JP7569296B2 - Radioactivity measuring device and method - Google Patents

Description

The present invention relates to an apparatus and method for measuring radiation and radioactivity.

Examples of conventional technology for measuring radiation and radioactivity are described in Patent Documents 1-3.

Patent Document 1 describes a radiation measurement method that quickly measures and evaluates radioactive contamination caused by controlled radioactive substances, even when controlled and other radioactive substances are mixed in the environment. In this method, an array of scintillator plates is placed opposite the measurement target, and the state of radioactive contamination can be determined by observing the distribution diagram of the counting rate.

Patent Document 2 describes a radioactivity measuring device that can measure the radioactivity intensity distribution in the depth direction of a measurement object with high accuracy. This device has multiple radiation detection units arranged in a row, and is inserted into the measurement object to determine an energy response function with respect to the depth direction, thereby distinguishing between radiation incident from a specific depth direction and radiation incident from other directions. Radiation incident from a direction other than the specific depth direction passes through the measurement object for a long distance before reaching the detection unit, and is therefore blocked, or its energy is reduced by Compton scattering. The device described in Patent Document 2 utilizes this phenomenon to measure the radioactivity intensity distribution in the depth direction from the response of the energy distribution.

Patent Document 3 describes a radiation measuring device that measures the amount of radiation near the surface of molten fuel with high precision. This device is equipped with a detector guide tube, a neutron shielding coating that covers the outer periphery of the detector guide tube to shield against neutrons, and a neutron detector placed in the hollow part of the detector guide tube, and detects the amount of radiation from the measurement target with high sensitivity.

JP 2016-151454 A JP 2020-109371 A JP 2016-121896 A

To measure radiation emitted from a target radioactive material present in a target area, a radioactivity measuring device is required to be able to detect radiation emitted from a specific radioactive material present in a specific direction. Radioactivity measuring devices may also be used to measure radiation or radioactivity in narrow spaces. It is preferable that a radioactivity measuring device used for measurements in narrow spaces is small and lightweight.

Conventional radiation measuring devices use radiation shielding materials and collimators to distinguish radiation emitted from a specific direction from radiation emitted from other directions. However, equipping a radiation measuring device with shielding materials and collimators is not preferable because it increases the size and weight of the device, making it difficult to perform measurements in narrow spaces.

The object of the present invention is to provide a lightweight and compact radioactivity measuring device that can measure radiation emitted from a target radioactive material present in a target area and distinguish it from radiation emitted from other radioactive materials, and a radioactivity measuring method using this radioactivity measuring device.

The radioactivity measuring device according to the present invention comprises a radiation detector having a plurality of detection elements arranged in one dimension to detect radiation, an operation control unit that moves the radiation detector, and a processing device having a data processing unit and a determination unit. The data processing unit creates an energy spectrum of radiation energy for each detection element from the radiation detected by the detection elements, uses the energy spectrum to identify the type of radiation and specify the nuclide emitting the radiation, and creates a count rate distribution for the detection elements for each radiation energy that identifies the type of radiation and the nuclide. The determination unit estimates the position of the source of the radiation based on the count rate distribution.

The radioactivity measurement method according to the present invention uses the radioactivity measurement device according to the present invention, defines the direction in which the detection elements are arranged as the length direction, brings the radiation detector close to the target area, which is the area to be measured, so that the length direction is perpendicular to the surface of the target area, detects the radiation with the radiation detector, and estimates the position of the radiation source.

The present invention provides a lightweight and compact radioactivity measuring device that can measure radiation emitted from a target radioactive material present in a target area and distinguish it from radiation emitted from other radioactive materials, and a radioactivity measuring method using this radioactivity measuring device.

FIG. 1 is a diagram showing the configuration of a radioactivity measuring device according to a first embodiment of the present invention. FIG. 1 is a diagram showing an example of the configuration of a radiation detector. FIG. 13 is a diagram showing an example of another configuration of a radiation detector. FIG. 2 is a diagram showing an example of a flowchart of a radioactivity measuring method according to the first embodiment of the present invention. FIG. 13 is a diagram showing an example of an energy spectrum. FIG. 13 is a diagram showing an example of count rate distribution. FIG. 13 is a graph showing another example of the distribution of count rates. FIG. 13 is a graph showing another example of the distribution of count rates. FIG. 13 is a graph showing another example of the distribution of count rates. FIG. 13 is a diagram showing an example of a flowchart of a method for estimating the position distribution and amount of radioactivity of a radiation source in a three-dimensional space. FIG. 13 is a diagram showing an example of a flowchart of another method for estimating the position distribution and amount of radiation of a radiation source in a three-dimensional space.

The radioactivity measuring device according to the present invention is equipped with a radiation detector having multiple detection elements arranged in one dimension, and creates an energy spectrum for each detection element from the radiation detected by the detection elements. From this energy spectrum, the type of radiation (e.g., neutrons and gamma rays) is identified, the nuclide emitting the radiation is specified, and a count rate distribution for the detection elements is created for each radiation energy (i.e., for each type of radiation and nuclide). The radioactivity measuring device according to the present invention estimates the position of the radiation source based on this count rate distribution, so that radiation emitted from the target radioactive material present in the target area can be measured separately from radiation emitted from other radioactive materials.

The radiation measuring device according to the present invention does not include a shielding material or a collimator, and is therefore lighter and smaller than conventional radiation measuring devices. Therefore, when using the radiation measuring device according to the present invention, measurements can be easily performed in narrow spaces.

Below, a radioactivity measuring device and a radioactivity measuring method according to an embodiment of the present invention will be described with reference to the drawings.

Figure 1 is a diagram showing the configuration of a radioactivity measuring device according to this embodiment. The radioactivity measuring device according to this embodiment includes a radiation detector 10, an arm 91, an operation control unit 90, a measurement signal processing unit 20, a processing device 100, an object observation device 70, a range finder 80, and an input/output device 60. The processing device 100 includes a data processing unit 30, a judgment unit 40, and an input/output unit 50. The radioactivity measuring device according to this embodiment is used to measure radioactive materials to be measured that are present in a measurement object 2. The measurement object 2 is, for example, a structure or an object housed in a container.

The radioactivity measuring device according to this embodiment can measure radiation 1a emitted from a radioactive material to be measured (hereinafter referred to as target radioactive material 4a) present in a region to be measured (hereinafter referred to as target region 3) in the measurement object 2, while distinguishing it from radiation 1b, 1c emitted from other radioactive materials 4b, 4c. The other radioactive materials 4b, 4c include, for example, radioactive materials other than the target radioactive material present in the target region 3, target radioactive materials present outside the target region 3, and radioactive materials other than the target radioactive material present outside the target region 3.

The radiation detector 10 is a long-type radiation detector that includes multiple detection elements 10a-10f arranged one-dimensionally (linearly) to detect radiation. While FIG. 1 shows the radiation detector 10 including six detection elements 10a-10f as an example, the number of detection elements included in the radiation detector 10 can be determined arbitrarily. The detection elements 10a-10f are elements that detect radiation and convert it into an electrical signal (e.g., a voltage value), and can be composed of, for example, scintillator elements or semiconductor elements.

In the following, the direction in which the multiple detection elements 10a to 10f are lined up in the radiation detector 10 is referred to as the length direction of the radiation detector 10. The direction in which the radiation detector 10 faces along its length direction is referred to as the front of the radiation detector 10. The direction perpendicular to the front direction of the radiation detector 10 is referred to as the side of the radiation detector 10.

The radiation detector 10 detects radiation emitted from a target radioactive material 4a present in a target region 3 in an air environment. The target radioactive material 4a is, for example, a radioactive material to be managed, and can include nuclear fuel. When the target radioactive material 4a is nuclear fuel, the radiation detector 10 detects fast neutrons and gamma rays emitted by specific nuclides (e.g., Eu-154, Cs-137, Cs-134) as the radiation to be measured.

The arm 91 is connected to the radiation detector 10 and moves the radiation detector 10.

The operation control unit 90 controls the arm 91 to move the radiation detector 10. When the radiation detector 10 measures the target area 3 and detects radiation emitted from the target radioactive material 4a, the operation control unit 90 controls the arm 91 to move the radiation detector 10 closer to the target area 3.

The measurement signal processing unit 20 extracts data related to the measurement of the radiation detector 10 from the electrical signals obtained by each of the detection elements 10a to 10f of the radiation detector 10. The measurement data includes, for example, a number (detection element ID) that identifies the detection elements 10a to 10f, the time when the radiation was detected (measurement time), and the value of the measured electrical signal (for example, a voltage value).

The data processing unit 30 receives data related to the measurement of the radiation detector 10 from the measurement signal processing unit 20, converts the electrical signal value (voltage value) into radiation energy, and creates a histogram of radiation energy for each of the detection elements 10a to 10f. In this way, the data processing unit 30 creates an energy spectrum (see FIG. 4, described later) for each of the detection elements 10a to 10f.

The data processing unit 30 separates the radiation measured by the radiation detector 10 into neutrons and gamma rays using the energy information of the created energy spectrum. Fast neutrons are extracted from the neutrons. The nuclide that emits the gamma rays is identified by the energy information. The data processing unit 30 creates a count rate distribution (FIGS. 5A-5D, described later) for the detection elements 10a-10f for each specific radiation energy that identifies the type of radiation and the nuclide. This specific energy is the energy of the radiation emitted by the target radioactive material 4a, and can be determined in advance. The count rate distribution indicates the distribution of count rates for the detection positions in the radiation detector 10, and is created for each specific energy, i.e., for each nuclide that emits gamma rays and for fast neutrons.

The object observation device 70 is a device that observes the measurement object 2, the target area 3, and the space that includes them. The object observation device 70 can observe, for example, the shapes of the measurement object 2 and the target area 3, the surface conditions of the measurement object 2 and the target area 3, and the positional relationship between the radiation detector 10 and the target area 3. The object observation device 70 can be configured, for example, with an optical camera or a point cloud acquisition device (for example, a device that uses a laser or LiDAR).

The range finder 80 is a device that measures the distance between the radiation detector 10 and the surface of the target area 3. The object observation device 70 can also function as the range finder 80.

The determination unit 40 determines the position and attitude of the arm 91 based on the observation results of the object observation device 70 and the distance measured by the rangefinder 80 (or the object observation device 70). Furthermore, the determination unit 40 estimates the position of the source of the radiation detected by the radiation detector 10 based on the count rate distribution created by the data processing unit 30, and estimates the amount (amount of radioactivity) of the target radioactive material 4a present in the target area 3 from the radiation emitted by the target radioactive material 4a present in the target area 3. The determination unit 40 can also determine whether or not a specified amount of radioactivity is present in the target area 3.

The input/output unit 50 is connected to the input/output device 60, and displays the processing results, data, and information obtained by the measurement signal processing unit 20, data processing unit 30, judgment unit 40, input/output unit 50, object observation device 70, rangefinder 80, and operation control unit 90 on the input/output device 60. For example, the input/output unit 50 displays the energy spectrum and counting rate distribution created by the data processing unit 30 on the input/output device 60. The input/output unit 50 also inputs information input by the user on the input/output device 60 (for example, instructions for the radioactivity measuring device and parameters related to the measurement).

The input/output device 60 includes at least one of a display 61, a mouse 62, and a keyboard 63. For example, the display 61 displays at least one of an energy spectrum and a counting rate distribution, and the mouse 62 and the keyboard 63 are used by the user to input parameters related to the measurement. The display 61 can also display information input by the user and data related to the measurement extracted by the measurement signal processing unit 20.

In the radioactivity measuring device according to this embodiment, the difference in detection values between the detection elements 10a to 10f decreases as the radiation detector 10 moves away from the target area 3, making it more difficult to grasp the difference in radiation energy due to the position of the detection elements 10a to 10f (the difference in radiation energy due to the detection position). For this reason, it is desirable for the distance between the radiation detector 10 and the target area 3 to be small relative to the size of the target area 3 (the size of the surface of the target area 3).

The radioactivity measuring device according to this embodiment can be used not only in an air environment but also underwater. When using the radioactivity measuring device according to this embodiment to measure radioactive materials underwater, it is necessary to take into consideration the shielding of neutrons and gamma rays underwater.

Figures 2A and 2B are diagrams showing an example of the configuration of a radiation detector 10. As an example, Figures 2A and 2B show a radiation detector 10 that includes a scintillator element 201 as a detection element.

The radiation detector 10 includes a plurality of scintillator elements 201 arranged one-dimensionally (linearly) and a light sensor 203. One light sensor 203 is provided for each scintillator element 201. The number of scintillator elements 201 and light sensors 203 can be determined arbitrarily.

In the radiation detector 10 shown in FIG. 2A, the optical sensor 203 is installed inside the radiation detector 10. In the radiation detector 10 shown in FIG. 2B, the optical sensor 203 is installed outside the radiation detector 10 and is optically connected to the scintillator elements 201 by optical fibers 204. Note that in FIG. 2B, in order to simplify the drawing, the optical sensors 203 connected to each scintillator element 201 are depicted as being represented by a single optical sensor 203.

The scintillator element 201, which is the detection element, can be made of any material, such as NaI, CsI, and CeBr3. When the radiation detector 10 measures neutrons, if it is made of a neutron detection scintillator element containing an element with a large neutron cross section, such as Li-6 or Cl-35, it can measure both neutrons and gamma rays while distinguishing between them. Instead of the scintillator element 201, the detection element can also be made of a semiconductor element, such as CZT, CdTe, and Ge.

The optical sensor 203 can be configured, for example, with a photomultiplier tube, a silicon photodiode, and an avalanche photodiode. If the detection element is configured with a semiconductor element, an electrode is installed instead of the optical sensor 203.

The radiation detector 10 may include a cover 202 made of a reflective material or a light-shielding material around the scintillator element 201. The radiation detector 10 may not include the cover 202, and the cover 202 is not necessary if the detection element is made of a semiconductor element.

In addition, to prevent saturation of the optical sensor 203 due to the amount of light, the radiation detector 10 may be provided with a light guide between the scintillator element 201 and the optical sensor 203, and the optical sensor 203 may detect the light dispersed by the light guide.

The radiation detector 10 does not have to have the configuration shown in Figures 2A and 2B and can have any configuration as long as the signal from the detection element that detected the radiation can be identified for each detection element and the position where the radiation was detected (the position of the detection element that detected the radiation) can be identified in one dimension.

Figure 3 is a diagram showing an example of a flowchart of the radioactivity measurement method according to this embodiment. In the radioactivity measurement method according to this embodiment, the radioactivity measurement device according to this embodiment is used to measure radiation 1a emitted from a target radioactive material 4a present in a target area 3 (Figure 1), and the amount of radioactivity in the target area 3 is estimated.

In step 301, the object observation device 70 acquires information about the measurement object 2 (e.g., the shape and surface condition of the measurement object 2) and divides the measurement object 2 into N target regions 3. Hereinafter, the N divided target regions 3 are represented as Ω _i (1≦i≦N) using the subscript i indicating the number of the target region 3. The object observation device 70 can arbitrarily determine the size and position of each of the N target regions Ω _i .

In step 302, the operation control unit 90 selects the i-th target region 3 (target region Ω _i ).

In step 303, the operation control unit 90 controls the arm 91 to move the radiation detector 10 along the length direction, and brings the radiation detector 10 closer to the target region Ω _i so that the length direction of the radiation detector 10 is perpendicular to the surface of the target region Ω _i . By the processing of step 303, the radiation detector 10 is positioned so that its length direction is perpendicular to the surface of the target region Ω _i . Note that "perpendicular to the surface of the target region Ω _i" does not have to be strictly perpendicular, and also includes a state that can be regarded as perpendicular.

In step 304, the radiation detector 10 performs a measurement on the target region Ω _i . The radiation detector 10 detects a voltage rise caused by an induced current generated by interaction with radiation in each of the detection elements 10a to 10f. This measurement can be ended based on the number of measurements and the measurement time designated by the user.

In step 305, the measurement signal processing unit 20 acquires data related to the measurement of the radiation detector 10 (e.g., detection element ID, measurement time, and measured voltage value) from the detection elements 10a to 10f, and records this data for each of the detection elements 10a to 10f.

In step 306, the data processing unit 30 creates an energy spectrum (FIG. 4) of radiation energy for each of the detection elements 10a to 10f from the radiation detected by the detection elements 10a to 10f. Specifically, the data processing unit 30 converts the voltage values measured by the detection elements 10a to 10f into radiation energy, and creates a histogram of radiation energy (i.e., the energy spectrum in FIG. 4) for each of the detection elements 10a to 10f. The data processing unit 30 then uses the energy spectrum to identify the type of radiation measured by the radiation detector 10 and specify the nuclide that emits the radiation. Specifically, the data processing unit 30 detects the peak of the energy spectrum, extracts fast neutrons and gamma rays from the radiation measured by the radiation detector 10, and specifies the nuclide that emits gamma rays (e.g., Eu-154, Cs-137, Cs-134).

In step 307, the data processing unit 30 creates a distribution of count rates (FIGS. 5A-5D) for the detection elements 10a-10f for each energy corresponding to the extracted fast neutrons and gamma rays. This count rate distribution is created for each of the nuclides that emit the extracted fast neutrons and gamma rays, and is a diagram showing the distribution of count rates relative to the position where the radiation was detected (the position of the detection element that detected the radiation), that is, a diagram showing how the count rate differs depending on the position of the detection element.

In step 307, the determination unit 40 can estimate the position of the radiation source in a plane perpendicular to the longitudinal direction of the radiation detector 10 based on the distribution of the count rates. The position of the radiation source is estimated for each of fast neutrons and nuclides that emit gamma rays. A method for estimating the position of the radiation source using the distribution of the count rates will be described later.

The determination unit 40 can estimate the position of the radiation source for fast neutrons and a specific nuclide that emits gamma rays, thereby determining the count rate of fast neutrons emitted in the target region Ω _i and the count rate of gamma rays emitted from a specific nuclide present in the target region Ω _i . In other words, the determination unit 40 can distinguish radiation 1a emitted from a target radioactive material 4a present in the target region 3 (Ω _i ) from other radiation 1b and 1c.

In step 308, the determination unit 40 estimates the amount of radioactivity in the target region Ω _i . However, it is difficult to estimate the position of the radiation source in a direction parallel to the length direction of the radiation detector 10 (i.e., the position of the radiation source in the depth direction of the measurement object 2) using the distribution of count rates. Therefore, the determination unit 40 estimates the amount of radioactivity with respect to depth by utilizing the attenuation of radiation in an object.

The amount of radioactivity A for the target radioactive material 4a that may be present in the target region 3 (Ω _i ) is expressed by the formula (1), as is conventionally known.
A≦4π(r ² + d ² )/ ^aεCedμ (1)
In formula (1), r is the distance between the surface of the target region 3 and the radiation detector 10, d is the position in the depth direction of the target region 3, a is an arbitrary constant, ε is the inherent sensitivity of the radiation detector 10 to the target radioactive material 4a, C is the count rate obtained by the radiation detector 10 for the target radioactive material 4a, and μ is the attenuation coefficient in the measurement object 2 including the target region 3. The attenuation coefficient μ is uniquely determined from the composition and density of the measurement object 2 and the energy of the radiation. For the composition and density of the measurement object 2, values that are known in advance can be used, or values that are estimated by measuring in advance using an object observation device 70 (for example, an optical camera or a point cloud acquisition device) can be used.

The determination unit 40 estimates the amount of radioactivity A of the target radioactive material 4a present in the target region _Ωi using formula (1). From formula (1), the upper limit (maximum value) of the amount of radioactivity A for the position d in the depth direction of the measurement object 2 can be obtained. Therefore, the determination unit 40 can estimate the upper limit of the amount of radioactivity A of the target radioactive material 4a present in the target region _Ωi .

Instead of using formula (1), the determination unit 40 may perform measurements while changing the radiation source position up to a position d in the depth direction of the target region _Ωi , and estimate the amount of radioactivity A of the target radioactive material 4a present in the target region _Ωi by converting the counting rate using a radioactivity conversion coefficient.

In step 309, the judgment unit 40 judges whether or not the amount of radioactivity A of the target radioactive material 4a present in the target region _Ωi estimated in step 308 is equal to or greater than a predetermined specified amount. If the amount of radioactivity A is less than the specified amount, the process of step 310 is executed.

If the amount of radioactivity A is equal to or greater than the specified amount, the number i is incremented by 1 (i=i+1) and the process returns to step 302. Then, the operation control unit 90 sets the i-th target region Ω _i , whose number i has been incremented by 1, as target region 3, and the radiation detector 10 performs measurement on the target region Ω _i .

In step 310, the user performs a predetermined operation on the region of interest Ω _i since the amount of radioactivity A in the region of interest Ω _i is less than a specified amount.

In step 311, the judgment unit 40 judges whether the number i has reached N, i.e., whether the processes from step 302 to step 309 have been performed for all target regions Ω _i (1≦i≦N). If i=N is not reached, the processes from step 302 to step 309 are repeated until i=N, and are performed for all target regions Ω _i (1≦i≦N). If the number i has reached N (i=N), the radiation measurement and radioactivity estimation are terminated in step 312.

Figure 4 is a diagram showing an example of an energy spectrum created by the data processing unit 30 in step 306 of Figure 3. The data processing unit 30 creates an energy spectrum for each of the detection elements 10a to 10f by converting the voltage values measured by the detection elements 10a to 10f into radiation energy and creating a histogram of radiation energy for each of the detection elements 10a to 10f. The energy spectrum shown in Figure 4 is an example of an energy spectrum for one detection element. The data processing unit 30 can create the energy spectrum using existing technology.

The data processing unit 30 detects a peak 601 in the energy spectrum, and uses information about the energy that gives this peak 601 to distinguish the radiation into types such as neutrons and gamma rays, and to identify nuclides that emit gamma rays (e.g., Eu-154, Cs-137, Cs-134). The data processing unit 30 can use existing technology, such as information measured by other radiation detectors, to identify fast neutrons from the energy spectrum. Information about the energy that gives the peak 601 can be calculated from the voltage values measured by the detection elements 10a to 10f, using a calibration curve prepared in advance.

Figures 5A, 5B, 5C, and 5D are diagrams showing an example of the count rate distribution created by the data processing unit 30 in step 307 of Figure 3. The data processing unit 30 creates a count rate distribution for each specific energy, i.e., for each of the nuclides that emit gamma rays identified in step 306 and for each of the fast neutrons.

This count rate distribution shows the distribution of count rates for detection positions in the radiation detector 10, i.e., how the count rate changes depending on the position of the detection elements in the radiation detector 10. In the radiation detector 10, multiple detection elements are arranged one-dimensionally (in a straight line). In the distributions of Figures 5A to 5D, the horizontal axis shows the position of the detection elements in the longitudinal direction of the radiation detector 10 (the direction in which the multiple detection elements are lined up). Note that Figures 5A to 5D show, as an example, the count rate distribution when the radiation detector 10 has 30 detection elements.

During measurement, the radiation detector 10 approaches the target region Ω i so that its length direction is perpendicular to the surface of the target region Ω _i . The detector element Ea that gives the leftmost count rate in the distributions of Figures 5A to 5D is the detector element that is closest to the target region Ω _i in the length direction of the radiation detector 10 among the detector elements of the radiation detector 10. In the distributions of Figures 5A to 5D, the detector elements that are located further to the right are located farther from _{the target region Ω i .} For example, the detector element Eb is the detector element that is located in the center of the radiation detector 10 in the length direction, and the detector element Ec is the detector element that is farthest from the target region Ω _i in the length direction of the radiation detector 10.

The count rate of the detection elements of the radiation detector 10 varies depending on the distance from the radiation source. Therefore, the distribution of the count rates varies depending on the positional relationship between the radiation detector 10 and the radiation source.

The count rate distribution shown in Fig. 5A is a distribution obtained when the target radioactive material 4a and radioactive material 4c shown in Fig. 1 are present as radiation sources. This count rate distribution has peaks at the positions of the forward-most detector element Ea (detector element Ea located closest to the target region _Ωi ) and the detector elements at the center in the longitudinal direction of the radiation detector 10 (detector element Eb and its surrounding detector elements). By separating the peaks by applying appropriate fitting to this count rate distribution using an exponential function, Gaussian function, or the like, the count rate distributions shown in Fig. 5B and Fig. 5C can be obtained.

In the count rate distribution shown in Fig. 5B, the count rate has a peak at the position of the detector element Ea located most forward (the detector element Ea located closest to the target region Ω _i ), and gradually decreases as the detector element moves away from the front (i.e., moves away from the target region Ω _i ). Such a distribution can be expressed, for example, by using an exponential function. When the count rate shows a distribution as shown in Fig. 5B, the source of the radiation that reached the radiation detector 10 is in the front, that is, the radiation detector 10 has detected radiation emitted from the target region Ω _i . Therefore, when the count rate shows a distribution as shown in Fig. 5B, it can be estimated that the radiation (this radiation is radiation of a specific energy) that gives this count rate distribution is radiation emitted from the target region Ω _i , that is, radiation emitted from the target radioactive material 4a present in the target region Ω _i .

In the count rate distribution shown in Fig. 5C, the count rate has a peak at the position of the detecting element (detecting element Eb and its surrounding detecting elements) located in the center in the longitudinal direction of the radiation detector 10. When the count rate shows a distribution as shown in Fig. 5C, the source of the radiation that reached the radiation detector 10 is on the side, that is, the radiation detector 10 has detected radiation emitted from a position in a direction perpendicular to the longitudinal direction of the radiation detector 10 (a region other than the target region _Ωi ). Therefore, when the count rate shows a distribution as shown in Fig. 5C, it can be estimated that the radiation that gives this count rate distribution (this radiation is radiation of a specific energy) is radiation emitted from a region other than the target region _Ωi , that is, radiation emitted from the radioactive material 4c.

When the counting rate shows a distribution as shown in Fig. 5B (for example, an exponential distribution), it can be estimated that the radiation source is in front of the radiation detector 10. When a radioactive material present in the target region _Ωi (target radioactive material 4a) and a radioactive material present outside the target region _Ωi (target radioactive material 4b) are present in front of the radiation detector 10, they can be distinguished from the spread of the counting rate distribution. In other words, it can be identified from the spread of the counting rate distribution whether the radioactive material is the target radioactive material 4a present in the target region _Ωi or the target radioactive material 4b present outside the target region _Ωi (target radioactive material 4b that is not the measurement target).

In the count rate distribution shown in FIG. 5B, the spread 702 of the count rate distribution can be calculated as the width of the count rate distribution at a reference value 701, which is calculated by multiplying the maximum count rate by a predetermined constant (e.g., 1/2).

The count rate distribution shown in FIG. 5D shows a distribution similar to the count rate distribution shown in FIG. 5B (e.g., an exponential distribution), and radiation is coming from the front of the radiation detector 10. The spread 703 of the count rate distribution in the count rate distribution shown in FIG. 5D is greater than the spread 702 of the count rate distribution in the count rate distribution shown in FIG. 5B.

When the spread 702 of the count rate distribution is small, as in the count rate distribution shown in Fig. 5B, it can be presumed that the radiation is emitted from the front of the radiation detector 10, i.e., from the target area _Ωi . Therefore, when the count rate shows a distribution with a small spread 702 as shown in Fig. 5B, it can be presumed that the radiation giving this count rate (radiation of a particular energy) is radiation emitted from the target radioactive material 4a in the target area _Ωi .

When the count rate distribution spread 703 is large, such as the count rate distribution shown in Fig. 5D, it can be estimated that the radiation is emitted from a position away from the target region _Ωi , rather than from directly in front of the radiation detector 10. Therefore, when the count rate shows a distribution with a large spread 703, such as the count rate distribution shown in Fig. 5D, it can be estimated that the radiation giving this count rate (radiation of a particular energy) is not radiation from the target radioactive material 4a in the target region _Ωi , but radiation emitted from the target radioactive material 4b present outside the target region _Ωi .

The threshold value for judging whether the spreads 702, 703 of the count rate distribution are large or small can be determined arbitrarily in advance. For example, a relationship between the position of the radioactive material that is the radiation source and the spreads 702, 703 of the count rate distribution can be obtained in advance by performing a simulation or an experiment, and the above threshold value can be determined based on this relationship. By using the threshold value determined in this manner, it is possible to more accurately distinguish whether the radioactive material is the target radioactive material 4a or a radioactive material that is not the measurement target (for example, the target radioactive material 4b that exists outside the target region Ω _i ).

There are cases where the radiation detector 10 cannot be disposed so that its length direction is perpendicular to the surface of the target region Ω _i . In such cases, the position and orientation of the radiation detector 10 with respect to the target region Ω _i are obtained by an object observation device 70 (for example, an optical camera or a point cloud acquisition device), and the influence of this position and orientation is included as an error in the spread of the counting rate distribution. By setting the reference value 701 and the threshold value with a margin of error in consideration of this error, it is possible to more accurately estimate whether a radioactive material is the target radioactive material 4a.

As described above, the data processing unit 30 identifies radiation emitted from the target radioactive material 4a using the energy spectrum in Fig. 4 in step 306 in Fig. 3, and creates the count rate distribution of Fig. 5A to Fig. 5D in step 307. The determination unit 40 uses the count rate distribution of Fig. 5A to Fig. 5D in step 307 to identify whether the radiation identified by the data processing unit 30 is radiation 1a emitted from a radioactive material present in the target region _Ωi .

In the radioactivity measurement method shown in the flowchart of Fig. 3, the amount of radioactivity in the target region _Ωi is estimated by estimating the position distribution of the radiation source in a plane perpendicular to the longitudinal direction of the radiation detector 10, i.e., in two-dimensional space. Below, a method for estimating the position distribution and amount of radioactivity of the radiation source in three-dimensional space will be described with reference to Figs. 6 and 7.

Figure 6 is a diagram showing an example of a flowchart of a method for estimating the position distribution and amount of radioactivity of a radiation source in three-dimensional space in the radioactivity measurement method according to this embodiment. The flowchart shown in Figure 6 adds the processes of steps 313 and 314 to the flowchart shown in Figure 3. The following mainly describes the differences between the flowchart shown in Figure 6 and the flowchart shown in Figure 3.

In the method shown in Fig. 6, the radiation detector 10 detects radiation by performing measurements on the target region Ω i at multiple angles at which its length direction is inclined relative to the surface of the target region Ω _i . When the radiation detector 10 is inclined obliquely relative to the surface of the target region Ω _i and measures at multiple inclination angles, the measured values change in the depth direction of the target region Ω _i . By utilizing this change in _{the measured values, it is possible to estimate the distribution of the radiation source in the depth direction of the target region Ω i (} similar to the principle of triangulation) and estimate the position distribution and amount of radioactivity of the radiation source in three-dimensional space. The radiation detector 10 can be inclined obliquely in its length direction relative to the surface of the target region Ω _i by the operation control unit 90.

In step 313, the operation control unit 90, after having brought the radiation detector 10 close to the target region _Ωi in step 303, controls the arm 91 to tilt the radiation detector 10 by an angle _θk with respect to the surface of the target region _Ωi . The angle _θk (1≦k≦M) indicates one of a plurality of (M) arbitrarily determined angles.

In step 307, the determination unit 40 creates a count rate distribution for measurements in which the radiation detector 10 is tilted at an angle θ _k, and estimates the position distribution of the radiation source in three-dimensional space from this count rate distribution.

In step 314, the determination unit 40 determines whether the subscript k of the angle θ _k has reached M, i.e., whether the processes of step 313 and steps 304 to 307 have been performed for all angles θ _k (1≦k≦M). If k=M is not reached, the processes of step 313 and steps 304 to 307 are repeated until k=M. That is, the radiation detector 10 performs measurements for a plurality of angles θ _k (1≦k≦M). If k has reached M (k=M), the process of step 308 is performed.

In step 308, the determination unit 40 estimates the amount of radiation A in a three-dimensional space including the target region Ω _i and its periphery based on the results of the tilted measurement by the radiation detector 10 and formula (1). The target region Ω _i and its periphery can be expressed in a voxel space. The determination unit 40 obtains the amount of radiation A (the upper limit _value of the amount of radiation A of the target radioactive material 4a) using the results of the measurement by the radiation detector 10 for a plurality of angles θ k and formula (1), and assigns the obtained amount of radiation A to the voxel values, thereby estimating the amount of radiation A in a three-dimensional space including the target region Ω _i and its periphery. Any method can be used to assign the amount of radiation A to the voxel values, and for example, the back projection method (BP method), the filtered back projection method (FBP method), and the maximum likelihood-expectation maximization method (MLEM method) can be used.

Figure 7 is a diagram showing an example of a flowchart of another method for estimating the position distribution and amount of radioactivity of a radiation source in three-dimensional space in the radioactivity measurement method according to this embodiment. The flowchart shown in Figure 7 adds steps 315 to 317 instead of steps 308 to 310 in the flowchart shown in Figure 3. The steps 315 to 317 are executed after step 311. Below, the flowchart shown in Figure 7 will be explained, focusing mainly on the differences from the flowchart shown in Figure 3.

In the method shown in Fig. 7, the radiation detector 10 detects radiation by performing measurements on the target region Ω _{i at multiple positions in a direction parallel to the surface of the target region Ω i with its length direction perpendicular to the surface of the target region Ω i. When measurements are performed at multiple positions on the target region Ω i , the measured} values change in the depth direction of the target region Ω _i . By utilizing this change in the measured values, it is possible to estimate the distribution of the radiation source in the depth direction of the target region Ω _i (similar to the principle of triangulation) and estimate the position distribution and amount of radiation of the radiation source in three-dimensional space. The radiation detector 10 can perform measurements at multiple positions in a direction parallel to the surface of the target region Ω _i by the operation control unit 90.

In step 304, the radiation detector 10 performs measurements on the target region Ω _i at a plurality of positions in a direction parallel to the surface of the target region Ω _i . The radiation detector 10 moves in a direction parallel to the surface of the target region Ω _i with its length direction perpendicular to the surface of the target region Ω _i to scan and measure the target region Ω _i . At this time, the radiation detector 10 may measure not only the target region Ω _i but also the region surrounding the target region Ω _i .

In step 307, the determination unit 40 creates a distribution of count rates for measurements at a plurality of positions of the radiation detector 10 for the target region Ω _i, and estimates the position distribution of the radiation source in three-dimensional space from this count rate distribution.

In step 315, the determination unit 40 estimates the amount of radioactivity for all target regions Ω _i (1≦i≦N) based on the results of measurements by the radiation detector 10 at a plurality of positions and formula (1). All target regions Ω _i can be expressed in a voxel space. The determination unit 40 obtains the amount of radioactivity A (the upper limit value of the amount of radioactivity A of the target radioactive material 4a) for all target regions Ω _i using the results of measurements by the radiation detector 10 and formula (1), and assigns the obtained amount of radioactivity A to the voxel values, thereby estimating the amount of radioactivity A in the three-dimensional space of all target regions Ω _i . Any method can be used to assign the amount of radioactivity A to the voxel values, and for example, the back projection method, the filtered back projection method, and the maximum likelihood estimation-expectation maximization method can be used.

In step 316, the determination unit 40 finds, from among all the target regions Ω _i , a target region Ω _i in which the amount of radioactivity A estimated in step 315 is less than a predetermined specified amount.

In step 317, the user performs a predetermined operation on the target region Ω _i obtained in step 316. The target region Ω _i obtained in step 316 is a region in which the amount of radioactivity A is less than a specified amount.

In the method shown in the flowchart of Fig. 6, the radiation detector 10 has its length direction inclined at an angle _θk with respect to the surface of the target region _Ωi , and measures the target region _Ωi at a plurality of angles _θk (1≦k≦M). In the method shown in the flowchart of Fig. 7, the radiation detector 10 measures the target region _Ωi at a plurality of positions in a direction parallel to the surface of the target region _Ωi . In the method shown in the flowchart of Fig. 3, it is difficult to estimate the position and amount of radiation of the radiation source in a direction parallel to the length direction of the radiation detector 10 (i.e., the depth direction of the measurement object 2). The methods shown in the flowcharts of Fig. 6 and Fig. 7 have the advantage that the position distribution and amount of radiation of the radiation source in a three-dimensional space can be estimated.

The method shown in the flowchart of Fig. 6 further has the advantage that it is possible to estimate the distribution of radioactivity with higher accuracy in the target region _Ωi to which the radiation detector 10 is approaching. The method shown in the flowchart of Fig. 7 has the advantage, compared to the method shown in the flowchart of Fig. 6, that it is possible to omit the process of tilting the radiation detector 10 by an angle _θk (step 313 in Fig. 6).

The radiation measuring device according to this embodiment can be used to detect any radioactive material or measure radioactivity, for example, to detect nuclear fuel or measure radioactivity. In addition, when the radiation measuring device according to this embodiment is provided with a plurality of radiation detectors 10, the radiation detectors 10 can be used as radiation detectors for X-ray CT. When the plurality of radiation detectors are the radiation detectors 10 of the radiation measuring device according to this embodiment and are radiation detectors provided in X-ray CT, it is possible to separate the components of radiation that are incident perpendicularly to the radiation detector 10 and the components of radiation that are scattered and detected inside the radiation detector 10 without using a collimator. Therefore, a radiation detector for X-ray CT using the radiation detectors 10 of the radiation measuring device according to this embodiment can measure X-rays by expanding the detection area two-dimensionally without a collimator, and therefore can obtain high throughput.

The present invention is not limited to the above-mentioned examples, and various modifications are possible. For example, the above-mentioned examples have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to an embodiment that includes all of the configurations described. It is also possible to replace part of the configuration of one example with the configuration of another example. It is also possible to add the configuration of another example to the configuration of one example. It is also possible to delete part of the configuration of each example, or to add or replace other configurations.

1a, 1b, 1c...radiation, 2...measurement object, 3...target area, 4a...target radioactive material, 4b, 4c...radioactive material, 10...radiation detector, 10a-10f...detection element, 20...measurement signal processing unit, 30...data processing unit, 40...determination unit, 50...input/output unit, 60...input/output device, 61...display, 62...mouse, 63...keyboard, 70...object observation device, 80...rangefinder, 90...motion control unit, 91...arm, 100...processing device, 201...scintillator element, 202...cover, 203...optical sensor, 204...optical fiber, 601...peak of energy spectrum, 701...reference value, 702, 703...spread of count rate distribution.

Claims (11)

a radiation detector that detects radiation and includes a plurality of detection elements that are arranged one-dimensionally;
A motion control unit that moves the radiation detector;
A processing device including a data processing unit and a determination unit;
Equipped with
The data processing unit includes:
creating an energy spectrum of radiation energy for each of the detection elements from the radiation detected by the detection elements;
Using the energy spectrum, identify the type of radiation and identify the nuclide emitting the radiation;
creating a distribution of count rates for the detection elements for each of the radiation energies that identifies the type of radiation and the nuclide;
the determination unit estimates a position of a source of the radiation based on the distribution of the count rates and a spread of the distribution of the count rates .
A radioactivity measuring device characterized by the above. If the direction in which the detection elements are arranged is defined as the length direction, and the direction along the length direction is defined as the forward direction,
the determination unit estimates that the radiation source is located in the front when the distribution of the count rate has a peak in the detection element located furthest in the front.
The radiation measuring device according to claim 1 . If the direction perpendicular to the forward direction is defined as the lateral direction,
the determination unit estimates that the radiation source is located on the side when the distribution of the count rate has a peak in the detection element located at the center in the length direction of the radiation detector.
The radiation measuring device according to claim 2 . The radiation detector is disposed by the operation control unit such that the length direction is perpendicular to a surface of a target area that is a measurement target area,
the determination unit estimates that the radiation detector has detected radiation emitted from the target region when the distribution of the count rate has a peak in the detection element located furthest to the front.
The radiation measuring device according to claim 2 . the determination unit estimates that the radiation detector has detected radiation emitted from a region other than the target region when the distribution of the count rates has a peak in the detection element located at a center portion in the longitudinal direction of the radiation detector.
The radiation measuring device according to claim 4. the determination unit infers that the radiation detector has detected radiation emitted from the target region when a spread of the distribution of the count rates is smaller than a predetermined threshold value.
The radiation measuring device according to claim 4. the radiation detector detects the radiation at a plurality of angles at which the length direction is inclined relative to a surface of a target area that is a measurement target area;
The radiation measuring device according to claim 2 . the radiation detector detects the radiation at a plurality of positions in a direction parallel to a surface of a target region, the length direction being perpendicular to the surface of the target region that is a region to be measured;
The radiation measuring device according to claim 2 . an input/output device for displaying at least one of the energy spectrum and the count rate distribution;
The radiation measuring device according to claim 1 . A plurality of the radiation detectors are provided,
The radiation detector is a radiation detector provided in an X-ray CT scanner.
The radiation measuring device according to claim 1 . Using the radioactivity measuring device according to claim 1,
The direction in which the detection elements are arranged is defined as a length direction,
the radiation detector is brought close to a target region, which is a region to be measured, so that the length direction is perpendicular to a surface of the target region, and the radiation is detected by the radiation detector to estimate a position of the radiation source;
Methods for measuring radioactivity.

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