Portable Gamma Irradiation System with 57Co, 133Ba, 137Cs, and 60Co for On-site Calibration of Environmental Radiation Monitoring Devices
Article information
Abstract
Background
A portable gamma irradiation system was developed for on-site calibration of environmental radiation monitoring devices. This system uses a portable collimator and gamma-ray sources such as 57Co, 133Ba, 137Cs, and 60Co, each with a nominal radioactivity of 10 MBq or less.
Materials and Methods
The air kerma rate produced by the system was measured using a spherical ionization chamber with a 10 L sensitive volume. This chamber was calibrated against the air kerma rate measured with primary standard ionization chambers. The system’s performance was evaluated by comparing the calibration results of the monitoring device with those obtained using a conventional method involving a standard ionization chamber for outdoor use.
Results and Discussion
The calibration coefficient of the spherical ionization chamber was within ±2% of the value of 3.029×103 Gy/C for photon energies ranging from 50 keV to 1,250 keV. The air kerma rate of the irradiation field produced by the portable system ranged from 0.17 μGy/hr to 4.7 μGy/hr, with an uncertainty between 2.4% and 10%. The calibration coefficients obtained using the portable system were consistent with those from the conventional method.
Conclusion
The portable gamma irradiation system offers significant advantages, including preventing damage to standard ionization chambers used outdoors and reducing measurement time compared to conventional methods. Due to its ease of construction and implementation, the system is expected to be useful not only for on-site calibration of monitoring devices but also in various radiation safety management scenarios.
Introduction
In Japan, numerous environmental radiation monitoring devices, referred to as “monitoring devices,” are continuously measuring radiation levels, particularly around nuclear facilities. As of February 2024, over 5,000 monitoring devices are in operation, managed by national and local governments [1]. Among these, approximately 4,000 are fixed monitoring posts, where the detector and processing unit are permanently affixed to buildings or the ground, making them unable to be removed for laboratory calibration. In contrast, portable monitoring posts and devices with detachable detection components can be calibrated at facilities specializing in low-dose rate calibration of environmental radiation [2, 3]. However, to ensure uninterrupted monitoring, on-site calibration is preferable for all types of monitoring devices.
The Japan Chemical Analysis Center (JCAC) has been conducting environmental radioactivity level surveys for 50 years [4]. In this endeavor, sealed 137Cs gamma-ray sources with a nominal activity of 10 MBq or less, along with a standard ionization chamber suitable for outdoor use, are transported to the monitoring device installation site. The monitoring device is then calibrated by comparing the outputs of the standard ionization chamber and the monitoring device. However, this calibration method, hereafter referred to as the standard ionization chamber carrying (SIC) method, poses risks of damaging standard ionization chambers during transport and outdoor use. It is also time-consuming due to the need for equipment alignment adjustments and warming up the standard ionization chamber before each calibration. Furthermore, since gamma sources are used without a collimator, measurement results may be influenced by scattered photons from surrounding objects [5]. In this project, additional 57Co, 133Ba, and 60Co sources are utilized to explore the energy characteristics of the monitoring device. Because gamma rays exist over a wide energy range in the environment [6, 7], and those from radionuclides released during an accident also have various energies [6–9], it is desirable for monitoring devices to have optimal energy characteristics. However, some monitoring devices, such as semiconductor-type and some pressurized ionization chamber-type detectors, have specific energy characteristics, making this characteristic test important [5].
Another on-site calibration method utilizes gamma-ray sources as the working standards, aiming to reduce the time required for calibration at the monitoring device installation site. When using gamma-ray sources for on-site calibration, a well-designed collimator is necessary to ensure the entire detector is irradiated with gamma rays while suppressing scattered radiation from surrounding objects. Zhang et al. [10] developed an apparatus for on-site calibration using 137Cs with a nominal activity of 460 MBq and tungsten alloy collimators, demonstrating its applicability for the on-site calibration of monitoring devices. However, such high-activity radioactive sources cannot be used in Japan unless specific locations are predetermined for their use. Additionally, expanding the use of other radionuclide sources is necessary to conduct on-site testing of the energy characteristics of monitoring devices.
In this study, a portable gamma irradiation system for on-site calibration has been developed. The system had a function of generating gamma-ray fields with four photon energies using a combination of 57Co, 133Ba, 137Cs, and 60Co gamma-ray sources and a portable lead collimator. The aim is to perform calibration and testing at any location in a short time using this system. The nominal radioactivity of the sources is 10 MBq or less to comply with Japanese regulations. The air kerma rate of the irradiation fields from these four gamma-ray sources was evaluated using a spherical ionization chamber calibrated against primary standards. Since the use of a small radioactive source requires measurement of a low-dose rate, a vibrating capacitor electrometer, suitable for measuring minute currents [11], was used for the spherical ionization chamber. As monitoring devices are typically calibrated using vertical irradiation while standard ionization chambers are calibrated using horizontal irradiation, the irradiation system can produce fields with beams in both directions. Additionally, to assess the performance of the irradiation system, the calibration results of the monitoring device were compared with those of the SIC method.
Materials and Methods
1. Portable Gamma Irradiation System
The portable gamma irradiation system developed in this study is depicted in Figs. 1 and 2, with photographs shown in Fig. 3. Fig. 1 illustrates the horizontal beam setup, which includes the collimator case housing the gamma-ray sources, a three-axis stage for adjusting source position, and a tripod. Fig. 2 depicts the vertical beam setup, utilizing two tripods to secure the frame for the collimator case adjuster. The source position is adjusted along the x-axis by moving the adjuster along the frame, while the frame’s position is adjusted in the y-axis direction using horizontal transmission stages. Fig. 4 presents a cross-section of the collimator case, which features cylindrical outer dimensions with a diameter of 96 mm, a length of 90 mm, and a weight of 10 kg. The gamma-ray source is positioned inside the collimator case at a fixed point using a polymethyl methacrylate adapter. The conical opening’s bottom diameter measures 42 mm, resulting in an irradiation field approximately 52 cm in diameter at a distance of 0.5 m from the radiation source and approximately 72 cm in diameter at 0.7 m. Given the low nominal radioactivity of gamma-ray sources used, a large-volume ionization chamber is necessary at close proximity to obtain sufficiently intense output. Hence, the collimator is designed to provide a sufficiently large irradiation field even at close distances.
Schematic of the horizontal beam setup for the portable irradiation system. Calibration of the irradiation fields produced by the portable gamma irradiation system using the spherical ionization chamber was conducted with this horizontal beam setup.
Schematic of the vertical beam setup for the portable irradiation system, used for calibrating on-site monitoring devices.
Pictures of the portable irradiation system. (A) Horizontal beam setup and (B) vertical beam setup.
Cross-section of the collimator case. The gamma-ray sources are secured with the radioisotope source adapter. In the horizontal beam setup, the collimator case is mounted on the 3-axis stage (see Fig. 1). In the vertical beam setup, it is installed in the collimator case adapter (see Fig. 2). PMMA, polymethyl methacrylate.
The distance between the source and detector was measured using a laser rangefinder (LR-TB5000C; KEYENCE). During measurement, the collimator case was replaced with the rangefinder such that the rangefinder’s origin aligns with the position of the radiation source inside the collimator case. The gamma-ray sources utilized in the irradiation system are detailed in Table 1. All sources listed in the table were manufactured by the Japan Isotope Association, which provides comprehensive information about their structure on their website. These gamma-ray sources are not restricted in terms of usage location.
Atomic Data of the Gamma-Ray Sources Used in the Portable Irradiation System
2. Calibration of the Spherical Ionization Chamber
The spherical ionization chamber with a 10 L sensitive volume (32003; PTW) was calibrated at the gamma and X-ray irradiation facilities of the National Metrology Institute of Japan [12–14]. Calibration coefficients were determined for various radiation qualities, including N-60, N-80, N-100, N-120, N-150, N-250, N-400, N-450, C-Cs, and C-Co [15]. These calibration coefficients NK,U for each radiation quality are expressed by the following Equation (1):
where K̇a,U are represents the air kerma rates in the irradiation field of the radiation quality U, I is the current from the spherical ionization chamber, and kac is the correction factor for atmospheric conditions. The current I was measured using a vibrating capacitor electrometer (MMA-II; Kawaguchi Electric Works). K̇a,U values were determined via measurements using a parallel plate-free air ionization chamber for the X-ray fields and a graphite wall cavity ionization chamber for the gamma-ray fields [12, 13]. The correction factor kac was derived from temperature and atmospheric pressure during measurement to correct the current to a value under reference conditions (temperature 20 °C, atmospheric pressure 1,013.125 hPa) [16]. Temperature and atmospheric pressure were measured using a combined pressure, humidity, and temperature transmitter (PTU303; VAISALA). The air kerma rate ranged from 1.4 μGy/hr to 9.1 μGy/hr for 137Cs and 60Co, and from 8.7 μGy/hr to 19 μGy/hr for the X-ray N series.
3. Calibration of the Irradiation Fields Produced with the Portable Gamma Irradiation System
The irradiation fields generated by a portable gamma irradiation system were calibrated at the calibration facility of the JCAC. Air kerma rates were determined at distances of 0.7 m for the sources, 57Co, 133Ba, 137Cs, and 60Co, listed in Table 1 using the spherical ionization chamber. As the distance increases, the irradiation field becomes more uniform and larger, but the dose rate decreases. The calibration distance was selected to maximize distance while still enabling the measurement of 57Co, which has the lowest air kerma rate among the radionuclides listed in Table 1. Calibration at 0.5 m for 137Cs was also conducted to assess dose rate characteristics. The air kerma rate K̇a(x; source), where x(m) denotes the distance between the source and the center of the ionization chamber, and the source refers to the radionuclide, was obtained as:
where NK,S represents the calibration coefficient of the spherical ionization chamber for each source, I′ is the measured current, k′ac represents the correction factor for the air conditions, and kic represents the correction factor for the difference between the irradiation condition of the spherical chamber and the gamma fields produced from the portable gamma irradiation system at calibration. kicwas determined using the calculation method with EGS5 Monte Carlo code [17]. The energy absorbed by the detection region in the ionization chamber under parallel-beam conditions, Epl, corresponding to the calibration of the spherical ionization chamber, and that under point-source conditions, Eps, corresponding to the calibration of the irradiation system were calculated. The kic ratio was determined by Epl to Eps. Initial photon energy was derived from gamma emission data found in the literature [18]. Cut-off energies for electrons and photons were 0.516 MeV and 0.01 MeV, respectively. The geometry of the ionization chamber considered only the walls and the central electrode, dimensions, and material published by the manufacturer [19]. Histories for parallel-beam conditions and point-source conditions were 1×109 and 1×1010, respectively, resulting in a statistical uncertainty of 0.15% or less for each calculation.
4. On-site Calibration of the Monitoring Post
A monitoring post in Marugame City, Kagawa Prefecture, underwent calibration to assess the performance of the irradiation system. A portable gamma irradiation system was employed under vertical beam conditions. An environmental radiation monitoring device utilizing a NaI scintillation detector (NAH01121-YYYYY-S/NAH02YY1-YYYYY-S; Fuji Electric Co. Ltd.) was exposed to radiation fields generated by 57Co, 133Ba, 137Cs, and 60Co at a distance of 0.7 m. Calibration coefficients were compared with those obtained using a standard ionization chamber for outdoor use, specifically a 21 L cylindrical ionization chamber (808-MR20 STD/V; NESCO).
Results and Discussion
1. Calibration Coefficient of the Spherical Ionization Chamber
Fig. 5 illustrates the calibration coefficient of the spherical ionization chamber as a function of photon energy. Within the displayed energy range, the calibration coefficient remained within 2% of the value for 137Cs, specifically 3.029× 103 Gy/C, consistent with the specified energy characteristics within ±3% [19]. The calibration coefficients for energy points ranging from 300 keV to 700 keV were higher than that for 60Co, likely due to primary photon attenuation by the walls of the ionization chamber, which are 2.75 mm thick polyoxymethylene and provide adequate build-up even for 60Co. Below 200 keV, the calibration coefficients were lower than those around 350 keV, attributed to increased signal from photoelectrons originating from the Al layer within the chamber walls, as reported in the literature [19]. Error bars indicate expanded uncertainties of the calibration coefficients, ranging from 2% to 3%. These uncertainties stem from variations in the signal from the spherical ionization chamber and the uncertainty in the air kerma rate of the irradiation fields. Calibration coefficients for each gamma-ray source used in the portable irradiation system were determined and detailed in Table 2. For 57Co and 133Ba, we utilized the calibration coefficients of N-150 and N-400, respectively, due to the close similarity between the mean energies of the X-ray fields and those of the gamma-ray sources. The uncertainties of the calibration coefficients for 57Co and 133Ba were estimated by combining the uncertainty of the calibration coefficients determined for the X-ray fields with that arising from the energy characteristics. The uncertainty due to energy characteristics was evaluated to be a relative standard uncertainty of 1%, based on the results shown in Fig. 5.
Calibration coefficient of the 10 L spherical ionization chamber as a function of photon energy. The horizontal dashed line represents the calibration coefficient for the 137Cs radiation field.
Calibration Coefficients for the Spherical Ionization Chamber for the Gamma-Ray Sources
2. Air Kerma Rate of the Gamma-Ray Field from the Irradiation System
Table 3 presents correction factors kic for the irradiation fields generated by the portable gamma irradiation system. Minimal variation in kic was observed when comparing measurements at the same distance, despite changes in the radiation source. Uncertainty was estimated assuming a rectangular distribution of correction widths. The kic values obtained were all less than 1, indicating that the absorbed energy under point-source conditions is greater than that under parallel-beam conditions. This discrepancy may stem from an overestimation of the kerma rate at the reference plane in a divergent photon field for cavity ionization chambers, where absorbed dose into the air cavity originates from electrons generated upstream of the cavity [20]. According to this argument, as the fluence of the primary photons follows the inverse square low, shorter distances result in greater overestimation, which was evident in the results—0.5 m showed a larger deviation from 1 m compared to that for 0.7 m. These findings highlight the importance of addressing irradiation field non-uniformity when calibrating monitoring devices equipped with large detectors like ionization chambers. If the correction amount is an order of magnitude smaller than the allowable uncertainty, its impact on calibration will be minimal. Therefore, for detectors similar in size to this 28 cm diameter ionization chamber, calibration should ideally occur at a distance of at least 0.5 m when an uncertainty of around 20% is required, and at least 0.7 m for an uncertainty of around 10%.
Correction Factor kic for Differences in Irradiation Conditions for the Portable Gamma Irradiation System
Table 4 presents the air kerma rates measured by the portable gamma irradiation system. For 137Cs, measurements were taken at two distances, and both values, when converted to the kerma rate at 1 m using the inverse square law, yielded 0.15 μGy/hr, suggesting the approximation of the inverse square law may be suitable for the 137Cs source. Table 5 details the uncertainty in measuring the air kerma rate for the portable gamma irradiation system. The uncertainty was estimated based on Equation (2). The relative standard uncertainty of NK,S was obtained by dividing the relative expanded uncertainty from Table 2 by a coverage factor of 2. Uncertainty in current measurement was derived from the standard deviations of repeated measurements during gamma-ray irradiation and background measurements. The uncertainty of k′ac was estimated based on uncertainties in temperature and pressure measurements. kic uncertainties were referenced from Table 3. Additional factors contributing to uncertainty, not explicitly listed in Equation (2), include humidity characteristic uncertainties based on current change relative to humidity [21], ranging from 30% to 70% during spherical ionization chamber calibration and air kerma rate measurement. Positioning uncertainties for both source and detector were considered with a ±1 mm rectangular distribution, assessing their relative impact on air kerma rate uncertainty. Significantly, uncertainties in current measurement strongly influenced air kerma rate uncertainty, particularly at low-dose rates. Furthermore, uncertainties in calibration coefficients of the spherical ionization chamber and kic also contributed significantly. These findings underscore the critical importance of a robust current measurement system, given the small magnitude of current values, and highlight the relevance of background measurement.
Air Kerma Rates for the Portable Gamma Irradiation System
Uncertainty Budget for Measuring the Air Kerma Rates for the Portable Gamma Irradiation System
3. Calibration Coefficient of the Monitoring Post
Table 6 presents the calibration coefficients of the monitoring posts determined using both the portable gamma irradiation system and the SIC method. The calibration coefficients obtained in this study fall within the uncertainty range of those obtained using the SIC method. Table 7 outlines the uncertainty budget for the calibration coefficient of the monitoring device. The uncertainty for K̇a(x; source) was taken from Table 5. Uncertainty in radioactivity decay correction from the date of K̇a(x; source) measurement to the calibration date was estimated considering the half-life, elapsed days, and their uncertainties. Temperature characteristic uncertainty was assessed based on manufacturer specifications. Positioning uncertainties for both source and detector were evaluated with a ±1 mm rectangular distribution, determining their relative contributions to the calibration coefficient. Uncertainty in the indicated value of the monitoring device was derived from standard deviations of repeated measurements during gamma-ray irradiation and background. Uncertainties in air kerma rate of the gamma-ray field and energy characteristics, particularly temperature dependence, also contributed significantly to calibration coefficient uncertainty. When estimating uncertainty, it is crucial to evaluate not only measurement variability but also understand equipment performance and operating conditions. The method developed in this study significantly reduces the time required to calibrate the monitoring device using the four sources, taking approximately 2.5 hours, compared to approximately 8 hours with the SIC method. As described above, it has been demonstrated that equivalent results to those of the SIC method can be achieved in a much shorter timeframe.
Calibration Coefficients of a Monitoring Post-determined Using the Portable Gamma Irradiation System and the SIC Method
Uncertainty Budget for Measuring the Calibration Coefficient of the Monitoring Device Based on NaI Scintillation Detector Using the Portable Gamma Irradiation System
Conclusion
A portable gamma irradiation system was developed for on-site calibration, employing a portable collimator and gamma-ray sources: 57Co, 133Ba, 137Cs, and 60Co, each with a nominal radioactivity of 10 MBq or 3.7 MBq. This system can irradiate in two directions: horizontal for measuring air kerma rates in irradiation fields, and vertical for calibrating monitoring devices. The air kerma rate of the radiation fields generated by the irradiation system was measured using a 10 L spherical ionization chamber. The measured air kerma rate ranged approximately from 0.17 μGy/hr to 4.7 μGy/hr, with uncertainties ranging from 2% to 10% depending on the air kerma rate. The calibration coefficients obtained using the portable irradiation system were consistent with those obtained using the SIC method within the uncertainty. This suggests that the developed method is suitable for testing the energy characteristics of monitoring devices. The calibration distances in the system are limited to 0.5 m and 0.7 m due to considerations regarding the influence of distance on measurements, as evaluated by the parameter kic. Further research from this perspective may enable calibration at any distance.
Utilizing the irradiation system for on-site calibration offers several advantages: it prevents damage to standard ionization chambers used outdoors and reduces measurement time compared to the SIC method. Additionally, since the developed method utilizes low-activity radioactive sources, it eliminates the need for large-scale equipment. This makes it feasible to implement on-site calibration of monitoring posts and manage various radiation safety scenarios more easily.
Notes
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Ethical Statement
This article does not contain any studies with human participants or animals performed by any of the authors.
Author Contribution
Conceptualization: Kurosawa T. Methodology: Tanaka H, Kurosawa T. Data curation: Kato M, Tanaka H. Formal analysis: Kato M. Funding acquisition: Tanaka H, Kurosawa T. Project administration: Kato M, Tanaka H. Investigation: all authors. Visualization: Kato M, Sugiyama M. Writing - original draft: Kato M. Writing - review & editing: all authors. Approval of final manuscript: all authors.
Acknowledgements
This study was supported in part by the Radiation Safety Research Promotion Fund, and by the Nuclear Facility Disaster Prevention Fund.