JP2020134316A - Air dose calculation method using flying body - Google Patents

Abstract

To provide a method and a device therefor capable of accurately measuring air dose near the ground without being affected by the situation of the measuring place such as undulations on the ground and the presence or absence of trees.SOLUTION: The air dose calculation method includes the steps of: obtaining measurement point data and calculation point data; calculating the detection probability for each measurement point and calculation point while considering the parameters of air attenuation, soil scattering attenuation, and forest attenuation derived for the measurement of radiation from the sky; calculating the radiation intensity value by repeating the calculation using the calculated detection probability value and following the ML-EM method; and converting the calculated radiation intensity value to the air dose rate using the air dose conversion factor.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method of calculating the amount of spatial radiation near the ground, such as a position 1 m above the ground, with high accuracy by using a flying object such as an unmanned helicopter.

As a method for safely and efficiently measuring the amount of air radiation near the ground over a vast area, a method for calculating air dose using an air vehicle such as an unmanned helicopter has been conventionally known (Patent Documents 1 and 2 and See Non-Patent Documents 1 and 2). In these methods, the air dose at a position 1 m above the ground is obtained based on the detection value of the radiation detector mounted on the flying object and the flight altitude, and the air dose is color-coded according to the magnitude of the air dose on the actual map. It is displayed in.

Japanese Unexamined Patent Publication No. 2014-145628 Japanese Unexamined Patent Publication No. 2014-145700

"Radiation measurement using an unmanned helicopter after the nuclear power plant accident" JAEA Research 2013-049 Sanada Yukihisa, Kondo Atsuya, Sugita Takeshi et al. "Radiation monitoring using an ununned helicopter in the evacuation zone around the Fukushima Daiichi nuclear power plant", EXPLORATION GEOPY, EXPLORATION GEOPY

However, in the radiation measurement from the sky, unlike the direct measurement on the ground, it is necessary to convert the measured value into the value of the air dose at 1 m above the ground. In the above-mentioned conventional method, when converting, the conversion is performed on the assumption that the ground surface is flat and the radiation sources are uniformly distributed. Therefore, if there are undulations in the terrain, a place with a large gradient of the radiation source, or a shield such as a tree, the system is different from the assumed system, so the radiation measurement value measured from the sky is set to the value of 1 m above the ground. When converted, it may differ significantly from the actual air dose value at a position 1 m above the ground.

Therefore, an object of the present invention is to provide a method capable of measuring an air dose near the ground with high accuracy without being influenced by the situation of the measurement place as described above.

The method of the present invention converts a radiation measurement value measured from the sky into a 1 m air dose value above the ground using topographical information. The calculation is performed using the radiation measurement point information (measurement count rate, measurement coordinates and altitude) and the topographical information (DEM, DSM) around the radiation measurement area at the conversion value calculation point. In the calculation, the air dose rate is calculated at the coordinates specified by the conversion value calculation point.

The method of the present invention is composed of an algorithm based on the ML-EM (Maximum Likelihood Expectation Maximization) method developed as a medical image reconstruction method. One of the features of the present invention is that the detection probability parameter (Cij described later) in the ML-EM method is optimized for radiation measurement from the sky.

The method of the present invention basically includes (1) acquisition of measurement point data and calculation point data, (2) calculation of detection probability for conversion, (3) calculation of radiation intensity value at conversion target point, and (4). It consists of four steps of conversion from radiation intensity value to air dose. More specifically, it looks like this:
(1) Acquire measurement point data consisting of latitude, longitude, height, and measurement count rate, and calculation point data consisting of latitude, longitude, and topography information around the radiation measurement area (DEM, DSM) at the conversion value calculation point.
(2) The detection probability is calculated for each measurement point and calculation point in consideration of the parameters of air attenuation, soil scattering attenuation, and forest attenuation derived for radiation measurement from the sky.
(3) Using the value calculated in (2) above, the calculation is repeated according to the method of the ML-EM method.
(4) Using the value calculated in (3) above, convert to the air dose rate by the air dose conversion coefficient.

More specifically, the present invention is a method of measuring an air dose at a constant altitude from the ground using an air vehicle equipped with at least a GPS and a radiation detector, wherein the latitude, longitude, height, and the like are described. The first step to acquire the measurement point data consisting of the measurement count rate from the radiation detector and the calculation point data consisting of the topography information around the radiation measurement area at the latitude, longitude, and conversion value calculation points, the acquired measurement point data and calculation points. The second step of calculating the detection probability by calculating the air attenuation coefficient from the distance of the data, the third step of introducing the calculated detection probability into the formula expressing the ML-EM method, and calculating the radiation intensity by iterative calculation, and the determination. It includes a fourth step of converting the radiation intensity into an air dose rate using an air dose conversion coefficient. According to another aspect of the present invention, the air vehicle is connected to at least a communication device that performs various data communication with GPS, a radiation detector, and the ground, and a program connected to each device and stored in an internal memory. It is an unmanned helicopter equipped with a computer that performs predetermined processing according to the above, and the program executes the first to fourth steps, and the execution result is transmitted in real time to a computer installed on the ground. It is configured to.

The computer installed on the ground displays the air dose map data color-coded according to the magnitude of the air dose based on the map data stored in advance in the memory and the data sent from the flying object to the ground. Create and display the map and the air dose at that location in color on the display device.

This method was developed for the purpose of converting the radiation measurement value measured from the sky using an unmanned helicopter or the like into a 1 m air dose value above the ground using topographical information, and is more than the conventional conversion method. It is possible to derive a value close to the value measured on the ground. The radiation intensity at the conversion target point can be estimated from the radiation count value, measurement position information, and surrounding topography information, and the air dose value can be calculated by using the air dose conversion count obtained from the relationship between the radiation intensity and the air dose rate. You can ask. Further, if the surrounding topographical information can be obtained not only by the radiation measurement from the sky but also by a 3D laser or the like, the radiation intensity can be estimated by this method also in the ground measurement and the indoor measurement.

The schematic diagram for demonstrating the air dose calculation method of this invention. The graph which shows the attenuation coefficient (PHITS calculation result) by air (distance). The graph which shows the attenuation coefficient (PHITS calculation result) by an angle. A graph showing the attenuation coefficient by forests. Detailed explanatory view of the calculation flow used in the air dose calculation method of this invention. The figure which shows the 1m air dose rate measurement result above the ground by a walking survey and the radiation measurement value from the sky by an unmanned helicopter 1m conversion result (area 1). The figure which shows the 1m air dose rate measurement result above the ground by a walking survey and the radiation measurement value from the sky by an unmanned helicopter 1m conversion result (area 2). Comparison of the conversion value by the conventional method and the ground measurement value in area 1 of FIG. Comparison of converted value by ML-EM method and ground measurement value in area 1 of FIG. Comparison of the conversion value by the conventional method and the ground measurement value in the area 2 of FIG. Comparison of converted value by ML-EM method and ground measurement value in area 2 of FIG.

First, the basic concept of the present invention will be described with reference to FIG. FIG. 1 is a diagram schematically showing the air dose calculation method of the present invention, and the figure on the left side facing the paper shows the main flow of the air dose calculation method adopted in the present invention. In addition, the figure on the right side of the paper is illustrated so that the parameters used in the above-mentioned air dose calculation method can be easily understood.
<Application of ML-EM method>
The greatest feature of the present invention is the ML used in the medical field as an image reconstruction method for PET examinations and the like by using radiation measurement data measured in the sky by using a radiation detector mounted on a moving body such as an unmanned helicopter. -EM (Maximum Likelihood-Expectation Maximization) method is used to incorporate parameters for aerial radiation measurement into a radiation measurement value at a height of 1 m above the ground. Here, the sequential equation of the ML-EM method is expressed as equation (1).

Here, k is the number of repeated calculations, j is the number of calculation points (mesh number), λj is the estimated calculation value in j, and B is the number of all calculation points. Further, i indicates the position number of the detector, yi is the radiation count value at the position i, and N is the number of all measurement points. Cij indicates the detection efficiency. For Cij, parameters considering the distance from j to i, the angle between j and i, the shielding effect by the terrain, and the shielding effect by the forest were used. For the calculation of Cij parameters, in addition to radiation measurement data by an unmanned helicopter, photographic survey data (DSM: Digital Surface Model height including trees and buildings) and ground surface elevation model (DEM:) of the board map information of the National Land Research Institute. Digital Elevation Model Ground surface height excluding trees and buildings) data was used. The detection efficiency Cij used in the ML-EM method is obtained by the following equation (2).

The detection efficiency Cij corresponds to the angle with the ground, which is calculated from the attenuation coefficient f (x) according to the distance calculated from the distance between the measurement point and the calculation point, and the angle between the measurement point and the ground at the calculation point. It is obtained from the attenuation coefficient, that is, the angle correction coefficient K (θ), and the attenuation coefficient M (h) due to the forest, which is calculated from the relationship between the measurement point and the tree height. The attenuation coefficient f (x) according to the distance is shown in equations (3) and (4).

For the attenuation coefficient by distance, the result of calculating the photon attenuation of the total energy count according to the distance of the 662 keV dotted line source emitted by Cs-137 by PHITS was used. The calculation result of the attenuation coefficient f (x) based on the distance is shown in FIG. In FIG. 2, the horizontal axis is the distance between the measurement point i and the calculation point j, and the vertical axis is the attenuation coefficient f (x). The attenuation coefficient f (x) is a power approximation up to a distance of 200 m, and an exponential approximation thereafter. Although the damping factor of f (x) differs depending on the energy of gamma rays, the frequency of calculating the value of x> 500 is low in the measurement at an altitude of 80 m or less. Therefore, this time, all calculations were made by attenuation with energy of Cs-137 (662 keV). The attenuation coefficient K (θ) according to the angle is shown in the equation (5).

For the attenuation coefficient by angle, the result of calculating the photon attenuation of the total energy count according to the distance of the 662 keV dotted line source emitted by Cs-137 by PHITS was used. The calculation result of the attenuation coefficient K (θ) depending on the angle is shown in FIG. In FIG. 3, the horizontal axis is the angle between the vector consisting of the measurement point i and the calculation point j and the soil surface, and the vertical axis is the attenuation coefficient K (θ). The attenuation coefficient K (θ) is a Gaussian approximation.

Attenuation coefficient due to forest is calculated from the ratio of 1m above ground conversion value and 1m above ground measurement value of the above-ground radiation measurement value by the conventional method, and DEM and DSM by measuring from the sky and from the ground in multiple forest areas. The coefficient was obtained from the relationship with the height of the tree. At that time, the tree height was taken as the average value of the forest (DSM) passage distances from j to i at a radius of 20 m directly below i. Figure 4 shows the relationship between the ratio of the 1m above-ground measurement value and the 1m above-ground measurement value and the tree height. In FIG. 4, the horizontal axis is the average tree height within a radius of 20 m from the measurement point, and the vertical axis is the attenuation coefficient M (h). From this result, the attenuation coefficient M (h) is defined as in the equation (6).

These calculation formulas were programmed using C language, and the radiation measurement data from the sky was analyzed. Figure 5 shows the detailed calculation flow.

The analysis range was set to a radius of 150 m from the measurement point in consideration of the influence from the surroundings. In addition, as a comparison target, an analysis was performed assuming a conventionally used plane model. To convert the estimated value obtained by the ML-EM method to air dose, obtain the dose conversion coefficient from the converted value obtained by the conventional method and the estimated value obtained by the ML-EM method, and obtain the air dose value of 1 m above the ground. Converted to. Each of the obtained values was interpolated using a commercially available GIS software (ArC GIS, manufactured by ESRI) using the kriging method, and a contour diagram was created.
<Radiation measurement method from the sky>
When measuring from the sky, Yamaha Motor Co., Ltd. autonomous flight type unmanned helicopter FAZER RG2 is used, and the total count rate and γ-ray energy spectrum including the direct gamma rays from the ground and the scattered rays by air are measured once per second. Continuous measurement was performed. A LaBr3 (Ce) (Lanthanum Bromide) scintillation detector was used as the radiation detector. The specifications of the unmanned helicopter and the radiation detector are shown in Table 1 and Table 2.

The flight altitude of the unmanned helicopter was set at 50 m above ground level. The radiation measured in the sky is an average of the gamma doses in a circle with a diameter of about 100 m centered directly under the unmanned helicopter. The acquired data is gamma ray data (counting rate) measured by a radiation detector every second, an energy spectrum, and position information by a corresponding DGPS (Differential Global Positioning System).

In order to verify the effect of the method according to the present invention, the same measurement was performed by a conventional method that does not use the ML-EM method.
<Data analysis method by the conventional method>
In order to obtain a coefficient for converting the gamma ray counting rate measured in the sky into the value of the air dose rate at a height of 1 m above the ground, the diameter is 200 m in a flat place where the air dose rate is relatively constant in the measurement area. Set up a circular test site. At the test site, air dose rate data at a height of 1 m above the ground was acquired in advance using a NaI survey meter. After that, an unmanned helicopter was hovered at an altitude of 50 m above the center of the test site, and the gamma ray count rate acquired at this altitude (reference altitude) was compared with the air dose rate on the ground of the test site, and the air dose rate was converted. The coefficient (CD: Conversion space: cps (μSv / h) -1) was calculated. Furthermore, hovering over the test site from 10 m to 100 m above the ground level every 10 m, the gamma ray count rate at each altitude is measured, and the relational expression between the ground level and the gamma ray count rate is obtained from the gamma ray count rate for each altitude. The correction coefficient (AF: Attitude factor: m-1) was calculated.

The gamma ray count rate acquired in the actual flight corrects the deviation between the altitude above ground level and the reference altitude by the altitude correction coefficient AF, and the air dose rate at a height of 1 m above the ground from the air dose rate conversion coefficient CD (μSv h-1). Converted to. The altitude to the ground was obtained by subtracting the digital elevation model DEM (Digital Elevation Model) data and geoid height of a 10 m mesh created by the Geospatial Information Authority of Japan from the GPS altitude determined by GPS.
<Measurement of ground dose rate by walking survey>
A walking survey was conducted on the ground to evaluate the validity of monitoring data from the sky. The walking survey includes the KURAMA-II (Kyoto University Radiation Mapping system) used in the walking survey conducted by the Nuclear Regulatory Agency's "Aggregation of distribution data of radioactive materials associated with the accident at the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Co., Inc." )It was adopted. KURAMA-II is composed of a CsI (Tl) scintillation detector (C12137 manufactured by Hamamatsu Photonics) using CsI (Tl) crystals of 13 mm x 13 mm x 20 mm and CompactRIO, and can correct the air dose rate with various parameters. It is possible. In the actual data acquisition method, the measurer carried the KURAMA-II system and the battery on his back, and acquired the air dose rate data measured every 3 seconds while walking and the position information by GPS.
<Conversion result>
The results of reconstructing the measurement results of the unmanned helicopter by the ML-EM method are shown in FIGS. 6 and 7. It can be seen that the gradient of the dose rate is clearly visible compared to the contour diagram created by the conventional method.

FIG. 6 shows the result of measuring the air dose rate of 1 m above the ground by a walking survey and the result of converting the radiation measured value from the sky by an unmanned helicopter into 1 m above the ground in the area 1 composed of forest and flat land. The flight altitude of the unmanned helicopter is 50 m, the speed is 2 m / s, and the measured line width is 10 m. From FIG. 6, in the conventional method, the forest part is shielded by trees and the value is low, but in the method of the present invention using the ML-EM method, the dose in the forest part is higher than the above-ground value as compared with the conventional method. It can be seen that the value is close to.

Further, FIG. 7 shows the results of measuring the air dose rate of 1 m above the ground by a walking survey and the results of converting the radiation measured values from the sky by an unmanned helicopter into 1 m above the ground in the area 2 composed of forests, residential areas, flat areas, etc. The flight altitude of the unmanned helicopter is 50 m, the speed is 5 m / s, and the measured line width is 50 m. The area is about 1.7 km x 1.0 km. From FIG. 7, in the conventional method, the area where the dose is low is affected by the ambient dose and becomes a high value, but in the method of the present invention using the ML-EM method, the value is closer to the ground value and the dose is low. It can be seen that it is converted as. In order to quantitatively evaluate the state change of the above-mentioned map, the results of comparison with the ground measurement values are shown in FIGS. 8a, 8b, 9a, and 9b. In order to carry out a quantitative comparison, the relative deviation (RD) is defined here by the following equation (7).

Here, i indicates the measurement point, G indicates the measurement result of the walking survey at the measurement point i, and V indicates the measurement result of the unmanned helicopter at the measurement point i. The results of comparing the measurement results of the walking survey and the measurement results of the unmanned helicopter for each measurement area and the frequency distribution of RD are shown in FIGS. 8a, 8b, 9a, and 9b. Looking at the average value of RD, the conversion result using the ML-EM method is closer to 0 than the conversion result by the conventional method. This result shows that the conversion by the ML-EM method is improved to a value closer to the ground value than the conventional method.

Both FIGS. 8a and 8b show a comparison between the measurement result of the air dose rate 1 m above the ground by the walking survey and the radiation measurement value 1 m above the ground conversion result by the unmanned helicopter in both areas 1. FIG. 8a shows a comparison between the converted value by the conventional method and the ground measured value, and FIG. 8b shows a comparison between the converted value by the ML-EM method and the ground measured value.

Further, FIGS. 9a and 9b both show a comparison between the measurement result of the air dose rate 1 m above the ground by the walking survey and the radiation measurement value 1 m above the ground conversion result by the unmanned helicopter in the area 2. FIG. 9a shows a comparison between the converted value by the conventional method and the ground measured value, and FIG. 9b shows a comparison between the converted value by the ML-EM method and the ground measured value.

The normalized mean square error NMSE (Normalized Mean Square Error) calculates the ground measurement value and the converted value by the above equation (8). The closer this value is to 0, the closer to the ground measurement value. It can be seen that in both area 1 and area 2, the converted value by the method of the present invention using the ML-EM method is smaller.

y ... Measurement point data λj ... Calculation point data
f (x)… Air attenuation coefficient
K (θ) ... Angle correction coefficient M (h) ... Forest attenuation coefficient Cij ... Detection probability λ ... Radiation intensity

Claims (8)

It is a method of measuring air dose at a certain altitude from the ground using an air vehicle equipped with at least GPS and a radiation detector.
The first step of acquiring measurement point data consisting of latitude, longitude, height, and measurement count rate from the radiation detector, and calculation point data consisting of topography information around the radiation measurement area at latitude, longitude, and conversion value calculation points.
The second step of calculating the detection probability by obtaining the air attenuation coefficient from the distance between the acquired measurement point data and the calculation point data,
The calculated detection probability is introduced into the formula expressing the ML-EM method, and the third step of calculating the radiation intensity by iterative calculation, and the calculated radiation intensity is converted into the air dose rate using the air dose conversion coefficient. 4 steps,
Air dosimetry method using an air vehicle consisting of. In the method according to claim 1, in the second step, the angle correction coefficient is obtained from the angle between the measurement point and the ground of the calculation point, and the detection probability is obtained from the integrated value of the air attenuation coefficient and the angle correction coefficient. An air dose measurement method using an air vehicle, which is characterized in that. The method according to claim 1 is characterized in that, in the second step, the forest attenuation coefficient is obtained from the relationship between the measurement point and the tree height, and the detection probability is obtained from the integrated value of the air attenuation coefficient and the forest attenuation coefficient. Air dose measurement method by the flying object. In the method according to claim 2, in the second step, the forest attenuation coefficient is obtained from the relationship between the measurement point and the tree height, and the detection probability is calculated from the integrated value of the air attenuation coefficient, the angle correction coefficient, and the forest attenuation coefficient. A method for measuring air dose by an air vehicle, which is characterized by obtaining. In the method according to any one of claims 1 to 4, the air vehicle is connected to at least a communication device that performs various data communication with GPS, a radiation detector, and the ground, and each device, and is stored in an internal memory. An unmanned helicopter equipped with a computer that performs predetermined processing according to a stored program. The program executes the first to fourth steps, and the execution result is a computer installed on the ground in real time. A method of measuring air dose by an air vehicle, characterized in that data is transmitted to a computer. In the method according to claim 5, the computer installed on the ground is color-coded according to the magnitude of air dose based on the map data stored in advance and the data sent from the flying object to the ground. An air dose measurement method using an air vehicle, which is characterized in that the air dose map data is created and displayed in color on a display device. It is a method of measuring air dose at a certain altitude from the ground using an air vehicle equipped with at least GPS and a radiation detector.
The measurement point data consisting of latitude, longitude, height, and the measurement count rate from the radiation detector, and the calculation point data consisting of the topography information around the radiation measurement area at the latitude, longitude, and conversion value calculation points are acquired.
Obtain the air attenuation coefficient from the distance between the acquired measurement point data and the calculation point data, the angle correction coefficient from the angle between the measurement point and the ground of the calculation point, and the forest attenuation coefficient from the relationship between the measurement point and the tree height, and integrate them. Calculate the detection probability from the value and
The calculated detection probability is introduced into the formula expressing the ML-EM method, and the radiation intensity is calculated by iterative calculation.
An air dose measurement method using an air vehicle, characterized in that the obtained radiation intensity is converted into an air dose rate using an air dose conversion coefficient. In the method according to claim 7, data such as an air dose rate obtained by the flying object is transmitted to a computer installed on the ground, and map data stored in advance in the memory of the computer and the flying object are used. Based on the data sent to the ground, the air dose map data color-coded according to the magnitude of the air dose is created and displayed in color on the display device of the computer installed on the ground. Air dose measurement method.

Images