JP2023048575A - Radioactivity measurement device and radioactivity measurement method - Google Patents

Abstract

A light-weight and compact radioactivity measuring device capable of measuring radiation emitted from a target radioactive substance existing in a target area while distinguishing it from radiation emitted from other radioactive substances. A radioactivity measuring apparatus according to the present invention includes a radiation detector 10 for detecting radiation having a plurality of detection elements 10a to 10f arranged in one dimension, a data processing unit 30, and a determination unit 40. A device 100 is provided. The data processing unit 30 creates an energy spectrum of radiation energy for each of the detection elements 10a to 10f from the radiation detected by the detection elements 10a to 10f, uses the energy spectrum to identify the type of radiation, and emits radiation. A nuclide is specified, and a count rate distribution for the detection elements 10a to 10f is created for each type of radiation and radiation energy for specifying the nuclide. The determination unit 40 estimates the position of the radiation source based on the count rate distribution. [Selection drawing] Fig. 1

Description

The present invention relates to devices and methods for measuring radiation and radioactivity.

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

Patent Document 1 discloses a radiation measurement method for measuring and evaluating radioactive contamination caused by controlled radioactive substances in a short period of time even when controlled radioactive substances and other radioactive substances coexist in the environment. is described. In this method, an array of scintillator plates is placed facing the object to be measured, and the state of radioactive contamination can be determined by observing the distribution pattern of the count rate.

Patent Literature 2 describes a radioactivity measuring device capable of measuring the radioactivity intensity distribution in the depth direction of a measurement object with high accuracy. This device is equipped with a plurality of radiation detectors arranged in a line, is inserted into the measurement object, and obtains an energy response function with respect to the depth direction, thereby detecting radiation incident from a specific depth direction and other radiation. Identify radiation incident from the direction of Radiation incident from a direction other than a specific depth direction passes through the measurement object for a long time before reaching the detection unit, and is either shielded, or has low energy due to Compton scattering. The device described in Patent Document 2 utilizes this phenomenon to measure the radioactivity intensity distribution in the depth direction from the energy distribution response.

Patent Literature 3 describes a radiation measuring device that accurately measures the radiation dose near the surface of molten fuel. This device is equipped with a detector guide tube, a neutron shielding coating that covers the outer periphery of the detector guide tube and shields neutrons, and a neutron detector that is placed in the hollow part of the detector guide tube. is detected with high sensitivity.

JP 2016-151454 A Japanese Patent Application Laid-Open No. 2020-109371 JP 2016-121896 A

In order to measure the radiation emitted from the target radioactive substance existing in the target area, the radioactivity measuring device is required to be able to detect the radiation emitted from the specific radioactive substance existing in a specific direction. be done. Radioactivity measuring devices are sometimes used to measure radiation and radioactivity in confined spaces. A radioactivity measuring device used for measurement in a narrow space is preferably small and lightweight.

In conventional radioactivity measurement devices, radiation shielding materials and collimators are used to distinguish radiation emitted from a specific direction from radiation emitted from directions other than the specific direction. However, if the radioactivity measuring device is equipped with a shielding material or a collimator, the size and weight of the radioactivity measuring device increase, which makes it difficult to measure in a narrow space, which is not preferable.

An object of the present invention is to provide a lightweight and compact radioactivity measuring device that can measure radiation emitted from a target radioactive substance existing in a target area while distinguishing it from radiation emitted from other radioactive substances. , to provide a radioactivity measuring method using this radioactivity measuring apparatus.

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

The radioactivity measuring method according to the present invention uses the radioactivity measuring device according to the present invention, the direction in which the detection elements are arranged is the length direction, and the radiation detector is positioned in the area to be measured in the length direction. A region of interest is approached perpendicular to the surface of the region of interest and the radiation is detected by the radiation detector to estimate the position of the source.

According to the present invention, a lightweight and compact radioactivity measuring device capable of measuring radiation emitted from a target radioactive substance present in a target area while distinguishing it from radiation emitted from other radioactive substances; A radioactivity measuring method using this radioactivity measuring device can be provided.

The figure which shows the structure of the radioactivity measuring apparatus by Example 1 of this invention. FIG. 4 is a diagram showing an example of the configuration of a radiation detector; FIG. 4 is a diagram showing another configuration example of a radiation detector; The figure which shows the example of the flowchart of the radioactivity measuring method by Example 1 of this invention. The figure which shows an example of an energy spectrum. The figure which shows an example of distribution of a counting rate. The figure which shows another example of distribution of a counting rate. The figure which shows another example of distribution of a counting rate. The figure which shows another example of distribution of a counting rate. The figure which shows the example of the flowchart of the method of estimating the position distribution of a radiation source in three-dimensional space, and the amount of radioactivity. The figure which shows the example of the flowchart of another method of estimating the position distribution and radioactivity amount of a radiation source in three-dimensional space.

A radioactivity measuring apparatus according to the present invention includes a radiation detector having a plurality of detector elements arranged in one dimension, and creates an energy spectrum for each detector element from the radiation detected by the detector elements. From this energy spectrum, the type of radiation (e.g., neutrons and gamma rays) is identified, the nuclide that emits radiation is identified, and the count rate for the detector element is determined for each radiation energy (i.e., for each radiation type and nuclide). Create a distribution. Since the radioactivity measuring apparatus according to the present invention estimates the position of the radiation source based on this count rate distribution, the radiation emitted from the target radioactive material existing in the target area is It can be measured separately from radiation emitted from radioactive substances.

Since the radioactivity measurement device according to the present invention does not include a shielding material or a collimator, it is lighter and smaller than conventional radioactivity measurement devices. Therefore, by using the radioactivity measuring apparatus according to the present invention, it is possible to easily perform measurement in a narrow space.

A radioactivity measuring apparatus and a radioactivity measuring method according to embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a radioactivity measuring apparatus according to this embodiment. The radioactivity measuring apparatus according to this embodiment includes a radiation detector 10 , an arm 91 , an operation control section 90 , a measurement signal processing section 20 , a processing device 100 , an object observation device 70 , a rangefinder 80 and an input/output device 60 . The processing device 100 includes a data processing section 30 , a determination section 40 and an input/output section 50 . The radioactivity measuring apparatus according to this embodiment is used for measuring the radioactive substance to be measured which exists in the measurement object 2 . The measurement object 2 is, for example, a structure or an object housed in a container.

The radioactivity measuring apparatus according to this embodiment measures a radioactive substance to be measured (hereinafter referred to as a target radioactive substance 4a) existing in a measurement target region (hereinafter referred to as a target region 3) in a measurement object 2. The radiation 1a emitted from the radioactive substances 4b and 4c can be distinguished from the radiation 1b and 1c emitted from the other radioactive substances 4b and 4c and measured. The other radioactive substances 4b and 4c include, for example, radioactive substances other than the target radioactive substances existing in the target region 3, target radioactive substances existing outside the target region 3, and target radioactive substances existing outside the target region 3. Radioactive substances other than radioactive substances are included.

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

The direction in which the plurality of detection elements 10a to 10f are arranged in the radiation detector 10 is hereinafter referred to as the lengthwise direction of the radiation detector 10. FIG. Also, the direction in which the radiation detector 10 faces along its length direction is referred to as the front of the radiation detector 10 . A direction perpendicular to the front direction of the radiation detector 10 is called the side of the radiation detector 10 .

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

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

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

The measurement signal processing unit 20 extracts data related to the measurement of the radiation detector 10 from the electrical signals obtained by the detection elements 10a to 10f of the radiation detector 10 respectively. The measurement data includes, for example, numbers (detection element IDs) identifying the detection elements 10a to 10f, times when radiation was detected (measurement times), and measured electrical signal values (for example, voltage values).

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 generates a radiation energy histogram for each of the detection elements 10a to 10f. to create The data processing unit 30 thus creates an energy spectrum (FIG. 4, which will be described later) for each of the detection elements 10a to 10f.

The data processing unit 30 divides 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. As for gamma rays, the emitted nuclide is specified by the energy information. The data processing unit 30 creates count rate distributions (FIGS. 5A to 5D described later) for the detection elements 10a to 10f for each specific radiation energy that specifies the type of radiation and the nuclide. This specific energy is the energy of radiation emitted by the target radioactive substance 4a, and can be determined in advance. The count rate distribution indicates the count rate distribution for detection positions in the radiation detector 10, and is created for each specific energy, that is, for each gamma-ray emitting nuclide and fast neutrons.

The object observation device 70 is a device that observes the measurement object 2, the target area 3, and the space including these. The object observation device 70 can observe, for example, the shapes of the measurement object 2 and the target region 3, the surface states of the measurement object 2 and the target region 3, and the positional relationship between the radiation detector 10 and the target region 3. The object observation device 70 can be composed of, for example, an optical camera or a point group acquisition device (for example, a device using a laser or LiDAR).

Rangefinder 80 is a device that measures the distance between radiation detector 10 and the surface of target region 3 . Object observer 70 can also serve as range finder 80 .

The determination unit 40 determines the position and posture of the arm 91 based on the observation result 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 radiation source of the radiation detected by the radiation detector 10 based on the count rate distribution created by the data processing unit 30, and the target radioactive material 4a existing in the target region 3 is The amount (radioactivity) of the target radioactive substance 4a present in the target area 3 is estimated from the emitted radiation. The determination unit 40 can also determine whether or not the target region 3 has a radioactivity amount equal to or greater than a specified amount.

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

Input/output device 60 includes at least one of display 61 , mouse 62 , and keyboard 63 . For example, the display 61 displays the energy spectrum and/or the count rate distribution, and the mouse 62 and keyboard 63 are used for user input of the parameters involved in the measurement. The display 61 can also display information entered by the user and data relating to the measurements extracted by the measurement signal processor 20 .

In the radioactivity measuring apparatus according to this embodiment, the farther away the radiation detector 10 is from the target region 3, the smaller the difference in the detection values among the detection elements 10a to 10f. It becomes difficult to grasp the difference (difference in radiation energy depending on detection position). Therefore, it is desirable that the distance between the radiation detector 10 and the target region 3 is smaller than the size of the target region 3 (size of the surface of the target region 3).

The radioactivity measuring device according to this embodiment can be used not only in an air environment but also in water. When measuring radioactive substances in water using the radioactivity measuring apparatus according to this embodiment, it is necessary to consider the shielding of neutrons and gamma rays in water.

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

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

In the radiation detector 10 shown in FIG. 2A, the photosensor 203 is installed inside the radiation detector 10 . In the radiation detector 10 shown in FIG. 2B, the photosensor 203 is installed outside the radiation detector 10 and optically connected to the scintillator element 201 by the optical fiber 204 . In FIG. 2B, for the sake of simplification, the photosensors 203 connected to the respective scintillator elements 201 are represented by one photosensor 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 configured with a neutron detecting scintillator element containing an element with a large neutron cross section such as Li-6 or Cl-35, both neutrons and gamma rays can be distinguished. can be measured. Instead of the scintillator element 201, the detection elements can also be composed of semiconductor elements such as CZT, CdTe, and Ge, for example.

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

The radiation detector 10 can 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 have the cover 202, and the cover 202 is unnecessary when the detection element is composed of a semiconductor element.

Further, the radiation detector 10 is provided with a light guide between the scintillator element 201 and the photosensor 203 in order to prevent saturation due to the amount of light from the photosensor 203. Even if the photosensor 203 detects the light dispersed by the light guide, good.

If the radiation detector 10 can identify the signals of the detection elements that have detected radiation for each detection element and can one-dimensionally specify the position where the radiation is detected (the position of the detection element that detected the radiation), then FIG. 2A and FIG. The configuration shown in 2B may not be provided, and any configuration may be provided.

FIG. 3 is a diagram showing an example of a flow chart of the radioactivity measurement method according to the present embodiment. In the radioactivity measuring method according to the present embodiment, the radioactivity measuring apparatus according to the present embodiment is used to measure the radiation 1a emitted from the target radioactive material 4a present in the target region 3 (FIG. 1). Estimate the amount of radioactivity.

In step 301 , the object observation device 70 acquires information about the measurement object 2 (for example, the shape and surface state of the measurement object 2 ) and divides the measurement object 2 into N target regions 3 . In the following, the divided N target regions 3 are expressed as Ω _i (1≦i≦N) using a subscript i indicating the number of the target regions 3 . The object observation device 70 can arbitrarily determine the size and position of each of the N target regions Ω _i .

At step 302, the operation control unit 90 selects the i-th target region 3 (target region Ω _i ). Note that the initial value of the number i is 1.

In step 303, the motion control unit 90 controls the arm 91 to move the radiation detector 10 along the length direction so that the length direction of the radiation detector 10 is perpendicular to the surface of the target area _Ωi . , bring the radiation detector 10 closer to the region of interest Ω _i . By the processing of step 303, the radiation detector 10 is arranged so that its length direction is perpendicular to the surface of the target area _Ωi . It should be noted that the term “perpendicular to the surface of the target region Ω _i” does not have to be strictly perpendicular, and includes a state that can be regarded as perpendicular.

At step 304, radiation detector 10 performs measurements on regions of interest Ω _i . The radiation detector 10 detects a voltage rise due to an induced current generated by interaction with radiation in each of the detection elements 10a to 10f. Note that this measurement can be terminated based on the number of measurements and the measurement time specified by the user.

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

At 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 (ie, the energy spectrum in FIG. 4) for each of the detection elements 10a to 10f. create. Then, the data processing unit 30 uses the energy spectrum to identify the type of radiation measured by the radiation detector 10, and specifies 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 emits gamma rays (for example, Eu-154, Cs- 137, Cs-134).

At step 307, the data processing unit 30 creates count rate distributions (FIGS. 5A to 5D) for the detection elements 10a to 10f for each energy corresponding to the extracted fast neutrons and gamma rays. This count rate distribution is created for each of the extracted fast neutrons and the extracted gamma-ray emitting nuclides, and the count rate distribution for the position where the radiation was detected (the position of the detector element that detected the radiation). Fig. 2 shows how the count rate varies with the position of the sensing element;

At step 307, the determining unit 40 can estimate the position of the radiation source in the plane perpendicular to the length direction of the radiation detector 10 based on the count rate distribution. The position of this source is estimated for each fast neutron and gamma-ray emitting nuclide. A method for estimating the position of the radiation source using the count rate distribution will be described later.

By estimating the position of the radiation source with respect to the fast neutrons and the specific nuclide that emits gamma rays, the determination unit 40 determines the count rate of the fast neutrons emitted in the target region Ω _i and the presence in the target region Ω _i It is possible to obtain the count rate of gamma rays emitted from a specific nuclide. That is, the determination unit 40 can distinguish the radiation 1a emitted from the target radioactive material 4a existing in the target region 3 (Ω _i ) from the other radiations 1b and 1c.

At step 308, the determination unit 40 estimates the amount of radioactivity in the target region _Ωi . However, the position of the radiation source in the direction parallel to the length direction of the radiation detector 10 (that is, the position of the radiation source in the depth direction of the measurement object 2) is difficult to estimate using the count rate distribution. be. Therefore, the determination unit 40 estimates the amount of radioactivity with respect to depth by utilizing the fact that radiation is attenuated in the object.

The radioactivity A for the target radioactive substance 4a that may exist in the target region 3 (Ω _i ) is expressed by the formula (1), as is conventionally known.
A≦4π(r ² +d ² )/aεCe ^dμ (1)
In equation (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, and ε is the target radioactivity. is the intrinsic sensitivity of the radiation detector 10 to the substance 4a, C is the count rate obtained by the radiation detector 10 to the target radioactive substance 4a, and μ is the attenuation coefficient at the measurement object 2 containing the target region 3; The attenuation coefficient μ is uniquely determined from the composition and density of the measurement object 2 and the radiation energy. For the composition and density of the measurement object 2, values known in advance may be used, or values estimated by measurement in advance using an object observation device 70 (for example, an optical camera or a point group acquisition device) may be used. can be done.

The determination unit 40 estimates the radioactivity amount A of the target radioactive material 4a present in the target region Ω _i using Equation (1). The upper limit value (maximum value) of the amount of radioactivity A with respect to the position d of the measurement object 2 in the depth direction is obtained from the formula (1). Therefore, the determination unit 40 can estimate the upper limit value of the radioactivity amount A of the target radioactive material 4a present in the target region _Ωi .

Instead of using the formula (1), the determination unit 40 measures while changing the radiation source position to the position d in the depth direction of the target region _Ωi , and converts from the count rate using the radioactivity conversion factor. The radioactivity A of the target radioactive substance 4a present in the target region _Ωi may be estimated.

At step 309, the determination unit 40 determines whether or not the radioactivity amount A of the target radioactive material 4a present in the target region Ω _i estimated at 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 area Ω _i whose number i is incremented by 1 as the target area 3, and the radiation detector 10 performs measurement on the target area Ω _i .

At step 310, the user performs a predetermined task on the target area Ω _i because the amount of radioactivity A in the target area Ω _i is less than the specified amount.

In step 311, the determination unit 40 determines whether or not the number i has reached N, that is, whether or not the processing from steps 302 to 309 has been performed for all target regions Ω _i (1≤i≤N). determine whether If i=N is not true, the process from step 302 to step 309 is repeated until i=N, and is performed for all regions of interest Ω _i (1≤i≤N). When the number i has reached N (i=N), at step 312, radiation measurement and radioactivity estimation are terminated.

FIG. 4 is a diagram showing an example of the energy spectrum created by the data processing section 30 in step 306 of FIG. 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 for each of the detection elements 10a to 10f, thereby obtaining an energy spectrum for each of the detection elements 10a to 10f. to create The energy spectrum shown in FIG. 4 is an example of the energy spectrum for one detector element. The data processing unit 30 can create an energy spectrum using existing technology.

The data processing unit 30 detects a peak 601 of the energy spectrum, uses information about the energy that gives this peak 601, and classifies radiation into ray types such as neutrons and gamma rays, as well as nuclide that emits gamma rays (for example, Eu-154, Cs-137, Cs-134). The data processing unit 30 can identify fast neutrons from the energy spectrum using information measured by other radiation detectors and using existing technology. 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 previously prepared calibration curve.

5A, 5B, 5C, and 5D are diagrams showing an example of the count rate distribution created by the data processing section 30 in step 307 of FIG. The data processing unit 30 creates a count rate distribution for each specific energy, that is, for each gamma-ray-emitting nuclide identified in step 306 and for each fast neutron.

This count rate distribution shows the count rate distribution for the detection positions in the radiation detector 10 , that is, how the count rate changes depending on the positions of the detection elements of the radiation detector 10 . In the radiation detector 10, a plurality of detection elements are arranged one-dimensionally (linearly). In the distributions of FIGS. 5A to 5D, the horizontal axis indicates the positions of the detection elements in the length direction of the radiation detector 10 (the direction in which the plurality of detection elements are arranged). 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 area Ω _i such that its length direction is perpendicular to the surface of the target area Ω _i . The detection element Ea that gives the leftmost count rate in the distributions of FIGS _. element. In the distributions of FIGS. 5A-5D, the detector elements located on the right side are located farther from the region of interest Ω _i . For example, the detection element Eb is the detection element positioned at the center in the length direction of the radiation detector 10, and the detection element Ec is positioned furthest from the target area Ω _i in the length direction of the radiation detector 10. It is a detection element.

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

The distribution of count rates shown in FIG. 5A is obtained when the target radioactive substance 4a and the radioactive substance 4c shown in FIG. 1 exist as radiation sources. This count rate distribution includes the frontmost detection element Ea (the detection element Ea closest to the target area Ω _i ), the detection element Ea at the center in the length direction of the radiation detector 10 (the detection element There is a peak at the position of Eb and its surrounding detection elements). Appropriate fitting using an exponential function, a Gaussian function, or the like is applied to this count rate distribution to separate the peaks, thereby obtaining the count rate distributions shown in FIGS. 5B and 5C.

In the count rate distribution shown in FIG. 5B, the count rate has a peak at the position of the frontmost detection element Ea (the detection element Ea closest to the target area _Ωi ), and the detection element moves away from the front. (ie, away from the region of interest Ω _i ). Such a distribution can be expressed using an exponential function, for example. When the count rate exhibits a distribution as shown in FIG. 5B, the source of the radiation coming to the radiation detector 10 is in front, i.e. the radiation detector 10 detected the radiation emitted from the target area Ω _i It will be. Therefore, if the count rate exhibits a distribution as shown in _FIG . That is, it can be estimated that the radiation is emitted from the target radioactive material 4a existing in the target area _Ωi .

In the count rate distribution shown in FIG. 5C, the count rate has a peak at the position of the detection element (the detection element Eb and its peripheral detection elements) in the central portion in the length direction of the radiation detector 10 . When the count rate exhibits a distribution as shown in FIG. We have detected radiation emitted from a position in the direction (a region that is not the region of interest Ω _i ). Therefore, if the count rate exhibits a distribution as shown in FIG. 5C, the radiation that gives this count rate distribution (the radiation is of a specific energy) is emitted from a region other than the region of interest Ω _i . It can be presumed to be the radiation emitted from the radioactive substance 4c.

If the count rate exhibits a distribution (for example, an exponential distribution) as shown in FIG. 5B, it can be estimated that the radiation source is in front of the radiation detector 10. If there are radioactive substances existing in the target area Ω _i (target radioactive substance 4a) and radioactive substances existing outside the target region Ω _i (target radioactive substance 4b) in front of the radiation detector 10, counting They can be distinguished from the spread of the rate distribution. That is, from the spread of the count rate distribution, the radioactive substance is either the target radioactive substance 4a existing in the target region Ω _i or the target radioactive substance 4b existing outside the target region Ω _i (the target radioactive substance 4b not to be measured). ).

In the distribution of the count rate shown in FIG. 5B, the spread 702 of the distribution of the count rate is obtained by multiplying the maximum value of the count rate by a predetermined constant (for example, 1/2) as a reference value 701, and this reference value 701 It can be obtained as the width of the distribution of the count rate in

The distribution of count rates shown in FIG. 5D shows a distribution (for example, an exponential distribution) similar to the distribution of count rates shown in FIG. Radiation is coming. The count rate distribution spread 703 in the count rate distribution shown in FIG. 5D is greater than the count rate distribution spread 702 in the count rate distribution shown in FIG. 5B.

If the spread 702 of the count rate distribution is small like the count rate distribution shown in FIG. 5B, it can be assumed that the radiation was emitted from the front front of the radiation detector 10, that is, the target area Ω _i . Therefore, if the count rate exhibits a distribution with a small spread 702 as shown in _FIG . can be estimated to be

If the spread 703 of the count rate distribution is large, as in the count rate distribution shown in FIG _. . Therefore, if the count rate exhibits a distribution with a large spread 703 as shown in _FIG . , is radiation emitted from the target radioactive material 4b existing outside the target region Ω _i .

A threshold value for determining whether the spreads 702 and 703 of the count rate distributions are large or small can be arbitrarily determined in advance. For example, simulations and experiments can be performed in advance to determine the relationship between the position of the radioactive material, which is the radiation source, and the spreads 702 and 703 of the count rate distribution, and the above thresholds can be determined based on this relationship. By using the threshold value determined in this way, it is possible to more easily determine whether the radioactive substance is the target radioactive substance 4a or a radioactive substance not to be measured (for example, the target radioactive substance 4b existing outside the target region Ω _i ). can be identified accurately.

The radiation detector 10 may not be arranged with its length direction perpendicular to the surface of the target area Ω _i . In such a case, the object observation device 70 (for example, an optical camera or a point group acquisition device) is used to determine the position and orientation of the radiation detector 10 with respect to the target area Ω _i , and the influence of this position and orientation on the distribution of the count rate is calculated. is included as an error in the spread of By setting the reference value 701 and the threshold with a margin in consideration of this error, it is possible to more accurately estimate whether or not the radioactive substance is the target radioactive substance 4a.

As described above, in step 306 of FIG. 3, the data processing unit 30 identifies the radiation emitted from the target radioactive material 4a using the energy spectrum of FIG. Create a distribution of the count rate of . In step 307, the determination unit 40 uses the count rate distributions of _FIGS . identify.

In the radioactivity measurement method shown in the flowchart of FIG. 3, the radioactivity amount in the target area Ω _i is estimated by estimating the position distribution of the radiation source in a plane perpendicular to the length direction of the radiation detector 10, that is, in a two-dimensional space. presume. A method for estimating the positional distribution of radiation sources and the amount of radioactivity in a three-dimensional space will be described below with reference to FIGS. 6 and 7. FIG.

FIG. 6 is a diagram showing an example of a flow chart of a method for estimating the positional distribution of radiation sources and the amount of radioactivity in a three-dimensional space in the radioactivity measurement method according to this embodiment. The flowchart shown in FIG. 6 has the processing of steps 313 and 314 added to the flowchart shown in FIG. In the following, differences of the flowchart shown in FIG. 6 from the flowchart shown in FIG. 3 will be mainly described.

In the method illustrated in FIG. 6, the radiation detector 10 detects radiation by performing measurements on the target area Ω _i at multiple angles oblique to the surface of the target area Ω _i with its length direction. When the radiation detector 10 tilts obliquely with respect to the surface of the target region Ω _i and measurements are made at a plurality of tilt angles, the measured values change in the depth direction of the target region Ω _i . Using the changes in these measured values, we can estimate the distribution of radiation sources in the depth direction of the target area Ω _i (similar to the principle of triangulation), and estimate the position distribution and radioactivity of the radiation sources in three-dimensional space. can do. The radiation detector 10 can be tilted in its longitudinal direction with respect to the surface of the target area Ω _i by the operation control section 90 .

In step 313, after bringing the radiation detector 10 closer to the target area _Ωi in step 303, the operation control unit 90 controls the arm 91 to move the radiation detector 10 to the surface of the target area _Ωi at an angle θ Tilt by _k . The angle θ _k (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 by an angle _θk , and estimates the position distribution of the radiation source in the three-dimensional space from this count rate distribution.

In step 314, the determination unit 40 determines whether or not the subscript k of the angle θ _k has reached M, that is, determines whether the subscript k of the angle θ _k (1≦k≦M) in steps 313 and 304 to 307 It is determined whether or not the processing of is performed. If k=M, then steps 313 and 304 through 307 are repeated until k=M. That is, the radiation detector 10 measures 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 determining unit 40 estimates the amount of radioactivity A in the three-dimensional space including the target region Ω _i and its surroundings based on the result of the tilted measurement of the radiation detector 10 and equation (1). The region of interest Ω _i and its surroundings can be represented in voxel space. The determination unit 40 obtains the radioactivity amount A (the upper limit of the radioactivity amount A of the target radioactive substance 4a) using the results measured by the radiation detector 10 for a plurality of angles θ _k and formula (1), and calculates the radioactivity amount By giving A to voxel values, it is possible to estimate the amount of radioactivity A in a three-dimensional space including the target region Ω _i and its surroundings. Any method can be used for the method of giving the radioactivity amount A to the voxel value, for example, back projection method (BP method, Back projection), filtered back projection method (FBP method, Filtered back projection), and A likelihood estimation-expectation maximization method (MLEM method, maximum likelihood-expectation maximization) can be used.

FIG. 7 is a diagram showing an example of a flow chart of another method for estimating the position distribution of the radiation source and the amount of radioactivity in the three-dimensional space in the radioactivity measurement method according to this embodiment. In the flowchart shown in FIG. 7, processing from step 315 to step 317 is added instead of the processing from step 308 to step 310 in the flowchart shown in FIG. Processing from step 315 to step 317 is executed after step 311 . In the following, the flowchart shown in FIG. 7 will be mainly described for the differences from the flowchart shown in FIG.

In the method shown in FIG. 7, the radiation detector 10, with _its longitudinal direction perpendicular to the surface of the target area _Ωi , is positioned at a plurality of positions in a direction parallel to the surface of the target area _Ωi . to detect radiation. Measurements at multiple positions with respect to the region of interest Ω _i result in varying measurements along the depth direction of the region of interest Ω _i . Using the changes in these measured values, we can estimate the distribution of radiation sources in the depth direction of the target area Ω _i (similar to the principle of triangulation), and estimate the position distribution and radioactivity of the radiation sources in three-dimensional space. can do. The radiation detector 10 can be measured at a plurality of positions in a direction parallel to the surface of the target area Ω _i by the motion controller 90 .

In step 304, the radiation detector 10 performs measurements on the region of interest Ω _i at a plurality of positions in a direction parallel to the surface of the region of interest Ω _i . The radiation detector 10 moves in a direction parallel to the surface of the target region Ω _i while its length direction is 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 area Ω _i but also the area around the target area Ω _i .

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

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 equation (1). do. All regions of interest Ω _i can be represented in voxel space. The determination unit 40 determines the radioactivity A (the upper limit of the radioactivity A of the target radioactive material 4a) using the results measured by the radiation detector 10 for all the target areas Ω _i and the formula (1), and determines the radiation By giving the amount of activity A to the voxel values, the amount of radioactivity A in the three-dimensional space of all target regions Ω _i can be estimated. Any method can be used as the method of giving the amount of radioactivity A to the voxel value, and for example, the back projection method, the filtered back projection method, and the maximum likelihood estimation-expectation value maximization method can be used.

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

At step 317 the user performs a predetermined task on the region of interest Ω _i determined at step 316 . The target area Ω _i obtained in step 316 is an area where the amount of radioactivity A is less than the specified amount.

In the method shown in the flow chart of FIG. 6, the radiation detector 10 is tilted with its longitudinal direction at an angle θ _k with respect to the surface of the target region Ω _i , and the target region is tilted at a plurality of angles θ _k (1≦k≦M). Measure _Ωi . In the method shown in the flow chart of FIG. 7, the radiation detector 10 measures the target area Ω _i at multiple positions in a direction parallel to the surface of the target area Ω _i . With the method shown in the flowchart of FIG. 3, it is difficult to estimate the radiation source position and radioactivity in the direction parallel to the length direction of the radiation detector 10 (that is, the depth direction of the measurement object 2). The method shown in the flow charts of FIGS. 6 and 7 has the advantage of estimating the positional distribution of radiation sources and the amount of radioactivity in a three-dimensional space.

The method shown in the flowchart of FIG. 6 also has the advantage that the distribution of the amount of radioactivity can be estimated with higher accuracy in the target area Ω _i to which the radiation detector 10 is approaching. The method shown in the flowchart of FIG. 7 has the advantage over the method shown in the flowchart of FIG. 6 that the process of tilting the radiation detector 10 by the angle θ _k (step 313 in FIG. 6) can be omitted.

The radioactivity measurement apparatus according to this embodiment can be used for detecting any radioactive substance and measuring radioactivity, for example, detecting nuclear fuel and measuring radioactivity. Moreover, when the radioactivity measuring apparatus according to the present embodiment includes 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 radioactivity measuring apparatus according to the present embodiment and are the radiation detectors provided in the X-ray CT, the radiation detector is perpendicular to the radiation detectors 10 without using a collimator. It is possible to separate the component of the radiation incident on the radiation detector 10 and the component of the radiation scattered and detected inside the radiation detector 10 . Therefore, the radiation detector for X-ray CT using the radiation detector 10 of the radioactivity measuring apparatus according to the present embodiment can measure X-rays by extending the detection area two-dimensionally without providing a collimator. , high throughput can be obtained.

It should be noted that the present invention is not limited to the above embodiments, and various modifications are possible. For example, the above embodiments have been described in detail in order to facilitate understanding of the present invention, and the present invention is not necessarily limited to aspects having all the described configurations. Also, part of the configuration of one embodiment can be replaced with the configuration of another embodiment. It is also possible to add the configuration of another embodiment to the configuration of one embodiment. Moreover, it is possible to delete a part of the configuration of each embodiment, or to add or replace another configuration.

1a, 1b, 1c... Radiation, 2... Measurement object, 3... Target area, 4a... Target radioactive substance, 4b, 4c... Radioactive substance, 10... Radiation detector, 10a to 10f... Detecting element, 20... Measurement signal processing unit , 30... Data processing unit 40... Judging unit 50... Input/output unit 60... Input/output device 61... Display 62... Mouse 63... Keyboard 70... Object observation device 80... Distance meter 90... Operation Control unit 91 Arm 100 Processor 201 Scintillator element 202 Cover 203 Optical sensor 204 Optical fiber 601 Energy spectrum peak 701 Reference value 702, 703 Count rate distribution spread.

Claims (11)

a radiation detector that includes a plurality of detection elements arranged one-dimensionally and that detects radiation;
an operation control unit that moves the radiation detector;
a processing device comprising a data processing unit and a determination unit;
with
The data processing unit
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 to identify the type of radiation and identify the nuclide that emits the radiation;
creating a count rate distribution for the detection element for each of the radiation energies that specify the type of radiation and the nuclide;
The determination unit estimates the position of the radiation source based on the count rate distribution.
A radioactivity measuring device characterized by: Assuming that the direction in which the detection elements are arranged is the length direction and the direction along the length direction is the front,
The determination unit estimates that the radiation source is in the front when the distribution of the count rate has a peak at the detection element that is the most in the front.
The radioactivity measuring device according to claim 1. Assuming that the direction perpendicular to the forward direction is the lateral direction,
The determination unit estimates that the radiation source is on the side when the count rate distribution has a peak at the detection element located at the center in the length direction of the radiation detector.
The radioactivity measuring device according to claim 2. The radiation detector is arranged by the operation control unit so that the length direction is perpendicular to the surface of the target area, which is the area to be measured,
The determination unit estimates that the radiation detector has detected radiation emitted from the target region when the count rate distribution has a peak at the most forward detection element.
The radioactivity measuring device according to claim 2. When the distribution of the count rate has a peak at the detection element located in the central portion of the radiation detector in the length direction, the determination unit determines that the radiation detector detects radiation emitted from a region other than the target region. presume to have detected
The radioactivity measuring device according to claim 4. The determination unit estimates that the radiation detector has detected radiation emitted from the target region when the spread of the count rate distribution is smaller than a predetermined threshold.
The radioactivity measuring device according to claim 4. The radiation detector detects the radiation at a plurality of angles at which the length direction and the surface of the target region, which is the region to be measured, are oblique.
The radioactivity measuring device according to claim 2. The radiation detector detects the radiation at a plurality of positions in a direction parallel to the surface of the target region, with the length direction perpendicular to the surface of the target region, which is the region to be measured.
The radioactivity measuring device according to claim 2. An input/output device that displays at least one of the energy spectrum and the count rate distribution,
The radioactivity measuring device according to claim 1. A plurality of the radiation detectors,
The radiation detector is a radiation detector provided in X-ray CT,
The radioactivity 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 the length direction,
The radiation detector is brought close to the target region so that the length direction is perpendicular to the surface of the target region, which is the region to be measured. estimating the position of the source,
Radioactivity measurement method.

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