JP6419448B2 - Contamination distribution measuring device for measuring the distribution of contamination by radioactive materials - Google Patents

Description

  The present invention relates to a contamination distribution measuring apparatus for measuring a contamination distribution due to radioactive substances.

  When a nuclear disaster such as a large-scale radioactive material leakage accident occurred at the Fukushima Daiichi Nuclear Power Station, the leaked radioactive material has a wide area with a radius of tens to hundreds of kilometers (or more). To spread. When such a nuclear disaster occurs, creating a map showing the distribution of the air dose rate (hereinafter referred to as the radiation dose rate map) estimates where and how much the diffused radioactive material is distributed. It is important to manage the exposure of residents and to estimate the diffusion status of radioactive materials, and to define evacuation areas and indoor evacuation areas for residents, and to establish various restricted areas to prevent residents from being exposed to unnecessary exposure. Useful to do. The creation of a radiation dose rate map is also very important in planning decontamination plans and environmental restoration plans for damaged areas. To deal with environmental pollution caused by radioactive substances (hereinafter also simply referred to as pollution), the following procedures are required: 1) grasping the distribution of radioactive substances in the environment, 2) accurate decontamination work, 3) confirmation and verification of decontamination. This is because it is common to step on.

  The air dose rate (or radiation dose rate) is the radiation dose per unit time of the target space. When the amount of radiation is measured by the amount of energy absorbed by the substance (absorbed dose), the air dose rate is expressed in Gy / h (gray / hour) using Gy (gray) which is the unit of absorbed dose. Is done. Alternatively, when the amount of radiation is measured by the magnitude of the biological effect (dose equivalent) due to the effects of exposure to the living body, the air dose rate is calculated using Sv (sievert), which is a unit of dose equivalent. It is expressed in h (sievert / hour). The dose equivalent is obtained by multiplying the absorbed dose by a coefficient corresponding to the influence on the living body.

  Radiation is not visible, and the chemical behavior of radioactive materials is the same as naturally occurring stable isotopes. Therefore, in order to measure the distribution of radioactive substances in the environment, it is necessary to use a radiation detector. Here, in order to perform decontamination, it is particularly important to measure the distribution of radioactive materials on the surface of the earth, but in order to accurately measure the distribution of radioactive materials on the surface of the earth, how to use a radiation detector The problem is whether it should be installed.

The following methods 1) to 3) are examples of methods for measuring the distribution of radioactive substances that have been performed conventionally.
1) A radiation detector is arranged near the ground surface (a distance of 1 cm or 5 cm away from the ground surface).
2) Surround the surface of the surface to be measured with a shield, and put a radiation detector in it to measure.
3) A radiation detector is arranged near the ground surface (distance 1 cm or 5 cm away from the ground surface) and at a certain height (50 cm to 1 m) from the ground surface, and the difference between the two detectors is read.
The first method is the most basic measurement method. The second and third measurement methods are those described in Non-Patent Documents 1 and 2, for Fukushima Prefecture and the Ministry of the Environment to confirm the distribution of radioactive substances at the time of decontamination, or It is recommended as a standard measurement method for verification of the distribution of radioactive material after decontamination.

JP2013-242180A

Fukushima "Guide for radiation dose reduction measures in living spaces" Ministry of the Environment “2013 Decontamination and Construction Common Specifications (4th Edition)”

  In any of the first to third measurement methods described above, the radiation detector is disposed close to the ground surface. This takes advantage of the fact that radiation contribution from contamination on the surface increases when the radiation detector is brought close to the surface. In this case, since the air dose rate near the surface increases due to contamination from the ground surface, the degree of contamination can be grasped with appropriate accuracy. When the ground surface is flat, continuous measurement is possible by moving the radiation detector close to the ground surface while keeping the radiation detector at a certain height (for example, 1 cm from the ground surface in Non-Patent Document 2). However, when working in an actual environment such as a park, a residential land, or a field, it is very difficult to place a detector close to the ground surface for measurement. This is because there are terrain problems such as stones, blocks of soil, crops, plants and other obstacles, slopes and steps. In a place where the ground surface is not flat, it can be said that it is practically impossible to perform continuous measurement while moving from the ground surface while maintaining a height of 1 cm (or 5 cm). Therefore, the procedure of adjusting and measuring the height of the radiation detector at a certain point, moving to the next point, adjusting the height of the radiation detector, and measuring is repeated.

  Generally, radiation detectors that measure air dose rates in the environment are not directional. For this reason, when a radiation detector is arranged close to the ground surface, it is affected not only by radioactive materials distributed on the ground surface but also by contamination of surrounding trees and buildings. Therefore, in order to measure more accurately, as in the second method described above, radiation from the radioactive material distributed on the ground surface can be efficiently selected and incident on the radiation detector. It is necessary to shield the surroundings. Here, in order to shield γ rays, it is preferable to use a material containing many elements having a large atomic number. Therefore, a lead block or the like is generally used as the above-described shield. The lead block has a high shielding effect against γ rays, but has a large specific gravity, and therefore requires a lot of labor to move and install the lead block.

  Under these circumstances, it is difficult to measure continuously while moving all over the section to be measured by the conventional method. Therefore, instead of measuring the area in a plane, determine the location that is supposed to cause the radiation dose increase in the area in consultation with the work supervisor, etc., and confirm the area by measuring at those points. Is generally completed (see Non-Patent Document 2). In this case, it is necessary to have someone who has experience in decontamination to select an appropriate point. However, there is no clear indicator as to which point in the compartment should be selected, and relies on the intuition of decontamination workers. Furthermore, there is concern that there will be no decontamination from the owner of the target section or the residents in the vicinity as well as the decontamination operator himself. In addition, the question remains as to whether the situation can be grasped when the situation changes and causes a dose increase after decontamination.

  In relation to the third method, Patent Document 1 discloses a rod-shaped device support rod, two radiation detectors mounted at different positions on the device support rod, and a GPS receiver. A gamma plotter H comprising a transmitter is disclosed. Radiation detectors are attached to the device support rod at approximately 1000 mm on the ground surface and at two locations near the ground surface. These measured values and the current position information acquired by the GPS receiver are associated with each other, and a server is connected by the transmitter. Sent to. In the gamma plotter H described in Patent Document 1, since the two radiation detectors are fixed at two positions having different heights of the device support rod, the device support rod is moved while being kept perpendicular to the ground surface. Thus, the measurement of the air dose rate based on the third measurement method can be continuously performed while moving. However, even in the gamma plotter H described in Patent Document 1, since one radiation detector is not located near the ground surface, it is an obstacle such as a stone, a lump of soil, a crop, a plant, or a slope. It is very difficult to make measurements on uneven terrain such as bumps and bumps.

  The object of the present invention is to measure the air dose rate continuously while moving easily even on obstacles such as stones, blocks of soil, crops, vegetation, etc., and uneven terrain such as slopes and steps. Another object of the present invention is to provide a portable contamination distribution measuring device capable of measuring the contamination distribution due to radioactive substances.

According to an aspect of the present invention, there is provided a contamination distribution measuring device for measuring a contamination distribution due to radioactive substances,
A first radiation detector and a second radiation detector for measuring an air dose rate;
A first shield disposed over the first radiation detector so as to shield radiation incident on the first radiation detector from an orientation other than the orientation;
The second radiation detector is opposed to the first direction so as to shield radiation incident on the second radiation detector from the first direction and to enter radiation from directions other than the first direction. A contamination distribution measuring device is provided comprising a second shield disposed over the surface.

  In the contamination distribution measuring apparatus of the present invention, the first shield covers the first radiation detector so as to shield radiation incident on the first radiation detector from a direction other than the predetermined direction. The first radiation detector can have directivity in a predetermined direction (for example, the surface direction). Thereby, a component (described later direct line component) that is incident on the first radiation detector from a predetermined direction while suppressing a component incident on the first radiation detector from a direction other than the predetermined direction (described later indirect line component). Can be measured efficiently. Further, the second shield covers the surface of the second radiation detector that faces the predetermined direction so as to shield the radiation incident on the second radiation detector from the predetermined direction. Thereby, the indirect line component incident on the second radiation detector can be efficiently measured while suppressing the direct line component incident on the second radiation detector from a predetermined direction. From the measured values of the air dose rate measured by these two radiation detectors, only a direct line component incident on the first radiation detector can be extracted from a predetermined direction and exists immediately below the predetermined direction. It is possible to identify contamination by radioactive materials.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to confirm the distribution of the radioactive substance at the time of decontamination, distribution of the radioactive substance after decontamination can be verified easily.

FIG. 1 is a schematic view showing a measurement principle for measuring a contamination distribution by a radioactive substance according to the present embodiment. FIG. 2 is a schematic diagram of the contamination distribution measuring apparatus 1. FIG. 3 is a schematic view of the first radiation detector 10. FIG. 4 is a schematic diagram of the KURAMA system 1. FIG. 5 is a schematic diagram of the NaI scintillation detector 10. FIG. 6 is an example of an air dose rate map displaying markers color-coded according to the size of the air dose rate on a three-dimensional map. FIG. 7 is an example of an air dose rate map in which markers color-coded according to the size of the air dose rate are displayed on a two-dimensional map. FIG. 8 is a schematic diagram of the KURAMA-II system 200. FIG. 9 shows an example of a pulse height spectrum. FIG. 10 is a photograph showing a state in which a person is carrying the entire pollution distribution measuring system 200A in a state where the whole is packed in a backpack. FIG. 11 is a map of A city in Fukushima Prefecture where the demonstration experiment was conducted. FIG. 12A is a plot of the air dose rate measured by the first radiation detector 10, and FIG. 12B is a plot of the air dose rate measured by the second radiation detector 20. Is. FIG. 13 (a) is a plot of the air dose rate measured by the first radiation detector 10 in the vicinity of the exercise plaza and the parking lot, and FIG. 13 (b) is the second radiation detector 20 at the same location. The measured air dose rate is plotted. FIG. 14A is a plot of the air dose rate measured by the first radiation detector 10 in the vicinity of the parking lot on the opposite side across the courtyard and road of the meeting place, and FIG. The air dose rate measured with the 2nd radiation detector 20 in the same place is plotted. FIG. 15A is a plot of the air dose rate measured by the first radiation detector 10 at the roadside portion of the road passing in front of the meeting place, and FIG. 15B is the second radiation detection at the same location. The air dose rate measured by the vessel 20 is plotted. FIG. 16 is a plot of changes over time for the measurements of the athletic park and its surroundings. Figure 17 is a sports park and the measurement of its periphery is air dose rate map of evaluating the local component [Phi _L. Figure 18 is a measurement of the peripheral meeting place, a spatial dose rate map of evaluating the local component [Phi _L. FIG. 19 is an air dose rate map in which the local component Φ _L is evaluated for the measurement of the road passing in front of the meeting place. FIG. 20 is a map of the local component Φ _L in the orchard. FIG. 21 is a plot of the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs measured by the Ge detector and the air dose rate measured by the first radiation detector 10. FIG. 22 is a plot of the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs measured with the Ge detector and the air dose rate measured with the second radiation detector 20. Figure 23 is a total amount of ¹³⁴ Cs and ¹³⁷ Cs measured by the Ge detector and (Bq), is a plot of the local component [Phi _L.

First, after describing the measurement principle of the contamination distribution measuring apparatus 1 for measuring the contamination distribution due to radioactive substances according to the present embodiment, a specific configuration will be described in detail.
<Outline of measurement principle>

In general, the air dose rate Φ at a point in the environment is the sum of the air dose rate Φ _L due to radiation from the source immediately below that point and the air dose rate Φ _F due to radiation from surrounding sources. As shown in the following mathematical formula (see FIG. 1).

Here, the contamination sources that cause the air dose rate Φ _F are distributed in various places around the place including a distant place. Therefore, assuming that a wide range of environmental pollution has occurred, the air dose rate Φ _F includes not only a component in which the radiation emitted from the contamination source directly enters the radiation detector (hereinafter also referred to as a direct line component), For example, it is considered that many components (hereinafter also referred to as indirect line components) that are incident on a radiation detector after being reflected and / or scattered by surrounding structures such as buildings and trees, the ground surface, and the atmosphere. The indirect line component does not enter the radiation detector from a specific direction but enters the radiation detector from all directions. From this, the air dose rate Φ _F can be approximated to a relatively uniform radiation field. In the following description, also referred to as "uniform components [Phi _F spatial dose rate""spatial dose rate [Phi _F" a, or simply "homogeneous component [Phi _F".

On the other hand, pollution sources directly below to be measured since it is a short distance, in the air dose rate [Phi _L considered direct ray component is the majority. In other words, radiation incident directly from the contamination source immediately below the radiation detector is considered to constitute a spatial dose rate [Phi _L. In the following description, "spatial dose rate [Phi _L" to "local components of the spatial dose rate [Phi _L", or simply referred to as "local component [Phi _L". In order to measure the contamination distribution, it is important to accurately measure the local component Φ _L. However, when extensive contamination occurs, the total amount of radioactive material from the source of contamination immediately below is the surrounding radioactive material. Therefore, the contribution of the radioactive material of the direct contamination source cannot be sufficiently determined unless the radiation detector is sufficiently close to the contamination source immediately below. For example, when assuming a ground surface that is uniformly contaminated only on the surface, the contribution of contamination of the ground surface in the range from immediately below to a radius of 1 m in the air dose rate measured at a height of 1 m is only about 10%. As described above, when the radiation detector is arranged at a high position from the ground surface, the ratio of the local component Φ _L caused by the radiation from the immediately lower contamination source to the uniform component Φ _F caused by the radiation from the surrounding contamination source is it becomes low, precisely it is difficult to measure the local component [Phi _L. Therefore, in the conventional method, the radiation detector is arranged near the ground surface so that the ratio of the local component Φ _L to the uniform component Φ _F is as high as possible.

In contrast, in the present invention, in view of the contribution of the direct ray components in the local component [Phi _L is large, this in order to efficiently measure the radiation detector having a directivity in the surface direction (hereinafter, the radiation It was decided to prepare a detector A). Specifically, a shield covering the portion of the radiation detector A other than the surface facing the ground surface is disposed around the radiation detector A so that radiation from directions other than the ground surface does not enter the radiation detector A. It was decided to. However, even in the radiation detector A having directivity in the ground surface direction, a part of the uniform component Φ _F should be mixed. This is because the radiation detector A has a portion that is not covered by the shield, and the uniform component Φ _F corresponding to the solid angle Ω _{A of the} portion that is not covered by the shield is the radiation detector A. It is because it will be measured by. Further, the shield does not always completely shield radiation such as gamma rays and X-rays. For example, when the thickness of the shield is not sufficient, part of the radiation passes through the shield and enters the radiation detector A. Considering these, when the shielding rate when the shielding body is attached to the radiation detector A is Q, the air dose rate Φ _A detected by the radiation detector _A is expressed by the following equation (Equation 2). Can be represented. The shielding rate Q represents the ratio of shielding by the shield when it is assumed that the radiation is uniformly incident from all solid angles, and the portion of the radiation detector A that is not covered by the shield. Depending on the solid angle Ω _A and the transmission probability of the radiation passing through the shield. Note that the shield is sufficiently thick, when transmission probability of γ-rays is close to zero, the value of (1-Q) in the following equation is substantially the same as the solid angle Omega _A.

Here, since the shielding rate Q of the shield can be calculated based on the shape and material of the shield or by performing a simulation, if the uniform component Φ _F can be accurately obtained, Based on the air dose rate Φ _A and the uniform component Φ _F measured by the radiation detector A, the local component Φ _L to be measured can be obtained. Therefore, consider that a radiation detector B having no directivity is prepared separately from the radiation detector A and the uniform component Φ _F is determined. When the radiation detector B does not have directivity, the air dose rate Φ _B measured by the radiation detector _B is the sum of the uniform component Φ _F and the local component Φ _L as described above. However, as can be seen from the example of the contribution of contamination of the ground surface in the range of 1 m radius from the bottom in the measurement at a position 1 m away from the ground surface when the pollution source is uniformly distributed on the ground surface, the radiation detector B If the height is about 1 m, the local component Φ _L included in the air dose rate Φ _B measured by the radiation detector B is smaller than the uniform component Φ _F , and thus can be ignored. That is, it can be seen that the air dose rate Φ _B measured using the radiation detector _B is almost equal to the uniform component Φ _F mainly caused by radiation from the surrounding contamination sources. From this, the uniform component Φ _F can be measured by disposing the radiation detector B having no directivity away from the ground surface.

Furthermore, as will be described later, by placing the radiation detector B directly above the shield, it is possible to efficiently shield direct line components from contamination of the ground surface directly below. In other words, the local component Φ _L included in the air dose rate Φ _B measured by the radiation detector B can be further reduced. In this state, by moving the radiation detectors A and B while keeping the height constant, the distribution of contaminants on the ground surface can be grasped.

As described above, the shielding rate Q of the shielding body can be obtained by calculation or simulation, but the inventors have found that the shielding rate Q can be obtained experimentally by the following method. When the measurement is performed at a location where the local component Φ _{L is} almost zero, the air dose rate Φ _A measured by the radiation detector _A is (1−Q) × Φ _{F and} is measured by the radiation detector B. The air dose rate Φ _B is Φ _F. Here, when calculating the ratio of the spatial dose rate _{Φ A (Φ A / Φ B} ) for the spatial dose rate [Phi _B, a (1-Q). From this, the air dose rate Φ _A measured by the radiation detector _A and the air dose rate Φ _B measured by the radiation detector B are measured at a place not directly below where there is no contamination by radioactive substances. By calculating the ratio, it is possible to experimentally obtain the shielding rate Q of the shielding body.
<Outline of contamination distribution measuring apparatus 1>

  Next, a specific configuration of the contamination distribution measuring apparatus 1 according to the present embodiment will be described with reference to the drawings. As shown in FIG. 2, the contamination distribution measuring apparatus 1 includes a first radiation detector 10, a second radiation detector 20, and a collimator 30 that limits the solid angle of radiation incident on the first radiation detector 10. Prepare mainly. Since the first radiation detector 10 and the second radiation detector 20 have the same structure, the first radiation detector 10 will be described below as an example.

  The first radiation detector 10 is the same as the radiation detector used in the KURAMA-II system 200 described later. The first radiation detector 10 is a CsI scintillation detector. As shown in FIG. 3, the CsI crystal 11, a photomultiplier tube optically connected to the CsI crystal 11, MPPC (Multi-Pixel Photon Counter). And a pulse signal output unit 13 that outputs a pulse signal output from the light receiving element 12. In the present embodiment, MMPC is adopted as the light receiving element 12. MMPC is a small-sized optical semiconductor element that can be used at room temperature, including multiple Geiger mode APD (avalanche photodiode) pixels. It can be operated at a lower voltage than a photomultiplier tube, and the influence of a magnetic field can be reduced. It has excellent characteristics that it is difficult to receive.

  The collimator 30 is made of a substantially cylindrical lead with a bottom having a thickness of about 1 cm, and the first radiation detector 10 has a front surface 11a of the CsI crystal 11 (a surface opposite to the light receiving element 12 of the CsI crystal 11). ) Except for the first radiation detector 10. Radiation (γ rays and X-rays) incident from the front surface 11 a of the CsI crystal 11 of the first radiation detector 10 is not blocked by the collimator 30, but radiation from other directions is blocked by the collimator 30. The

  As shown in FIG. 2, in a state where the collimator 30 and the first radiation detector 10 are combined, the front surface 11a of the CsI crystal 11 of the first radiation detector 10 faces the ground surface side (the lower side in FIG. 2). Are arranged as follows. The second radiation detector 20 is disposed above the collimator 30 in a state of being separated by about 15 cm to 30 cm. Thus, by arranging the second radiation detector 20 above the collimator 30, it is possible to efficiently shield the direct line component from the contamination of the ground surface directly incident on the second radiation detector 20. In addition, the distance between the 2nd radiation detector 20 and the collimator 30 can be adjusted suitably as needed. For example, the collimator 30 and the second radiation detector 20 may be disposed so as to contact each other, and the distance between them may be 15 cm or less, or 30 cm or more.

Since the collimator 30 is attached to the first radiation detector 10, radiation incident from other than the front surface 11 a of the CsI crystal 11 of the first radiation detector 10 can be shielded by the collimator 30. Then, as described above, by directing the front surface 11a of the CsI crystal 11 of the first radiation detector 10 to the ground surface, the collimator 30 reduces the uniform component Φ _F caused by the radiation from the surrounding contamination sources, while The local component Φ _L resulting from the radiation from the contamination source can be measured efficiently. In other words, it is possible to increase the ratio of the local component Φ _L caused by the radiation from the immediately lower contamination source to the uniform component Φ _F caused by the radiation from the surrounding contamination source.

In the second radiation detector 20 arranged further above the collimator 30, the local component Φ _L caused by the radiation from the contamination source immediately below is collimated by the collimator 30, contrary to the case of the first radiation detector 10. It is possible to efficiently measure the uniform component Φ _F caused by the radiation from the surrounding contamination sources while reducing the. In other words, the ratio of the local component Φ _L caused by the radiation from the immediately lower contamination source to the uniform component Φ _F caused by the radiation from the surrounding contamination source can be kept low.

Thus, since the collimator 30 of this embodiment is arrange | positioned with respect to the 1st radiation detector 10 so that the 1st radiation detector 10 may be enclosed except the front surface 11a of the CsI crystal | crystallization 11, It has a function of suppressing the uniform component Φ _F caused by radiation from the contamination source. At the same time, the second radiation detector 20 is arranged so as to shield the front surface 11a of the CsI crystal 11 from the ground surface, so that it has a function of suppressing the local component Φ _L caused by radiation from the contamination source immediately below. doing. Then, the air dose rate measured by the first radiation detector 10 is subtracted by subtracting a value obtained by multiplying the air dose rate measured by the second radiation detector 20 by the solid angle of the first radiation detector 10. It is possible to calculate the local component Φ _L caused by radiation from the contamination source.

  Here, in the contamination distribution measuring apparatus 1 of the present embodiment, the radiation detector 10 to which the collimator 20 is attached does not necessarily have to be placed in close contact with the vicinity of the ground surface. For example, in a state of being separated from the ground surface by about 1 m. Measurements can be made. Therefore, when measuring in an actual environment (for example, parks, residential land, fields), the ground surface is not flat due to obstacles such as stones, blocks of soil, crops, vegetation, slopes, steps, etc. Measurement can be easily performed even at a place.

  Although the contamination distribution measuring apparatus 1 of this embodiment can be used alone, KURAMA is a system for producing an air dose rate map developed at Kyoto University led by one of the inventors. It can be used in combination with the -II system 200. Note that KURAMA is an abbreviation of “Kyoto University Radiation Mapping”.

Here, the KURAMA-II system 200 is an improvement of the KURAMA system 100 developed by one of the inventors. Therefore, before describing the KURAMA-II system 200, first, the KURAMA system 100 as a basis thereof will be described, and then, the KURAMA-II system 200 will be described focusing on differences from the KURAMA system 100.
<Outline of KURAMA System 100>

  As shown in FIG. 4, the KURAMA system 100 uses a NaI scintillation detector 110 as a radiation measuring device for measuring an air dose rate, and a GPS that acquires current position information using a global positioning system (GPS system). A unit 120 (position information acquisition mechanism), a data processing system 130 for processing data (dose rate data and position data) acquired by the NaI scintillation detector 110 and the GPS unit 120, and an analog output from the NaI scintillation detector 110 Are mainly provided with an interface unit 140 for A / D converting the data into the data processing system 130 and a data transmission unit 150 for transmitting data processed by the data processing system 130 to a server 190 described later.

  As shown in FIG. 5, the NaI scintillation detector 110 includes a NaI crystal 111 doped with thallium (Tl) as an emission center and a photomultiplier tube (not shown) optically connected to the NaI crystal 111. A main body in which a cylindrical measuring unit 112 disposed inside and a high-voltage power source for applying a predetermined voltage to the photomultiplier tube and an electronic circuit for shaping an output signal from the photomultiplier tube and classifying the wave height are arranged Unit 113 and cable 114 that electrically connects measurement unit 112 and main body unit 113 are mainly provided. The main body 113 includes a meter 113a that displays a measured dose (air dose rate) of radiation (γ rays or X-rays) per unit time, a range switch 113b that switches a measurement range of the NaI scintillation detector 110, and a measurement An output unit 113c for outputting an analog voltage signal corresponding to the air dose rate thus provided is provided.

  Here, when radiation such as γ rays or X-rays is incident on the inside of the NaI crystal 111, high energy electrons may be emitted due to the interaction between the atoms constituting the NaI crystal 111 and the radiation. The emitted high-energy electrons lose energy while exciting surrounding atoms, but scintillation light is emitted from the excited surrounding atoms. In other words, the kinetic energy of the emitted high-energy electrons is converted into scintillation light. The amount of this scintillation light depends on the magnitude of the kinetic energy of the emitted electrons, and the magnitude of the kinetic energy of the emitted electrons depends on the magnitude of the interaction between the atoms constituting the NaI crystal 111 and the radiation. Dependent. From this, by measuring the amount of scintillation light emitted from the NaI crystal 111, the magnitude of energy lost inside the crystal due to the interaction of radiation such as γ rays and X-rays with the atoms constituting the NaI crystal 111. Can be requested. Specifically, the amount of scintillation light emitted from the NaI crystal 111 is measured by a photomultiplier tube (not shown) optically connected to the NaI crystal 111. The output signal from the photomultiplier tube is subjected to signal processing by an electronic circuit disposed in the main body 113, and the air dose rate is obtained. The measured air dose rate is displayed on the meter 113a, and an analog voltage signal (for example, 0 to 10 mV) corresponding to the magnitude of the air dose rate is output from the output unit 113c.

  The interface unit 140 mainly includes an operational amplifier 141 that amplifies the analog voltage signal output from the output unit 113c, and an AD converter 142 that converts the analog voltage signal amplified by the operational amplifier 141 into a digital signal. The operational amplifier 141 amplifies a low voltage analog voltage signal of, for example, 0 to 10 mV to an analog voltage signal of 0 to 10 V.

  A digital signal (hereinafter, referred to as an air dose rate signal) that is AD-converted by the AD converter 142 of the interface unit 140 and that includes information on the air dose rate, and an output signal (hereinafter referred to as a GPS signal) that includes information on the current position. , Referred to as a GPS output signal) is input to the data processing system 130. The data processing system 130 extracts information on the current position (measurement position data) from the air dose rate signal and the GPS output signal, and extracts air dose rate data at the position. Then, air dose rate map data (radiation dose rate map data) is created by associating measurement position data (for example, latitude information and longitude information) with air dose rate data, and this is recorded in a text file. In this text file, air dose rate map data (that is, measurement position data and air dose rate data at that position) is additionally updated at intervals of 1 to 10 seconds.

  The text file in which the air dose rate map data is sequentially described is transmitted to the server 190 by the data transmission unit 150. Files placed on the server 190 can also be shared with other users.

  As described above, since the measurement position data and the air dose rate data associated therewith are described in the text file, the air dose rate map can be created using this data. Specifically, for example, a white map may be prepared, and markers (for example, dots, etc.) color-coded according to the magnitude of the air dose rate may be displayed at locations corresponding to the measurement positions on the white map. Alternatively, a contour map may be created.

Further, the measurement position data and the air dose rate data can be combined with electronic map data (for example, Google Earth (registered trademark) of Google). In this case, using the Google Earth (registered trademark) as described above, the air dose rate map in which markers color-coded according to the size of the air dose rate are displayed on the three-dimensional map (see FIG. 6). ) And an air dose rate map (see FIG. 7) in which markers colored by the size of the air dose rate are displayed on a two-dimensional map can be formed. .
<Outline of KURAMA-II System 200>

  Furthermore, led by the present inventor, the above-described KURAMA system 100 has been improved not only to collect air dose rate data but also to be able to simultaneously measure γ-ray energy, and the following KURAMA-II System 200 was developed.

  Hereinafter, the KURAMA-II system 200 will be described with reference to FIG. Note that the difference from the above-described KURAMA system 1 will be described, and the same components are denoted by the same reference numerals and description thereof will be omitted.

  As shown in FIG. 8, the KURAMA-II system 200 includes a CsI scintillation detector 210, a GPS unit 120, a data processing system 130, an interface unit 240, a data transmission unit 150, and a controller 260. As will be described later, the interface unit 140 is realized by a Compact RIO system of National Instrument. In the above-described KURAMA system 100, the data processing system 130 is realized by a program that is activated on a light-weight and small notebook personal computer called a netbook. The KURAMA-II system 200 is realized by a program that is activated in a small PC module mounted on the Compact RIO system that constitutes the interface unit 240. The GPS unit 120 and the data transmission unit 150 are also small modules that are attached to the PC module via a USB or the like. The controller 260 is also realized by the same PC module. That is, the KURAMA-II system 200 substantially includes a Compact RIO system that constitutes the interface unit 240, the data processing system 130, the controller 260, the CsI scintillation detector 210, the GPS unit 120, the data transmission unit 150, and the like. And a small module. These devices (units) can be arranged in the same housing. Therefore, compared with the KURAMA system 100, it is remarkably excellent in portability, and since it is almost unnecessary for the user to connect each unit, it is possible to suppress the occurrence of troubles such as erroneous connection.

  In the KURAMA-II system 200, a CsI scintillation detector 210 is employed instead of the NaI scintillation detector 110. The reason is as follows. In general, it is known that the interaction between a substance and γ rays increases as the atomic number of the element constituting the substance increases. Here, the atomic number of Na is 11, whereas the atomic number of Cs is 55. Therefore, when comparing the NaI scintillation detector and the CsI scintillation detector, the CsI scintillation detector is more γ-rayed. It can be said that the sensitivity to is high. Therefore, in the KURAMA-II system 200, by adopting the CsI scintillation detector 210 instead of the NaI scintillation detector 110, the entire system is smaller and lighter than the KURAMA system 1 while maintaining the sensitivity to γ rays. I was able to.

  The interface unit 240 includes an amplifier (pulse shaping amplifier) 241 that amplifies the pulse signal output from the pulse signal output unit 213, and an AD converter that converts the output signal (analog signal) of the amplifier 141 into a digital signal (digital value). 242, FPGA 243, and CPU 244. Note that FPGA 243 is an abbreviation for Field Programmable Gate Array, and is an integrated circuit that can be configured by a purchaser or designer after manufacturing. In the KURAMA-II system 200, a Compact RIO system manufactured by National Instrument is adopted as the interface unit 140. The above-described amplifier 241, AD converter 242, FPGA 243, and CPU 244 are mounted as modules mounted on the Compact RIO system. As described above, the CPU 244 is realized by a PC module mounted on the Compact RIO system, and this PC module also serves as the data processing system 130 and the controller 260. Here, the Compact RIO system is a reconfigurable embedded control and acquisition system. For example, a program for directly controlling each module using LabVIEW (registered trademark) can be easily created. Further, in the KURAMA-II system 200, by using the FPGA 243, based on the digital signal output from the AD converter 242, the peak height discrimination of the pulse and the noise level discrimination are performed by software. Realized. Information on the peak height of the pulse obtained by the FPGA 243 is sent to the CPU 244, and the CPU 244 can create a pulse height spectrum as shown in FIG.

In the KURAMA-II system 200, a plurality of radiation detectors can also be used. Further, the KURAMA-II system 200 can be configured to be driven by a battery as a configuration for walking, and a high-precision DGPS (differential GPS) can also be used as a GPS. A KURAMA-II system configured for walking (hereinafter referred to as a walking KURAMA-II system) is lightweight and excellent in portability, and for example, can be measured while a person is walking on the back.
<Outline of contamination distribution measurement system 200A and its verification experiment>

In the walking type KURAMA-II system 200 that is configured to be driven by a battery and adopts a high-precision DGPS (differential GPS) as a GPS, the present inventors replace the CsI scintillation detector 210 with the above contamination. By incorporating the distribution measuring apparatus 1, a contamination distribution measuring system 200A was produced. Since the contamination distribution measuring system 200A includes the above-described contamination distribution measuring apparatus 1, it is possible to easily calculate the local component Φ _L caused by radiation from the contamination source immediately below. Further, since the walking type KURAMA-II system 200 is employed, the mapping of the contamination distribution can be easily measured by performing measurement while moving. For example, as shown in the photograph of FIG. 10, a person can carry out measurement while carrying the entire contamination distribution measuring system 200A in a backpack.

  In order to demonstrate the contamination distribution measurement system 200A according to the present embodiment, the present inventors conducted a measurement test of the contamination distribution measurement system 200A in A city, Fukushima Prefecture. In the exercise plaza and the parking lot, the meeting place, and the road in front of the meeting place shown in FIG. 11, the measurement was performed by the measurer moving on foot with the contamination distribution measurement system 200 </ b> A packed in the backpack and carrying it. . In addition, since the walking type KURAMA-II system is adopted, an attendant can monitor in real time with Google Earth (registered trademark) on a PC during measurement. Further, the measurer himself also uses a tablet terminal, for example, to monitor the latitude and longitude of the current position and the measured values of the two radiation detectors 10 and 20 in real time using graphs and numerical values displayed on the terminal screen. be able to.

FIGS. 12A and 12B show the air dose rate measured by the first radiation detector 10 having directivity in the ground direction by the collimator 30 and the second radiation detector 20 having no directivity. Is a plot of the air dose rate measured in. According to this, although the air dose rate of the 1st radiation detector 10 does not rise so much, the place where the air dose rate of the 2nd radiation detector 20 seems to be high is seen. This suggests that the air dose rate may be greatly influenced by the contribution other than the direct source. That is, even when the ground surface immediately below is not contaminated, it is considered that the uniform component Φ _F may be high due to, for example, contamination of buildings or trees nearby.

  FIGS. 13A and 13B are plots of the air dose rate measured in the vicinity of the exercise plaza and the parking lot. Here, FIG. 13A plots the air dose rate measured by the first radiation detector 10 having directivity in the ground surface direction by the collimator 30, and FIG. 13B shows the directivity. The air dose rate measured by the 2nd radiation detector 20 which does not have is plotted. There is a lawn planted in the center of the sports plaza, which has already been decontaminated and used on a daily basis. The decontamination of the surrounding area is not yet complete. Reflecting this, in FIGS. 13A and 13B, it can be seen that the air dose rate in the exercise square is lower than that in the vicinity of the parking lot.

  FIGS. 14A and 14B are plots of air dose rates measured in the vicinity of the parking lot on the opposite side across the courtyard of the meetinghouse and the road. FIG. 14A is a plot of the air dose rate measured by the first radiation detector 10 having directivity in the ground direction by the collimator 30, and FIG. 14B does not have directivity. The air dose rate measured by the second radiation detector 20 is plotted. The courtyard's courtyard is unpaved and partially covered with grass or pebbles.

  FIGS. 15A and 15B are plots of air dose rates measured at the roadside portion of the road passing in front of the meeting place. FIG. 15A is a plot of the air dose rate measured by the first radiation detector 10 having directivity in the ground direction by the collimator 30, and FIG. 15B has no directivity. The air dose rate measured by the second radiation detector 20 is plotted. In addition, the road is paved, and the state where it is sandwiched between the edge of the mountain where the tree grows or the mountain continues about 150m south from the meeting place.

  From the above measurement results, the surface contamination was considered. 13 to 15, the air dose rate measured by the radiation detectors 10 and 20 shows a similar tendency, but the air dose rate measured by the first radiation detector 10 is almost equal to the second radiation. It can be seen that the air dose rate measured by the detector 20 is lower. This is because the front surface 11a of the CsI crystal 11 of the radiation detector 10 is directed to the ground surface, and the portion other than the front surface 11a of the CsI crystal 11 of the radiation detector 10 is covered with a lead collimator 30. It is thought to be caused by things.

  FIG. 16 is a plot of changes over time for the measurements of the athletic park and its surroundings. The lower graph shows the change over time of the measurement values of the radiation detectors 10 and 20, and the upper graph shows the change over time of the ratio of the measurement values of the first radiation detector 10 to the measurement values of the second radiation detector 20. Is shown.

When this is seen, the measured value of the air dose rate measured by the radiation detectors 10 and 20 generally shows a similar behavior, and the first radiation detection with respect to the measured value of the second radiation detector 20. It can be seen that the ratio of the measured values of the vessel 10 is about 0.4 to 0.5. It is considered that 70 to 80% of gamma rays from above and from the side are blocked by shielding by the collimator 30 attached to the first radiation detector 10. Considering this, if there is no contamination immediately below and only the uniform component Φ _F is measured, the ratio of the measurement values of the first radiation detector 10 and the second radiation detector 20 is 0. It can be understood that it is about 4 to 0.5. That is, under the conditions of this measurement, it was found that the value of (1-Q) in the above mathematical formula (Formula 2) is about 0.4 to 0.5.

  In addition, when FIG. 16 is seen, the place where the ratio of the measured value of the 1st radiation detector 10 with respect to the measured value of the 2nd radiation detector 20 is increasing greatly is seen. This is because the increase rate of the measurement value of the first radiation detector 10 having directivity in the ground surface direction is larger than the increase rate of the measurement value of the second radiation detector 20 having no directivity at that time. It means that In other words, it means that the contribution from the surface of the earth has increased, that is, there was contamination immediately below. For example, paying attention to the measurement value for 10 minutes with 10:30 in FIG. 16, the second radiation detector 20 observes peaks having almost the same height before 10:30 and after 10:30. On the other hand, when the ratio of the measurement value of the first radiation detector 10 to the measurement value of the second radiation detector 20 is seen, a peak of about 0.6 height is seen before 10:30. Thus, after 10:30, no noticeable peak was seen, and it remained at about 0.4. Similar fluctuations can be confirmed at around 10:50 to 55, even after 11:00 to 11:10. In addition, there are points where the measurement value of the first radiation detector 10 is substantially the same as or higher than the measurement value of the second radiation detector 20, and it can be seen that there are points where the contribution from the ground surface is extremely high.

Based on these, FIG. 17 shows an evaluation of the local component Φ _L from the relational expression shown in the above mathematical formula (Formula 2). In addition, it evaluated that the ratio of the measured value of the 1st radiation detector 10 with respect to the measured value of the 2nd radiation detector 20 was 0.4. As described above, the ratio of the measurement value of the first radiation detector 10 to the measurement value of the second radiation detector 20 was approximately 0.4 to 0.5. Among these, the ratio was 0.4. This is because it is considered that the contribution from the ground is the smallest with respect to the measurement value of the first radiation detector 10, and is considered to be the closest when only a uniform field exists at the measurement point.

When the evaluation of the surface contamination shown in FIG. 17 is compared with FIG. 13 obtained by a general air dose rate map, as shown in the following (A) to (D), a tendency different from a simple air dose rate map Can be read.
(A) There is no major contamination on the measurement path in the exercise plaza.
(B) Although the air dose rate is high in the north-south direction route on the east side of the exercise square, there is no suggestion of large contamination.
(C) The slope on the west side of the athletic park is relatively contaminated, while the concrete staircase with the same slope is less polluted.
(D) There is a small hot spot in the parking lot adjacent to the athletic park, and the degree of contamination in the parking lot is mottled.
Such a result may well explain the case where it is often said that the air dose rate will not decrease as expected even after decontamination. For example, the tendency of (B) suggests that the cause is not contamination on the ground surface even in a place where the air dose rate is high. The north-south direction route on the east side of the exercise square where the air dose rate is high is on the gutter, and if it is conventionally estimated that contaminated earth and sand are accumulated, it tends to be decontaminated. There is a high possibility that the dose rate will not decrease even after decontamination.

  The results of applying the same technique to meetinghouses and roads are shown in FIGS. Although the contamination is relatively uniformly distributed in the meeting place of FIG. 18, the contamination corresponding to the entrance / exit of the car is low. This is thought to be because the contaminated soil has been removed and removed by repeated passage of cars and people at the doorway. In addition, in FIG. 19, although the road has an increased dose rate over the entire section, the contribution from the ground surface is as low as about athletic parks in about half of the road portion. This suggests that the contamination of the mountains approaching the road is likely to increase the air dose rate on the road, rather than the surface being contaminated. In addition, there are places where the contribution of the ground surface is high, but it is thought that there is a possibility that the outflow of contaminated soil from the mountain is accumulated on the surface of gutters and dirt roads.

  As described above, it has been found that by using the contamination distribution measurement system 200A according to the present embodiment, effective information can be obtained for grasping the distribution state of contamination on the ground surface. As described above, the contamination distribution measuring system 200 </ b> A can be measured by the measurer carrying the detector in a rucksack. Therefore, the measurement can be performed very easily as compared with the conventional measurement in which at least one radiation detector is arranged near the ground surface. By using the contamination distribution measuring system 200A according to the present embodiment, it is possible to easily perform the planning before decontamination and the effect confirmation after decontamination, and greatly contribute to the improvement of the efficiency and certainty of the decontamination work. can do.

Furthermore, the present inventors compare the local component Φ _L measured by the contamination distribution measurement system 200A according to the present embodiment with the contamination density measured by sampling a part of the actual topsoil, and the present embodiment An experiment was conducted to verify the usefulness of the contamination distribution measurement system 200A.

In an orchard in B city, Fukushima Prefecture, the local component Φ _L was measured in the same manner as described above, and a map of the local component Φ _L as shown in FIG. 20 was created. Then, to include the high point and low point of the local component [Phi _L, it was selected six representative points. The selected representative points are displayed with square markers in FIG. The numbers shown in the vicinity of each marker are identification numbers for identifying measurement points by the contamination distribution measurement system 200A.

At each representative point, hundreds of grams were collected using topsoil as a sample, and γ-rays emitted from the sample were measured with a Ge detector to determine the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs. Note that the Ge detector has high energy resolution, and the number of becquerels of ¹³⁴ Cs and ¹³⁷ Cs in the sample can be individually evaluated. However, in the air dose rate measurement in the contamination distribution measurement system 200A, the contribution of γ rays from ¹³⁴ Cs to the air dose rate and the contribution of γ rays from ¹³⁷ Cs to the air dose rate are not individually evaluated. Therefore, in order to make a comparison with the air dose rate measured by the contamination distribution measurement system 200A, the total amount of ¹³⁴ Cs and ¹³⁷ Cs is determined by the Ge detector.

FIG. 21 shows the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs measured by the Ge detector and the air dose rate measured by the first radiation detector 10 having directivity in the ground direction by the collimator 30. A plot is shown. FIG. 22 shows a plot of the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs measured by the Ge detector and the air dose rate measured by the second radiation detector 20 having no directivity. When these are seen, when the air dose rate is measured using the first radiation detector 10 or the second radiation detector 20 alone, the total amount of ¹³⁴ Cs and ¹³⁷ Cs contained in the topsoil actually sampled from the ground surface. It was found that there was not a very good correlation with. On the other hand, FIG. 23 shows a plot of the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs measured by the Ge detector and the local component Φ _L calculated by the above method. Looking at this, the local component Φ _L obtained by the above-described procedure is clearly more practical than the case where the air dose rate is measured using the first radiation detector 10 or the second radiation detector 20 alone. It was found that there was a good correlation with the total amount of ¹³⁴ Cs and ¹³⁷ Cs contained in the topsoil sampled from the ground surface. This indicates that the local component Φ _L obtained by the above-described procedure is useful for evaluating the total amount (Bq) of ¹³⁴ Cs and ¹³⁷ Cs contained in the actual topsoil. However, two representative points (166 and 367) of the six representative points seem to be out of correlation. For example, it is sufficient between the local component Φ _L and the total amount of ¹³⁴ Cs and ¹³⁷ Cs included in the topsoil. It cannot be said that a good correlation is observed. The reason for this is that while the measured value of the contamination distribution measuring system 200A looks at the average contamination in the field of view of the first radiation detector 10 (with a radius of about 1 m), the collected topsoil has a sampling point radius of 10 to 20 cm. This is considered to be a reflection of uneven contamination within the field of view. According to the knowledge of the inventors, it can be improved by sampling the topsoil at a plurality of points within the range of the field of view of the first radiation detector 10 and evaluating the average contamination density.

  In the above embodiment, the collimator 30 is formed of lead, but it is not necessarily formed of lead. In view of the fact that the radiation shielding effect is higher when the element having a large atomic number includes a larger amount as described above, it is preferable to form it with lead, but for weight reduction, for example, a collimator is made with iron or the like. Also good. In addition, the shape of the collimator 30 can be changed as needed. In the above embodiment, the collimator 30 covers a portion other than the surface facing the ground surface of the first radiation detector and shields the surface facing the ground surface of the second radiation detector 20 from the ground surface. The invention is not limited to such a configuration. For example, a collimator that covers a portion other than the surface facing the ground surface of the first radiation detector and a collimator that shields the surface facing the ground surface of the second radiation detector 20 from the ground surface can be provided separately.

  In the above embodiment, the first radiation detector 10 is configured to have directivity in the ground surface direction by the collimator 30, but the direction in which the directivity is provided only needs to be directed to the direction in which contamination is desired to be detected. It is not necessarily limited to the surface direction. For example, when it is desired to perform measurement on a surface different from the ground surface, such as a wall surface of a building, a collimator may be arranged so as to have directivity in the direction facing the surface.

In the above embodiment, the CsI scintillation detector is used as the radiation detector, but the present invention is not necessarily limited to such a configuration. For example, other scintillation detectors such as a NaI scintillation detector or a BaF ₂ scintillation detector can be used. Alternatively, a gas detector such as a GM tube or a semiconductor detector such as a Ge detector can be used.

  In the above embodiment, the moving measurer carries the contamination distribution measurement system 200A, but the present invention is not limited to such a configuration. For example, the contamination distribution measuring system 200A can be mounted on other moving means such as a car, a bicycle, a motorcycle, and the measurement can be performed while moving.

DESCRIPTION OF SYMBOLS 1 Contamination distribution measuring apparatus 10 1st radiation detector 20 2nd radiation detector 30 Collimator 200A Contamination distribution measuring system

Claims (5)

A contamination distribution measuring device for measuring the distribution of contamination by radioactive substances,
A first radiation detector and a second radiation detector for measuring an air dose rate;
A first shield disposed over the first radiation detector so as to shield radiation incident on the first radiation detector from a direction other than the first direction;
The second radiation detector is opposed to the first direction so as to shield radiation incident on the second radiation detector from the first direction and to enter radiation from directions other than the first direction. A contamination distribution measuring device comprising: a second shield disposed over the surface. The first shield also serves as the second shield,
The said 2nd radiation detector is arrange | positioned on the opposite side to the said 1st radiation detector on both sides of the said 1st shielding body regarding the said 1st direction. Contamination distribution measuring device. Furthermore, position information acquisition means for acquiring position data relating to the current position;
Map data creation means for creating radiation dose rate map data by associating the air dose rate data measured by the first and second radiation detectors and the position data at the point where the dose rate data was measured;
3. A control unit that controls the first and second radiation detectors, the position information acquisition unit, and the map data generation unit to automatically collect the radiation dose rate map data. The contamination distribution measuring device described in 1. The first shield detects the radiation incident on the first radiation detector from a second direction opposite to the first direction and the first radiation detection from an arbitrary third direction orthogonal to the first direction. Disposed over the first radiation detector so as to shield radiation incident on the vessel;
The said 2nd radiation detector is comprised so that it may have omni-directionality regarding the said 2nd direction and the said arbitrary 3rd direction, The Claim 1 characterized by the above-mentioned. Contamination distribution measuring device. The second radiation detector is not provided with a shield that shields radiation incident on the second radiation detector from the second direction and the arbitrary third direction. 4. The contamination distribution measuring device according to 4.

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