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A portable internal contamination monitor with dual detectors for screening in a large-scale radiological incident
Applied Radiation and Isotopes 154 (2019) 108858
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A portable internal contamination monitor with dual detectors for screening in a large-scale radiological incident
T
⁎
Lei Wanga, , Yi-ni Wua, Jie Panc, Jing Ningc, Xin Liua, Xue-lan Yuana, Ting Zhanga, Wei Lua, ⁎⁎ Xian-guo Tuob, a
Chengdu University of Technology, Chengdu, 610059, China Sichuan University of Science&Engineering, Zigong, 643000, China c Beijing Institute of Radiation Medicine, Beijing, 100850, China b
H I GH L IG H T S
MDA of the portable internal contamination monitor had been test by a Solid BOMAN Phantom for China Reference Adult Men and reached excellent • The detection limits for Cs-137. • The monitor with compact structure and low cost can be applied in the rapid test for large-scale screening, especially in the high population density areas.
A R T I C LE I N FO
A B S T R A C T
Keywords: Internal contamination Radiologic incident Large-scale screening Digital gamma spectrometer
In the case of a radiological incident, large numbers of affected people should get rapid internal contamination screening, so a portable internal contamination monitor for large-scale application has been developed. It comprises dual detectors, a digital gamma spectrometer, and analysis software. Experiments carried out with a Chinese adult man model. Because of the inadequate shielding and poor detector resolution, the monitor is not sensitive to the lower energy emitters. However, it shows the excellent performance for the emitters above 661 keV. MDA for Cs-137, Y-88, and Co-60 reached 320Bq, 300Bq, and 530Bq in 5-min measurement. Due to the strong mobility, considerable detection limit, and low cost, the monitor can be applied to the rapid internal contamination screening in a radiological incident.
1. Introduction The radiologic incident at nuclear power plant and the nuclear terrorist attack have become the potential radiological crisis. A radiological incident involves the release of large amounts of artificial radionuclides into the public environment, so it could be very necessary to fulfill a rapid internal radioactive contamination measurement for a large group of contaminated individuals in an inner cordon (Chunsheng Li et al., 2016; Scuffham et al., 2016; Youngman, 2015; Hosoda et al., 2013; Manger et al., 2012). Due to the high population density in the big cities and areas close to the nuclear plants in China, a large-scale radiological triage is a great practical challenge in the radiological incident (Nomura et al., 2016; Declan Butler, 2017). Although the low-background-level whole-body counter (WBC) is regularly used to monitor the internal contamination of individual, only
⁎
a few of centers for disease control and prevention in big cities are equipped with WBC in China. On the other hand, the nuclear medicine machines, such as Gamma cameras, single photon emission computed tomography/computed tomography (SPECT/CT), mainly equipped in the top-tier 3A hospitals, are designed for the diagnostic purpose and need further complex calibration for the internal contamination detection. Due to the lack of mobility and availability of whole-body counters and nuclear medicine machines, the diagnostic equipment can't be promptly used in the rapid measurement for a large-scale application. However, we should pay more attention to the rapid response and large-scale quick deployment of the initial internal contamination screening than the sensitivities of the instrument. Some simple portable instruments with single detector, such as NaI detector, plastic detector, HPGe detector, have been developed for the internal contamination screening (Wi-Ho Ha et al., 2017; Thompson
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (X.-g. Tuo).
⁎⁎
https://doi.org/10.1016/j.apradiso.2019.108858 Received 13 April 2019; Received in revised form 9 August 2019; Accepted 13 August 2019 Available online 13 August 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
et al., 2011; Muikku and Rahola, 2007). These measurement systems comprised general gamma counter or spectrometer. Even after calibration, the measurement process of the general software often involves several independent steps, and the measurement data needs further processing to get the final result. At the same time, the portable instruments with semiconductor detector, such as HPGe detector, are very expensive. So we developed a portable internal contamination monitor which has a professional analysis software operated simply with a wizard after calibration. It also has considerable Minimum Detection Activity (MDA) and low cost and be suitable for large-scale application.
Table 1 The activities of the nuclides in the sBCAM.
2. Materials and methods
Nuclides
Energy/keV
Activity/Bq
Uncertainty of the relative expansion (k = 2)/%
Am-241 Cd-109 Co-57 Ce-139 Hg-203 Sn-113 Cs-137 Y-88
59.5 88 122.1 165.9 279.2 391.7 661.7 898 1836.1 1173.2 1332.5
5.69E+03 5.66E+04 1.24E+03 8.44E+02 2.03E+03 1.19E+03 2.22E+03 1.71E+03
5.6 5.4 6.0 6.0 5.4 5.8 5.8 4.4
3.36E+03
5.2
Co-60
2.1. sBCAM model The sBCAM(Solid BOMAB Phantom for China Reference Adult Men), designed by China Institute of Radiation Protection, represents a Chinese adult man with 170 cm height and 70 kg weight. The various size solid right circular and elliptical cylinders made the different part of body such as human's head, neck, arm, chest, abdomen, thigh and leg. The model was made of the ultra-high molecular weight polyethylene (UPE) with the density of 0.98 g/cm3 ± 5%. Compared with the soft tissue, the relative deviation of the mass attenuation coefficient of UPE is less than 3% when the energy of the photon varies from 100 keV to 3 MeV. A total of 1962 solid point sources are placed evenly in the sBCAM. The dispersion of the point sources is better than 2%. The sBCAM can simulate the uniform distribution of the radioactive source pretty well, as shown in Fig. 1. There are 9 radioactive nuclides, such as Am-241,Cd-109, Co-57, Ce-139, Hg-203, Sn-113,Cs-137, Y-88, and Co-60 in the sBCAM model. The activities of the nuclides in the sBCAM are listed in Table 1. 2.2. Structure of a portable internal contamination monitor
Fig. 2. The structure of the portable internal contamination monitor (PICM) designed in this paper.
A portable internal contamination monitor (PICM) was designed, as shown in Fig. 2. It comprises a measurement unit and dual detectors. Each detector is a 3″ NaI(Tl) detector surrounded by a Lead shield with 6 mm thickness. The measurement unit mainly consists of a digital gamma spectrometer sealed in an Aluminum case, slider tracks, regulating handles, lithium battery, charging interface, and USB interface. The spacing between the dual detectors can be adjusted by regulating handles according to the chest width of the person measured. In this study, we only employed one adult male phantom to investigate the detection limit, so we fixed the facing between detectors in the experiments. The digital gamma spectrometer, based on Field-Programmable Gate Array (FPGA), comprises a high-speed Analog to Digital Converter, a digital signal processing unit, a digital baseline recovery, a trapezoidal filter, a peak identification module, and a histogram memory module, as illustrated in Fig. 3.
2.3. Analysis software An analysis software working with PICM, mainly includes a data acquisition module, a hardware parameters module, an energy calibration module, an efficiency calibration module, a full energy peak calculation module, a region of interest calculation module, an activity calculation module, and a database. The three-tier architecture and the function diagram of the analysis software are shown in Fig. 4 and Fig. 5. 2.4. Experiments We performed experiments in the internal irradiation physics laboratory of China Institute for Radiation Protection (CIRP). Firstly, PICM was utilized to acquire the natural background in the laboratory. And then the radioactivity was measured in sBCAM and the distances between PICM and sBCAM varied from 0 to 15 cm with an interval of 5 cm, as shown in Fig. 6. 3. Results Fig. 7 presents the spectra of natural background acquired by PICM in 5 h in the internal irradiation physics laboratory. Peaks of K-40 at 1460.8 keV and Tl-208 at 2614 keV are significant. The 6 mm-Lead shield also reduced the count rate caused by the background radiation, especially in the low energy region. Fig. 8 demonstrates the gamma spectral of sMBCAM acquired by PICM in natural environment. Peaks of artificial nuclides, including Cs137 at 662 keV, Y-88 at 898 keV, Co-60 at 1173.2 keV and 1332.5 keV, are recognized easily. But the nuclides such as Am-24, Cd-109, Co-57,
Fig. 1. The sBCAM, designed by China Institute of Radiation Protection, represents a Chinese adult man with 170 cm height and 70 kg weight. 2
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Fig. 3. The digital gamma spectrometer.
emitters below 661 keV, such as Mo-99(140.5 keV) Ce-141(145 keV),I131(365 keV), but also the emitters above 661 keV, such as Cs137(662 keV),Cs-134(604 keV,795 keV),Zr-75(724 keV,756 keV),Mn55(834.85 keV),Co-60(1173 keV,1332 keV). Although the monitor proposed is not sensitive to the peaks bellow 661 keV, it still can be used to detect some important nuclides released in radiologic incidents, such as Cs-137,Cs-134,Zr-75,Mn-55,Co-60, and play an important role in on-site nuclear emergency rescue. Under each condition in Table 2, each experiment was repeated 12 times, and the average of the counts was used to calculate the corresponding MDAs. Actually, according to Table 2, at 662 keV, the MDA with the 6 mm-Lead shield is slightly better than that without 6 mmLead shield, and there are no significantly different between them from 898 keV to 1332 keV. So the 6 mm-Lead shield has no remarkable affection on MDAs above 662 keV. There are 3 reasons mostly determining the MDAs in Table 2. (1) The detection efficiency of the NaI detector for the source decreases as the ray energy increases, (2) the attenuation coefficient of Lead for gamma-ray from 662 keV to about
Ce-139, Hg-203, Sn-113 evenly distributed in the sMBCAM, emitted low-energy gamma rays which can be more dramatically attenuated by the model's material than higher gamma rays emitted by Cs-137, Y-88, Co-60, and overwhelmed by natural background. So we didn't get the peaks of Am-241, Cd-109, Co-57, Ce-139, Hg-203, Sn-113 in Fig. 8. Fig. 9 presents the contribution of the natural background in ROIs of Cs137, Y-88, Co-60 at 662 keV, 898 keV, 1173.2 keV and 1332.5 keV. It shows that the natural background has been reduced considerably in the ROIs of Cs-137, which suggests the partial Lead shield may help improve MDA at this point. The net count rate in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV) is shown in Fig. 10. The measured value at the distance of 0 cm are higher than ones at other distances. The MDA obtained by the monitor with and without 6 mm-Lead shields at 0–15 cm distance, as shown in Fig. 11, Fig. 12 and Table 2. Due to the inadequate shielding and poor detector resolution, the monitor did not obtain the peaks below 661 keV. However, the radionuclides released in the radiologic incidents include not only the
Fig. 4. The three-tier architecture of the analysis software. 3
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Fig. 5. The function diagram of the analysis software.
1 MeV quickly decreases as a function of energy, and then it tends to be flat in the 1MeV–1.5 MeV, (3) the close detection geometry. From 662 keV to about 1.0 MeV, gamma rays are easier to go through the phantom and reach the detectors, and the rapid decrease of the attenuation coefficient of Lead makes more rays penetrate the shielding material, which can largely improve the detection efficiency with and without Lead shield, especially in the close detection geometry. Therefore, from 662 keV to about 1.0 MeV, the increased detection efficiency dominants the MDAs and get the MDA to reach to the lowest point at 898 keV in Table 2. From 1.0 MeV to about 1.5 MeV, the attenuation curve of the Lead shield tends to be flat. At the same time, the detector's detection efficiency for high-energy rays continues to decrease as a function of energy, which dramatically decreases the detection efficiency for the high-energy nuclides in the phantom. Therefore, the MDAs, from 898 keV to 1332 keV, increase gradually in Table 2. According to Table 2, MDA without 6 mm-Lead shield at 1173 keV was a little bit higher than that at 1332 keV, this is mainly because the background count rates without Lead shield near 1173 keV are higher than that near 1332 keV, as shown in Fig. 8(b)(black) and Fig. 9(red). The higher background principally makes the MDA without Lead shield at 1173 keV higher than that at 1332 keV. MDAs for Cs-137, Y-88, and Co-60 reached 320, 300, and 530 Bq respectively at the distance of 0 cm in our experiments. MDAs for Cs137 from several research groups are listed in Table 3 (Scuffham et al., 2016; Youngman, 2015). In the different applications, the measure times listed in Table 3 are significantly different. This is because the measurement speed and detection limit are contradictory in the detection of the nuclear emergency condition. Researchers usually need to keep a balance between them. In our study, we hope to complete one test in 5 min, so we focused on the 5-min detection limit. If 100 monitors were deployed to the on-site at the same time, 6000 individuals would be tested within 5 h. It can significantly improve screening efficiency. The standing posture was employed in our work, which was different from the sitting posture adopted in other studies, as shown in Fig. 6. When the monitor was close to the upper body of the sBCAM, it obtained a good minimum detectable activity for Cs-137 uniformly distributed in the sBCAM. In fact, the close geometry means at a short time after intake activity in the lungs will dominate efficiency. The close geometry is likely to overestimate the internal activity, which could involve to some extent more contaminated individuals accepting further test by the whole-body counter (WBC) or lung counter (LC) in professional labs and hospitals. However, it still can exclude the vast majority of individuals without internal contamination and the number
Fig. 6. The PICM was employed to measure the radioactivity in sBCAM.
Fig. 7. Spectral of natural background in the internal irradiation physics laboratory acquired by PICM with or without 6 mm Lead shield.
4
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Fig. 8. Spectral acquired by PICM by measuring the artificial radionuclides in the sMBCAM. (a) Dual detectors with Lead shield. (b) Dual detectors without 6 mm Lead shield.
Fig. 9. The natural background count rates registered by PICM with and without Lead shield in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV).
Fig. 11. MDA of Cs-137, Y-88, and Co-60 at different measurement distances with Lead shield in 5-min measurements.
Fig. 12. MDA of Cs-137, Y-88, and Co-60 at different measurement distances without Lead shield in 5-min measurements.
Fig. 10. Net count rates in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV) registered by PCIM with 6 mm-Lead shield at different distances.
5
528 585 604 657
530 594 625 681
Co-60 (1332 keV)
0 5 10 15
505 575 658 719
575 607 660 880
of contaminated individuals screened can be acceptable to professional labs and hospitals. If the monitor proposed in this paper was used to measure the lung internal contamination, the spacing between the dual-detectors can be adjusted by regulating handles to make two detectors aim at the double lungs of individuals. The close measurement geometry can offer high detection efficiency. In lung measurement, the user also needs a lung model to calibrate the monitor. Therefore, if the monitor employed to measure the lung internal contamination, it can get higher detection efficiency and lower detection limit. Considering the 3 “NaI detector widely used in nuclear radiation measurement community is low cost and easy to get, and the portable measurement system developed in this paper costs less than $10,000, and has a significant low-cost advantage in large-scale deployment. 4. Conclusion In order to meet the large-scale monitoring for internal contamination in the potential radiological emergencies, a portable internal contamination monitor has been developed. The monitor includes dual detectors (each of them is a 3″ NaI detector surrounded by a Lead shield with 6 mm thickness), digital gamma spectrometer, and an analysis software. PICM was employed to measure the radioactivity in the sBCAM. It provides desirable detection limits for Cs-137, Y-88 and Co-60 distributed in the whole-body. MDA for Cs-137, Y-88 and Co-60 achieved 320, 300, and 530 Bq respectively in 5-min measurement. Because of the inadequate shielding and poor detector resolution, the monitor is not sensitive to the lower energy emitters. However, it still can be used to detect some important nuclides released in radiological incidents, such as Cs-137,Cs-134,Zr-75,Mn-55,Co-60. Due to the compact structure and low cost, the monitor can be applied in the rapid test for large-scale screening, especially in the high population density areas. Acknowledgements We thank China Institute of Radiation Protection for providing the sBCAM(Solid BOMAB Phantom for China Reference Adult Men) and help in our experiments. This study also benefits from the major scientific instruments and equipment development project of the Ministry of Science and Technology, People's Republic of China (Contract Grant No.2012YQ180118). This work also was supported by the National 6
c
0 5 10 15
b
Co-60 (1173 keV)
a
300 311 314 332
Smith et al. (1994) Lahham and Fulop (1997) Castagnet et al. (2007) Castagnet et al. (2007) Muikku and Rahola (2007) Youngman (2015) Reported in this paper(2019)
300 299 301 309
0.0003 0.006 0.07 0.001 0.02 2 0.0036
0 5 10 15
18 450 5000 100 1200 120000 320
Y-88 (898 keV)
5 Nal(TI) Detectors each 10 × 15 cm Diameter Nal(TI) 12.5 × 12.5 cm Diameter Nal(TI) 5 × 5 cm Diameter Nal(TI) 7.6 × 13 × 41 cm Nal(TI) 5.1 × 5.1 cm Diameter Nal(TI) 0.25 × 3.2 cm Diameter 2 NaI(Tl) Detectors each 7.6 × 7.6 cm Diameter
320 320 337 358
Well shielded Laboratory based Well shielded Mobile Well shielded Mobile Well shielded Mobile Mobile Gamma-ray Spectrometer Hand-held Instrument Portable internal contamination monitor with 6 mm Lead shield
360 379 395 405
Approximate Minimum detectable does, mSvb
0 5 10 15
Dla, Bq
Cs-137 (662 keV)
Measurement time(mins)
with Lead shields
Detector type and size
without Lead shields
Reference
MDA
System
Distance(cm)
Table 3 Reported detection limits for Cs-137 and effective dose corresponding to whole body activity at the level of the detection limit (Youngman et al., 2015).
Nuclides
45 10 10 10 1.7 0.3c 5
Table 2 The MDA obtained by the monitor with and without 6 mm-Lead shields at 0–15 cm distance in 5-min measurements.
For the 95% confidence level. Assumes whole body activity at the DL and intake by acute inhalation occurred 1 d before measurement. Dose calculated for adults, assuming Activity Median Aerodynamic Diameter of 5 μm, absorption Type F. Instantaneous count rate is indicated but the reading should be viewed for about 20 s to obtain an approximate average value.
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Natural Science Foundation of China(Grant No.41874121) and Technology Support Program of Sichuan province (No.18JY0181).
Dosim. 105 (1), 101–108. Muikku, M., Rahola, T., 2007. Improvement of the measuring equipment used in the assessment of internal doses in emergency situation. Radiat. Prot. Dosim. (127), 277–281. Nomura, S., Tsubokura, M., Furutani, T., Hayano, R.S., Kami, M., Kanazawa, Y., et al., 2016. Dependence of radiation dose on the behavioral patterns among school children. J. Radiat. Res. 57 (1), 1–8. Scuffham, J.W., Yip-Braidley, M., Shutt, A.L., et al., 2016. Adapting clinical gamma cameras for body monitoring in the event of a large-scale radiological incident. Soc. Radiol. Prot. (36), 363–381. Smith, J., Marsh, J., Etherington, G., Shutt, A., et al., 1994. Evaluation of a high purity germanium detector. Radiat. Prot. Dosim. (53), 73–75. Thompson, N.J., Youngman, M.J., Moody, J., et al., 2011. Radiation Monitoring Units: Planning and Operational Guidance. HPA Centre for Radiation, Chemical and Environmental Hazards. Ha, Wi-Ho, Kim, Jong Kyung, Jin, Young Woo, et al., 2017. Estimation of counting efficiencies of a portable NaI detector using Monte Carlo simulation for thyroid measurement following nuclear accidents. J. Radiol. Prot. 635–641. Youngman, M.J., 2015. Review of methods to measure internal contamination in an emergency. J. Radiol. Prot. (35), R1–R15.
References Castagnet, X., Amabile, J.C., Cazoulat, A., et al., 2007. Diagnosis of internal radionuclide contamination by mobile laboratories. Radiat. Prot. Dosim. (125), 469–471. Li, Chunsheng, Ansari, Armin, Etherington, George, 2016. Managing internal radiation contamination following an emergency: identification of gaps and priorities. Radiat. Prot. Dosim. (171), 78–84. Butler, Declan, 2017. Reactors, Residents and Risk. http://www.nature.com/news/ 2011/110421/full/472400a.html. Hosoda, Masahiro, Tokonami, Shinji, Akiba, Suminori, et al., 2013. Estimation of internal exposure of the thyroid to 131I on the basis of 134Cs accumulated in the body among evacuees of the Fukushima Daiichi Nuclear Power Station accident. Environ. Int. (61), 73–76. Lahham, A., Fulop, M., 1997. A mobile whole body counter for accident and occupational monitoring of internal contamination. Radiat. Prot. Dosim. (71), 41–45. Manger, R.P., Hertel1, N.E., Burgett, E.A., et al., 2012. Using handheld plastic scintillator detectors to triage individuals exposed to a radiological dispersal device. Radiat. Prot.
7
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A portable internal contamination monitor with dual detectors for screening in a large-scale radiological incident
T
⁎
Lei Wanga, , Yi-ni Wua, Jie Panc, Jing Ningc, Xin Liua, Xue-lan Yuana, Ting Zhanga, Wei Lua, ⁎⁎ Xian-guo Tuob, a
Chengdu University of Technology, Chengdu, 610059, China Sichuan University of Science&Engineering, Zigong, 643000, China c Beijing Institute of Radiation Medicine, Beijing, 100850, China b
H I GH L IG H T S
MDA of the portable internal contamination monitor had been test by a Solid BOMAN Phantom for China Reference Adult Men and reached excellent • The detection limits for Cs-137. • The monitor with compact structure and low cost can be applied in the rapid test for large-scale screening, especially in the high population density areas.
A R T I C LE I N FO
A B S T R A C T
Keywords: Internal contamination Radiologic incident Large-scale screening Digital gamma spectrometer
In the case of a radiological incident, large numbers of affected people should get rapid internal contamination screening, so a portable internal contamination monitor for large-scale application has been developed. It comprises dual detectors, a digital gamma spectrometer, and analysis software. Experiments carried out with a Chinese adult man model. Because of the inadequate shielding and poor detector resolution, the monitor is not sensitive to the lower energy emitters. However, it shows the excellent performance for the emitters above 661 keV. MDA for Cs-137, Y-88, and Co-60 reached 320Bq, 300Bq, and 530Bq in 5-min measurement. Due to the strong mobility, considerable detection limit, and low cost, the monitor can be applied to the rapid internal contamination screening in a radiological incident.
1. Introduction The radiologic incident at nuclear power plant and the nuclear terrorist attack have become the potential radiological crisis. A radiological incident involves the release of large amounts of artificial radionuclides into the public environment, so it could be very necessary to fulfill a rapid internal radioactive contamination measurement for a large group of contaminated individuals in an inner cordon (Chunsheng Li et al., 2016; Scuffham et al., 2016; Youngman, 2015; Hosoda et al., 2013; Manger et al., 2012). Due to the high population density in the big cities and areas close to the nuclear plants in China, a large-scale radiological triage is a great practical challenge in the radiological incident (Nomura et al., 2016; Declan Butler, 2017). Although the low-background-level whole-body counter (WBC) is regularly used to monitor the internal contamination of individual, only
⁎
a few of centers for disease control and prevention in big cities are equipped with WBC in China. On the other hand, the nuclear medicine machines, such as Gamma cameras, single photon emission computed tomography/computed tomography (SPECT/CT), mainly equipped in the top-tier 3A hospitals, are designed for the diagnostic purpose and need further complex calibration for the internal contamination detection. Due to the lack of mobility and availability of whole-body counters and nuclear medicine machines, the diagnostic equipment can't be promptly used in the rapid measurement for a large-scale application. However, we should pay more attention to the rapid response and large-scale quick deployment of the initial internal contamination screening than the sensitivities of the instrument. Some simple portable instruments with single detector, such as NaI detector, plastic detector, HPGe detector, have been developed for the internal contamination screening (Wi-Ho Ha et al., 2017; Thompson
Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (X.-g. Tuo).
⁎⁎
https://doi.org/10.1016/j.apradiso.2019.108858 Received 13 April 2019; Received in revised form 9 August 2019; Accepted 13 August 2019 Available online 13 August 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
et al., 2011; Muikku and Rahola, 2007). These measurement systems comprised general gamma counter or spectrometer. Even after calibration, the measurement process of the general software often involves several independent steps, and the measurement data needs further processing to get the final result. At the same time, the portable instruments with semiconductor detector, such as HPGe detector, are very expensive. So we developed a portable internal contamination monitor which has a professional analysis software operated simply with a wizard after calibration. It also has considerable Minimum Detection Activity (MDA) and low cost and be suitable for large-scale application.
Table 1 The activities of the nuclides in the sBCAM.
2. Materials and methods
Nuclides
Energy/keV
Activity/Bq
Uncertainty of the relative expansion (k = 2)/%
Am-241 Cd-109 Co-57 Ce-139 Hg-203 Sn-113 Cs-137 Y-88
59.5 88 122.1 165.9 279.2 391.7 661.7 898 1836.1 1173.2 1332.5
5.69E+03 5.66E+04 1.24E+03 8.44E+02 2.03E+03 1.19E+03 2.22E+03 1.71E+03
5.6 5.4 6.0 6.0 5.4 5.8 5.8 4.4
3.36E+03
5.2
Co-60
2.1. sBCAM model The sBCAM(Solid BOMAB Phantom for China Reference Adult Men), designed by China Institute of Radiation Protection, represents a Chinese adult man with 170 cm height and 70 kg weight. The various size solid right circular and elliptical cylinders made the different part of body such as human's head, neck, arm, chest, abdomen, thigh and leg. The model was made of the ultra-high molecular weight polyethylene (UPE) with the density of 0.98 g/cm3 ± 5%. Compared with the soft tissue, the relative deviation of the mass attenuation coefficient of UPE is less than 3% when the energy of the photon varies from 100 keV to 3 MeV. A total of 1962 solid point sources are placed evenly in the sBCAM. The dispersion of the point sources is better than 2%. The sBCAM can simulate the uniform distribution of the radioactive source pretty well, as shown in Fig. 1. There are 9 radioactive nuclides, such as Am-241,Cd-109, Co-57, Ce-139, Hg-203, Sn-113,Cs-137, Y-88, and Co-60 in the sBCAM model. The activities of the nuclides in the sBCAM are listed in Table 1. 2.2. Structure of a portable internal contamination monitor
Fig. 2. The structure of the portable internal contamination monitor (PICM) designed in this paper.
A portable internal contamination monitor (PICM) was designed, as shown in Fig. 2. It comprises a measurement unit and dual detectors. Each detector is a 3″ NaI(Tl) detector surrounded by a Lead shield with 6 mm thickness. The measurement unit mainly consists of a digital gamma spectrometer sealed in an Aluminum case, slider tracks, regulating handles, lithium battery, charging interface, and USB interface. The spacing between the dual detectors can be adjusted by regulating handles according to the chest width of the person measured. In this study, we only employed one adult male phantom to investigate the detection limit, so we fixed the facing between detectors in the experiments. The digital gamma spectrometer, based on Field-Programmable Gate Array (FPGA), comprises a high-speed Analog to Digital Converter, a digital signal processing unit, a digital baseline recovery, a trapezoidal filter, a peak identification module, and a histogram memory module, as illustrated in Fig. 3.
2.3. Analysis software An analysis software working with PICM, mainly includes a data acquisition module, a hardware parameters module, an energy calibration module, an efficiency calibration module, a full energy peak calculation module, a region of interest calculation module, an activity calculation module, and a database. The three-tier architecture and the function diagram of the analysis software are shown in Fig. 4 and Fig. 5. 2.4. Experiments We performed experiments in the internal irradiation physics laboratory of China Institute for Radiation Protection (CIRP). Firstly, PICM was utilized to acquire the natural background in the laboratory. And then the radioactivity was measured in sBCAM and the distances between PICM and sBCAM varied from 0 to 15 cm with an interval of 5 cm, as shown in Fig. 6. 3. Results Fig. 7 presents the spectra of natural background acquired by PICM in 5 h in the internal irradiation physics laboratory. Peaks of K-40 at 1460.8 keV and Tl-208 at 2614 keV are significant. The 6 mm-Lead shield also reduced the count rate caused by the background radiation, especially in the low energy region. Fig. 8 demonstrates the gamma spectral of sMBCAM acquired by PICM in natural environment. Peaks of artificial nuclides, including Cs137 at 662 keV, Y-88 at 898 keV, Co-60 at 1173.2 keV and 1332.5 keV, are recognized easily. But the nuclides such as Am-24, Cd-109, Co-57,
Fig. 1. The sBCAM, designed by China Institute of Radiation Protection, represents a Chinese adult man with 170 cm height and 70 kg weight. 2
Applied Radiation and Isotopes 154 (2019) 108858
L. Wang, et al.
Fig. 3. The digital gamma spectrometer.
emitters below 661 keV, such as Mo-99(140.5 keV) Ce-141(145 keV),I131(365 keV), but also the emitters above 661 keV, such as Cs137(662 keV),Cs-134(604 keV,795 keV),Zr-75(724 keV,756 keV),Mn55(834.85 keV),Co-60(1173 keV,1332 keV). Although the monitor proposed is not sensitive to the peaks bellow 661 keV, it still can be used to detect some important nuclides released in radiologic incidents, such as Cs-137,Cs-134,Zr-75,Mn-55,Co-60, and play an important role in on-site nuclear emergency rescue. Under each condition in Table 2, each experiment was repeated 12 times, and the average of the counts was used to calculate the corresponding MDAs. Actually, according to Table 2, at 662 keV, the MDA with the 6 mm-Lead shield is slightly better than that without 6 mmLead shield, and there are no significantly different between them from 898 keV to 1332 keV. So the 6 mm-Lead shield has no remarkable affection on MDAs above 662 keV. There are 3 reasons mostly determining the MDAs in Table 2. (1) The detection efficiency of the NaI detector for the source decreases as the ray energy increases, (2) the attenuation coefficient of Lead for gamma-ray from 662 keV to about
Ce-139, Hg-203, Sn-113 evenly distributed in the sMBCAM, emitted low-energy gamma rays which can be more dramatically attenuated by the model's material than higher gamma rays emitted by Cs-137, Y-88, Co-60, and overwhelmed by natural background. So we didn't get the peaks of Am-241, Cd-109, Co-57, Ce-139, Hg-203, Sn-113 in Fig. 8. Fig. 9 presents the contribution of the natural background in ROIs of Cs137, Y-88, Co-60 at 662 keV, 898 keV, 1173.2 keV and 1332.5 keV. It shows that the natural background has been reduced considerably in the ROIs of Cs-137, which suggests the partial Lead shield may help improve MDA at this point. The net count rate in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV) is shown in Fig. 10. The measured value at the distance of 0 cm are higher than ones at other distances. The MDA obtained by the monitor with and without 6 mm-Lead shields at 0–15 cm distance, as shown in Fig. 11, Fig. 12 and Table 2. Due to the inadequate shielding and poor detector resolution, the monitor did not obtain the peaks below 661 keV. However, the radionuclides released in the radiologic incidents include not only the
Fig. 4. The three-tier architecture of the analysis software. 3
Applied Radiation and Isotopes 154 (2019) 108858
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Fig. 5. The function diagram of the analysis software.
1 MeV quickly decreases as a function of energy, and then it tends to be flat in the 1MeV–1.5 MeV, (3) the close detection geometry. From 662 keV to about 1.0 MeV, gamma rays are easier to go through the phantom and reach the detectors, and the rapid decrease of the attenuation coefficient of Lead makes more rays penetrate the shielding material, which can largely improve the detection efficiency with and without Lead shield, especially in the close detection geometry. Therefore, from 662 keV to about 1.0 MeV, the increased detection efficiency dominants the MDAs and get the MDA to reach to the lowest point at 898 keV in Table 2. From 1.0 MeV to about 1.5 MeV, the attenuation curve of the Lead shield tends to be flat. At the same time, the detector's detection efficiency for high-energy rays continues to decrease as a function of energy, which dramatically decreases the detection efficiency for the high-energy nuclides in the phantom. Therefore, the MDAs, from 898 keV to 1332 keV, increase gradually in Table 2. According to Table 2, MDA without 6 mm-Lead shield at 1173 keV was a little bit higher than that at 1332 keV, this is mainly because the background count rates without Lead shield near 1173 keV are higher than that near 1332 keV, as shown in Fig. 8(b)(black) and Fig. 9(red). The higher background principally makes the MDA without Lead shield at 1173 keV higher than that at 1332 keV. MDAs for Cs-137, Y-88, and Co-60 reached 320, 300, and 530 Bq respectively at the distance of 0 cm in our experiments. MDAs for Cs137 from several research groups are listed in Table 3 (Scuffham et al., 2016; Youngman, 2015). In the different applications, the measure times listed in Table 3 are significantly different. This is because the measurement speed and detection limit are contradictory in the detection of the nuclear emergency condition. Researchers usually need to keep a balance between them. In our study, we hope to complete one test in 5 min, so we focused on the 5-min detection limit. If 100 monitors were deployed to the on-site at the same time, 6000 individuals would be tested within 5 h. It can significantly improve screening efficiency. The standing posture was employed in our work, which was different from the sitting posture adopted in other studies, as shown in Fig. 6. When the monitor was close to the upper body of the sBCAM, it obtained a good minimum detectable activity for Cs-137 uniformly distributed in the sBCAM. In fact, the close geometry means at a short time after intake activity in the lungs will dominate efficiency. The close geometry is likely to overestimate the internal activity, which could involve to some extent more contaminated individuals accepting further test by the whole-body counter (WBC) or lung counter (LC) in professional labs and hospitals. However, it still can exclude the vast majority of individuals without internal contamination and the number
Fig. 6. The PICM was employed to measure the radioactivity in sBCAM.
Fig. 7. Spectral of natural background in the internal irradiation physics laboratory acquired by PICM with or without 6 mm Lead shield.
4
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Fig. 8. Spectral acquired by PICM by measuring the artificial radionuclides in the sMBCAM. (a) Dual detectors with Lead shield. (b) Dual detectors without 6 mm Lead shield.
Fig. 9. The natural background count rates registered by PICM with and without Lead shield in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV).
Fig. 11. MDA of Cs-137, Y-88, and Co-60 at different measurement distances with Lead shield in 5-min measurements.
Fig. 12. MDA of Cs-137, Y-88, and Co-60 at different measurement distances without Lead shield in 5-min measurements.
Fig. 10. Net count rates in ROIs of Cs-137(662 keV), Y-88(898 keV), Co60(1173.2 keV, 1332.5 keV) registered by PCIM with 6 mm-Lead shield at different distances.
5
528 585 604 657
530 594 625 681
Co-60 (1332 keV)
0 5 10 15
505 575 658 719
575 607 660 880
of contaminated individuals screened can be acceptable to professional labs and hospitals. If the monitor proposed in this paper was used to measure the lung internal contamination, the spacing between the dual-detectors can be adjusted by regulating handles to make two detectors aim at the double lungs of individuals. The close measurement geometry can offer high detection efficiency. In lung measurement, the user also needs a lung model to calibrate the monitor. Therefore, if the monitor employed to measure the lung internal contamination, it can get higher detection efficiency and lower detection limit. Considering the 3 “NaI detector widely used in nuclear radiation measurement community is low cost and easy to get, and the portable measurement system developed in this paper costs less than $10,000, and has a significant low-cost advantage in large-scale deployment. 4. Conclusion In order to meet the large-scale monitoring for internal contamination in the potential radiological emergencies, a portable internal contamination monitor has been developed. The monitor includes dual detectors (each of them is a 3″ NaI detector surrounded by a Lead shield with 6 mm thickness), digital gamma spectrometer, and an analysis software. PICM was employed to measure the radioactivity in the sBCAM. It provides desirable detection limits for Cs-137, Y-88 and Co-60 distributed in the whole-body. MDA for Cs-137, Y-88 and Co-60 achieved 320, 300, and 530 Bq respectively in 5-min measurement. Because of the inadequate shielding and poor detector resolution, the monitor is not sensitive to the lower energy emitters. However, it still can be used to detect some important nuclides released in radiological incidents, such as Cs-137,Cs-134,Zr-75,Mn-55,Co-60. Due to the compact structure and low cost, the monitor can be applied in the rapid test for large-scale screening, especially in the high population density areas. Acknowledgements We thank China Institute of Radiation Protection for providing the sBCAM(Solid BOMAB Phantom for China Reference Adult Men) and help in our experiments. This study also benefits from the major scientific instruments and equipment development project of the Ministry of Science and Technology, People's Republic of China (Contract Grant No.2012YQ180118). This work also was supported by the National 6
c
0 5 10 15
b
Co-60 (1173 keV)
a
300 311 314 332
Smith et al. (1994) Lahham and Fulop (1997) Castagnet et al. (2007) Castagnet et al. (2007) Muikku and Rahola (2007) Youngman (2015) Reported in this paper(2019)
300 299 301 309
0.0003 0.006 0.07 0.001 0.02 2 0.0036
0 5 10 15
18 450 5000 100 1200 120000 320
Y-88 (898 keV)
5 Nal(TI) Detectors each 10 × 15 cm Diameter Nal(TI) 12.5 × 12.5 cm Diameter Nal(TI) 5 × 5 cm Diameter Nal(TI) 7.6 × 13 × 41 cm Nal(TI) 5.1 × 5.1 cm Diameter Nal(TI) 0.25 × 3.2 cm Diameter 2 NaI(Tl) Detectors each 7.6 × 7.6 cm Diameter
320 320 337 358
Well shielded Laboratory based Well shielded Mobile Well shielded Mobile Well shielded Mobile Mobile Gamma-ray Spectrometer Hand-held Instrument Portable internal contamination monitor with 6 mm Lead shield
360 379 395 405
Approximate Minimum detectable does, mSvb
0 5 10 15
Dla, Bq
Cs-137 (662 keV)
Measurement time(mins)
with Lead shields
Detector type and size
without Lead shields
Reference
MDA
System
Distance(cm)
Table 3 Reported detection limits for Cs-137 and effective dose corresponding to whole body activity at the level of the detection limit (Youngman et al., 2015).
Nuclides
45 10 10 10 1.7 0.3c 5
Table 2 The MDA obtained by the monitor with and without 6 mm-Lead shields at 0–15 cm distance in 5-min measurements.
For the 95% confidence level. Assumes whole body activity at the DL and intake by acute inhalation occurred 1 d before measurement. Dose calculated for adults, assuming Activity Median Aerodynamic Diameter of 5 μm, absorption Type F. Instantaneous count rate is indicated but the reading should be viewed for about 20 s to obtain an approximate average value.
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Applied Radiation and Isotopes 154 (2019) 108858
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Natural Science Foundation of China(Grant No.41874121) and Technology Support Program of Sichuan province (No.18JY0181).
Dosim. 105 (1), 101–108. Muikku, M., Rahola, T., 2007. Improvement of the measuring equipment used in the assessment of internal doses in emergency situation. Radiat. Prot. Dosim. (127), 277–281. Nomura, S., Tsubokura, M., Furutani, T., Hayano, R.S., Kami, M., Kanazawa, Y., et al., 2016. Dependence of radiation dose on the behavioral patterns among school children. J. Radiat. Res. 57 (1), 1–8. Scuffham, J.W., Yip-Braidley, M., Shutt, A.L., et al., 2016. Adapting clinical gamma cameras for body monitoring in the event of a large-scale radiological incident. Soc. Radiol. Prot. (36), 363–381. Smith, J., Marsh, J., Etherington, G., Shutt, A., et al., 1994. Evaluation of a high purity germanium detector. Radiat. Prot. Dosim. (53), 73–75. Thompson, N.J., Youngman, M.J., Moody, J., et al., 2011. Radiation Monitoring Units: Planning and Operational Guidance. HPA Centre for Radiation, Chemical and Environmental Hazards. Ha, Wi-Ho, Kim, Jong Kyung, Jin, Young Woo, et al., 2017. Estimation of counting efficiencies of a portable NaI detector using Monte Carlo simulation for thyroid measurement following nuclear accidents. J. Radiol. Prot. 635–641. Youngman, M.J., 2015. Review of methods to measure internal contamination in an emergency. J. Radiol. Prot. (35), R1–R15.
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