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TL tandem systems for the determination of effective energy in X radiation beams
ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 788–790
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
TL tandem systems for the determination of effective energy in X radiation beams Me´rcia L. Oliveira a, Ana F. Maia b, a b
~ Nacional de Energia Nuclear, Av. Professor Luiz Freire, 200, 50740-540 Recife/PE, Brazil Centro Regional de Ciˆencias Nucleares do Nordeste (CRCN/CNEN-NE), Comissao ~ Cristo ~ ´vao–SE, Departamento de Fı´sica, Universidade Federal de Sergipe, Av. Marechal Rondon, s/n, 49100-000 Sao Brazil
a r t i c l e in f o
a b s t r a c t
Keywords: Thermoluminescent dosimetry Tandem systems Energy dependence Absorbed dose
Tandem systems usually comprise two detectors with different radiation energy responses and are utilized to determine the effective energy of unknown radiation beams. The aim of this paper was to evaluate the influence of absorbed dose on tandem curves obtained for different TL materials. In the studied dose interval, the results demonstrate that there is no significant influence of dose on the tandem curves. Therefore, the reliability of these systems for the determination of radiation beam effective energy was confirmed. & 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tandem systems are very useful for the determination of the effective energy in unknown radiation beams. This method is based on the fact that materials with different effective atomic numbers present different responses when exposed to the same radiation beam. Some works have shown the feasibility of tandem systems based on ionization chambers (Caldas, 1991; Costa and Caldas, 2003a; Costa and Caldas, 2003b; Maia and Caldas, 2006). However, because of the diversity of thermoluminescence (TL) materials, TL dosimeters (TLD) are widely utilized to make up these systems (Gorbics and Attix, 1968; Spurny et al., 1973; Rossiter, 1975; Da Rosa and Nette, 1988; Miljanie et al., 1999). Tandem systems have been utilized to improve dose calculations (once the energy dependence of the detector is taken into account) (Rossiter, 1975; Da Rosa and Nette, 1988; Miljanie et al., 1999), or as an important tool in quality control programs (replacing the conventional and time-consuming technique of determining the effective energy from half-value layer measurements) (Costa and Caldas, 2003a; Costa and Caldas, 2003b; Maia and Caldas, 2006). Usually, tandem curves are determined for one dose condition and are applied arbitrarily for several values of dose. However, the energy dependence of TLDs can be affected by this parameter. Therefore, the aim of this study is to evaluate the influence of absorbed dose on tandem curves obtained for different TL materials. Low doses, from 3 to 10 mGy, typical of diagnostic radiology applications, for various radiation beams, were utilized. Corresponding author. Tel./fax: + 55 79 21056848.
E-mail addresses: [email protected] (M.L. Oliveira), [email protected] (A.F. Maia). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.01.001
In this paper, calibration curves and tandem curves were obtained for different absorbed dose values. Besides, a blind test was performed to confirm the usefulness of tandem curves to determine the effective energy of radiation beams.
2. Materials and methods In this work, three types of TL materials were utilized: CaSO4:Dy (Zeff =14.4), produced at the Instituto de Pesquisas Energe´ticas e Nucleares, Brazil; LiF:Mg,Cu,P, named TLD-100H, (Zeff = 8.2) and LiF:Mg,Ti, named TLD-100, (Zeff = 8.2), produced by Bicron–Harshaw, USA. The conventional CaSO4:Dy pellets are 6.0 mm in diameter and 0.8 mm thick (Campos, 1983; Campos, 1988; Campos and Lima, 1986; Campos, 1993). The TLD-100H samples are 3.6 mm in diameter and 0.5 mm thick, while the TLD-100 samples are 0.3 0.3 0.9 mm3. To select the pellets, an initial lot of 100 CaSO4:Dy pellets and 50 TLD-100 and TLD-100H samples were tested to determine the uniformity of response. The maximum variation in TL response allowed was 3%. Therefore, for the following studies, 39 CaSO4:Dy pellets, 37 TLD-100 and 28 TLD-100H samples were utilized. The sensibility factor was determined for each pellet. A Harshaw 5500 readout system was utilized for TL measurements. The samples were irradiated at dose intervals from 0.5 to 10 mGy and calibration curves were obtained for each radiation quality. The results for doses below 3 mGy were not reproducible. The thermal treatments applied were 300 1C for 15 min for the CaSO4:Dy pellets and 400 1C for 3 h and 100 1C for 1 h for the TLD-100 samples (Muniz et al., 1996). The TLD-100H samples were not submitted to any thermal treatment.
ARTICLE IN PRESS M.L. Oliveira, A.F. Maia / Applied Radiation and Isotopes 68 (2010) 788–790
The samples were irradiated with ISO 4037 (1996) radiation protection standard beams, with X-ray energies from 48 to 118 keV, and gamma ray energies of 662 keV (137Cs) and 1.2 MeV (60Co). The characteristics of the radiation beams are listed in Table 1. The reference system is composed of an ionization chamber NE model 2575, serial number 518 (600 cm3), and an NE electrometer, model Farmer 2670, serial number 148.
3. Results Five calibration curves were obtained for each TL material (one for each radiation quality). For each curve, a linear fit was applied to verify the linearity of the TL response with air kerma. The parameters of the fit are listed in Table 2. These results confirm the highly linear behavior of TLDs in this dose interval. Besides, the variation with angular coefficient obtained is in accordance with the expected energy dependence: much more accentuated for CaSO4:Dy pellets than for TLD-100 and TLD-100H samples. The energy dependence curves were obtained for each material and at each absorbed dose, as presented in Fig. 1. It can be observed that the behavior of each material is really similar between doses. The overall behavior of the curves obtained for each material are in agreement with the results found in the literature, including the under-response of the TLD-100H samples at energies close to 100 keV (Zoetelief et al., 2000; Chang et al. 2001; Yang et al., 2002; Moscovitch and Horowitz, 2007). The energy dependence was evaluated and the extreme values were compared according to Eq. (1). The values obtained are listed in Table 3. The results indicate the possibility of establishing a tandem system composed of CaSO4:Dy pellets and TLD-100 samples or CaSO4:Dy pellets and TLD-100H samples, because of the difference in the energy dependences. Energy Dependence ð%Þ ¼
789
Tandem curves were determined for both pairs of materials, and at each dose value. As expected from the energy dependence results, the tandem curves did not present any significant variation. A tandem curve, obtained using the mean values, is presented in Fig. 2. The maximum percentage standard deviation was 5.9%, which proves the similarity among the curves. Therefore, it can be concluded that it is possible to use only one curve in this dose interval. The tandem curves obtained are useful for energies up to 118 keV. In this energy range, the curves have a steep dependence, which is necessary for discriminating radiation beams with energies that are close together. The differences in the ratios of response of subsequent points on the curves in this energy range
ðMajor ResponseMinor ResponseÞ 100% Major Response
ð1Þ Fig. 1. Energy dependence curves obtained for CaSO4:Dy (full line), TLD-100 (dashed line) and TLD-100H (dotted line) dosimeters, using various values of doses.
Table 1 Characteristics of radiation qualities. Radiation quality
Voltage (kV)
Half-value layer (mmCu)
Effective energy (keV)
Air kerma rate (mGy/min)
N60 N100 N150 137 Cs 60 Co
58 97 145.7 – –
0.24 1.11 2.36 – –
48 83 118 662 1250
2.71 0.67 2.52 30780a 2163b
a b
Table 3 Energy dependence of the TL materials.
Reference date: 02/15/2007. Reference date: 01/15/2007.
Absorbed dose (mGy)
CaSO4:Dy (%)
TLD-100 (%)
TLD-100H (%)
3 5 7 10
91.2 91.4 91.7 91.4
50.9 43.5 43.2 45.8
38.0 35.4 38.8 35.8
Table 2 Parameters of the linear fita applied to the calibration curves obtained. Radiation quality
N60 N100 N150 137 Cs 60 Co a
CaSO4:Dy
TLD-100
TLD-100H
Angular coefficient (B)
Correlation coefficient (R)
Angular coefficient (B)
Correlation coefficient (R)
Angular coefficient (B)
Correlation coefficient (R)
599.3 210.1 107.4 53.7 51.2
0.9991 0.9999 1.0000 0.9998 1.0000
8.29 5.85 5.36 4.86 4.56
0.9993 0.9992 1.0000 0.9994 0.9979
25.75 19.28 16.31 20.96 21.47
0.9994 0.9997 0.9988 0.9998 0.9994
Linear regression: y= Bx.
ARTICLE IN PRESS 790
M.L. Oliveira, A.F. Maia / Applied Radiation and Isotopes 68 (2010) 788–790
CaSO4:Dy/TLD-100H tandem system, indicating that both systems can be utilized to perform effective energy determinations of unknown radiation beams.
102
Ratio of Response
TLD100 TLD100H
4. Conclusions The results obtained in this study prove the reliability of thermoluminescent tandem systems for the determination of the effective energy in radiation protection radiation beams. Besides, the tandem curves obtained at different doses did not present significant variation. Further efforts should be made to improve the energy discrimination at low energies.
101
Acknowledgments
100
102
103
Effective Energy (keV) Fig. 2. Tandem curves obtained using CaSO4:Dy pellets and the TLDs in caption.
Table 4 Determination of effective energy of unknown X radiation beams. Value obtained (keV) CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H
40.1 43.0 44.2 48.0 83.9 79.4 107.8 102.3 115.0 112.5 138.5 136.8
Nominal energy (keV) 48 57 83 104 118 137
The authors acknowledge Dr. L. Campos for providing the CaSO4:Dy pellets and Dr. E. C. Vilela for providing the TLD-100 and TLD-100H samples. The authors are thankful to Ms. R.S. Galindo for the TL measurements.
References Difference (%) 17 10 23 16 1.1 4.3 3.7 1.6 2.5 4.7 1.1 0.1
were always higher than 24.7%. However, the difference between the points at 662 and 1250 keV were much lower, showing that this system has a limited region of applicability. The overall uncertainties were estimated according to the ISO GUM (1995) recommendations. The estimated values (1s) are 8.3% and 8.7% for tandem curves using CaSO4:Dy and TLD-100, and CaSO4:Dy and TLD-100 H, respectively. Finally, a blind test was performed to confirm the usefulness of the tandem systems for the determination of the effective energies of unknown radiation fields. To that end, pairs of CaSO4:Dy and TLD-100, and CaSO4:Dy and TLD-100H were irradiated simultaneously with 6 different X radiation qualities, with effective energies varying from 48 to 137 keV. After readout procedures, the measurement ratios were calculated and the radiation beam effective energies were determined from the tandem curves. The results are showed in Table 4. The tandem systems show adequate performance except at the two lower energies of 48 and 57 keV. In the first case (48 keV), the beam has a narrow spectrum, while, in the second (57 keV), it has a wide spectrum. In both cases, the contribution of photons below 50 keV is very significant. Therefore, these results indicate that the TL tandem system is less efficient for radiation beams with a higher contribution of energy photons below 50 keV. Besides, no significant difference was observed between energy values obtained with the CaSO4:Dy/TLD-100 tandem system and the
Caldas, L.V.E., 1991. A sequential tandem system of ionization chambers for effective energy determination of X-radiation fields. Radiat. Prot. Dosim. 36 (1), 47–50. Campos, L.L., 1983. Preparation of CaSO4:Dy TL single crystals. J. Lumin. 28 (4), 481–483. Campos, L.L., Lima, M.F., 1986. Dosimetric properties of CaSO4:Dy teflon pellets produced at IPEN. Radiat. Prot. Dosim. 14 (4), 333–335. Campos, L.L., 1988. Determination of TL parameters of CaSO4:Dy produced at Instituto de Pesquisas Energe´ticas e Nucleares (IPEN). Appl. Radiat. Isot. 39 (3), 233–236. Campos, L.L., 1993. Graphite mixed CaSO4:Dy TL dosemeters for beta radiation dosimetry. Radiat. Prot. Dosim. 48 (2), 205–207. Chang, J.K., Nam, Y.M., Kim, J.L., Chang, S.Y., Kim, B.H., 2001. Calculated energy dependence of CaSO4:Dy TL phosphor and phosphor embedded Teflon for X and gamma rays. Radiat. Meas. 33 (5), 675–678. Costa, A.M., Caldas, L.V.E., 2003a. Response characteristics of a tandem ionization chamber in standard X-ray beams. Appl. Radiat. Isot. 58 (4), 495–500. Costa, A.M., Caldas, L.V.E., 2003b. A special ionisation chamber for quality control of diagnostic and mammography X-ray equipment. Radiat. Prot. Dosim. 104 (1), 41–45. Da Rosa, L.A.R., Nette, P.H., 1988. Thermoluminescent dosimeters for exposure assessment in gamma or X radiation fields with unknown spectral distribution. Int. J. Radiat. Appl. Instrum. Part A 39 (3), 191–197. Gorbics, S.G., Attix, F.H., 1968. LiF and CaF2:Mn thermoluminescent dosimeters in tandem. Int. J. Appl. Radiat. Isot. 19 (2), 81–89. ISO 4037-1, 1996. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—Part 1: radiation characteristics and production methods. International Organization for Standardization, Gene ve. ISO-GUM, 1995. Guide to the expression of uncertainty in measurement. International Organization for Standardization, Gene ve. Maia, A.F., Caldas, L.V.E., 2006. A simple method for evaluation of half-value layer variation in CT equipment. Phys. Med. Biol. 51 (6), 1595–1601. Miljanie, S., Vekie, B., Martineie, R., 1999. Determination of X-ray effective energy and absorbed dose using CaF2:Mn and LiF:Mg,Ti thermoluminescence dosemeters. Radiat. Prot. Dosim. 85 (1–4), 381–384. Moscovitch, M., Horowitz, Y.S., 2007. Thermoluminescent materials for medical applications: LiF:Mg,Ti and LiF:Mg,Cu,P. Radiat. Meas. 41 (1), S71–S77. Muniz, J.L., Hernandez Verduzco, R., Delgado, A., 1996. A comparison of the TLD-100 dosimetric performance using different annealing procedures and glow curve analysis. Radiat. Prot. Dosim. 66 (1–4), 273–277. Rossiter, M.J., 1975. The use of precision thermoluminescence dosimetry for intercomparison of absorbed dose. Phys. Med. Biol. 20 (5), 735–746. Spurny, Z., Milu, C., Racoveanu, N., 1973. Comparison of X-ray beams using thermoluminescent dosemeters. Phys. Med. Biol. 18 (2), 276–278. Yang, J.S., Kim, D.Y., Kim, J.L., Chang, S.Y., Nam, Y.M., Park, J.W., 2002. Thermoluminescence characteristics of Teflon embedded CaSO4:Dy TLD. Radiat. Prot. Dosim. 100 (1–4), 337–340. Zoetelief, J., Julius, H.W., Christensen, P., 2000. Recommendations for patient dosimetry in diagnostic radiology using TLD. Topical Report, EUR 19604, European Commission.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
TL tandem systems for the determination of effective energy in X radiation beams Me´rcia L. Oliveira a, Ana F. Maia b, a b
~ Nacional de Energia Nuclear, Av. Professor Luiz Freire, 200, 50740-540 Recife/PE, Brazil Centro Regional de Ciˆencias Nucleares do Nordeste (CRCN/CNEN-NE), Comissao ~ Cristo ~ ´vao–SE, Departamento de Fı´sica, Universidade Federal de Sergipe, Av. Marechal Rondon, s/n, 49100-000 Sao Brazil
a r t i c l e in f o
a b s t r a c t
Keywords: Thermoluminescent dosimetry Tandem systems Energy dependence Absorbed dose
Tandem systems usually comprise two detectors with different radiation energy responses and are utilized to determine the effective energy of unknown radiation beams. The aim of this paper was to evaluate the influence of absorbed dose on tandem curves obtained for different TL materials. In the studied dose interval, the results demonstrate that there is no significant influence of dose on the tandem curves. Therefore, the reliability of these systems for the determination of radiation beam effective energy was confirmed. & 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tandem systems are very useful for the determination of the effective energy in unknown radiation beams. This method is based on the fact that materials with different effective atomic numbers present different responses when exposed to the same radiation beam. Some works have shown the feasibility of tandem systems based on ionization chambers (Caldas, 1991; Costa and Caldas, 2003a; Costa and Caldas, 2003b; Maia and Caldas, 2006). However, because of the diversity of thermoluminescence (TL) materials, TL dosimeters (TLD) are widely utilized to make up these systems (Gorbics and Attix, 1968; Spurny et al., 1973; Rossiter, 1975; Da Rosa and Nette, 1988; Miljanie et al., 1999). Tandem systems have been utilized to improve dose calculations (once the energy dependence of the detector is taken into account) (Rossiter, 1975; Da Rosa and Nette, 1988; Miljanie et al., 1999), or as an important tool in quality control programs (replacing the conventional and time-consuming technique of determining the effective energy from half-value layer measurements) (Costa and Caldas, 2003a; Costa and Caldas, 2003b; Maia and Caldas, 2006). Usually, tandem curves are determined for one dose condition and are applied arbitrarily for several values of dose. However, the energy dependence of TLDs can be affected by this parameter. Therefore, the aim of this study is to evaluate the influence of absorbed dose on tandem curves obtained for different TL materials. Low doses, from 3 to 10 mGy, typical of diagnostic radiology applications, for various radiation beams, were utilized. Corresponding author. Tel./fax: + 55 79 21056848.
E-mail addresses: [email protected] (M.L. Oliveira), [email protected] (A.F. Maia). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.01.001
In this paper, calibration curves and tandem curves were obtained for different absorbed dose values. Besides, a blind test was performed to confirm the usefulness of tandem curves to determine the effective energy of radiation beams.
2. Materials and methods In this work, three types of TL materials were utilized: CaSO4:Dy (Zeff =14.4), produced at the Instituto de Pesquisas Energe´ticas e Nucleares, Brazil; LiF:Mg,Cu,P, named TLD-100H, (Zeff = 8.2) and LiF:Mg,Ti, named TLD-100, (Zeff = 8.2), produced by Bicron–Harshaw, USA. The conventional CaSO4:Dy pellets are 6.0 mm in diameter and 0.8 mm thick (Campos, 1983; Campos, 1988; Campos and Lima, 1986; Campos, 1993). The TLD-100H samples are 3.6 mm in diameter and 0.5 mm thick, while the TLD-100 samples are 0.3 0.3 0.9 mm3. To select the pellets, an initial lot of 100 CaSO4:Dy pellets and 50 TLD-100 and TLD-100H samples were tested to determine the uniformity of response. The maximum variation in TL response allowed was 3%. Therefore, for the following studies, 39 CaSO4:Dy pellets, 37 TLD-100 and 28 TLD-100H samples were utilized. The sensibility factor was determined for each pellet. A Harshaw 5500 readout system was utilized for TL measurements. The samples were irradiated at dose intervals from 0.5 to 10 mGy and calibration curves were obtained for each radiation quality. The results for doses below 3 mGy were not reproducible. The thermal treatments applied were 300 1C for 15 min for the CaSO4:Dy pellets and 400 1C for 3 h and 100 1C for 1 h for the TLD-100 samples (Muniz et al., 1996). The TLD-100H samples were not submitted to any thermal treatment.
ARTICLE IN PRESS M.L. Oliveira, A.F. Maia / Applied Radiation and Isotopes 68 (2010) 788–790
The samples were irradiated with ISO 4037 (1996) radiation protection standard beams, with X-ray energies from 48 to 118 keV, and gamma ray energies of 662 keV (137Cs) and 1.2 MeV (60Co). The characteristics of the radiation beams are listed in Table 1. The reference system is composed of an ionization chamber NE model 2575, serial number 518 (600 cm3), and an NE electrometer, model Farmer 2670, serial number 148.
3. Results Five calibration curves were obtained for each TL material (one for each radiation quality). For each curve, a linear fit was applied to verify the linearity of the TL response with air kerma. The parameters of the fit are listed in Table 2. These results confirm the highly linear behavior of TLDs in this dose interval. Besides, the variation with angular coefficient obtained is in accordance with the expected energy dependence: much more accentuated for CaSO4:Dy pellets than for TLD-100 and TLD-100H samples. The energy dependence curves were obtained for each material and at each absorbed dose, as presented in Fig. 1. It can be observed that the behavior of each material is really similar between doses. The overall behavior of the curves obtained for each material are in agreement with the results found in the literature, including the under-response of the TLD-100H samples at energies close to 100 keV (Zoetelief et al., 2000; Chang et al. 2001; Yang et al., 2002; Moscovitch and Horowitz, 2007). The energy dependence was evaluated and the extreme values were compared according to Eq. (1). The values obtained are listed in Table 3. The results indicate the possibility of establishing a tandem system composed of CaSO4:Dy pellets and TLD-100 samples or CaSO4:Dy pellets and TLD-100H samples, because of the difference in the energy dependences. Energy Dependence ð%Þ ¼
789
Tandem curves were determined for both pairs of materials, and at each dose value. As expected from the energy dependence results, the tandem curves did not present any significant variation. A tandem curve, obtained using the mean values, is presented in Fig. 2. The maximum percentage standard deviation was 5.9%, which proves the similarity among the curves. Therefore, it can be concluded that it is possible to use only one curve in this dose interval. The tandem curves obtained are useful for energies up to 118 keV. In this energy range, the curves have a steep dependence, which is necessary for discriminating radiation beams with energies that are close together. The differences in the ratios of response of subsequent points on the curves in this energy range
ðMajor ResponseMinor ResponseÞ 100% Major Response
ð1Þ Fig. 1. Energy dependence curves obtained for CaSO4:Dy (full line), TLD-100 (dashed line) and TLD-100H (dotted line) dosimeters, using various values of doses.
Table 1 Characteristics of radiation qualities. Radiation quality
Voltage (kV)
Half-value layer (mmCu)
Effective energy (keV)
Air kerma rate (mGy/min)
N60 N100 N150 137 Cs 60 Co
58 97 145.7 – –
0.24 1.11 2.36 – –
48 83 118 662 1250
2.71 0.67 2.52 30780a 2163b
a b
Table 3 Energy dependence of the TL materials.
Reference date: 02/15/2007. Reference date: 01/15/2007.
Absorbed dose (mGy)
CaSO4:Dy (%)
TLD-100 (%)
TLD-100H (%)
3 5 7 10
91.2 91.4 91.7 91.4
50.9 43.5 43.2 45.8
38.0 35.4 38.8 35.8
Table 2 Parameters of the linear fita applied to the calibration curves obtained. Radiation quality
N60 N100 N150 137 Cs 60 Co a
CaSO4:Dy
TLD-100
TLD-100H
Angular coefficient (B)
Correlation coefficient (R)
Angular coefficient (B)
Correlation coefficient (R)
Angular coefficient (B)
Correlation coefficient (R)
599.3 210.1 107.4 53.7 51.2
0.9991 0.9999 1.0000 0.9998 1.0000
8.29 5.85 5.36 4.86 4.56
0.9993 0.9992 1.0000 0.9994 0.9979
25.75 19.28 16.31 20.96 21.47
0.9994 0.9997 0.9988 0.9998 0.9994
Linear regression: y= Bx.
ARTICLE IN PRESS 790
M.L. Oliveira, A.F. Maia / Applied Radiation and Isotopes 68 (2010) 788–790
CaSO4:Dy/TLD-100H tandem system, indicating that both systems can be utilized to perform effective energy determinations of unknown radiation beams.
102
Ratio of Response
TLD100 TLD100H
4. Conclusions The results obtained in this study prove the reliability of thermoluminescent tandem systems for the determination of the effective energy in radiation protection radiation beams. Besides, the tandem curves obtained at different doses did not present significant variation. Further efforts should be made to improve the energy discrimination at low energies.
101
Acknowledgments
100
102
103
Effective Energy (keV) Fig. 2. Tandem curves obtained using CaSO4:Dy pellets and the TLDs in caption.
Table 4 Determination of effective energy of unknown X radiation beams. Value obtained (keV) CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H CaSO4:Dy/TLD-100 CaSO4:Dy/TLD-100H
40.1 43.0 44.2 48.0 83.9 79.4 107.8 102.3 115.0 112.5 138.5 136.8
Nominal energy (keV) 48 57 83 104 118 137
The authors acknowledge Dr. L. Campos for providing the CaSO4:Dy pellets and Dr. E. C. Vilela for providing the TLD-100 and TLD-100H samples. The authors are thankful to Ms. R.S. Galindo for the TL measurements.
References Difference (%) 17 10 23 16 1.1 4.3 3.7 1.6 2.5 4.7 1.1 0.1
were always higher than 24.7%. However, the difference between the points at 662 and 1250 keV were much lower, showing that this system has a limited region of applicability. The overall uncertainties were estimated according to the ISO GUM (1995) recommendations. The estimated values (1s) are 8.3% and 8.7% for tandem curves using CaSO4:Dy and TLD-100, and CaSO4:Dy and TLD-100 H, respectively. Finally, a blind test was performed to confirm the usefulness of the tandem systems for the determination of the effective energies of unknown radiation fields. To that end, pairs of CaSO4:Dy and TLD-100, and CaSO4:Dy and TLD-100H were irradiated simultaneously with 6 different X radiation qualities, with effective energies varying from 48 to 137 keV. After readout procedures, the measurement ratios were calculated and the radiation beam effective energies were determined from the tandem curves. The results are showed in Table 4. The tandem systems show adequate performance except at the two lower energies of 48 and 57 keV. In the first case (48 keV), the beam has a narrow spectrum, while, in the second (57 keV), it has a wide spectrum. In both cases, the contribution of photons below 50 keV is very significant. Therefore, these results indicate that the TL tandem system is less efficient for radiation beams with a higher contribution of energy photons below 50 keV. Besides, no significant difference was observed between energy values obtained with the CaSO4:Dy/TLD-100 tandem system and the
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