Air Dose Rates and 134Cs/137Cs Measurement Results by the Radi-Probe ( From September 25, 2012 to October 7, 2012 )
- This data was created based on the results of the vehicle-borne survey conducted by MEXT from September 25 to October 7, 2012.
- The measurement used the Radio Probe developed by the National Institute of Radiological Sciences.
- Regarding probes, we used HDS-100GN CsI(Tl) spectrometer by Mirion Technologies, TCS-172B by Hitachi-Aloka Medical and Falcon5000 Portable HPGe-Based Radionuclide Identifier by Canberra. Air dose rates were measured using the HDS-100GN and TCS-172B. 134Cs/137Cs ratios were measured using Falcon5000.
- The air dose rate measured in the monitoring car and the air dose rate outside the car differ because of shielding by the chassis of the monitoring car. The monitoring car used in this survey has been used before the hydrogen explosion of the Fukushima Dai-ichi NPP around the former Offsite Center and contamination was confirmed in its wheel housings. Therefore correction formula was derived by comparing air dose rate outside the car with air dose rate inside the car to compensate the shielding of the chassis and background including wheel housing contamination.
■Air dose rate measurement
While the monitoring car was in motion, the data was obtained by running HDS-100GN continuously. At the calibration point, measurement was conducted by both HDS-100GN and TCS-172B to compare air dose rates inside and outside the car. The calibrated data as a value of TCS-172B at a height of 1 m above the ground outside the car was obtained by conversion based on the measured data comparison between HDS-100GN and TCS-172B. First, Din, measured data by HDS-100GN inside the car, was converted to D’in, measured data by TCS-172B inside the car.
Conversion equation from D’in to the air dose rate outside the car (D’out , as a value of TCS-172B) is as follows.
If D’in is equal to or larger than 1µSv/h, conversion equation is as follows.
Using either of equations above, the air dose rate at a height of 1 m above the ground outside the car was derived.
■134Cs/137Cs ratio measurement
Gamma spectrum analysis of the per unit time obtained by Falcon5000 was conducted with the following procedure. - ① Find gamma ray photopeaks of 134Cs, 137Cs and 40K by the secondary differentiation method. Convert channel values of pulse height analyzer to energy. It is difficult to distinguish peaks in the low dose area therefore peak search was conducted with data 15 minutes before and after sampling.
- ② Photopeak values vary by change of detector response due to ambient temperature change or unstable power supply, therefore energy calibration was conducted using major photopeaks (134Cs and 40K).
- ③ The energy resolution (FWHM) of Falcon5000 is mostly independent on energy level and equal to or less than 5 keV (average 4.1 keV) in the rage of 60 - 1333 keV. Therefore a tentative energy resolution of 5 keV was defined.
- ④ Regarding gamma-ray peaks of nuclides, +/- 2.5 times of the tentative FWHM were set as borders and then background of the peak position was defined by linear approximation. True FWHM of the peak was derived. A larger value either the true FWHM or the tentative FWHM was assigned to "w" and the peak area was derived using "w". This method was applied to calculate the peak area even if a temperature drift takes place within an accumulation period.
- ⑤ For 137Cs, the peak range was defined between -1.5w and 1.5w with center 662keV. For 134Cs, the peak range was defined between -1.5w and 1.5w with center 799keV which is the center of combined peaks 796 keV and 802 keV. Ranges from -3w to -1.5w and from 1.5w and 3w were assigned to the baseline in order to derive the background. Peak area (the number of gamma-ray) of each peak was calculated by Covell's method. Net counts of 662 keV of 137Cs and 796 keV of 134Cs were derived. The peaks of 796 keV and 802 keV of 134Cs overlap therefore only the 796 keV peak of 134Cs was selected based on gamma-ray emission ratio and was calculated.
- ⑥ Surface concentration of contamination (Bq/cm2) can be calculated from each photopeak count. The following assumptions and corrections are needed.
- ( a ) The surface on the infinite plane is assumed to be contaminated and radioactive cesium concentration decreases exponentially along depth direction from the surface. Average 1 g/cm2 of the surface is contaminated.
- ( b ) A Ge detector unit is cylindrical and it is empirically known that photopeak efficiency is almost symmetric with respect to the axis of the cylinder. Here, based on axial symmetry assumption and laboratory measurement results of photopeak efficiency for a calibration source, a direction-dependent model (axial symmetry model) of photopeak efficiency of Falcon5000 was created and then used.
Surface concentration of contamination is expressed in equation (3).
Here, E: Gamma-ray energy, NE: Net peak number count derived in ④ and ⑤ above, bE: Background counting rate (peak component), t: counting time (live time), kE: Geometric factor including the direction-dependency of photopeak efficiency of Falcon5000 and the effect of depth distribution of contamination, sE: Gamma-ray transmission of the vehicle chassis, ηE: Counting height correction factor between the inside/outside the vehicle, πr20: Anterior cross-section of Ge detector unit, ε0E: Photopeak efficiency, γE: Gamma-ray emission probability per decay of relevant nuclide.
Substituting 796 keV of 134Cs and 662 keV of 137Cs into equation (3), surface concentration of contamination was derived. 134Cs/137Cs ratio was derived as seen in equation (4).
Coefficients used in this analysis | Energy | 662 keV | 796 keV | bE[cps] | 2.13 | 1.28 | k | 0.773 | 0.824 | sη | 35.3 % | 36.8 % | r0[cm] | 3 | ε0 | 15.6 % | 12.9 % | γE | 85.1 % | 85.5 % |
- ⑦ The 134Cs/137Cs ratios and its statistical errors were registered when statistical error less than 10 percent.
Air Dose Rates and 134Cs/137Cs Measurement Results by the Radi-Probe ( From December 13, 2011 to December 21, 2011 )
- This data was created based on the results of the vehicle-borne survey conducted by MEXT from December 13 to December 21, 2011.
- The measurement used the Radio Probe developed by the National Institute of Radiological Sciences.
- Regarding probes, we used an HDS-100GN CsI(Tl) spectrometer by Mirion Technology and a Falcon5000 Portable HPGe-Based Radionuclide Identifier by Canberra. Air dose rates were measured using the HDS-100GN and 134Cs/137Cs ratios were measured using Falcon5000.
- The air dose rate measured in the vehicle and the air dose rate at a height of 1 m outside the vehicle differ because of shielding by the chassis of the vehicle. The vehicle used in this survey has been used before the hydrogen explosion of the Fukushima Dai-ichi NPP around the former Offsite Center and contamination was confirmed in its wheel housings. Therefore correction formula was derived by comparing air dose rate at a height of 1 m outside the vehicle with air dose rate inside the car to compensate the shielding of the chassis and background including wheel housing contamination.
■Air dose rate measurement
The air dose rate outside the monitoring car "D" was derived by substituting the air dose rate obtained by HDS-100GN in the vehicle "Dm" into the equation (1).
(1)
where, sd: transmission of the vehicle chassis, κ: caliblation factor of HDS-100GN, c: correction factor to compensate the contamination by the vehicle. sd = 0.617, κ=1.04, c=-0.08 µSv/h.
■134Cs/137Cs ratio measurement
Gamma spectrum analysis of the per unit time obtained by Falcon5000 was conducted with the following conditions and procedure based on "Gamma-ray Spectrometry Using Germanium Semiconductor Detectors" of MEXT's Radiation Measurement Method Series No.7, and "Method of the in-situ measurement using germanium semiconductor detectors" of MEXT's Radiation Measurement Method Series No.33. - ① Find photopeaks of 134Cs,137Cs and 40K by the secondary differentiation method. Convert channel values of pulse height analyzer to energy. It is difficult to distinguish peaks in the low dose area therefore peak search was conducted with data 15 minutes before and after sampling.
- ② Photopeak values vary by change of detector response due to ambient temperature change or unstable power supply, therefore energy calibration was conducted using major photopeaks (134Cs and 40K).
- ③ Full width at half maximum (FWHM) of 40K's peak was determined and the FWHM was defined as a tentative energy resolution of the sampling point.
- ④ Regarding gamma-ray peaks of other nuclides, +/- 2.5 times of the tentative FWHM were set as borders and then background of the peak position was defined by linear approximation. True FWHM of the peak was derived and then peak area (from -1.4*FWHM to 1.4*FWHM) and baseline (from -2.8*FWHM to -1.4*FWHM, and from 1.4*FWHM to 2.8*FWHM) were defined. Using these values peak area (the number of gamma-ray) was calculated by Covell's method.
- ⑤ Net counts of 662 keV of 137Cs and 796 keV of 134Cs were derived. Peaks of 796 keV and 802 keV of 134Cs overlap therefore only 796 keV of 134Cs was selected based on gamma-ray emission ratio and was calculated.
- ⑥ Surface concentration of contamination (Bq/cm2) can be calculated from each photopeak count. The following assumptions and corrections are needed.
- ( a ) The surface on the infinite plane is assumed to be contaminated and radioactive cesium concentration decreases exponentially along depth direction from the surface. Average 1 g/cm2 of the surface is contaminated.
- ( b ) Ge detector has isotropic sensitivity independent on gamma-ray energy.
- ( c ) Transmission of vehicle chassis and background due to vehicle contamination are corrected using the coefficient that is mentioned in later section.
Surface concentration of contamination is expressed in equation (2)
(2)
Here, E: Gamma-ray energy, NE: Net peak number count derived in ④ and ⑤ above, bE: Background counting rate (peak component). T: counting time, kE: Geometric factor, sE: Gamma-ray transmission of the vehicle chassis, ηE: Counting height correction factor between inside/outside the vehicle, πr2eff: Effective area of Ge detector, εE: Photopeak efficiency of Falcon5000, γE: Gamma-ray emission probability per decay of relevant nuclide.
Substituting 796 keV of 134Cs and 662 keV of 137Cs into equation (2), 134Cs/137Cs ratio was derived as seen in equation (3).
(3)
Coefficients used in this analysis | 662 keV | 796 keV | bE | 2.58 cps | 2.13 cps | k662/k796 | 0.966 | (s662 η662)/ (s796 η796) | 0.935 | ε662/ ε796 | 1.22 | γE | 85.1 % | 85.5 % |
- Blank in the columns "134Cs/137Cs" and "Statistic Error": Accurate data were not obtained when the air dose rate was low (below 0.26 µSv/h). To keep the statistic error below 10%, data in with an air dose rate equal to or above 0.26 µSv/h were registered.
