|Year : 2016 | Volume
| Issue : 2 | Page : 68-74
Development and performance evaluation of HPGe detector-based shadow shield bed whole body counter
IS Singh1, Rajesh Sankhla1, DD Rao1, Abhay Kumar2, AK Sinha2, KS Pradeepkumar1
1 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
2 Centre for Design and Manufacture, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
|Date of Web Publication||13-Sep-2016|
I S Singh
Radiation Safety Systems Division, Bhabha Atomic Research Center, Trombay, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
An HPGe detector-based shadow shield bed whole body counter has been developed for internal contamination monitoring of radiation workers. The system is calibrated for the measurement of internally deposited radionuclides which emits photon energies >100 keV. Its performance characteristics are compared with NaI(Tl) detector-based whole body counter. It is observed that due to superior energy resolution of the HPGe-based system, identification and quantification of internally deposited radionuclides in workers is superior than NaI(Tl) detector-based system. The 40 K contents of 15 persons were measured using this system. It is found that the total body potassium varies from 0.82 gk kg −1 to 2.6 gk kg −1 of the body weight.
Keywords: HPGe detector, internal dosimetry, Na(Tl) detector, whole body counter
|How to cite this article:|
Singh I S, Sankhla R, Rao D D, Kumar A, Sinha A K, Pradeepkumar K S. Development and performance evaluation of HPGe detector-based shadow shield bed whole body counter. Radiat Prot Environ 2016;39:68-74
|How to cite this URL:|
Singh I S, Sankhla R, Rao D D, Kumar A, Sinha A K, Pradeepkumar K S. Development and performance evaluation of HPGe detector-based shadow shield bed whole body counter. Radiat Prot Environ [serial online] 2016 [cited 2022 Jul 5];39:68-74. Available from: https://www.rpe.org.in/text.asp?2016/39/2/68/190394
| Introduction|| |
Whole body monitoring of occupational radiation workers is an important part of the radiation protection program in nuclear research establishments and industry. For detection and measurement of internal contamination due to various gamma emitting fission and activation products such as 131 I,132 Te,133 Cs,134 Cs,137 Cs,60 Co,95 Zr-95 Nb,106 Ru,124 Sb, and 125 Sb on a routine basis, shadow shield bed and shielded chair whole body counters are used widely throughout the world.,,,, The Internal Dosimetry Section (IDS) of Radiation Safety Systems Division has been engaged in the development, design, fabrication, installation, and commissioning of whole body radioactivity counters.,,, Shadow shield and shielded chair whole body counters were developed in late sixties, specifically for routine monitoring of radiation workers. Recently, we have also developed standing geometry quick scan whole body counter incorporating two large size NaI (Tl) (406 mm × 127 mm × 76 mm) detector for monitoring large number of radiation workers as well as member of public in case of any radiological or nuclear emergency. The NaI(Tl) detectors are still popular for the body activity measurement, because of its good efficiency, moderate energy resolution, and lower cost of operation. Taking this into the consideration, NaI(Tl) detector-based whole body counters are used at various nuclear facilities in the country. These counters are adequate enough if internal contamination occurs from three or four radionuclides with well separated photon energy. However, there may be situations in which exposure to workers can occur from a mixture of radionuclides with overlapping energy band. In such a case, NaI(Tl)-based counter has limited use due to its poor energy resolution and identification and quantification of radionuclides become difficult and it can be misinterpreted or incorrectly analyzed. Such situations can be handled conveniently using HPGe detector-based whole body counters. The HPGe detector has superior energy resolution; can significantly enhance the capabilities for better identification and quantification of internally deposited radionuclides in workers. Taking this task into account, earlier we have developed actinide lung/organ monitoring system incorporating an array of three 70 mm dia. HPGe detector for the in vivo measurement of low energy photon (<200 keV) emitters specially Pu/Am and U. In this work, we have developed a new shadow shield bed whole body counter (SSBWBC) incorporating 100% relative efficiency (RE) HPGe detector for in vivo monitoring of workers handling high energy photon (>100 keV) emitters, especially various fission and activation products. The efficiency calibration of system is carried out for the measurement of internally deposited radionuclides, which emits photon energies >100 keV. Whole body monitoring of 15 persons were carried out to study the variation of background in various energy region of interest and for the estimation of body potassium content. Moreover, its performance characteristics are compared with existing NaI(Tl) detector based whole body counter.
| Materials and Methods|| |
Detector and associated electronics
The new SSBWBC is fabricated using mild steel plates to accommodate p-type 100% RE HPGe detector (Model NO. GCD-100 220 U-down 30 L, Make: M/s Baltic Scientific Instruments) having crystal diameter of 81.5 mm and length of 81.1 mm in shadow shield scanning geometry [Figure 1]. The detector is configured in inverted U-type cryostat arrangement integrated with 30 L Dewar. The detector is cooled by liquid nitrogen Dewar having holding time of about 15 days. The detector output is connected to preamplifier, multi-channel analyzer (MCA) and computer. The positive bias voltage of 4200 V is given to the detector through inbuilt power supply of the MCA. A 64 K channel MCA (M/s Itech Instrument Model Orion) with Interwinner 7.0 gamma ray spectroscopy software (M/s Tomcom Software Inc.) is used for the spectrum acquisition and analysis. The measured full width at half maximum (FWHM) of the system is 1.47 keV at 661.6 keV photo peak of 137 Cs and 1.9 keV at 1332 keV photo-peak of 60 Co.
The SSBWBC is designed to shield the HPGe detector all around except its viewing face. The LN2 Dewar of the detector is mounted on a support fixed to the shadow shield such that position of the detector is in the middle of shadow shield geometry. [Figure 1] shows 3D model of the whole body counter. The shield is fabricated using mild steel plates of 1.27 cm thickness, 40.6 cm width and of varying lengths, stacked to form 365 cm long side shields of SSBWBC. The bottom shield serves as a platform for the movement of the bed, which is motorized for continuous movement. Total shield was made of more than 208 plates of 13 different sizes, weighing about 4.5 tons. The shield thickness at the center is about 15.2 cm and it gradually decrease toward end, at the extreme end its thickness is about 3.8 cm. At the bottom, shield thickness is 20.3 cm at the center and 3.8 cm at the extreme end. The long platform that results as a consequence of shadow-shield arrangement helps to accommodate a linear scanning geometry. The detector is provided with 15 cm mild steel shielding al around except detector window. The design of shield is such that no gamma-ray photon from environment can reach the detector directly without passing through a 15 cm of effective shield thickness. The photon emitted from the subject, shield material, and those scattered from the subject or shadow shield only can reach the detector without attenuation.
Bed drive and its control system
For bed movement, a human–machine interface (HMI) controlled new bed drive mechanism with variable bed speed is used in place of chain and pulley mechanism which was used in earlier systems. This consists of a servo motor for bed movement with handling capacity of 300 kg, encoder, linear motion (LM) guide, sensors at different locations for profile scanning, limit switches, HMI control and suitable software to control on/off with spectroscopy system. The LM guide system is rugged and produces negligible noise and is smooth during motion of the bed. The installed LM guide-based bed drive mechanism provides forward and reverse motion with variable scanning speed from 2 to 30 min. Therefore, based on the requirement, monitoring period of the worker can be changed. A manual pushbutton and speed control mechanism is also provided with bed drive control to operate the system without HMI.
Calibration of system with BOttle Manikin ABsorption phantom
In order to estimate the activity of a radionuclide present in the body using whole body counting system, the response of the detector has to be calibrated for that particular radionuclide. This is accomplished by placing a known amount of activity of the radionuclide in an anthropometric phantom constructed to simulate the human body and assessing the detector response to the emitted photons in a standard reproducible counting geometry to be used for subject counting.
The energy calibration of HPGe detector was carried out using 133 Ba,137 Cs, and 60 Co sources. The energy calibration of the system is kept 0.5 keV/channel. Efficiency calibration of the system was carried out in the range of 80–1500 keV using 133 Ba (80, 303, 356, 384 keV),137 Cs (661.6 keV) and 60 Co (1173 and 1332 keV) sources. Efficiency calibration was carried out with a BOttle Manikin ABsorption (BOMAB) phantom shown in [Figure 2] representing an average Indian radiation worker. The phantom is 166 cm in length and weighs 66 kg when fully filled with water. Sources of a particular radionuclide, each one having its strength in proportion to the weight of the body part, were used to simulate the uniform distribution in the human body. Each source was kept at the geometric center of the respective block in the phantom.,, Efficiencies for the energy range of 80–1500 keV were interpolated using a fit obtained from measured efficiency. The measured and interpolated efficiency of the system is shown in the [Figure 3]. This shows the efficiency of the system increases with photon energy up to about 300 keV, then efficiency of the system decreases with energy. This is due to absorption of low energy photon in phantom and detector window. Therefore, care should be taken while calibrating system in the low energy range (<300 keV) using BOMAB phantom.
|Figure 2: BOttle Manikin ABsorption phantom representing an Indian reference worker of weight 66 kg and height 166 cm|
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|Figure 3: Detection efficiency of shadow shield bed whole body counter incorporating 100% relative efficiency HPGe detector obtained with 133Ba, 137Cs and 60Co uniformly distributed in water filled BOttle Manikin ABsorption phantom|
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Determination of minimum detectable activity
The minimum detectable activity (MDA) of the system at 95% confidence level is evaluated using following equation:
Where σb is the standard deviation in the normal subject background counts, E (counts photon −1) is the counting efficiency, I is the emission probability of the photon, and T is counting time.
| Results and Discussion|| |
A new SSBWBC incorporating an HPGe detector and associated electronics has been installed at IDS Laboratories in Bhabha Atomic Research Center (BARC) Hospital. [Figure 4] shows the new system with person in monitoring geometry. The new bed driving mechanism with variable bed speed is incorporated in the new system. The esthetic look of the new whole body counter has also been improved. [Table 1] gives the basic properties of HPGe detector employed in shadow shield geometry. [Figure 5] and [Figure 6] show 133 Ba,137 Cs, and 60 Co spectrum obtained with HPGe and NaI(Tl) detector-based whole body counter. The measured FWHM of HPGe and NaI(Tl) detector at 1332 keV photon energy is 1.9 and 55 keV, respectively. This shows the energy resolution of HPGe detector is about 30 times better than NaI (Tl) detector, therefore peaks with few keV photon energy difference can be resolved easily. Thus radionuclide identification capability of the system is enhanced significantly. The system is calibrated using known amount of 133 Ba,137 Cs, and 60 Co uniformly distributed in water filled BOMAB phantom. The 133 Ba (356 keV) is used to evaluate the efficiency of the system for 131 I (364.5 keV) measurement.
|Figure 4: The shadow shield bed whole body counter incorporating 100% relative efficiency HPGe detector|
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|Figure 5: Pulse height spectrum of 133Ba, 137Cs and 60Co obtained with HPGe detector based shadow shield bed whole body counter|
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|Figure 6: Pulse height spectrum of 133Ba, 137Cs and 60Co obtained with NaI(Tl) detector based shadow shield bed whole body counter|
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[Table 2] gives the calibration factors and MDA of HPGe and 102 mm diameter × 76 mm thick NaI (Tl) detector. The evaluated calibration factors of HPGe-based new system for 131 I,137 Cs, and 60 Co were 8.7, 8.4, and 8.7 cpm kBq -1, respectively, and corresponding values for NaI (Tl) based SSBWBC are 60.0, 32.3, and 24.5 cpm kBq -1, respectively. The MDA of the system is evaluated using background counts of an uncontaminated person monitored in scanning geometry. Figure 7] and [Figure 8] show the background spectra of an uncontaminated person and water filled BOMAB phantom, respectively. The 40 K photo peak at 1460 keV is clearly visible in background spectrum of an uncontaminated person. For uniformly distributed radioactivity in person, MDA of the new system is 150, 100, and 80 Bq for 131 I,137 Cs, and 60 Co, respectively, for a counting time of 15 min which is the small fraction of annual limit of intake for most of the fission and activation products encountered in nuclear fuel cycle. The MDA of NaI(Tl) based SSBWBC for measurement of internally deposited 131 I,137 Cs, and 60 Co were 215, 210, and 190 Bq, respectively, for a counting time of 15 min. The calibration factor of NaI (Tl) detector is about 2–5 times higher than HPGe detector, but MDA of HPGe is about two times better than NaI (Tl) detector for the same counting time. This is because of a superior energy resolution, high peak to Compton ratio (84) and lower background in the energy region of interest for HPGe detector. The new system has superior energy resolution and allows unambiguous identification and quantification of intake from mixture of radionuclides whose spectral features are separated by few keV. This will be also useful as an adjunct to scintillation detectors to facilitate the identification of radionuclides in case of exposure with unknown radionuclides in any nuclear and/or radiological emergency. The experimental detection efficiency of HPGe detector obtained with radionuclides uniformly distributed in BOMAB phantom is shown in [Figure 3] in respect of energy verses counts per photon. Using this, efficiency of the system for any radionuclide can be interpolated.
|Table 2: Comparison of calibration factors and minimum detectable activity of shadow shield bed whole body counter with HPGe and NaI(Tl) detector for counting time of 15 min|
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|Figure 7: Background spectrum of an uncontaminated person (weight 73 kg, height 173 cm) using new shadow shield bed whole body counter for counting time of 15 min|
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|Figure 8: Background spectrum of water filled BOttle Manikin ABsorption phantom using new shadow shield bed whole body counter for counting time of 15 min|
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The HPGe-based whole body monitors have already proven to be more useful than NaI (Tl)-based whole body counter in the case of nuclear incident occurred in the world viz., nuclear accident of Chernobyl or Fukushima Daiichi nuclear power plant. In these cases, exposure to the workers and public from mixture of radionuclides has occurred. It is reported that, after Fukushima Daiichi Nuclear Power Plant accident, in vivo measurements were performed on 550 foreigners returning home in their respective countries. Intake of radioactive material was detected in 208 persons and the committed effective doses were <1 mSv in all the cases. The measured radionuclides in these persons were 134 Cs (604.7 and 795.8 keV),137 Cs (661.6 keV),131 I (364.5 and 637.0 keV),132 I (667.7 and 772.6 keV), and 132 Te (228.2 keV). Both HPGe and NaI (Tl) detector-based WBCs were used for the measurement. The NaI (Tl) based WBCs were not able to identify and discriminate some of the above radionuclides. The HPGe detector-based system has shown as an excellent option for discriminating all the gamma emissions in the spectrum when all the expected radionuclides (radiotellurium, radioiodine, radiocesium) were present in the contaminated person being monitored.
In vivo measurement of 40 K
The whole body counters were used earlier to assess the total lean body mass by measuring 40 K content in person, which is 0.012% of the natural potassium. The potassium is known to be distributed uniformly throughout the body such as cesium and its content in the body depends on the individual. Therefore, in vivo measurement of 15 nonradiation workers of different age, weight, and sex is carried out for the estimation of 40 K content. [Figure 7] shows the background spectrum of an uncontaminated worker obtained with HPGe detector-based whole body counter. The 1460 keV photo peak of 40 K is clearly visible in the spectrum. The counting efficiency of the system for 40 K (1460 keV) is extrapolated using efficiency (counts photon −1) graph shown in the [Figure 3]. After yield correction for 1460 keV gamma of 40 K, the evaluated counting efficiency is 0.9 cpm kBq -1. Using this efficiency 40 K content in all 15 persons was estimated. [Table 3] shows the physiological parameters and measured body potassium contents in all monitored person. It is observed that the total potassium content in the measured person varies with the body weight. The body potassium content varies from 0.82 gk kg −1 to 2.6 gk kg −1 of the body weight. It is known that adipose tissue has much less potassium contents than the lean body mass which contains about 2.663 gk kg −1., The lower potassium content in some of the measured person indicates higher adipose content. The adipose contents of the individual can be measured by some techniques like bioelectrical impendence instrument. A study can be conducted to measure total body potassium and adipose contents in of persons. The correlation between lean body mass and total body potassium content in the person can give valuable information on the health condition of the Indian.
|Table 3: Physiological parameters and measured body potassium content of persons|
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| Conclusions|| |
A new SSBWBC incorporating 100% RE HPGe detector has been developed for assessing internal contamination of radiation workers due to the incorporated gamma emitting radionuclides. In the case of exposure to workers handling several radionuclides in laboratories or in the case of any radiological/nuclear accident, possibility of intake of multiple radionuclides cannot be ruled out. For such situations, existing NaI(Tl) detector-based whole body counters are unable to identify and quantify deposited radionuclides and HPGe detector-based system will be more useful for in vivo monitoring of the workers. Due to its higher operational cost and maintenance, the HPGe detector-based system may not be installed at each nuclear facility. The HPGe-based system can be used as an adjunct to NaI(Tl) detectors to facilitate superior identification of radionuclides. Afterward, subsequent follow-up measurement of the identified radionuclides can be carried out using NaI(Tl) based whole body counters. Thus, commissioning of HPGe detector-based WBC has enhanced our capability significantly for the superior identification and quantification of radionuclides in case of exposure to workers and public.
The authors are thankful to the staff of Centre for Design and Manufacture, BARC, and D. Toppo, RSSD, BARC, for their help in the fabrication of the system.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3]