TY - JOUR
AU - Zhang, Xinyu
AB - Abstract Ambient dose equivalent dosemeters are widely distributed in nuclear facilities for routine continuous monitoring or nuclear accident emergency monitoring, they are inconvenient to be disassembled and sent to the metrology laboratory for calibration. To ensure the accuracy of such dosemeters, the research of in-situ calibration method has been carried out. Ambient dose equivalent secondary standard ionization chamber and portable gamma ray irradiation facility have been designed for in-situ calibration. The experiments of in-situ calibration were carried out in five typical sites located in China Institute of Atomic Energy, results showed that the relative deviation between in-situ calibration factors and metrology laboratory calibration factors are within 5%.The combined standard uncertainty of the in-situ calibration factor is 2.8%, and the expanded uncertainty is 5.6% (k=2).The in-situ calibration method can meet the calibration requirements of the fixed ambient dose equivalent dosemeters. Fixed X and gamma-rays dose ratemeters are widely distributed in nuclear reactors can be divided into 2 types, one type involving air kerma Ka is used for the continuous monitoring of nuclear facilities, the spent fuel storage plants and retirement facilities, and estimating the running state of the nuclear facilities for an early warning of a possible nuclear accident purpose. Another type involving ambient dose equivalent H*(10) is used for routine continuous monitoring or nuclear accident emergency monitoring of the fuel element operation area, evaporator chambers, circular corridor area with large flow of people and solid waste treatment plants, and estimating effective dose E to personnel for protection purpose. Fixed X and gamma radiation dosemeters are inconvenient to be disassembled and sent to the metrology laboratory for calibration. To ensure the accuracy of such dosemeters, the research of in-situ calibration technology needs to be carried out. Although the secondary standard ionization chambers are commercially available like the TW30013 ionization chamber produced by PTW in Germany has good energy response, repeatability and stability. But such products as free-air ionization chamber, applies only to the laboratory under standard temperature and humidity conditions. The in-situ calibration environment is complex, the temperature and humidity variation are greater than that in a standard laboratory, even with dust in some environments. So, this type of ionization chamber is not suitable for dose measurement in the in-situ calibration process. In addition, although some commercially available dosemeters such as 6150 AD and FH40G have good repeatability and stability, the energy response and angular response cannot meet the requirements of in-situ calibration. Therefore, it is necessary to independently develop a secondary standard ionization chamber with an energy response deviation of not more than 5% in the range of 80 keV-1.25 MeV and with an angular response deviation of no more than 5% in the range of 0°-360°. In order to improve the environmental adaptability and sensitivity of the secondary standard ionization chamber, a closed high-pressure ionization chamber has been developed. Therefore, a secondary standard ionization chamber of ambient dose equivalent radiation was developed by Monte Carlo simulation combining with experiments, and a portable gamma ray irradiation field was established, which solved the problem of in-situ calibration of ambient dose equivalent irradiation ratemeters. SECONDARY STANDARD IONIZATION CHAMBERS OF AMBIENT DOSE EQUIVALENT The “conversion coefficient method”, “spectroscopy method” and “secondary standard ionization chamber method” are internationally considered as three common methods for determination of ambient dose equivalent radiation of X-rays and gamma-rays(1, 2). In conversion factor method, ambient dose equivalent radiation is obtained by multiplying conversion coefficient and air kerma of X-ray or gamma-ray determined by standard device(2). The conversion coefficients |${k}_k^{\ast}(10;R)$| from air kerma Ka to ambient dose equivalent radiation H*(10) of photons with different energies are recommended by ISO4037-3 X and gamma reference radiation for calibration dosemeters and doseratemeters 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. The conversion coefficients are closely related to the energy of X-rays and gamma-rays, that is, the conversion coefficients change with energies, especially in the case of low-energy photons (under 30 keV). The conventional truth value of H*(10) at the reference point is impossibly accurately obtained through the conversion coefficient method, considering the complex environment of in-situ calibration and the large amount of low-energy scattered radiation in the portable gamma-ray irradiation field. In “spectroscopy method”, the energy deposition spectrum of filtered X rays is measured by spectrometer, and the flux spectrum distribution of filtered X rays is obtained by spectrum analysis, then the conventional truth value of H*(10) can be obtained by multiplying the conversion coefficients and spectrum results. The uncertainty of measurement results by spectroscopy method is small, but the spectrum analysis is complex, which is not applicable to in-situ determination of H*(10)(3). In “secondary standard ionization chamber method”, H*(10) of portable gamma ray irradiation field can be determined by energy compensated cavity ionization chamber developed and calibrated in the standard radiation field(4–6). The secondary standard ionization chamber method is suitable for determination of H*(10) of portable gamma ray in the process of in-situ calibration with the advantages of simple operation, good repeatability, and small measurement uncertainty, etc. Principle Gamma rays enter the ionization chamber detector then interact and ionize with gas molecules or atoms in the chamber to produce secondary electrons, which gaining enough energy will continue to interact and ionize with other gas molecules or atoms. The positive and negative ions generated by ionization will respectively move towards the poles under the action of electric field. The ionic charge at the collecting pole is sent to the electrometer and data acquisition system through signal cable, and then converted into H*(10) after quantitative measurement. The energy-compensated secondary standard ionization chamber was developed as a standard device for the transmission of H*(10) of portable gamma-ray irradiation field, based on the previous research experience of secondary standard ionization chamber(7) and the Monte Carlo simulation method, making its energy response curve similar to the curve of |${k}_k^{\ast}(10;R)$| changing with energy(8),as shown in Fig. 1. The energy-compensated secondary standard ionization chamber can be used to determine H*(10) of the portable gamma-ray irradiation field after calibration in reference radiation field. Figure 1 Open in new tabDownload slide Air Kerma-Ambient Dose Equivalent Conversion Coefficients. Figure 1 Open in new tabDownload slide Air Kerma-Ambient Dose Equivalent Conversion Coefficients. Monte Carlo simulation The simplified calculation model of spherical ambient dose equivalent secondary standard ionization chamber was established by Monte Carlo program (GEANT4), mainly including ionized outdoor wall (high pressure electrode), collecting pole compensation slice (tin, 1.5 mm thick) and high purity argon gas in the sensitive zone, etc., as shown in Fig. 2. A point-shaped radioactive source is used in the Monte Carlo model. The collimated beam source is formed after collimation. The opening angle of the collimating diaphragm is 14.5°. The sensitive volume center of the ionization chamber is 1 m away from the radioactive source. The experiment layout of the laboratory radiation dosimetry characteristic test is the same. The radius of the high-voltage pole is 5 cm, the wall thickness of that is respectively 0.3 mm, 0.5 mm, 0.8 mm, 1 mm and 1.5 mm, the radius of collection pole is 1.5 cm, and the wall thickness of that is 0.5 mm. The high-voltage pole and collecting pole are made of stainless steel, the sensitivity zone is 8 × 105 Pa argon. The energy response simulation results of ionization chambers with different wall thicknesses are shown in Fig. 3. The relative response factor of the ordinate in Figure 3 refers to the response |${R}_E$| (Dose reading/conventional true value) of the ionization chamber in the range of 30 keV to 1.25 MeV divided by the response |${R}_{\mathrm{Cs}}$| of the ionization chamber to Cs-137 gamma rays. Figure 2 Open in new tabDownload slide Monte Carlo Model of Spherical Ambient Dose Equivalent Ionization Chamber. Figure 2 Open in new tabDownload slide Monte Carlo Model of Spherical Ambient Dose Equivalent Ionization Chamber. Figure 3 Open in new tabDownload slide Energy Response Curves of Secondary Standard Ionization Chamber with Different Wall Thicknesses. Figure 3 Open in new tabDownload slide Energy Response Curves of Secondary Standard Ionization Chamber with Different Wall Thicknesses. The thinner the outer wall is, the higher energy response of the ionization chamber at about 100 keV under the condition of constant pressure in the sensitive zone, as shown in Fig. 3. The response of the ionization chamber with 0.3 mm wall thickness at 100 keV is 700% of that at 662 keV. The energy response of ionization chamber at 100 keV is significantly reduced with appropriate increase in the outer wall thickness of the high-voltage pole of the ionization chamber, meanwhile, the lower energy detection threshold of the ionization chamber having an undesired increase. In order to ensure energy detection threshold of the secondary standard ionization chamber is sufficiently low, the thickness of the ionization chamber outer wall should be less than 1 mm. Since there is 8 × 105 Pa high-purity argon in the sensitive area of the ionization chamber, considering the stability and safety of the ionization chamber, the ambient dose equivalent secondary standard ionization chamber was advisedly developed with a wall thickness of 0.8 mm, with the addition of some appropriate energy compensation method. The Monte Carlo method is used to calculate the energy response of the ionization chamber, with tin energy compensation slices of different areas and different thicknesses applied to the outer wall. According to the Monte Carlo calculation results, the energy response of the ionization chamber is best when the thickness of the tin compensation slices is 1.5 mm and the compensation slices area is 70% of the surface area of the ionization chamber. The relative response in the range of 80 keV to 1.25 MeV does not exceed 5%. The relative response refers to the response divided by the response in Cs-137 gamma rays. Experimental result The spherical ambient dose equivalent secondary standard ionization chamber was optimized combined with Monte Carlo simulation results, and the sensitive volume of the ionization chamber was 500 cm3. The structure of the ionization chamber are shown in Fig. 4 and Fig 5. Figure 4 Open in new tabDownload slide Appearance Drawing of Spherical Ambient Dose Equivalent Secondary Standard Ionization Chamber. Figure 4 Open in new tabDownload slide Appearance Drawing of Spherical Ambient Dose Equivalent Secondary Standard Ionization Chamber. Characteristics of the secondary standard ionization chamber were studied in the standard laboratory of X and gamma ray air kerma in China Institute of Atomic Energy. Our laboratory is a member of secondary standard dosimetry laboratories of the IAEA. The conventional true values in the reference field have been measured using a primary standard spherical ionization chambers, the uncertainty of the true value is less than 3%(k = 2). The reference point of the ambient dose equivalent secondary standard ionization chamber was located at 1 m away from the radioactive source or focus of the X-ray tube, and the geometric center of the ionization chamber coincided with the reference point of the radiation field in energy response experiments. The output power of the X-ray machine was adjusted to ensure that the dose rate at the reference point was around 2 mGy/h, for avoiding the influence of different response of the ionization chamber to dose rates. The same dose rate is obtained by adjusting the distance between the ionization chamber and the source in the gamma ray reference radiation field. N-40, N-60, N-80, N-100, N-120, N-150, N-200 N-250, N-300 in N series X-ray reference radiation quality and S-Cs (662 keV), S-Co (1250 keV) of gamma-ray radiation quality were respectively selected as the energy point in energy response tests. The energy response curve of the ambient dose equivalent secondary standard ionization chamber is shown in Fig. 6. The relative response factor of the ordinate in Figure 6 refers to the response |${R}_E$| (Dose reading/conventional true value) of the ionization chamber in the range of 30 keV to 1.25 MeV divided by the response |${R}_{\mathrm{Cs}}$| of the ionization chamber to Cs-137 gamma rays. Figure 5 Open in new tabDownload slide Structure Drawing of Spherical Ambient Dose Equivalent Secondary Standard Ionization Chamber. Figure 5 Open in new tabDownload slide Structure Drawing of Spherical Ambient Dose Equivalent Secondary Standard Ionization Chamber. Figure 6 Open in new tabDownload slide Energy Response Curves of Ambient dose Equivalent Secondary Standard Ionization Chambers. Figure 6 Open in new tabDownload slide Energy Response Curves of Ambient dose Equivalent Secondary Standard Ionization Chambers. The energy response curve of the ambient dose equivalent secondary standard ionization chamber has a good agreement with the air kerma—ambient dose equivalent conversion coefficients curve shown in Fig. 6. The maximum deviation of the two curves is less than 5% within the range of 80 keV to 1.25 MeV. The ambient dose equivalent secondary standard ionization chamber can be used for in-situ calibration of fixed ambient dose equivalent radiation ratemeters, considering the measurement range of dose rate of the ionization chamber was 0.1 μSv/h ~ 100 Sv/h, meeting the design requirements. The leakage current of the ionization chamber is less than 10fA and the annual stability is less than 3% under the working voltage -400 V. Since the ionization chamber adopts a sealed design, changes in external temperature and air pressure will not affect the gas density in the sensitive area of the ionization chamber, so no correction is required. PORTABLE GAMMA-RAY IRRADIATION FACILITY The establishment of the reference radiation field is based on radioactive sources, which is essential in calibration of gamma-ray irradiation dosemeters. Multiple isotopic radioactive sources with different activities (60Co, 137Cs, 241Am, etc.) are required in the metrological laboratory to establish multiple reference radiations for dosemeter calibration, according to ISO4037.1–2019(1) X and gamma reference radiation for calibrating dosemeters and doseratemeters and for determining their response as a function of photon energy Part1: Radiation characteristics and production methods and other relevant standards. A portable gamma-ray irradiation facility was developed to producing reference radiation for in-situ calibration of fixed ambient dose equivalent dosemeters. The portable gamma-ray irradiation facility consists of 137Cs radioactive source, scattering chamber, lift shutter and lead shielding, as shown in Fig. 7 and Fig. 8. Figure 7 Open in new tabDownload slide Appearance Drawing of Portable Gamma Ray Irradiation Facility. Figure 7 Open in new tabDownload slide Appearance Drawing of Portable Gamma Ray Irradiation Facility. Figure 8 Open in new tabDownload slide Structure Drawing of Portable Gamma Ray Irradiation Facility. Figure 8 Open in new tabDownload slide Structure Drawing of Portable Gamma Ray Irradiation Facility. The size of the portable irradiation facility is: 235 mm × 160 mm × 185 mm, the thickness of the lead shielding is 67.5 mm, the portable gamma-ray irradiation device has a built-in 137Cs radioactive source with activity of 1.85 × 109 Bq for radiation production, the scattered contribution of that can be effectively reduced by the scattering chamber. When the shutter is closed, the maximum leakage dose rate of the portable device is 2.5 μSv/h on the surface of the irradiation facility. To begin irradiation, lift the shutter and open the lead plug. Uniformity The PTW32005 ionization chamber (30 mL sensitive volume) combined with the UNIDOS-E ratemeter was used to measure the uniformity of the radiation field at 0.5 m away from the source, and the measurement results are shown in Fig. 9. Figure 9 Open in new tabDownload slide Uniformity of Portable Gamma Ray Radiation Field. Figure 9 Open in new tabDownload slide Uniformity of Portable Gamma Ray Radiation Field. The uniformity of the radiation field within a radius of 3 cm at 0.5 m away from the portable gamma-ray irradiation device was better than 3.1%, as shown in Fig. 9. Scattered radiation The portable gamma-ray irradiation facility is placed in the center of the laboratory. The wall has a negligible scattered contribution when the distance to the radioactive source is more than 5 m. The PTW32002 ionization chamber combined with the UNIDOS-E ratemeter was used to measure the axial ambient dose equivalent rate of the portable gamma-ray irradiation field. The measurement results are shown in table 1. Table 1 Dose rate measurement results of portable gamma-ray irradiation field Distance/m . Measured value of ambient dose equivalent |${H}^{\ast }(10)$|/μSv|$\cdot $|h−1 . Air attenuation correction factor . Corrected ambient dose equivalent |${H}^{\ast,, }(10)$|/μSv|$\cdot $|h−1 . Calculated value of ambient dose equivalent |${H}^{\ast, }(10)$|/μSv|$\cdot $|h−1 . Relative deviation between |${H}^{\ast, }(10)$| and |${H}^{\ast,, }(10)$|/% . 1.0 187.9 1.00000 187.9 187.9 0 1.5 81.24 0.99534 81.61 83.51 −2.3 2.0 45.28 0.99070 45.70 46.98 −2.7 2.5 29.86 0.98609 30.29 30.07 0.7 3.0 20.68 0.98149 21.07 20.88 0.9 3.5 15.26 0.97692 15.62 15.34 1.8 4.0 11.92 0.97237 12.26 11.75 4.3 Distance/m . Measured value of ambient dose equivalent |${H}^{\ast }(10)$|/μSv|$\cdot $|h−1 . Air attenuation correction factor . Corrected ambient dose equivalent |${H}^{\ast,, }(10)$|/μSv|$\cdot $|h−1 . Calculated value of ambient dose equivalent |${H}^{\ast, }(10)$|/μSv|$\cdot $|h−1 . Relative deviation between |${H}^{\ast, }(10)$| and |${H}^{\ast,, }(10)$|/% . 1.0 187.9 1.00000 187.9 187.9 0 1.5 81.24 0.99534 81.61 83.51 −2.3 2.0 45.28 0.99070 45.70 46.98 −2.7 2.5 29.86 0.98609 30.29 30.07 0.7 3.0 20.68 0.98149 21.07 20.88 0.9 3.5 15.26 0.97692 15.62 15.34 1.8 4.0 11.92 0.97237 12.26 11.75 4.3 Open in new tab Table 1 Dose rate measurement results of portable gamma-ray irradiation field Distance/m . Measured value of ambient dose equivalent |${H}^{\ast }(10)$|/μSv|$\cdot $|h−1 . Air attenuation correction factor . Corrected ambient dose equivalent |${H}^{\ast,, }(10)$|/μSv|$\cdot $|h−1 . Calculated value of ambient dose equivalent |${H}^{\ast, }(10)$|/μSv|$\cdot $|h−1 . Relative deviation between |${H}^{\ast, }(10)$| and |${H}^{\ast,, }(10)$|/% . 1.0 187.9 1.00000 187.9 187.9 0 1.5 81.24 0.99534 81.61 83.51 −2.3 2.0 45.28 0.99070 45.70 46.98 −2.7 2.5 29.86 0.98609 30.29 30.07 0.7 3.0 20.68 0.98149 21.07 20.88 0.9 3.5 15.26 0.97692 15.62 15.34 1.8 4.0 11.92 0.97237 12.26 11.75 4.3 Distance/m . Measured value of ambient dose equivalent |${H}^{\ast }(10)$|/μSv|$\cdot $|h−1 . Air attenuation correction factor . Corrected ambient dose equivalent |${H}^{\ast,, }(10)$|/μSv|$\cdot $|h−1 . Calculated value of ambient dose equivalent |${H}^{\ast, }(10)$|/μSv|$\cdot $|h−1 . Relative deviation between |${H}^{\ast, }(10)$| and |${H}^{\ast,, }(10)$|/% . 1.0 187.9 1.00000 187.9 187.9 0 1.5 81.24 0.99534 81.61 83.51 −2.3 2.0 45.28 0.99070 45.70 46.98 −2.7 2.5 29.86 0.98609 30.29 30.07 0.7 3.0 20.68 0.98149 21.07 20.88 0.9 3.5 15.26 0.97692 15.62 15.34 1.8 4.0 11.92 0.97237 12.26 11.75 4.3 Open in new tab The distributed measurements of ambient dose equivalent of the portable gamma-ray irradiation field corrected for air attenuation are show in Table 1. The measured value of the air ambient dose equivalent |${H}^{\ast,, }(10)$| within 5% in the radiation field (after correcting the air attenuation) is inversed squared to the distance from the center of the radioactive source to the effective center of the detector, which meets the design requirements of the international standard ISO4037.1–2019(1) and can be used for in-situ calibration experiments. However, the scattered radiation caused by the wall at the reference point should be considered because the calibrated dosemeter is usually installed close to the wall in the actual in-situ calibration. The scattered contribution caused by the wall was explored by the developed ambient dose equivalent secondary standard ionization chamber, with the diminishing distance between the detector and the wall, while the distance between the detector and the radioactive source is constant. The scattered contribution from the wall was determined by the measurement results of the detector, as shown in table 2. Table 2 Scattered contribution caused by wall to portable gamma ray radiation field Number . Distance between the detector and the wall/cm . Measured value of ambient dose rate/μSv|$\cdot $||${\mathrm{h}}^{-1}$| . Normalized measured value of ambient dose rate/% . 1 60 44.67 100.00 2 50 44.70 100.07 3 40 45.05 100.85 4 20 46.11 103.23 5 7 47.33 105.95 Number . Distance between the detector and the wall/cm . Measured value of ambient dose rate/μSv|$\cdot $||${\mathrm{h}}^{-1}$| . Normalized measured value of ambient dose rate/% . 1 60 44.67 100.00 2 50 44.70 100.07 3 40 45.05 100.85 4 20 46.11 103.23 5 7 47.33 105.95 Open in new tab Table 2 Scattered contribution caused by wall to portable gamma ray radiation field Number . Distance between the detector and the wall/cm . Measured value of ambient dose rate/μSv|$\cdot $||${\mathrm{h}}^{-1}$| . Normalized measured value of ambient dose rate/% . 1 60 44.67 100.00 2 50 44.70 100.07 3 40 45.05 100.85 4 20 46.11 103.23 5 7 47.33 105.95 Number . Distance between the detector and the wall/cm . Measured value of ambient dose rate/μSv|$\cdot $||${\mathrm{h}}^{-1}$| . Normalized measured value of ambient dose rate/% . 1 60 44.67 100.00 2 50 44.70 100.07 3 40 45.05 100.85 4 20 46.11 103.23 5 7 47.33 105.95 Open in new tab The scattered radiation at the reference point needs to be corrected in the in-situ calibration experiment considering that the scattered contribution caused by wall increases with the decrease of the distance from the reference point to the wall. Dose rate at 7 cm away from the wall increases by about 6% compared with that at 60 cm away from the wall, as shown in table 2. In-situ calibration experiment The calibrated dosemeter was irradiated directly by radioactive source, with laser collimation system and the adjustable height of the portable gamma-ray irradiation device. The calibrated dosemeter respectively receives the radiation from the direct beam provided by the portable gamma-ray irradiation facility and the scattered radiation from the wall, as shown in Fig. 10. Figure 10 Open in new tabDownload slide Diagram of in-situ Calibration Figure 10 Open in new tabDownload slide Diagram of in-situ Calibration The reference values of dose rate at the location of the calibrated dosemeter can be determined in two ways, one of which is using the ambient dose equivalent secondary standard ionization chamber in an open space to determine the ambient dose equivalent rate at the reference point, with the scattered radiation corrected based on the data provided in table 2. Another way is determining the dose equivalent rate at the reference point on the spot of in-situ calibration with the ambient dose equivalent secondary standard ionization chamber parallel to the calibrated dosemeter. The first method has a simplified in-situ calibration operation, with the scattered radiation at the calibration location restrictedly from the wall, which cannot solve the in-situ calibration of the special installation location (complex scattered radiation). The second method is suitable for in-situ calibration under complex scattering conditions, for requirements of in-situ determination of ambient dose equivalent true value using secondary standard ionization chamber. The calibration factor N is the ratio of H*(10) true value to the read-out value M of the dosemeter. The first method and the second method were both carried out for in-situ calibration experiments in a nuclear facility of China Institute of Atomic Energy. In the first situation, considering the fixed ambient dose equivalent dosemeters are installed without other scattered objects except the wall. The ambient dose equivalent secondary standard ionization chamber is used to determine the dose rate of a certain point in the portable gamma-ray irradiation field, with the scattered radiation corrected. In the second situation, it is very difficult to correct in complex scattered radiation situation. The secondary standard ionization chamber was fixed parallel to the calibrated dosemeter and measured the ambient dose equivalent true value. The calibration factor N is the ratio of H*(10) true value to the read-out value M of the calibrated dosemeter. After in-situ calibration, the fixed ambient dose equivalent radiation dosemeters were disassembled and sent to metrology laboratory for calibration. The difference between in-situ calibration factors and metrology laboratory calibration factors were shown in table 3. The five dosemeters in table 3 were the same model, but the installation positions are different. No. 1 to No. 3 dosemeters were installed on the wall with no scattering object. There was no other scattered radiation except the wall, so the first method has been used for calibration. No. 4 and No. 5 were installed on the wall with other scattering objects (tubes and facilities), so the second method has been used for calibration. All of the calibration experiments have been carried out in China Institute of Atomic Energy. Table 3 Experiment results of in-situ calibration Number . In-situ calibration factor . Laboratory calibration factor . Relative deviation/% . 1 0.937 0.930 0.75 2 0.920 0.959 –4.07 3 1.005 1.040 –3.37 4 0.940 0.971 –3.19 5 1.011 0.990 2.12 Number . In-situ calibration factor . Laboratory calibration factor . Relative deviation/% . 1 0.937 0.930 0.75 2 0.920 0.959 –4.07 3 1.005 1.040 –3.37 4 0.940 0.971 –3.19 5 1.011 0.990 2.12 Open in new tab Table 3 Experiment results of in-situ calibration Number . In-situ calibration factor . Laboratory calibration factor . Relative deviation/% . 1 0.937 0.930 0.75 2 0.920 0.959 –4.07 3 1.005 1.040 –3.37 4 0.940 0.971 –3.19 5 1.011 0.990 2.12 Number . In-situ calibration factor . Laboratory calibration factor . Relative deviation/% . 1 0.937 0.930 0.75 2 0.920 0.959 –4.07 3 1.005 1.040 –3.37 4 0.940 0.971 –3.19 5 1.011 0.990 2.12 Open in new tab The portable gamma-ray irradiation facility combined with ambient dose equivalent secondary standard ionization chamber is a better solution to the problem of in-situ calibration of fixed ambient dose equivalent dosemeter, since the relative deviation between the in-situ calibration factors of the fixed ambient dose equivalent radiation dosemeters at five locations and the calibration factors obtained in the laboratory are within 5%, as shown in table 3. UNCERTAINTY The ambient dose equivalent secondary standard ionization has been used to measure the dose rate at the reference point of the radiation field, and the scattered radiation from the wall was corrected. In measurements, the statistical fluctuations, distance positioning and scattered radiation can contribute to the uncertainty of the conventional true value of the ambient dose equivalent, as shown in Table 4. Table 4 Uncertainty of the conventional true value of ambient dose equivalent Factors . Relative standard uncertainty u/% . Uncertainty type . Statistical fluctuation 0.2 A Calibration factor 2.2 B Uniformity of radiation field 1.0 B Positioning distance 0.3 B Scattered radiation 1.0 B Combined standard uncertainty uc1 2.7 Factors . Relative standard uncertainty u/% . Uncertainty type . Statistical fluctuation 0.2 A Calibration factor 2.2 B Uniformity of radiation field 1.0 B Positioning distance 0.3 B Scattered radiation 1.0 B Combined standard uncertainty uc1 2.7 Open in new tab Table 4 Uncertainty of the conventional true value of ambient dose equivalent Factors . Relative standard uncertainty u/% . Uncertainty type . Statistical fluctuation 0.2 A Calibration factor 2.2 B Uniformity of radiation field 1.0 B Positioning distance 0.3 B Scattered radiation 1.0 B Combined standard uncertainty uc1 2.7 Factors . Relative standard uncertainty u/% . Uncertainty type . Statistical fluctuation 0.2 A Calibration factor 2.2 B Uniformity of radiation field 1.0 B Positioning distance 0.3 B Scattered radiation 1.0 B Combined standard uncertainty uc1 2.7 Open in new tab The ambient dose equivalent at 1 m from the source in the portable gamma-ray reference radiation field was measured continuously 20 times by the secondary standard ionization chamber. According to formula (1). $$\begin{equation} V=\frac{1}{\overline{M}}\sqrt{\frac{1}{n\left(n-1\right)}\sum_{i=1}^n{\left({M}_i-\overline{M}\right)}^2}\times 100\% \end{equation}$$(1) The type A uncertainty of the statistical fluctuation of the measurement results is 0.2%. The secondary standard ionization chamber was calibrated in the gamma-ray reference radiation field of China Institute of Atomic Energy, and the uncertainty of calibration factor is 2.2%, which is type B uncertainty. The secondary standard ionization chamber is a closed type ionization chamber, the temperature |$T$| and the air pressure |$P$| have no necessity to be corrected in measurements, so the uncertainty caused by the temperature and air pressure in measurements is not considered. The uncertainty caused by the nonuniformity of the radiation field is mainly related to the positioning accuracy of the secondary standard ionization chamber in the direction perpendicular to the axis of the radiation field. Combined with the experimental results of the uniformity of the radiation field, it is estimated to be 1.0%, which is type B uncertainty. The sensitive volume center of the secondary standard ionization chamber cannot be accurately located at the 1 m reference point in the radiation field. The uncertainty caused by the distance error is estimated to be 0.3%, which is type B uncertainty. Since the scattered radiation from the wall needs to be corrected during in-situ calibration (As shown in Table 3, the correction factor for scattered radiation at 7 cm from the wall is 6%), it is necessary to consider the uncertainty to the reference value in correction. It is estimated to be 1.0%, which is type B uncertainty. The combined standard uncertainty of the conventional true value of the ambient dose equivalent at 1 m is given by equation (2). $$\begin{equation} {u}_{C1}=\sqrt{u_1^2+{u}_2^2+\cdots +{u}_n^2} \end{equation}$$(2) It can be seen from Table 4 that the combined standard uncertainty of the conventional true value of ambient dose equivalent at the reference point of the portable gamma-ray radiation field is 2.7%. During in-situ calibration, in addition to the uncertainty caused by the conventional true value of the ambient dose equivalent at the reference point, the uncertainty caused by the statistical fluctuations in the readings of the calibrated dosemeter should also be considered. The uncertainty of the in-situ calibration factor of the fixed radiation dosemeter is combined by two factors shown in Table 5. Table 5 Uncertainty of in-situ calibration factor Factors . Relative standard uncertainty u/% . Uncertainty type . Conventional true value 2.7 B Statistical fluctuations of the calibrated dosemeter 0.5 A Combined standard uncertainty 2.8 Factors . Relative standard uncertainty u/% . Uncertainty type . Conventional true value 2.7 B Statistical fluctuations of the calibrated dosemeter 0.5 A Combined standard uncertainty 2.8 Open in new tab Table 5 Uncertainty of in-situ calibration factor Factors . Relative standard uncertainty u/% . Uncertainty type . Conventional true value 2.7 B Statistical fluctuations of the calibrated dosemeter 0.5 A Combined standard uncertainty 2.8 Factors . Relative standard uncertainty u/% . Uncertainty type . Conventional true value 2.7 B Statistical fluctuations of the calibrated dosemeter 0.5 A Combined standard uncertainty 2.8 Open in new tab It can be seen from Table 5 that the combined standard uncertainty of the in-situ calibration factor is 2.8%, and the expanded uncertainty is 5.6% (k = 2). CONCLUSION Ambient dose equivalent secondary standard ionization chambers and portable gamma ray irradiation facility were developed combining the Monte Carlo method to solve in-situ calibration problems in the nuclear facilities. The experiments of in-situ calibration were carried out in five typical sites located in China Institute of Atomic Energy, results showed that the relative deviation between in-situ calibration factors and standard laboratory calibration factors are within 5%. The in-situ calibration method can meet the calibration requirements of the fixed ambient dose equivalent dosemeters. References 1. International Organization for Standardization . X and gamma reference radiations for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy—Part1: radiation characteristics and production method[R] . ( Geneva : ISO Publication Office ) ( 2019 ). Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC 2. International Organization for Standardization . 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 ene [R] . ( Geneva : ISO Publication Office ) ( 2019 ). Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC 3. Ankerhold , U. , Behrens , R. and Ambrosi , P. 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A Chamber for Determining the Conventionally True Value of Hp(10) and H*(10) Needed by Calibration Laboratories . Radiation Protection Dosimetry 96 , 133 – 137 ( 2001 ). Google Scholar OpenURL Placeholder Text WorldCat 7. Fei GAO ， Xuefu XIAO and Ning NI. Monte Carlo Optimization Design of High-Pressure Ionization Chamber Energy Response Characteristics[J] . Nuclear Electronics & Detection Technology , 2014 , 6 : 788 – 793 . Google Scholar OpenURL Placeholder Text WorldCat 8. International Commission on Radiation Units and Measurements . Determination of Dose Equivalents Resulting from External Radiation Sources. ICRU Report 39 . ( Bethesda, MD : ICRU Publications ) ( 1985 ). Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC © The Author(s) 2021. Published by Oxford University Press. All rights reserved. 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TI - The in-situ Calibration Method for Ambient dose Equivalent Dosemeters
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncab130
DA - 2021-09-08
UR - https://www.deepdyve.com/lp/oxford-university-press/the-in-situ-calibration-method-for-ambient-dose-equivalent-dosemeters-6gF5Zbu4x3
SP - 1
EP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -