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Comparison of LiF (TLD-100 and TLD-100H) detectors for extremity monitoring
Radiation Measurements 43 (2008) 646 – 650 www.elsevier.com/locate/radmeas
Comparison of LiF (TLD-100 and TLD-100H) detectors for extremity monitoring L. Freire a,b , A. Calado a , J.V. Cardoso a , L.M. Santos a , J.G. Alves a,∗ a Departamento de Protecção Radiológica e Segurança Nuclear, Instituto Tecnológico e Nuclear, E.N. 10, 2683-953 Sacavém, Portugal b Laboratório de Medicina Nuclear, Lda, Atomedical, Rua Helena Félix, 11D, 1600-121 Lisboa, Portugal
Abstract In this work the results aimed at assessing the performance of two types of LiF detectors, TLD-100 and TLD-100H, used in the context of extremity dosimetry are presented. Each detector variety was studied for reproducibility, batch homogeneity, residual dose, linearity and energy dependence using, when appropriate, the 90 Sr/90 Y radiation source built-in one of the Harshaw 6600 readers, the ISO narrow X-ray beams of N30, N40, N60, N80, N100 and N120 or the gamma radiations of 137 Cs and 60 Co. Two calibration energies (N120 and 137 Cs) were also used. The reproducibility and linearity results indicate that both LiF:Mg,Ti and LiF:Mg,Cu,P performed equally well. However, LiF:Mg,Cu,P presents a higher residual signal. In terms of energy dependence, LiF:Mg,Cu,P shows less variation than LiF:Mg,Ti particularly when N120 is used as calibration radiation. This seems to be a more realistic setup since the energy of the most frequently used radioisotopes in Nuclear Medicine departments with single photon emission computed tomography (SPECT) use gamma radiation energies closer to N120 than to 137 Cs. © 2008 Elsevier Ltd. All rights reserved. Keywords: Individual monitoring; Extremity dosemeters; Thermoluminescence
1. Introduction/scope The individual monitoring service (IMS) of ITN-DPRSN is presently performing whole body monitoring to around 3000 workers in Portugal. ITN has recently felt the need to provide extremity monitoring for workers in the field of Nuclear Medicine at ITN, hospitals and private clinics as well as for radiologists in interventional procedures. Two LiF varieties of extremity dosemeters LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) of the EXT-RAD type, for use with the Harshaw 6600 readers, were compared in terms of reproducibility, residual signal, stability of the element correction coefficients, batch homogeneity, linearity and energy dependence. Both 137 Cs and the ISO N120 (ISO 4037-1, 1996) reference radiations were used as calibration energies in this study. The former was used following the indications contained in (ISO 12794, 2000) and also in national requirements (DL 167, 2002). The latter was chosen because the energy of the most frequently ∗ Corresponding author. Tel.: +351 219 946 297; fax: +351 219 941 995.
E-mail address: [email protected] (J.G. Alves). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.12.013
used radionuclides in Nuclear Medicine departments performing single photon emission computed tomography (SPECT) is closer to the mean energy of N120. In this paper, some of the tests performed with both dosemeter varieties will be presented and analyzed. 2. Materials and methods The IMS dosimetry system at ITN-DPRSN is based on two Harshaw 6600 readers. One of the readers incorporates a 90 Sr/90 Y irradiator that is used for the determination of individual element correction coefficients (ECCs), for the irradiation of quality control dosemeters to be read interspaced with field dosemeters during readouts and for experiments that do not require special irradiation conditions. Two hundred EXT-RAD dosemeters, 100 of each LiF variety (TLD-100 and TLD-100H), were purchased from Thermo Electronic Corporation (USA) for these experiments. All detectors followed an initialization procedure before they were considered ready for use, which consisted of 10 cycles of irradiation and readout. ECCs were determined using the 90 Sr/90 Y irradiator. All dosemeters, which are identified as calibration
L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650 Table 1 Reading cycle parameters for the two LiF detectors
1.05 TLD-100 TLD-100H
10 s at 130 ◦ C 15 300 13.3
6 s at 140 ◦ C 15 250 10
or field dosemeters, were individually calibrated following the suggestions found in the readers and software manuals (Harshaw-Bicron, 1992, 1994) and criteria defined in-house. The parameters of the reading cycles used are mentioned in Table 1 and were selected as follows: for TLD-100, the reading cycle is the same as for the whole body detectors; for TLD-100H, the same heating rate was used but the preheating temperature and the highest temperature attained were chosen from the observation of the glow curves. The reproducibility and the residual signal of each type of detectors were studied for five detectors of each variety, chosen at random, which were irradiated and read out for 10 consecutive times, in sequence. The irradiation was carried out with the 90 Sr/90 Y irradiator and the irradiation dose corresponded to approximately 5 mSv. This experiment was also used to evaluate the stability of the ECC of the five detectors. Following the determination of the ECC for all the detectors of each type, the corresponding distributions were assessed. Two calibration energies were always considered, corresponding to the ISO N120 and the 137 Cs reference radiations (ISO 4037-1, 1996). All irradiations were carried out in terms of Hp (0.07) using the ISO rod phantom which consists of a right circular cylinder of PMMA with a diameter of 19 mm and a length of 300 mm (ISO 4037-3, 1999). The reader calibration factors (RCF) for both radiation beams and for both LiF species were periodically evaluated during the experiment to assess the stability of the reading system. For the linearity and energy dependence experiments, sets of five randomly selected detectors of TLD-100 and TLD-100H were always used, inserted into pouches and rings, irradiated and read at the same time with the respective reading cycle, so that the results are directly comparable. Linearity for both the 137 Cs and ISO N120 radiations was tested for the following dose values: 1, 2, 5, 10, 20, 50 and 100 mSv. For the energy dependence experiments, the dosemeters were irradiated to 5 mSv and the following ISO reference radiations were used: N30, N40, N60, N80, N100, N120, 137 Cs and 60 Co (ISO 4037-1, 1996). Since RCF values are available for 137 Cs and N120, the energy dependence results were normalized to these two calibration energies. 3. Results and discussion 3.1. Reproducibility and residual signal The reproducibility of each TLD type, following the 10 irradiation cycles, is presented in Fig. 1 where the results were
1.00 Reproducibility, a.u.
TLD-100
0.95 1.05 TLD-100H 1.00
0.95 1
2
3
4
5
6
7
8
9
10
Irradiation cycle Fig. 1. Reproducibility following 10 irradiation cycles for the two detectors.
6 5 Residual signal, %
Pre-annealing Heating rate (◦ C s−1 ) Maximum temperature attained (◦ C) Reading cycle duration (s)
647
4
TLD-100H
3 TLD-100
2 1 0 1
2
3
4
5
6
7
8
9
10
Irradiation cycle Fig. 2. Residual signals for the two detector types.
normalized to the first cycle. Open circles refer to TLD-100, whereas closed circles to TLD-100H. Error bars are ±1. The results in Fig. 1 show that both the detectors present reproducible results along the 10 irradiation cycles. All values including the error bars are contained by ±2% guides around unity. In Fig. 2, the average residual signal expressed in percentage of the irradiation value measured in Fig. 1 is presented for the same cycles. The same symbols as in Fig. 1 were used. As expected, the residual signals of TLD100H are higher than those of TLD-100 and correspond to about 5% of the readout value. The residuals for TLD-100 correspond to about 1–1.5%. The higher residual value of TLD-100H, when compared to the one of TLD-100, is a well-known feature of the former. Though these results were obtained using the 90 Sr/90 Y internal irradiator, the residual was found to be slightly higher than expected. Second readouts were considered and implemented. Following the linearity test described in Section 3.3 and performed in the dose range 1–100 mSv, the residual signal
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L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650
Fig. 3. ECC distribution in the batch: TLD-100 (light) and TLD-100H (dark).
evaluated (2nd read) after the 100 mSv irradiation in the 137 Cs beam yielded 0.91 and 0.09 mSv, respectively, for TLD-100H and TLD-100. The results are in accord to ISO 12794 (2000), although more easily met with TLD-100 than with TLD-100H. Further tests on the importance of the residual signal as a function of dose and on the evaluation of the detection limits will be carried out for both TLD varieties. The ECCs of the five dosemeters used in this experiment varied between 0.85 and 1.25. Each individual ECC remained stable throughout the 10 cycles (results not shown in the paper). This statement is valid for both materials. 3.2. Distribution of the ECC The distributions of the ECC for each batch of dosemeters are presented in Fig. 3. Each batch of TLD is composed of 100 dosemeters. Although the number of circulating dosemeters is relatively small, the TLD-100 detectors follow a well-shaped Gaussian distribution. However, the TLD-100H dosemeters show a different distribution, which might be explained due to the relatively small number of dosemeters in the batch. Nevertheless, assuming normal distributions for both TLD materials, the estimated variability is 10% for TLD-100 and 9% for TLD-100H. The results for TLD-100H are similar to that reported by others for the same material but for a larger population of detectors (Mariotti et al., 2006). Typical batch homogeneity reported by manufacturers for the LiF:Mg,Cu,P detector (Moscovitch, 1999) is generally higher than the reported value. 3.3. Linearity The results corresponding to the linearity measurements in both 137 Cs and N120 radiation beams are presented in Table 2. Both materials show linear behavior as indicated by the quality of the fit. The results in the 137 Cs beam are better as they are closer to unity. The intercept for the TLD-100H is half the value of the intercept for TLD-100.
Table 2 Linear regression parameters Radiation beam
Dosemeter type
Slope
Intercept
137 Cs
TLD-100 TLD-100H
0.99 0.98
0.40 0.23
N120
TLD-100 TLD-100H
1.04 1.05
−0.03 −0.01
R2 0.9998 0.99997 0.999998 0.9999
Table 3 Reader calibration factors (102 nC mSv−1 ) Reference radiation
TLD-100
TLD-100H
137 Cs
0.1224 (±2.7%) 0.1694 (±3.9%)
0.3170 (±3.0%) 0.4399 (±1.3%)
N120
The linearity results obtained in the ISO N120 beam are approximately 4–5% higher than expected. The intercepts are considerably good and definitely better than for 137 Cs. However, this may be due to the increment of the slope as compared with the 137 Cs results. The reading system with both LiF materials presents a linear behavior in either calibration energies. 3.4. Stability of the reading system The experiments presented were carried out over one year. During this period the reader calibration factors were evaluated four times. The average results obtained for each detector and calibration beam are presented in Table 3. As would have been expected, TLD-100H is more efficient in terms of light emission than TLD-100 independently of the calibration energy. For N120, both materials are also more light efficient than for 137 Cs. During the experimental period, the relative standard deviations of the average reader calibration factors show that the reading system remained stable.
L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650
2.5
2.5 DL 167, 2002
DL 167, 2002
2.0
2.0 ISO12794, 2000
1.5
1.0
0.5
Energy response
Energy response
649
ISO 12794, 2000 1.5
1.0
0.5
0.0
0.0 10
100
1000
Mean energy, keV
10
100
1000
Mean energy, keV
Fig. 4. Energy response of TLD-100 and TLD-100H normalized to 137 Cs without build-up layer: TLD-100 (open circles), TLD-100H (closed circles).
Fig. 6. Energy response of TLD-100 and TLD-100H normalized to 137 Cs with build-up layer: TLD-100 (open circles), TLD-100H (closed circles). Results for TLD-100 normalized to N120 (open triangles).
2.5 DL 167, 2002
Energy response
2.0
ISO 12794, 2000 1.5
1.0
0.5
0.0 10
100 Mean energy, keV
1000
Fig. 5. Energy response of TLD-100 and TLD-100H normalized to N120: TLD-100 (open circles), TLD-100H (closed circles).
3.5. Energy dependence For both detector types the energy dependence of the response at normal incidence was studied. The N30, N40, N60, N80, N100, N120, 137 Cs and 60 Co beams were used and in the last two cases the irradiations were performed with and without the respective build-up layer for attaining conditions of electronic equilibrium. The results were normalized to 137 Cs without the build-up layer, N120 and 137 Cs with build-up layer and are, respectively, presented in Figs. 4–6. In all figures the 0.5 and 1.5 guides allowed by the standard on extremity dosemeters (ISO 12794, 2000), and the 0.5 and 2.0 guides allowed by national requirements (DL 167, 2002) are shown. Again, the same symbols were used: open circles for TLD-100 and closed circles for TLD-100H. Fig. 6 contains one more curve that will be explained later on.
The results presented in Fig. 4 are normalized to 137 Cs without the build-up layer and are all within the guides allowed by national requirements. However, mean energies below N100 (∼ 83 keV) are also all above the ISO 12794 upper guide, for both materials. Considering the normalization to N120 radiation presented in Fig. 5, similar shaped curves were obtained for both materials when compared with Fig. 4 but less spread. The results are well within the guides of the more restrictive ISO 12794 and obviously the guides relative to the national requirements. This statement is valid for all energies except for 60 Co. In this case the measurement underestimates the true dose below the acceptance criteria for both ISO 12794 and national requirements. In previous Figs. 4 and 5 the 137 Cs and 60 Co irradiations were performed without the respective build-up layers. Although (ISO 12794, 2000) is not explicit on their use (ISO 4037-3, 1999) states that when necessary a PMMA build-up plate (. . .) should be used. The experiment was carried out considering the respective build-up layers for the 137 Cs and 60 Co beams and the results in Fig. 6 are normalized to 137 Cs with build-up layer (open and closed circles, as above). Several differences are observed in Figs. 4 and 6 for both TLD varieties. The overall data are less spread as a consequence of the increase in yield of both 137 Cs and 60 Co results clearly suggesting that the previous measurement was performed with lack of electronic equilibrium. On the other hand, the N120, 137 Cs and 60 Co results are closer, particularly in the case of TLD-100H, where the variation does not exceed 3%. For TLD-100, energies below N80 (∼ 65 keV) are above the ISO 12794 upper guide but below the national requirements, and comments similar to the ones made for Fig. 4 are also applicable. However, for TLD-100H all the measurements fit between the guides of both documents. The normalization to the N120 radiation is only important for the TLD-100 results which are presented in Fig. 6 as open
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L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650
triangles. In this case all the data are well within the guides too. For low energies TLD-100 overestimates the true dose by as much as 36% for the case of N30. For higher energies it underestimates the true dose value by 21% for the case of 60 Co. All the results are within the maximum allowed variation ranges set by both ISO 12794 and the national requirements.
However, if positron emission tomography (PET) applications are of concern the system should be calibrated in terms of 137 Cs to avoid unnecessary underestimations of the measured dose. In this paper, only X-ray and gamma radiations were studied. However, the dosemeters’ response should also be tested with beta particle reference radiation fields.
4. Conclusion
References
In this paper, LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) extremity dosemeters of the EXT-RAD type were tested for reproducibility, residual signal, linearity and energy dependence. The experiments were simultaneously carried out with both LiF materials so that the results are directly comparable. The reading system with either detector presents a linear behavior and both materials performed equally well to the tests. As would have been expected LiF:Mg,Cu,P showed an increased residual signal relative to LiF:Mg,Ti. However, further tests are required to evaluate the importance of the residual as a function of the dose and the detection limits of the system. LiF:Mg,Ti, presents a wider energy dependence than LiF:Mg,Cu,P. For both detectors the best results are obtained with the N120 calibration beam, particularly if no build-up layer is used for the irradiations with 137 Cs and 60 Co. In reference conditions, that is, using build-up layers for the higher energies, LiF:Mg,Cu,P performs better than LiF:Mg,Ti, as it shows a minor variation range for the energies studied. Using N120 as a calibration energy seems to be a more realistic setup since the energy of the most frequently used radioisotopes in Nuclear Medicine departments (with SPECT) show average gamma radiation energies closer to N120 than to 137 Cs.
Decreto-Lei no. 167, 2002. Diário da República. Imprensa Nacional—Casa da Moeda, 18 July 2002. Harshaw-Bicron, 1992. TLD Radiation Evaluation and Management System (TLD-REMS) User’s Manual for use with TLD 8800 & 6600 Card Readers. REMS-0-U-0492-006. Bicron, Saint-Gobain/Norton Industrial Ceramics Corporation, Solon, OH, USA. Harshaw-Bicron, 1994. Model 6600E Automatic TLD Workstation User’s Manual. Publication no. 6600-E-U-0294-001. Bicron, Saint-Gobain/Norton Industrial Ceramics Corporation, Solon, OH, USA. 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, Geneva. ISO 4037-3, 1999. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 3: calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. International Organization for Standardization, Geneva. ISO 12794, 2000. Nuclear energy–radiation protection–individual thermoluminescence dosemeters for extremities and eyes. International Organization for Standardization, Geneva. Mariotti, F., Uleri, G., Fantuzzi, E., 2006. Batch homogeneity of LiF(Mg,Cu,P)-GR200 and LiF(Mg,Cu,P)-MCP-NS TL detectors for use as extremity dosemeters at ENEA Personal Dosimetry Service. Radiat. Prot. Dosim. 120 (1–4), 283–288. Moscovitch, M., 1999. Personnel dosimetry using LiF:Mg,Cu,P. Radiat. Prot. Dosim. 85 (1–4), 49–56.
Comparison of LiF (TLD-100 and TLD-100H) detectors for extremity monitoring L. Freire a,b , A. Calado a , J.V. Cardoso a , L.M. Santos a , J.G. Alves a,∗ a Departamento de Protecção Radiológica e Segurança Nuclear, Instituto Tecnológico e Nuclear, E.N. 10, 2683-953 Sacavém, Portugal b Laboratório de Medicina Nuclear, Lda, Atomedical, Rua Helena Félix, 11D, 1600-121 Lisboa, Portugal
Abstract In this work the results aimed at assessing the performance of two types of LiF detectors, TLD-100 and TLD-100H, used in the context of extremity dosimetry are presented. Each detector variety was studied for reproducibility, batch homogeneity, residual dose, linearity and energy dependence using, when appropriate, the 90 Sr/90 Y radiation source built-in one of the Harshaw 6600 readers, the ISO narrow X-ray beams of N30, N40, N60, N80, N100 and N120 or the gamma radiations of 137 Cs and 60 Co. Two calibration energies (N120 and 137 Cs) were also used. The reproducibility and linearity results indicate that both LiF:Mg,Ti and LiF:Mg,Cu,P performed equally well. However, LiF:Mg,Cu,P presents a higher residual signal. In terms of energy dependence, LiF:Mg,Cu,P shows less variation than LiF:Mg,Ti particularly when N120 is used as calibration radiation. This seems to be a more realistic setup since the energy of the most frequently used radioisotopes in Nuclear Medicine departments with single photon emission computed tomography (SPECT) use gamma radiation energies closer to N120 than to 137 Cs. © 2008 Elsevier Ltd. All rights reserved. Keywords: Individual monitoring; Extremity dosemeters; Thermoluminescence
1. Introduction/scope The individual monitoring service (IMS) of ITN-DPRSN is presently performing whole body monitoring to around 3000 workers in Portugal. ITN has recently felt the need to provide extremity monitoring for workers in the field of Nuclear Medicine at ITN, hospitals and private clinics as well as for radiologists in interventional procedures. Two LiF varieties of extremity dosemeters LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) of the EXT-RAD type, for use with the Harshaw 6600 readers, were compared in terms of reproducibility, residual signal, stability of the element correction coefficients, batch homogeneity, linearity and energy dependence. Both 137 Cs and the ISO N120 (ISO 4037-1, 1996) reference radiations were used as calibration energies in this study. The former was used following the indications contained in (ISO 12794, 2000) and also in national requirements (DL 167, 2002). The latter was chosen because the energy of the most frequently ∗ Corresponding author. Tel.: +351 219 946 297; fax: +351 219 941 995.
E-mail address: [email protected] (J.G. Alves). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.12.013
used radionuclides in Nuclear Medicine departments performing single photon emission computed tomography (SPECT) is closer to the mean energy of N120. In this paper, some of the tests performed with both dosemeter varieties will be presented and analyzed. 2. Materials and methods The IMS dosimetry system at ITN-DPRSN is based on two Harshaw 6600 readers. One of the readers incorporates a 90 Sr/90 Y irradiator that is used for the determination of individual element correction coefficients (ECCs), for the irradiation of quality control dosemeters to be read interspaced with field dosemeters during readouts and for experiments that do not require special irradiation conditions. Two hundred EXT-RAD dosemeters, 100 of each LiF variety (TLD-100 and TLD-100H), were purchased from Thermo Electronic Corporation (USA) for these experiments. All detectors followed an initialization procedure before they were considered ready for use, which consisted of 10 cycles of irradiation and readout. ECCs were determined using the 90 Sr/90 Y irradiator. All dosemeters, which are identified as calibration
L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650 Table 1 Reading cycle parameters for the two LiF detectors
1.05 TLD-100 TLD-100H
10 s at 130 ◦ C 15 300 13.3
6 s at 140 ◦ C 15 250 10
or field dosemeters, were individually calibrated following the suggestions found in the readers and software manuals (Harshaw-Bicron, 1992, 1994) and criteria defined in-house. The parameters of the reading cycles used are mentioned in Table 1 and were selected as follows: for TLD-100, the reading cycle is the same as for the whole body detectors; for TLD-100H, the same heating rate was used but the preheating temperature and the highest temperature attained were chosen from the observation of the glow curves. The reproducibility and the residual signal of each type of detectors were studied for five detectors of each variety, chosen at random, which were irradiated and read out for 10 consecutive times, in sequence. The irradiation was carried out with the 90 Sr/90 Y irradiator and the irradiation dose corresponded to approximately 5 mSv. This experiment was also used to evaluate the stability of the ECC of the five detectors. Following the determination of the ECC for all the detectors of each type, the corresponding distributions were assessed. Two calibration energies were always considered, corresponding to the ISO N120 and the 137 Cs reference radiations (ISO 4037-1, 1996). All irradiations were carried out in terms of Hp (0.07) using the ISO rod phantom which consists of a right circular cylinder of PMMA with a diameter of 19 mm and a length of 300 mm (ISO 4037-3, 1999). The reader calibration factors (RCF) for both radiation beams and for both LiF species were periodically evaluated during the experiment to assess the stability of the reading system. For the linearity and energy dependence experiments, sets of five randomly selected detectors of TLD-100 and TLD-100H were always used, inserted into pouches and rings, irradiated and read at the same time with the respective reading cycle, so that the results are directly comparable. Linearity for both the 137 Cs and ISO N120 radiations was tested for the following dose values: 1, 2, 5, 10, 20, 50 and 100 mSv. For the energy dependence experiments, the dosemeters were irradiated to 5 mSv and the following ISO reference radiations were used: N30, N40, N60, N80, N100, N120, 137 Cs and 60 Co (ISO 4037-1, 1996). Since RCF values are available for 137 Cs and N120, the energy dependence results were normalized to these two calibration energies. 3. Results and discussion 3.1. Reproducibility and residual signal The reproducibility of each TLD type, following the 10 irradiation cycles, is presented in Fig. 1 where the results were
1.00 Reproducibility, a.u.
TLD-100
0.95 1.05 TLD-100H 1.00
0.95 1
2
3
4
5
6
7
8
9
10
Irradiation cycle Fig. 1. Reproducibility following 10 irradiation cycles for the two detectors.
6 5 Residual signal, %
Pre-annealing Heating rate (◦ C s−1 ) Maximum temperature attained (◦ C) Reading cycle duration (s)
647
4
TLD-100H
3 TLD-100
2 1 0 1
2
3
4
5
6
7
8
9
10
Irradiation cycle Fig. 2. Residual signals for the two detector types.
normalized to the first cycle. Open circles refer to TLD-100, whereas closed circles to TLD-100H. Error bars are ±1. The results in Fig. 1 show that both the detectors present reproducible results along the 10 irradiation cycles. All values including the error bars are contained by ±2% guides around unity. In Fig. 2, the average residual signal expressed in percentage of the irradiation value measured in Fig. 1 is presented for the same cycles. The same symbols as in Fig. 1 were used. As expected, the residual signals of TLD100H are higher than those of TLD-100 and correspond to about 5% of the readout value. The residuals for TLD-100 correspond to about 1–1.5%. The higher residual value of TLD-100H, when compared to the one of TLD-100, is a well-known feature of the former. Though these results were obtained using the 90 Sr/90 Y internal irradiator, the residual was found to be slightly higher than expected. Second readouts were considered and implemented. Following the linearity test described in Section 3.3 and performed in the dose range 1–100 mSv, the residual signal
648
L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650
Fig. 3. ECC distribution in the batch: TLD-100 (light) and TLD-100H (dark).
evaluated (2nd read) after the 100 mSv irradiation in the 137 Cs beam yielded 0.91 and 0.09 mSv, respectively, for TLD-100H and TLD-100. The results are in accord to ISO 12794 (2000), although more easily met with TLD-100 than with TLD-100H. Further tests on the importance of the residual signal as a function of dose and on the evaluation of the detection limits will be carried out for both TLD varieties. The ECCs of the five dosemeters used in this experiment varied between 0.85 and 1.25. Each individual ECC remained stable throughout the 10 cycles (results not shown in the paper). This statement is valid for both materials. 3.2. Distribution of the ECC The distributions of the ECC for each batch of dosemeters are presented in Fig. 3. Each batch of TLD is composed of 100 dosemeters. Although the number of circulating dosemeters is relatively small, the TLD-100 detectors follow a well-shaped Gaussian distribution. However, the TLD-100H dosemeters show a different distribution, which might be explained due to the relatively small number of dosemeters in the batch. Nevertheless, assuming normal distributions for both TLD materials, the estimated variability is 10% for TLD-100 and 9% for TLD-100H. The results for TLD-100H are similar to that reported by others for the same material but for a larger population of detectors (Mariotti et al., 2006). Typical batch homogeneity reported by manufacturers for the LiF:Mg,Cu,P detector (Moscovitch, 1999) is generally higher than the reported value. 3.3. Linearity The results corresponding to the linearity measurements in both 137 Cs and N120 radiation beams are presented in Table 2. Both materials show linear behavior as indicated by the quality of the fit. The results in the 137 Cs beam are better as they are closer to unity. The intercept for the TLD-100H is half the value of the intercept for TLD-100.
Table 2 Linear regression parameters Radiation beam
Dosemeter type
Slope
Intercept
137 Cs
TLD-100 TLD-100H
0.99 0.98
0.40 0.23
N120
TLD-100 TLD-100H
1.04 1.05
−0.03 −0.01
R2 0.9998 0.99997 0.999998 0.9999
Table 3 Reader calibration factors (102 nC mSv−1 ) Reference radiation
TLD-100
TLD-100H
137 Cs
0.1224 (±2.7%) 0.1694 (±3.9%)
0.3170 (±3.0%) 0.4399 (±1.3%)
N120
The linearity results obtained in the ISO N120 beam are approximately 4–5% higher than expected. The intercepts are considerably good and definitely better than for 137 Cs. However, this may be due to the increment of the slope as compared with the 137 Cs results. The reading system with both LiF materials presents a linear behavior in either calibration energies. 3.4. Stability of the reading system The experiments presented were carried out over one year. During this period the reader calibration factors were evaluated four times. The average results obtained for each detector and calibration beam are presented in Table 3. As would have been expected, TLD-100H is more efficient in terms of light emission than TLD-100 independently of the calibration energy. For N120, both materials are also more light efficient than for 137 Cs. During the experimental period, the relative standard deviations of the average reader calibration factors show that the reading system remained stable.
L. Freire et al. / Radiation Measurements 43 (2008) 646 – 650
2.5
2.5 DL 167, 2002
DL 167, 2002
2.0
2.0 ISO12794, 2000
1.5
1.0
0.5
Energy response
Energy response
649
ISO 12794, 2000 1.5
1.0
0.5
0.0
0.0 10
100
1000
Mean energy, keV
10
100
1000
Mean energy, keV
Fig. 4. Energy response of TLD-100 and TLD-100H normalized to 137 Cs without build-up layer: TLD-100 (open circles), TLD-100H (closed circles).
Fig. 6. Energy response of TLD-100 and TLD-100H normalized to 137 Cs with build-up layer: TLD-100 (open circles), TLD-100H (closed circles). Results for TLD-100 normalized to N120 (open triangles).
2.5 DL 167, 2002
Energy response
2.0
ISO 12794, 2000 1.5
1.0
0.5
0.0 10
100 Mean energy, keV
1000
Fig. 5. Energy response of TLD-100 and TLD-100H normalized to N120: TLD-100 (open circles), TLD-100H (closed circles).
3.5. Energy dependence For both detector types the energy dependence of the response at normal incidence was studied. The N30, N40, N60, N80, N100, N120, 137 Cs and 60 Co beams were used and in the last two cases the irradiations were performed with and without the respective build-up layer for attaining conditions of electronic equilibrium. The results were normalized to 137 Cs without the build-up layer, N120 and 137 Cs with build-up layer and are, respectively, presented in Figs. 4–6. In all figures the 0.5 and 1.5 guides allowed by the standard on extremity dosemeters (ISO 12794, 2000), and the 0.5 and 2.0 guides allowed by national requirements (DL 167, 2002) are shown. Again, the same symbols were used: open circles for TLD-100 and closed circles for TLD-100H. Fig. 6 contains one more curve that will be explained later on.
The results presented in Fig. 4 are normalized to 137 Cs without the build-up layer and are all within the guides allowed by national requirements. However, mean energies below N100 (∼ 83 keV) are also all above the ISO 12794 upper guide, for both materials. Considering the normalization to N120 radiation presented in Fig. 5, similar shaped curves were obtained for both materials when compared with Fig. 4 but less spread. The results are well within the guides of the more restrictive ISO 12794 and obviously the guides relative to the national requirements. This statement is valid for all energies except for 60 Co. In this case the measurement underestimates the true dose below the acceptance criteria for both ISO 12794 and national requirements. In previous Figs. 4 and 5 the 137 Cs and 60 Co irradiations were performed without the respective build-up layers. Although (ISO 12794, 2000) is not explicit on their use (ISO 4037-3, 1999) states that when necessary a PMMA build-up plate (. . .) should be used. The experiment was carried out considering the respective build-up layers for the 137 Cs and 60 Co beams and the results in Fig. 6 are normalized to 137 Cs with build-up layer (open and closed circles, as above). Several differences are observed in Figs. 4 and 6 for both TLD varieties. The overall data are less spread as a consequence of the increase in yield of both 137 Cs and 60 Co results clearly suggesting that the previous measurement was performed with lack of electronic equilibrium. On the other hand, the N120, 137 Cs and 60 Co results are closer, particularly in the case of TLD-100H, where the variation does not exceed 3%. For TLD-100, energies below N80 (∼ 65 keV) are above the ISO 12794 upper guide but below the national requirements, and comments similar to the ones made for Fig. 4 are also applicable. However, for TLD-100H all the measurements fit between the guides of both documents. The normalization to the N120 radiation is only important for the TLD-100 results which are presented in Fig. 6 as open
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triangles. In this case all the data are well within the guides too. For low energies TLD-100 overestimates the true dose by as much as 36% for the case of N30. For higher energies it underestimates the true dose value by 21% for the case of 60 Co. All the results are within the maximum allowed variation ranges set by both ISO 12794 and the national requirements.
However, if positron emission tomography (PET) applications are of concern the system should be calibrated in terms of 137 Cs to avoid unnecessary underestimations of the measured dose. In this paper, only X-ray and gamma radiations were studied. However, the dosemeters’ response should also be tested with beta particle reference radiation fields.
4. Conclusion
References
In this paper, LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) extremity dosemeters of the EXT-RAD type were tested for reproducibility, residual signal, linearity and energy dependence. The experiments were simultaneously carried out with both LiF materials so that the results are directly comparable. The reading system with either detector presents a linear behavior and both materials performed equally well to the tests. As would have been expected LiF:Mg,Cu,P showed an increased residual signal relative to LiF:Mg,Ti. However, further tests are required to evaluate the importance of the residual as a function of the dose and the detection limits of the system. LiF:Mg,Ti, presents a wider energy dependence than LiF:Mg,Cu,P. For both detectors the best results are obtained with the N120 calibration beam, particularly if no build-up layer is used for the irradiations with 137 Cs and 60 Co. In reference conditions, that is, using build-up layers for the higher energies, LiF:Mg,Cu,P performs better than LiF:Mg,Ti, as it shows a minor variation range for the energies studied. Using N120 as a calibration energy seems to be a more realistic setup since the energy of the most frequently used radioisotopes in Nuclear Medicine departments (with SPECT) show average gamma radiation energies closer to N120 than to 137 Cs.
Decreto-Lei no. 167, 2002. Diário da República. Imprensa Nacional—Casa da Moeda, 18 July 2002. Harshaw-Bicron, 1992. TLD Radiation Evaluation and Management System (TLD-REMS) User’s Manual for use with TLD 8800 & 6600 Card Readers. REMS-0-U-0492-006. Bicron, Saint-Gobain/Norton Industrial Ceramics Corporation, Solon, OH, USA. Harshaw-Bicron, 1994. Model 6600E Automatic TLD Workstation User’s Manual. Publication no. 6600-E-U-0294-001. Bicron, Saint-Gobain/Norton Industrial Ceramics Corporation, Solon, OH, USA. 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, Geneva. ISO 4037-3, 1999. X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—part 3: calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. International Organization for Standardization, Geneva. ISO 12794, 2000. Nuclear energy–radiation protection–individual thermoluminescence dosemeters for extremities and eyes. International Organization for Standardization, Geneva. Mariotti, F., Uleri, G., Fantuzzi, E., 2006. Batch homogeneity of LiF(Mg,Cu,P)-GR200 and LiF(Mg,Cu,P)-MCP-NS TL detectors for use as extremity dosemeters at ENEA Personal Dosimetry Service. Radiat. Prot. Dosim. 120 (1–4), 283–288. Moscovitch, M., 1999. Personnel dosimetry using LiF:Mg,Cu,P. Radiat. Prot. Dosim. 85 (1–4), 49–56.