##### 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.
^{134}Cs/^{137}Cs 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, D_{in}, 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.

**■**^{134}Cs/^{137}Cs 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
^{134}Cs,^{137}Cs and^{40}K 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 (
^{134}Cs and^{40}K). - 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
^{137}Cs, the peak range was defined between -1.5w and 1.5w with center 662keV. For^{134}Cs, 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^{137}Cs and 796 keV of^{134}Cs were derived. The peaks of 796 keV and 802 keV of^{134}Cs overlap therefore only the 796 keV peak of^{134}Cs 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/cm
^{2}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, N_{E}: Net peak number count derived in ④ and ⑤ above, b_{E}: Background counting rate (peak component), t: counting time (live time), k_{E}: Geometric factor including the direction-dependency of photopeak efficiency of Falcon5000 and the effect of depth distribution of contamination, s_{E}: Gamma-ray transmission of the vehicle chassis, η_{E}: Counting height correction factor between the inside/outside the vehicle, πr^{2}_{0}: Anterior cross-section of Ge detector unit, ε^{0}_{E}: Photopeak efficiency, γ_{E}: Gamma-ray emission probability per decay of relevant nuclide.

Substituting 796 keV of^{134}Cs and 662 keV of^{137}Cs into equation (3), surface concentration of contamination was derived.^{134}Cs/^{137}Cs ratio was derived as seen in equation (4).

Coefficients used in this analysisEnergy 662 keV 796 keV b _{E}[cps]2.13 1.28 k 0.773 0.824 sη 35.3 % 36.8 % r _{0}[cm]3 ε ^{0}15.6 % 12.9 % γ _{E}85.1 % 85.5 %

- ( 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/cm
- ⑦ The
^{134}Cs/^{137}Cs ratios and its statistical errors were registered when statistical error less than 10 percent.

- ① Find gamma ray photopeaks of