et al., 2001). Based on these previous studies by Nam et al. after all, the sensitivity and reusability of the pellet type LiF:Mg,Cu,Na,Si TL detectors was been improved by the modification of dopants concentration and parameters of preparing procedure. The optimum concentration of dopants for pellet type LiF:Mg,Cu,Na,Si TL detectors was investigated as Mg: 0.2 mol%, Cu: 0.05 mol%, Na: 0.9 mol% and Si: 0.9 mol% (Lee et al., 2002). The objective of this study is to design and develop the personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detector by taking advantage of its sensitivity, tissueequivalency and energy dependencies to allow the measurement of personal dose equivalent, Hp ðdÞ: Through the radiation exposure test for this TL detector, design properties of dosimeter were optimized and proposed, and then the dosimeter developed by proposed design was evaluated the energy response characteristics for 20–662 keV photon range, in terms of TL output per unit Hp ð10Þ and Hp ð0:07Þ; as defined by ICRU (1993).
Newly Developed LiF:Mg,Cu,Na,Si Chinese GR-200A (LiF:Mg,Cu,P)
8000 7000 6000 5000 4000 3000 2000 1000 0 50
100
150
200
250
300
Temperature (°C)
Fig. 1. Glow-curve and comparison TL intensity of newly developed LiF:Mg,Cu,Na,Si and Chinese GR-200A (Mg,Cu,P). Note that intensity of LiF:Mg,Cu,Na,Si is approximately 15% higher than that of GR-200A.
LiF:Mg,Ti (TLD-100) LiF:Mg,Cu,Na,Si (KAERI) LiF:Mg,Cu,P (MCP-N)
1.4
Relative Response
88
1.2
1.0
0.8
0.6 10
100
Effective Photon Energy (keV)
2.1. Sintered LiF:Mg,Cu,Na,Si TL detector
Fig. 2. Photon energy response of LiF:Mg,Cu,Na,Si TL detector, LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (MCP-N, Poland) in bare condition. Responses are normalized to 662 keV photons from a 137Cs source.
The TL detector used in this study is solid pellet type LiF:Mg,Cu,Na,Si detectors in shape of disk having a diameter of 4.5 mm and a thickness of 0.8 mm, white in color, were developed and fabricated through cold pressing and sintering the powder at 825 C at Korea Atomic Energy Research Institute (KAERI). It is made from TL powder with concentration of dopants: Mg: 0.2 mol%, Cu: 0.05 mol%, Na: 0.9 mol% and Si: 0.9 mol% (Lee et al., 2002). The total signal and glow curve structure of LiF:Mg,Cu,Na,Si TL detector were measured and analyzed and then compared with commercialized LiF:Mg,Cu,P TL detector (GR-200A, China). The typical measured glow curves of both TL detectors were represented in Fig. 1. This figure shows that the sensitivity of developed
LiF:Mg,Cu,Na,Si TL detector is about 15% higher than that of LiF:Mg,Cu,P TL detector (GR-200A, China) that is widely used in the world. Energy dependence of TL material is a key characteristic factor and play an important role in designing the dosimeter. In Fig. 2 typical photon energy dependence of LiF:Mg,Cu,Na,Si TL detector and other common TL detectors in bare condition are presented. The energy response as a function of the mean photon energy is normalized to 662 keV photons from a 137Cs source. The energy dependence of LiF:Mg,Ti (TLD-100) detector and that of LiF:Mg,Cu,P (MCP-N) detectors were simultaneously presented to compare the results of LiF:Mg,Cu,Na,Si TL detector. The data of LiF:Mg,Ti
2. Materials and methods
ARTICLE IN PRESS H. Jung et al. / Applied Radiation and Isotopes 59 (2003) 87–93
was referred to the reference of Horowitz (1984) and that of LiF:Mg,Cu,P (MCP-N) detectors was referred to the research by Budzanowski et al. (2001). In the research by Budzanowski et al. (2001), he has presented the energy response of LiF:Mg,Cu,Na,Si TL detector ahead of being modified and being set the optimum dopants concentration to improve the sensitivity and reusability and that of LiF:Mg,Cu,P (MCP-N). Therefore, the energy dependence of LiF:Mg,Cu,Na,Si detector in this study differs from the result of Budzanowski. While the energy response of LiF:Mg,Ti is different from two others, the energy response between LiF:Mg,Cu,P and LiF:Mg,Cu,Na,Si has a similar curve. However, LiF:Mg,Cu,Na,Si TL detector shows that it reduces the variation of response between 30 and 662 keV, compared with MCP-N. 2.2. Irradiation experiments The radiation exposure test of the energy response for TL detectors before designing the dosimeter and the verification for proposed and developed dosimeter were carried out at KAERI using 137Cs source and X-ray beam (ANSI X-ray beam code M30, M60, M100, M150, H150) produced to meet the Korea national X-ray test standard which is based on ANSI N13.11 standard (ANSI, 1993; Kim et al., 1997). Specifications for each X-ray techniques used in this experiment are given in Table 1. The reference X-ray fields constructed in KAERI are consisted of two type of X-ray machines, the low energy system and the medium system. Low energy X-ray system is 3.0 kW grade (Pantak, HF-75c) and beryllium plate of 1.0 mm is installed as inherent filters. Medium energy X-ray system is 3.2 kW grade (Phillips, MG325) and inherent filter consists of Be (4 mm)+Al (1.5 mm). In the first stage of experiment, irradiation test was accomplished in free air condition without a phantom to look into the energy dependence of the TL detectors in case of behind various filters. This experiment is to select the filter material and to optimize the filter configuration of the TL dosimeter. The TL detector and filters are placed at 200 cm away from the radiation source.
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The irradiation test geometry of the proposed TL dosimeter after design the LiF:Mg,Cu,Na,Si TL dosimeter and the experimental conditions are shown in Fig. 3. The dosimeter was modeled at 200 cm away from the radiation source, mounted at the center of a tissue equivalent phantom, facing towards the source. The phantom was assumed as a homogeneous polymethylmethacrylate (PMMA) slab with a mass density of 1.19 g/cm3, with a front profile of 30 30 cm2, and a thickness of 15 cm. After the irradiation experiment, the TELEDYNE TLD Reader System 310 with resistive heating systems was used for readout.
3. Results and discussion 3.1. Energy response of LiF:Mg,Cu,Na,Si TL detector under the filters Analysis and investigation of the various dosimetric properties and characteristics for this new TL detector are indispensable for its application to the practical personal dosimeter. The photon energy response, especially, is more important property than any other properties to develop the dosimeter. Therefore, in this study the energy response of sintered LiF:Mg,Cu,Na,Si detector developed at the KAERI was tested through
Fig. 3. A schematic geometrical configuration of irradiation experiments of the dosimeter.
Table 1 Properties of X-ray technique used for irradiation experiments at KAERI X-ray Beam Code
ANSI ANSI ANSI ANSI ANSI
M30 M60 M100 M150 H150
Eeff (keV)
19.4 35.2 51.2 73.0 118.3
H.V. (kVp)
30 60 100 150 150
Current (mA)
10 5 3 3 20
Ka (mGy/h)
842.4 326.6 193.7 230.4 31.5
Conversion coefficient (Sv/Gy) Hp ð10Þ=Ka
Hp ð0:07Þ=Ka
0.44 1.17 1.61 1.90 1.80
1.03 1.38 1.59 1.76 1.68
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filter or thin filter. The result shows that copper is adequate for the discrimination of photon energy because it could appropriately attenuate the low energy photon in terms of energy response. Table 2 shows the dosimetric properties of LiF:Mg,Cu,Na,Si TL detector in terms of energy response and the fundamental data for the development of a personal radiation dosimeter using this TL detector.
the experiment in case of being placed behind various filters before the design and proposal of the dosimeter. Filters used in this experiment were selected in consideration of the common filter material of widely used LiF TLD. This response results are the basic data for selecting the filter materials and these results is helpful to develop the TL dosimeter holder. This experiment for energy response of LiF:Mg,Cu,Na,Si detector behind various filter is carried out in-air without phantom to investigate the energy response of this TL detector for various candidate filter materials. Table 2 shows the photon energy response of the LiF:Mg,Cu,Na,Si TL detector with various filters. As mentioned above, various filter materials used in this experiments are based on the holder of common LiF TL dosimeters. As shown in Table 2, the energy response shows a decreasing trend with the increase of atomic number of filter material in low energy photon. In case of thin plastic filter, Teflon 1 mm thickness, the energy response for ANSI X-ray beam M30 is lower than that of bare pellet without filter. In case of high atomic number filter or thick filter, the energy response below 100 keV is much lower than that of low atomic number
3.2. Design and fabrication of LiF:Mg,Cu,Na,Si dosimeter The personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detector was designed and fabricated based on the results of energy response of TL detector in bare condition and with various filter materials. The configurations of the dosimeter are tabulated in Table 3 and the picture of LiF:Mg,Cu,Na,Si TL detectors and developed new dosimeter are presented in Fig. 4. A typical TLD card consists of four LiF:Mg,Cu,Na,Si TL detectors of radius 2.25 mm circle, encapsulated between two sheets of Teflon 0.05 mm (10 mg/cm2) thick and mounted on an aluminum substrate. The dosimeter
Table 2 Photon energy response of LiF:Mg,Cu,Na,Si TL detector with various filter materials X-ray Beam Code
Photon energy response of LiF:Mg,Cu,Na,Si Plastic (Teflon)
ANSI ANSI ANSI ANSI ANSI 137 Cs
M30 M60 M100 M150 H150
Aluminum
Copper
Tin
1 mm
4 mm
0.5 mm
2 mm
0.1 mm
0.5 mm
0.5 mm
0.63 0.98 1.03 0.96 0.91 1.00
0.30 0.81 1.02 0.93 0.87 1.00
0.44 0.91 1.00 1.00 0.90 1.00
0.12 0.54 0.88 0.93 0.91 1.00
0.07 0.46 0.84 0.96 0.98 1.00
0.02 0.12 0.41 0.74 0.97 1.00
0.02 0.06 0.21 0.52 0.97 1.00
Table 3 Characteristic of filter system for proposed TL dosimeter Area
Filter material
Density (g/cm3)
Thickness of filter (mm)
Total density thickness (mg/cm2)
A1
Teflon (card)
2.2
0.05
10
A2
Teflon (card) ABS plastic PTFE
2.2 1.04 2.2
0.05 1.0 4.0
995
A3
Teflon (card) ABS plastic Copper
2.2 1.04 8.96
0.05 1.0 0.5
563
A4
Teflon (card) ABS plastic
2.2 1.04
0.05 3.0
312
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1.4
Design Limit Relative Response
1.2
1.0
0.8
0.6
10
100
1000
Effective Photon Energy (keV)
Fig. 4. LiF:Mg,Cu,Na,Si TL detector and the developed TL dosimeter.
case is made of 1.0 mm thick ABS (polyacrylonitrilebutadiene-styrene) plastic material which is one of the most reliable and stable plastic among many plastic materials in condition of radiation field. The effect of the plastic dosimeter case was also considered in the analysis of the holder design in each area of the dosimeter. Each detector/filter combination performs a specific function, as follows: For open window area A1, a TLD pellet with 0.05 mm Teflon encapsulation determines the shallow dose. The filtration for this element is 10 mg/cm2. No additional filter material is needed to measure the shallow dose in A1 area since the density thickness of Teflon encapsulation of TL pellet is 10 mg/cm2. Another TLD with 995 mg/cm2 combined PTFE/ABS filtration (104 mg/cm2 ABS+880 mg/cm2 PTFE filters) measures the deep dose in A2 area. The third TLD covered with 104 mg/cm2 ABS plastic and 450 mg/cm2 copper filtration is used for low energy photon discrimination. The last TLD is covered with 312 mg/cm2 ABS plastic for energy discrimination for beta particles.
3.3. Energy dependence of proposed dosimeter with respect to dose equivalent, Hp ðdÞ As mentioned in a previous section, the new dosimeter designed in this study has four main regions for photon radiation dosimetry. Each of these areas contains a typical set of filters to estimate a personal dose equivalent, Hp ðdÞ and dosimetric information. The energy dependence of proposed TL dosimeter was obtained to verify the ability to evaluate the dose equivalent and to present more information such as response ratio. The deep dose area (A2) was designed to measure the photon dose at a tissue depth of 10 mm, Hp ð10Þ: As mentioned in advance, PTFE plastic filter with its about
Fig. 5. Energy response of the deep dose area (A2) with personal dose equivalent, Hp ð10Þ: Results are relative to 137Cs. This area (A2) provide relative response values in the range between 0.78 and 1.08 over the photon energy range from 20 to 662 keV.
4 mm thickness was utilized based on experimental results and the consideration about other requirements of filter systems. To meet the purpose of this area, the filters used should be able to cutoff beta radiation. The experimental response results of dosimeter in the deep dose are shown in Fig. 5. The responses are normalized to the 137Cs response. Each relative response value represents mean value of eight dosimeters. As shown in Fig. 5, A2 area appears to provide relative response values in the range between 0.78 and 1.08 over the photon energy range from 20 to 662 keV. This is within the 730% design limits required by the ISO (1984) standard. Given the responses in terms of absorbed dose with Hp ð10Þ are proportional to the dosimeter reading, therefore, without application of any sophisticated algorithm this design would enable us to determine the deep dose equivalent more easily. In Fig. 6, the energy response for the open window area in terms of Hp ð0:07Þ are shown. The responses are normalized to the 137Cs response. These area response values are between 0.82 and 1.16 over the photon energy range from 20 to 662 keV. This is also within the design limit of the ISO standard. 3.4. Angular dependence The angular dependence for the deep dose area, A2 of the LiF:Mg,Cu,Na,Si TL dosimeter in terms of Hp ð10Þ for two X-ray beams with a effective energy of 35.2, 118.3 and 662 keV photon from 137Cs is shown in Fig. 7. The responses are normalized to normal incidence (0 ). The response for each angle of incidence represents the mean of four measurements, and the error bars correspond to 1 SD.
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1.4
Design Limit
Relative Response
1.2
1.0
0.8
0.6
10
100
1000
Effective Photon Energy (keV)
Fig. 6. Energy response of the open window area (A1) with personal dose equivalent, Hp ð0:07Þ: Results are relative to 137Cs. This area (A1) provides relative response values in the range between 0.82 and 1.16 over the photon energy range from 20 to 662 keV.
1.0 0.9 0.8
Response
0.7 0.6 0.5
137
Cs (662keV) ANSI X-ray H150 (118.3keV) ANSI X-ray M60 (35.2keV)
0.4 0.3 0.2 0
20
40
60
80
100
Angle of incidence (deg)
Fig. 7. The angular dependence for the Hp ð10Þ element of the LiF:Mg,Cu,Na,Si TL dosimeter, normalized to response at normal incidence (0), for three X-ray beams with a effective energy of 35.2, 118.3 and 662 keV from 137Cs. The response for each angle of incidence represents the mean of four measurements, and the error bars correspond to 1 SD.
As shown in Fig. 7, for 137Cs a slight decrease in angular response was seen within the 60 , but the response after 60 decreased rapidly and reached 0.8 at 90 . As expected, the high-energy photon of 137Cs has less angular dependence than the low energy photon such as X-ray beam M60.
4. Conclusion A personal dosimeter based on a newly developed TL material LiF:Mg,Cu,Na,Si was developed for personal
radiation monitoring. It turned out that this TL dosimeter with LiF:Mg,Cu,Na,Si TL detector is able to estimate personal dose equivalent value, Hp ðdÞ; based on the irradiation experiments for energy response of dosimeter design proposed by pre-experiment approach. In view of energy response results, the proposed TL dosimeter meets the 730% design limits required by the ISO (1984) standard and it shows good characteristics for a personal radiation monitoring. The TL dosimeter proposed in this study has, therefore, a feasibility to be utilized in common dosimetry market. Although the feasibility of the LiF:Mg,Cu,Na,Si TL dosimeter was demonstrated in some radiation fields, more elaborating works for the dosimeter such as beta discrimination and dose calculation algorithm, etc., are needed to make the proposed TL dosimeter more stable and reliable before being applied for routine dose measurements. References American National Standard Institute, 1993. American National Standard for Dosimetry—Personal dosimetry performance criteria for testing, ANSI N13.11. Bilski, P., Budzanowski, M., Olko, P., 1997. Dependence of LiF:Mg,Cu,P (MCP-N) glow curve structure on dopant composition and thermal treatment. Radiat. Prot. Dosim. 69, 187–198. Bos, A.J.J., 2001. High sensitivity thermoluminescence dosimetry. Nucl. Instrum. Methods Phys. Res. B. 184, 3–28. Budzanowski, M., Kim, J.L., Nam, Y.M., Chang, S.Y., Bilski, P., Olko, P., 2001. Dosimetric properties of sintered LiF:Mg,Cu,Na,Si TL detectors. Radiat. Meas. 33, 537–540. Doh, S.H., Chu, M.C., Chung, W.H., Kim, H.J., Kim, D.S., Kang, Y.H., 1989. Preparation of LiF (Mg,Cu,Na,Si) phosphor and its thermoluminescent characteristics. Korean Appl. Phys. 2, 425–431. Horowitz, Y.S., 1984. Thermoluminescence and Thermoluminescent Dosimetry. CRC Press Inc., Boca Raton, FL. Horowitz, Y.S., 1993. LiF:Mg, Ti versus LiF:Mg, Cu, P: the competition heat up. Radiat. Prot. Dosim. 47 (1), 135–141. ICRU, 1993. Quantities and units in radiation protection dosimetry, ICRU Report 51. ISO, 1984. Personal and Environmental Thermoluminescence Dosimeters, ISO Standards DP 8034. Kim, H.J., Chung, W.H., Doh, S.H., Chu, M.C., Kim, D.S., Kang, Y.H., 1989. Thermoluminescence dosimetric properties of LiF(Mg,Cu,Na,Si). J. Korean Phys. Soc. 22, 415–420. Kim, J.L., Kim, B.H., Chang, S.Y., Lee, J.K., 1997. Establishment of ANSI 13.11 X-ray radiation fields for personal dosimetry performance test by computational and experiment. Environ. Health Perspect. 105 (6), 1417–1422. Lee, J.I., Kim, J.L., Yang, J.S., Kim, B.Y., Nam, Y.M., 2002. Development of highly sensitive LiF:Mg,Cu,Na,Si TL detector. The First Asian and Oceanic Congress for Radiation Protection (AOCRP-1). Nam, Y.M., Kim, J.L., Chang, S.Y., Kim, G.D., 1998. The study of glow curves for LiF:Mg, Cu, Na, Si phosphor with different dopant concentrations. Ungyong Mulli 11, 578–583.
ARTICLE IN PRESS H. Jung et al. / Applied Radiation and Isotopes 59 (2003) 87–93 Nam, Y.M., Kim, J.L., Chang, S.Y., 1999. Dependence of glow curve structure on the concentration of dopants in LiF:Mg,Cu,Na,Si phosphor. Radiat. Prot. Dosim. 84, 231–234. Nam, Y.M., Kim, J.L., Chang, S.Y., Doh, S.H., 2000. Fabrication of LiF:Mg, Cu, Na, Si pellets by sintering process. Saemulli 40 (3), 189–193. Nam, Y.M., Kim, J.L., Chang, S.Y., 2001. Dosimetric properties of LiF:Mg,Cu,Na,Si TL pellets. J. Korean Assoc. Radiat. Prot. 26 (1), 1–6.
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Vij, D.R., 1993. Thermoluminescent Material. Prentice-Hall, Englewood Cliffs, NJ. Wang, S., Chen, G., Wu, F., Li, Y., Zha, Z., Zhu, J., 1986. Newly developed highly sensitive LiF (Mg,Cu,P) TL chips with high signal-to-noise ratio. Radiat. Prot. Dosim. 14 (3), 223–227. Zha, Z., Wang, S., Shen, W., Zhu, J., Cai, G., 1993. Preparation and characteristics of LiF:Mg,Cu,P thermoluminescent material. Radiat. Prot. Dosim. 47 (1/4), 111–118.
Applied Radiation and Isotopes 59 (2003) 87–93
A personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detectors for photon fields Haiyong Junga,*, Kun Jai Leea, Jang-Lyul Kimb a
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea b Health Physics Department, Korea Atomic Energy Research Institute (KAERI), P.O. Box 105, Yuseong, Daejeon 305-600, South Korea
Abstract A new personal thermoluminescence (TL) dosimeter for photon fields using LiF:Mg,Cu,Na,Si TL detector was developed by taking advantage of its dosimetric properties including energy dependencies. Solid pellet type LiF:Mg,Cu,Na,Si detector was developed and fabricated at Korea Atomic Energy Research Institute (KAERI) and has been studied on its dosimetric properties such as TL grow curve, dose response, energy response and reusability. Its dosimetric properties show the feasibility of application of LiF:Mg,Cu,Na,Si TL detector to personal dosimetry fields. A new dosimeter using LiF:Mg,Cu,Na,Si TL detector was designed and tested through irradiation experiments. This multi-element TL dosimeter allows the measurement of a personal dose equivalent Hp ðdÞ in photon fields. Based on the experimental results of the proposed dosimeter, it was demonstrated that a personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detector is appropriate to estimate personal dose equivalent for wide range energy of photon fields. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: LiF:Mg,Cu,Na,Si; Thermoluminescence; Personal dosimeter; Filter system
1. Introduction Thermoluminescence dosimetry is the most widely used technology for evaluating the personal and environmental radiation exposure. Lithium fluo ride (LiF) is a well-known thermoluminescent (TL) dosimeter used in environmental and personal monitoring due to its high sensitivity, stability and tissueequivalency. The TLD material based on LiF that has been studied most extensively is LiF:Mg,Ti, which is widely used in personal dosimetry and available on the market under trade names like TLD-100 and its variations, TLD-600 and TLD-700 which contain different concentrations of lithium isotopes (Vij, 1993). Since the introduction of LiF:Mg,Ti (TLD-100) many new types of TL materials *Corresponding author. Tel.: +82-42-869-3858; fax: +8242-869-3810. E-mail address: [email protected] (H. Jung).
have been developed and used to evaluate the personal dose equivalent in various radiation fields. Research towards developing more sensitive and more effective thermoluminescence materials has been accomplished (Wang et al., 1986; Zha et al., 1993; Horowitz, 1993; Bilski et al., 1997; Bos, 2001). In Korea, Doh et al. (1989) introduced powder type of lithium fluoride doped with magnesium, copper, sodium and silicon, and accomplished the study on its characteristic about a wide range of dopant concentration in the 1980s. After the proposal of LiF doped with four dopants by Doh et al. and Kim et al. (1989) investigated the main dosimetric properties of powder type LiF:Mg,Cu,Na,Si which the response for low energy photons was higher than that of LiF:Mg,Cu,P. These researches were, however, just the introduction and the feasibility study of new TL material. In the 1990s, more concrete researches on the powder type LiF:Mg,Cu,Na,Si TL phosphor have been accomplished by Korea Atomic Energy Research Institute (KAERI) (Nam et al., 1998,
0969-8043/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-8043(03)00120-9
ARTICLE IN PRESS H. Jung et al. / Applied Radiation and Isotopes 59 (2003) 87–93 9000
Relative TL Intensity (a. u.)
1999). Powder type TL phosphor has about twice the sensitivity in comparison with LiF:Mg,Cu,P (Nam et al., 1999). However, powder type TL phosphor has many disadvantages for the practical handling of the material. Therefore, it is needed to develop the suitable shaped solid type TL detector for application of practical dosimetry fields. During the last few years the LiF:Mg,Cu,Na,Si TL material has been studied for practical pellet type TL detector by Nam et al., the dosimetry group of health physics department in KAERI (Nam et al., 2000). But the sensitivity of developed pellet type TL detectors was not exceed 50% of Chinese GR-200A, and had poor reusability that is 10% decrease of the readout values after a reuse of 8 times (Nam et al., 2001). Based on these previous studies by Nam et al. after all, the sensitivity and reusability of the pellet type LiF:Mg,Cu,Na,Si TL detectors was been improved by the modification of dopants concentration and parameters of preparing procedure. The optimum concentration of dopants for pellet type LiF:Mg,Cu,Na,Si TL detectors was investigated as Mg: 0.2 mol%, Cu: 0.05 mol%, Na: 0.9 mol% and Si: 0.9 mol% (Lee et al., 2002). The objective of this study is to design and develop the personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detector by taking advantage of its sensitivity, tissueequivalency and energy dependencies to allow the measurement of personal dose equivalent, Hp ðdÞ: Through the radiation exposure test for this TL detector, design properties of dosimeter were optimized and proposed, and then the dosimeter developed by proposed design was evaluated the energy response characteristics for 20–662 keV photon range, in terms of TL output per unit Hp ð10Þ and Hp ð0:07Þ; as defined by ICRU (1993).
Newly Developed LiF:Mg,Cu,Na,Si Chinese GR-200A (LiF:Mg,Cu,P)
8000 7000 6000 5000 4000 3000 2000 1000 0 50
100
150
200
250
300
Temperature (°C)
Fig. 1. Glow-curve and comparison TL intensity of newly developed LiF:Mg,Cu,Na,Si and Chinese GR-200A (Mg,Cu,P). Note that intensity of LiF:Mg,Cu,Na,Si is approximately 15% higher than that of GR-200A.
LiF:Mg,Ti (TLD-100) LiF:Mg,Cu,Na,Si (KAERI) LiF:Mg,Cu,P (MCP-N)
1.4
Relative Response
88
1.2
1.0
0.8
0.6 10
100
Effective Photon Energy (keV)
2.1. Sintered LiF:Mg,Cu,Na,Si TL detector
Fig. 2. Photon energy response of LiF:Mg,Cu,Na,Si TL detector, LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (MCP-N, Poland) in bare condition. Responses are normalized to 662 keV photons from a 137Cs source.
The TL detector used in this study is solid pellet type LiF:Mg,Cu,Na,Si detectors in shape of disk having a diameter of 4.5 mm and a thickness of 0.8 mm, white in color, were developed and fabricated through cold pressing and sintering the powder at 825 C at Korea Atomic Energy Research Institute (KAERI). It is made from TL powder with concentration of dopants: Mg: 0.2 mol%, Cu: 0.05 mol%, Na: 0.9 mol% and Si: 0.9 mol% (Lee et al., 2002). The total signal and glow curve structure of LiF:Mg,Cu,Na,Si TL detector were measured and analyzed and then compared with commercialized LiF:Mg,Cu,P TL detector (GR-200A, China). The typical measured glow curves of both TL detectors were represented in Fig. 1. This figure shows that the sensitivity of developed
LiF:Mg,Cu,Na,Si TL detector is about 15% higher than that of LiF:Mg,Cu,P TL detector (GR-200A, China) that is widely used in the world. Energy dependence of TL material is a key characteristic factor and play an important role in designing the dosimeter. In Fig. 2 typical photon energy dependence of LiF:Mg,Cu,Na,Si TL detector and other common TL detectors in bare condition are presented. The energy response as a function of the mean photon energy is normalized to 662 keV photons from a 137Cs source. The energy dependence of LiF:Mg,Ti (TLD-100) detector and that of LiF:Mg,Cu,P (MCP-N) detectors were simultaneously presented to compare the results of LiF:Mg,Cu,Na,Si TL detector. The data of LiF:Mg,Ti
2. Materials and methods
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was referred to the reference of Horowitz (1984) and that of LiF:Mg,Cu,P (MCP-N) detectors was referred to the research by Budzanowski et al. (2001). In the research by Budzanowski et al. (2001), he has presented the energy response of LiF:Mg,Cu,Na,Si TL detector ahead of being modified and being set the optimum dopants concentration to improve the sensitivity and reusability and that of LiF:Mg,Cu,P (MCP-N). Therefore, the energy dependence of LiF:Mg,Cu,Na,Si detector in this study differs from the result of Budzanowski. While the energy response of LiF:Mg,Ti is different from two others, the energy response between LiF:Mg,Cu,P and LiF:Mg,Cu,Na,Si has a similar curve. However, LiF:Mg,Cu,Na,Si TL detector shows that it reduces the variation of response between 30 and 662 keV, compared with MCP-N. 2.2. Irradiation experiments The radiation exposure test of the energy response for TL detectors before designing the dosimeter and the verification for proposed and developed dosimeter were carried out at KAERI using 137Cs source and X-ray beam (ANSI X-ray beam code M30, M60, M100, M150, H150) produced to meet the Korea national X-ray test standard which is based on ANSI N13.11 standard (ANSI, 1993; Kim et al., 1997). Specifications for each X-ray techniques used in this experiment are given in Table 1. The reference X-ray fields constructed in KAERI are consisted of two type of X-ray machines, the low energy system and the medium system. Low energy X-ray system is 3.0 kW grade (Pantak, HF-75c) and beryllium plate of 1.0 mm is installed as inherent filters. Medium energy X-ray system is 3.2 kW grade (Phillips, MG325) and inherent filter consists of Be (4 mm)+Al (1.5 mm). In the first stage of experiment, irradiation test was accomplished in free air condition without a phantom to look into the energy dependence of the TL detectors in case of behind various filters. This experiment is to select the filter material and to optimize the filter configuration of the TL dosimeter. The TL detector and filters are placed at 200 cm away from the radiation source.
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The irradiation test geometry of the proposed TL dosimeter after design the LiF:Mg,Cu,Na,Si TL dosimeter and the experimental conditions are shown in Fig. 3. The dosimeter was modeled at 200 cm away from the radiation source, mounted at the center of a tissue equivalent phantom, facing towards the source. The phantom was assumed as a homogeneous polymethylmethacrylate (PMMA) slab with a mass density of 1.19 g/cm3, with a front profile of 30 30 cm2, and a thickness of 15 cm. After the irradiation experiment, the TELEDYNE TLD Reader System 310 with resistive heating systems was used for readout.
3. Results and discussion 3.1. Energy response of LiF:Mg,Cu,Na,Si TL detector under the filters Analysis and investigation of the various dosimetric properties and characteristics for this new TL detector are indispensable for its application to the practical personal dosimeter. The photon energy response, especially, is more important property than any other properties to develop the dosimeter. Therefore, in this study the energy response of sintered LiF:Mg,Cu,Na,Si detector developed at the KAERI was tested through
Fig. 3. A schematic geometrical configuration of irradiation experiments of the dosimeter.
Table 1 Properties of X-ray technique used for irradiation experiments at KAERI X-ray Beam Code
ANSI ANSI ANSI ANSI ANSI
M30 M60 M100 M150 H150
Eeff (keV)
19.4 35.2 51.2 73.0 118.3
H.V. (kVp)
30 60 100 150 150
Current (mA)
10 5 3 3 20
Ka (mGy/h)
842.4 326.6 193.7 230.4 31.5
Conversion coefficient (Sv/Gy) Hp ð10Þ=Ka
Hp ð0:07Þ=Ka
0.44 1.17 1.61 1.90 1.80
1.03 1.38 1.59 1.76 1.68
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filter or thin filter. The result shows that copper is adequate for the discrimination of photon energy because it could appropriately attenuate the low energy photon in terms of energy response. Table 2 shows the dosimetric properties of LiF:Mg,Cu,Na,Si TL detector in terms of energy response and the fundamental data for the development of a personal radiation dosimeter using this TL detector.
the experiment in case of being placed behind various filters before the design and proposal of the dosimeter. Filters used in this experiment were selected in consideration of the common filter material of widely used LiF TLD. This response results are the basic data for selecting the filter materials and these results is helpful to develop the TL dosimeter holder. This experiment for energy response of LiF:Mg,Cu,Na,Si detector behind various filter is carried out in-air without phantom to investigate the energy response of this TL detector for various candidate filter materials. Table 2 shows the photon energy response of the LiF:Mg,Cu,Na,Si TL detector with various filters. As mentioned above, various filter materials used in this experiments are based on the holder of common LiF TL dosimeters. As shown in Table 2, the energy response shows a decreasing trend with the increase of atomic number of filter material in low energy photon. In case of thin plastic filter, Teflon 1 mm thickness, the energy response for ANSI X-ray beam M30 is lower than that of bare pellet without filter. In case of high atomic number filter or thick filter, the energy response below 100 keV is much lower than that of low atomic number
3.2. Design and fabrication of LiF:Mg,Cu,Na,Si dosimeter The personal thermoluminescence dosimeter using LiF:Mg,Cu,Na,Si detector was designed and fabricated based on the results of energy response of TL detector in bare condition and with various filter materials. The configurations of the dosimeter are tabulated in Table 3 and the picture of LiF:Mg,Cu,Na,Si TL detectors and developed new dosimeter are presented in Fig. 4. A typical TLD card consists of four LiF:Mg,Cu,Na,Si TL detectors of radius 2.25 mm circle, encapsulated between two sheets of Teflon 0.05 mm (10 mg/cm2) thick and mounted on an aluminum substrate. The dosimeter
Table 2 Photon energy response of LiF:Mg,Cu,Na,Si TL detector with various filter materials X-ray Beam Code
Photon energy response of LiF:Mg,Cu,Na,Si Plastic (Teflon)
ANSI ANSI ANSI ANSI ANSI 137 Cs
M30 M60 M100 M150 H150
Aluminum
Copper
Tin
1 mm
4 mm
0.5 mm
2 mm
0.1 mm
0.5 mm
0.5 mm
0.63 0.98 1.03 0.96 0.91 1.00
0.30 0.81 1.02 0.93 0.87 1.00
0.44 0.91 1.00 1.00 0.90 1.00
0.12 0.54 0.88 0.93 0.91 1.00
0.07 0.46 0.84 0.96 0.98 1.00
0.02 0.12 0.41 0.74 0.97 1.00
0.02 0.06 0.21 0.52 0.97 1.00
Table 3 Characteristic of filter system for proposed TL dosimeter Area
Filter material
Density (g/cm3)
Thickness of filter (mm)
Total density thickness (mg/cm2)
A1
Teflon (card)
2.2
0.05
10
A2
Teflon (card) ABS plastic PTFE
2.2 1.04 2.2
0.05 1.0 4.0
995
A3
Teflon (card) ABS plastic Copper
2.2 1.04 8.96
0.05 1.0 0.5
563
A4
Teflon (card) ABS plastic
2.2 1.04
0.05 3.0
312
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Design Limit Relative Response
1.2
1.0
0.8
0.6
10
100
1000
Effective Photon Energy (keV)
Fig. 4. LiF:Mg,Cu,Na,Si TL detector and the developed TL dosimeter.
case is made of 1.0 mm thick ABS (polyacrylonitrilebutadiene-styrene) plastic material which is one of the most reliable and stable plastic among many plastic materials in condition of radiation field. The effect of the plastic dosimeter case was also considered in the analysis of the holder design in each area of the dosimeter. Each detector/filter combination performs a specific function, as follows: For open window area A1, a TLD pellet with 0.05 mm Teflon encapsulation determines the shallow dose. The filtration for this element is 10 mg/cm2. No additional filter material is needed to measure the shallow dose in A1 area since the density thickness of Teflon encapsulation of TL pellet is 10 mg/cm2. Another TLD with 995 mg/cm2 combined PTFE/ABS filtration (104 mg/cm2 ABS+880 mg/cm2 PTFE filters) measures the deep dose in A2 area. The third TLD covered with 104 mg/cm2 ABS plastic and 450 mg/cm2 copper filtration is used for low energy photon discrimination. The last TLD is covered with 312 mg/cm2 ABS plastic for energy discrimination for beta particles.
3.3. Energy dependence of proposed dosimeter with respect to dose equivalent, Hp ðdÞ As mentioned in a previous section, the new dosimeter designed in this study has four main regions for photon radiation dosimetry. Each of these areas contains a typical set of filters to estimate a personal dose equivalent, Hp ðdÞ and dosimetric information. The energy dependence of proposed TL dosimeter was obtained to verify the ability to evaluate the dose equivalent and to present more information such as response ratio. The deep dose area (A2) was designed to measure the photon dose at a tissue depth of 10 mm, Hp ð10Þ: As mentioned in advance, PTFE plastic filter with its about
Fig. 5. Energy response of the deep dose area (A2) with personal dose equivalent, Hp ð10Þ: Results are relative to 137Cs. This area (A2) provide relative response values in the range between 0.78 and 1.08 over the photon energy range from 20 to 662 keV.
4 mm thickness was utilized based on experimental results and the consideration about other requirements of filter systems. To meet the purpose of this area, the filters used should be able to cutoff beta radiation. The experimental response results of dosimeter in the deep dose are shown in Fig. 5. The responses are normalized to the 137Cs response. Each relative response value represents mean value of eight dosimeters. As shown in Fig. 5, A2 area appears to provide relative response values in the range between 0.78 and 1.08 over the photon energy range from 20 to 662 keV. This is within the 730% design limits required by the ISO (1984) standard. Given the responses in terms of absorbed dose with Hp ð10Þ are proportional to the dosimeter reading, therefore, without application of any sophisticated algorithm this design would enable us to determine the deep dose equivalent more easily. In Fig. 6, the energy response for the open window area in terms of Hp ð0:07Þ are shown. The responses are normalized to the 137Cs response. These area response values are between 0.82 and 1.16 over the photon energy range from 20 to 662 keV. This is also within the design limit of the ISO standard. 3.4. Angular dependence The angular dependence for the deep dose area, A2 of the LiF:Mg,Cu,Na,Si TL dosimeter in terms of Hp ð10Þ for two X-ray beams with a effective energy of 35.2, 118.3 and 662 keV photon from 137Cs is shown in Fig. 7. The responses are normalized to normal incidence (0 ). The response for each angle of incidence represents the mean of four measurements, and the error bars correspond to 1 SD.
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1.4
Design Limit
Relative Response
1.2
1.0
0.8
0.6
10
100
1000
Effective Photon Energy (keV)
Fig. 6. Energy response of the open window area (A1) with personal dose equivalent, Hp ð0:07Þ: Results are relative to 137Cs. This area (A1) provides relative response values in the range between 0.82 and 1.16 over the photon energy range from 20 to 662 keV.
1.0 0.9 0.8
Response
0.7 0.6 0.5
137
Cs (662keV) ANSI X-ray H150 (118.3keV) ANSI X-ray M60 (35.2keV)
0.4 0.3 0.2 0
20
40
60
80
100
Angle of incidence (deg)
Fig. 7. The angular dependence for the Hp ð10Þ element of the LiF:Mg,Cu,Na,Si TL dosimeter, normalized to response at normal incidence (0), for three X-ray beams with a effective energy of 35.2, 118.3 and 662 keV from 137Cs. The response for each angle of incidence represents the mean of four measurements, and the error bars correspond to 1 SD.
As shown in Fig. 7, for 137Cs a slight decrease in angular response was seen within the 60 , but the response after 60 decreased rapidly and reached 0.8 at 90 . As expected, the high-energy photon of 137Cs has less angular dependence than the low energy photon such as X-ray beam M60.
4. Conclusion A personal dosimeter based on a newly developed TL material LiF:Mg,Cu,Na,Si was developed for personal
radiation monitoring. It turned out that this TL dosimeter with LiF:Mg,Cu,Na,Si TL detector is able to estimate personal dose equivalent value, Hp ðdÞ; based on the irradiation experiments for energy response of dosimeter design proposed by pre-experiment approach. In view of energy response results, the proposed TL dosimeter meets the 730% design limits required by the ISO (1984) standard and it shows good characteristics for a personal radiation monitoring. The TL dosimeter proposed in this study has, therefore, a feasibility to be utilized in common dosimetry market. Although the feasibility of the LiF:Mg,Cu,Na,Si TL dosimeter was demonstrated in some radiation fields, more elaborating works for the dosimeter such as beta discrimination and dose calculation algorithm, etc., are needed to make the proposed TL dosimeter more stable and reliable before being applied for routine dose measurements. References American National Standard Institute, 1993. American National Standard for Dosimetry—Personal dosimetry performance criteria for testing, ANSI N13.11. Bilski, P., Budzanowski, M., Olko, P., 1997. Dependence of LiF:Mg,Cu,P (MCP-N) glow curve structure on dopant composition and thermal treatment. Radiat. Prot. Dosim. 69, 187–198. Bos, A.J.J., 2001. High sensitivity thermoluminescence dosimetry. Nucl. Instrum. Methods Phys. Res. B. 184, 3–28. Budzanowski, M., Kim, J.L., Nam, Y.M., Chang, S.Y., Bilski, P., Olko, P., 2001. Dosimetric properties of sintered LiF:Mg,Cu,Na,Si TL detectors. Radiat. Meas. 33, 537–540. Doh, S.H., Chu, M.C., Chung, W.H., Kim, H.J., Kim, D.S., Kang, Y.H., 1989. Preparation of LiF (Mg,Cu,Na,Si) phosphor and its thermoluminescent characteristics. Korean Appl. Phys. 2, 425–431. Horowitz, Y.S., 1984. Thermoluminescence and Thermoluminescent Dosimetry. CRC Press Inc., Boca Raton, FL. Horowitz, Y.S., 1993. LiF:Mg, Ti versus LiF:Mg, Cu, P: the competition heat up. Radiat. Prot. Dosim. 47 (1), 135–141. ICRU, 1993. Quantities and units in radiation protection dosimetry, ICRU Report 51. ISO, 1984. Personal and Environmental Thermoluminescence Dosimeters, ISO Standards DP 8034. Kim, H.J., Chung, W.H., Doh, S.H., Chu, M.C., Kim, D.S., Kang, Y.H., 1989. Thermoluminescence dosimetric properties of LiF(Mg,Cu,Na,Si). J. Korean Phys. Soc. 22, 415–420. Kim, J.L., Kim, B.H., Chang, S.Y., Lee, J.K., 1997. Establishment of ANSI 13.11 X-ray radiation fields for personal dosimetry performance test by computational and experiment. Environ. Health Perspect. 105 (6), 1417–1422. Lee, J.I., Kim, J.L., Yang, J.S., Kim, B.Y., Nam, Y.M., 2002. Development of highly sensitive LiF:Mg,Cu,Na,Si TL detector. The First Asian and Oceanic Congress for Radiation Protection (AOCRP-1). Nam, Y.M., Kim, J.L., Chang, S.Y., Kim, G.D., 1998. The study of glow curves for LiF:Mg, Cu, Na, Si phosphor with different dopant concentrations. Ungyong Mulli 11, 578–583.
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Vij, D.R., 1993. Thermoluminescent Material. Prentice-Hall, Englewood Cliffs, NJ. Wang, S., Chen, G., Wu, F., Li, Y., Zha, Z., Zhu, J., 1986. Newly developed highly sensitive LiF (Mg,Cu,P) TL chips with high signal-to-noise ratio. Radiat. Prot. Dosim. 14 (3), 223–227. Zha, Z., Wang, S., Shen, W., Zhu, J., Cai, G., 1993. Preparation and characteristics of LiF:Mg,Cu,P thermoluminescent material. Radiat. Prot. Dosim. 47 (1/4), 111–118.