Muon measurement device, and muon measurement method

Abstract

To enable a weight of a specific substance an object contains to be measured appropriately, utilizing a muon passing through the object even under a radiation environment.SOLUTION: A muon measurement device is configured to have: a muon trajectory detector 11 that is provided in a facing position across an object 1 of a measurement object, and measures a muon μ passing through the object to detect a muon trajectory; a Cerenkov detector 12 that radiates Cerenkov light to measure the Cerenkov light, eliminates an event by a radioactive ray by setting sensitivity with respect to the muon higher than sensitivity with respect to the radioactive ray as for the radiation of the Cerenkov light, and selects an event by the muon; a trajectory determination unit 14 that determines the event by the muon on the basis of a measurement result of the Cerenkov detector, and as for the determined event by the muon, reconfigures the muon trajectory on the basis of a measurement value of the muon trajectory detector and analyzes muon scattering; and a weight determination unit 15 that obtains a weight of a specific substance the object contains on the basis of the muon scattering.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] An embodiment of the present invention relates to a muon measurement device and a muon measurement method that operate in a radiation environment. [Background technology]

[0002] Nuclear fuels such as uranium are subject to strict international management controls in terms of nuclear security. In particular, spent fuel from nuclear reactors contains large amounts of fissile components and is radioactive, so it requires strict management with measures such as shielding and cooling. Existing burnup measurement devices are used when spent fuel from nuclear reactors is received at reprocessing facilities. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2-222884 [Patent Document 2] JP 2017-161465 A [Patent Document 3] Patent Publication No. 2021-189050 [Patent Document 4] JP 2022-187538 A Summary of the Invention [Problem to be solved by the invention]

[0004] Several methods have been proposed for measuring the weight of nuclear material contained in nuclear fuel or fuel debris for which prior information is lacking. In particular, Patent Document 2 proposes a method for measuring the weight of nuclear fuel contained in a material of unknown composition by measuring the scattering angle distribution of cosmic ray muons, which have a strong penetrating power. In addition, Patent Document 3 proposes a method for improving the accuracy of estimating the weight of nuclear fuel contained in a material by measuring gamma rays γ and neutron rays in addition to cosmic ray muons.

[0005] However, general radiation detectors, including muon measuring devices, generate noise in a radiation environment. In other words, these proposals have the risk of adverse effects due to noise when measuring highly radioactive materials. In a muon measuring device, muons pass through in a straight line, so noise and muon signals can be distinguished from each other based on the linearity of the muon trajectory. However, in a high radiation environment, a large amount of noise is generated, and the number of noise events becomes predominant, making it difficult to distinguish between muon events and noise events. In addition, if the noise is measured in a linear position, a false trajectory may be erroneously measured.

[0006] Although it is easy to shield charged particles, it is difficult to shield gamma rays, and they are prone to generating noise in muon measurement devices due to Compton scattering, etc. In particular, when radioactive waste has high radioactivity and emits large amounts of gamma rays, noise from gamma rays becomes a problem when using muon measurement devices.

[0007] The embodiments of the present invention have been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a muon measurement device and a muon measurement method that can preferably measure the weight of a specific substance contained in an object by utilizing muons that pass through the object even in a radiation environment. [Means for solving the problem]

[0008] A muon measurement device in an embodiment of the present invention is a muon measurement device that operates in a radiation environment, and is characterized by comprising: muon trajectory detectors that are provided at opposing positions sandwiching an object to be measured and that measure muons passing through the object to detect a muon trajectory; a Cherenkov detector that radiates and measures Cherenkov light and sets the sensitivity to the muons for the radiation of the Cherenkov light higher than the sensitivity to radiation, thereby eliminating events caused by the radiation and selecting events caused by the muons; a trajectory determination unit that determines the event caused by the muon based on the measurement result of the Cherenkov detector and reconstructs the muon trajectory for the determined event caused by the muon based on the measurement value of the muon trajectory detector to analyze muon scattering; and a weight determination unit that determines the weight of a specific substance contained in the object based on the muon scattering.

[0009] A muon measurement method in an embodiment of the present invention is a muon measurement method performed in a radiation environment, characterized by comprising the steps of: muon trajectory detectors provided at opposing positions sandwiching an object to be measured measuring muons passing through the object to detect a muon trajectory; a Cherenkov detector that sets its sensitivity to the muons for radiation to be higher than its sensitivity to radiation, thereby eliminating events caused by radiation and selecting events caused by the muons, emitting and measuring Cherenkov light; a trajectory determination unit determining the event caused by the muon based on the measurement result of the Cherenkov detector and reconstructing the muon trajectory for the determined event caused by the muon based on the measurement value of the muon trajectory detector to analyze muon scattering; and a weight determination unit determining the weight of a specific substance contained in the object based on the muon scattering. Effect of the Invention

[0010] According to an embodiment of the present invention, it is possible to suitably measure the weight of a specific substance contained in an object by utilizing muons that pass through the object even in a radiation environment. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a configuration diagram showing a muon measurement device according to a first embodiment. [Diagram 2] 2 is a flowchart of a muon measurement method executed by the muon measurement device of FIG. 1. [Diagram 3] 2 is a graph showing the relationship between the minimum energy of radiation at which a emitter of a Cherenkov detector can emit Cherenkov light and the refractive index of the emitter. [Figure 4] FIG. 2 is a perspective view illustrating the radiation angle of Cherenkov light. [Diagram 5] 1 is a graph showing the relationship between the weight of nuclear material contained in an object and the muon scattering angle. [Figure 6] 13 is a graph showing the number of photons of Cherenkov light radiated for each refractive index of a radiator of a Cherenkov detector in a muon measurement device according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [A] First embodiment (FIGS. 1 to 5) Fig. 1 is a configuration diagram showing a muon measurement device according to the first embodiment. Fig. 2 is a flowchart of a muon measurement method executed by the muon measurement device of Fig. 1. As shown in Figs. 1 and 2, the muon measurement device 10 operates under a radiation environment, and includes a muon trajectory detector 11 that measures muons μ passing through an object 1 to be measured and detects a muon trajectory, a Cherenkov detector 12 that radiates and measures Cherenkov light, a data accumulation unit 13, a trajectory determination unit 14 that determines an event caused by muons μ and analyzes muon scattering based on the muon trajectory, and a weight determination unit 15 that derives the weight of a specific material (e.g., nuclear material) contained in the object 1.

[0013] As shown in FIG. 1, an object 1 to be measured is, for example, waste (for example, fuel debris) containing nuclear fuel as nuclear material generated in a nuclear power plant or the like, or spent nuclear fuel.

[0014] At least one pair of muon trajectory detectors 11 are installed at opposing positions across the object 1, and measure muons μ, which are charged particles passing through the object 1, to detect the muon trajectory. That is, the muon trajectory detector 11 measures muons μ of cosmic rays flying from the sky to the ground, and detects the muon trajectory. Note that the muons μ are not limited to cosmic ray muons μ, and may be artificially generated muons μ. As the muon trajectory detector 11, a device in which drift tube detectors using ionized gas are stacked, or a device in which multi-channel scintillation detectors are stacked, is used. The muon trajectory detector 11 may also include a shield 16 on the object 1 side to shield radiation emitted from the object 1.

[0015] The Cherenkov detector 12 is configured to include a emitter 12A with a refractive index n that emits Cherenkov light, and an optical sensor 12B that measures the Cherenkov light emitted from the emitter 12A. The Cherenkov detector 12 excludes events caused by radiation and selects events caused by muons μ by setting the refractive index n of the emitter 12A so that the sensitivity to muons μ is higher than the sensitivity to radiation (e.g., gamma rays γ). For example, the refractive index n of the emitter 12A is set to a value equal to or higher than the refractive index value corresponding to the minimum energy at which muons μ can emit Cherenkov light, and lower than the refractive index value corresponding to the minimum energy at which radiation (gamma rays γ, beta rays, etc.) can emit Cherenkov light.

[0016] In other words, the Cherenkov detector 12 uses a phenomenon called Cherenkov radiation to eliminate events caused by radiation (such as gamma rays γ and beta rays) and select events caused by muons μ. Here, Cherenkov radiation refers to a phenomenon in which, when a charged particle (including muons μ and radiation) moves in a material (radiator 12A) with a refractive index n at a speed ratio β=v / c (v: charged particle speed [m / s], c: speed of light in vacuum [m / s]), Cherenkov light is emitted when the speed ratio β of the charged particle exceeds the speed of light in the material: c / n.

[0017] Cosmic ray muons μ have high energy and therefore have a speed ratio β close to 1, radiating Cherenkov light. Meanwhile, Fig. 3 shows the relationship between the minimum energy at which Cherenkov light can be emitted and the refractive index n of emitter 12A for radiation such as electrons and gamma rays γ. According to the relationship shown in Fig. 3, by appropriately setting the refractive index n of emitter 12A, it is possible to set conditions for the minimum energy at which Cherenkov light can be emitted for radiation such as electrons and gamma rays γ, and to select whether or not to emit Cherenkov light.

[0018] In addition, when the energy of the charged particles (including muons μ and radiation) is high enough that the speed ratio β can be approximated as 1, the number of photons per unit length of the emitted Cherenkov light is dN / dx=2πz 2 α(1-1 / n 2 )∫dλ / λ 2 ...(1) Here, N is the number of photons of the Cherenkov light, n is the refractive index of the emitter 12A, α is the fine structure constant, z is the charge of the charged particle, λ is the wavelength of the Cherenkov light, and x is the infinitesimal length.

[0019] By performing an integration of equation (1) over the sensitive area of ​​a typical photodetector, the number of photons per unit length of Cherenkov light emitted for a charged particle (including muons μ and radiation) is given by dN / dx=475z 2 (1-1 / n 2 ) [photon / cm]······(2) Thus, when the refractive index n of the radiator 12A is increased, the amount of emitted Cherenkov light increases.

[0020] As shown in formula (2), when the refractive index n of the emitter 12A is increased, the total amount of Cherenkov light increases, but the minimum energy at which Cherenkov light can be emitted decreases as shown in Fig. 3. For this reason, in order to increase the sensitivity to muons μ while decreasing the sensitivity to radiation such as gamma rays γ, it is necessary to select an appropriate refractive index value for the refractive index n of the emitter 12A.

[0021] For example, the minimum value of the refractive index corresponding to the minimum energy at which a 1 GeV muon μ can emit Cherenkov light is 1.0046, which is sufficiently low, so that the radiator 12A has a high sensitivity to muons μ and emits Cherenkov light by setting its refractive index n to 1.0046 or more. On the other hand, since the minimum energy at which a gamma ray γ from Cs-137 can emit Cherenkov light is 0.662 MeV, for example, if the refractive index n of the radiator 12A is set to less than 1.17, which corresponds to the refractive index of 0.662 MeV, the radiator 12A has a low sensitivity to the gamma ray γ from Cs-137 and does not emit Cherenkov light.

[0022] In addition, since the minimum energy at which gamma rays γ from Co-60 can emit Cherenkov light is 1.332 MeV, if the refractive index n of the radiator 12A is set to less than 1.05, the refractive index value corresponding to 1.332 MeV, the sensitivity of the radiator 12A to the gamma rays γ from Co-60 decreases and the radiator 12A does not emit Cherenkov light. Furthermore, since the minimum energy at which beta rays emitted by Y-90, the daughter nucleus of Sr-90, can emit Cherenkov light is 2.28 MeV, if the refractive index n of the radiator 12A is set to less than 1.017, the refractive index value corresponding to 2.28 MeV, the sensitivity of the radiator 12A to the beta rays from Sr-90 decreases and the radiator 12A does not emit Cherenkov light.

[0023] 4, the Cherenkov light is emitted in a cone shape at a radiation angle θ with respect to the traveling direction of the charged particle, which is expressed by the following equation (3). The radiation angle θ of this Cherenkov light is determined according to the speed (energy) of the charged particle incident on the radiator 12A. cosθ=1 / (nβ) (3)

[0024] Therefore, it is possible to obtain the muon trajectory of muons μ as charged particles from the distribution 17 of detection positions where the Cherenkov light radiated at the radiation angle θ by the Cherenkov detector 12 is detected by the optical sensor 12B. Even if the Cherenkov detector 12 is equipped with a simple optical sensor 12B that does not obtain the distribution 17 of detection positions of the radiated Cherenkov light, it is possible to obtain the arrival positions of the muons μ (corresponding to the above-mentioned detection positions) by dividing the Cherenkov detector 12 in the planar direction as shown by the two-dot chain line in FIG.

[0025] Furthermore, as shown in FIG. 1, when the radiator 12A that emits Cherenkov light is made of, for example, a gas, the refractive index n can be changed by adjusting the pressure. When the radiator 12A is made of a solid, various refractive indices n can be realized by using a substance called aerogel.

[0026] 1 accumulates data relating to the muon detection signal and muon trajectories, which are measurement values ​​from the muon trajectory detector 11. Furthermore, the data accumulation unit 13 accumulates data such as the presence or absence of detection of Cherenkov light as a measurement result including the measurement values ​​from the Cherenkov detector 12, and a distribution 17 of the detection positions of Cherenkov light.

[0027] The trajectory determination unit 14 shown in FIG. 1 determines an event caused by a muon μ based on the measurement results of the Cherenkov detector 12, and for this determined event caused by a muon μ, reconstructs the muon trajectory based on the measurement values ​​of the muon trajectory detector 11 to determine changes in the muon trajectory, and analyzes muon scattering from the changes in the muon trajectory.

[0028] First, the determination of an event caused by muon μ will be described. First, the trajectory determination unit 14 requests that the information on the detection position and detection time of the Cherenkov light detected by the Cherenkov detector 12 match the information on the detection position and detection time of the muon μ measured by the muon trajectory detector 11. Next, the trajectory determination unit 14 limits and regulates the analysis space in the muon trajectory detector 11 based on the information on the distribution 17 of the detection positions of the muons μ detected by the Cherenkov detector 12. Then, the trajectory determination unit 14 analyzes the detection signal of muon μ from the muon trajectory detector 11 based on the measurement time of the Cherenkov light from the Cherenkov detector 12 to determine an event caused by muon μ excluding events caused by radiation, and calculates the muon trajectory.

[0029] Next, we will describe the analysis of muon scattering by the trajectory determination unit 14. The trajectory determination unit 14 reconstructs the above-mentioned calculated muon trajectory to obtain a change in the muon trajectory, and analyzes and derives muon scattering (i.e., muon scattering angle and muon scattering position) from the change in the muon trajectory.

[0030] Here, muons μ have a high penetrating power, and many of the muons μ that are incident on the object 1 to be measured pass through the material. Furthermore, muons μ cause multiple Coulomb scattering depending on the composition and shape of the material. The average scattering angle θ0 of this muon scattering is shown in the following formula (4) and varies depending on the radiation length X0 specific to the atomic number of the material.

[0031]

number

[0032] In equation (4), p is the momentum of the muon μ, βc is the speed of the muon μ, z is the charge of the muon μ, and x is the distance that the muon μ travels through the material. Since the radiation length X0 differs depending on the material, it is possible to estimate the material present at the coordinates where muon scattering occurred from the analyzed average scattering angle θ0.

[0033] The weight determination unit 15 shown in FIG. 1 derives the weight of a specific substance (e.g., a nuclear substance) contained in the target 1 based on the muon scattering analyzed by the trajectory determination unit 14, particularly the muon scattering angle (average muon scattering angle), from the relationship between the weight of the specific substance and the muon scattering angle.

[0034] Here, muon scattering has the property that it is largely scattered by heavy elements with high atomic numbers. Therefore, the weight of the nuclear material contained in the object 1 is the dominant factor in determining the magnitude of muon scattering in the object 1 containing a large amount of nuclear material. FIG. 5 shows an example of the relationship between the nuclear material weight (uranium weight) and muon scattering (average muon scattering angle). Since the value of muon scattering increases with an increase in the weight of the nuclear material, the analysis of muon scattering by the trajectory determination unit 14 enables the weight determination unit 15 to measure the weight of the nuclear material contained in the object 1.

[0035] However, muon scattering is not sensitive to differences in scattering due to differences in isotopes of the same element, so it cannot distinguish between U-238 and U-235, for example. Therefore, the results of analysis using muon scattering are the total amounts of heavy elements such as uranium and plutonium, regardless of whether they are fissile or not.

[0036] Next, a muon measurement method executed by the muon measurement device 10 configured as described above will be described with reference to FIG. The muon trajectory detector 11 measures muons μ passing through the target object 1 in a radiation environment and detects the muon trajectory (S1). In parallel with this step S1, the Cherenkov detector 12 radiates Cherenkov light to measure the incoming muons μ, but does not radiate Cherenkov light to radiation (S2).

[0037] The trajectory determination unit 14 determines an event caused by a muon μ based on the measurement result of the Cherenkov detector 12, and calculates a muon trajectory based on this determination (S3). Furthermore, the trajectory determination unit 14 reconstructs this calculated muon trajectory, and analyzes and derives muon scattering (muon scattering angle, muon scattering position) (S4).

[0038] Next, the weight determination unit 15 calculates and determines the weight of the specific material (for example, nuclear material) contained in the target 1 based on the muon scattering angle (average muon scattering angle) derived by the trajectory determination unit 14 (S5).

[0039] As configured as above, the first embodiment provides the following effect (1). (1) The muon trajectory detector 11 measures muons μ passing through the object 1 to detect the muon trajectory, the Cherenkov detector 12, which emits and measures Cherenkov light, selects the muon μ event by eliminating events of radiation (gamma rays γ, beta rays, etc.) in a radiation environment, and the trajectory determination unit 14 determines the event caused by the muon μ based on the measurement results of the Cherenkov detector 12, and for the determined event caused by the muon μ, reconstructs the muon trajectory based on the measurement value of the muon trajectory detector 11 to analyze the muon scattering. This makes it possible to suitably measure the weight of a specific substance (e.g., nuclear material) contained in the object 1 based on the muon scattering by the trajectory determination unit 14, particularly the muon scattering angle, without being affected by noise due to radiation.

[0040] [B] Second embodiment (FIG. 6) 6 is a graph showing the number of photons of Cherenkov light emitted by a radiator of a Cherenkov detector in a muon measurement system according to the second embodiment for each refractive index of the radiator. In the second embodiment, the same reference numerals as in the first embodiment are used for the same parts as in the first embodiment, and the description thereof will be simplified or omitted.

[0041] The muon measurement device 20 (FIG. 1) of the second embodiment differs from the first embodiment in the configuration of the refractive index n of the emitter 21A in the Cherenkov detector 21. The refractive index n of this emitter 21A is set to be equal to or higher than the refractive index value corresponding to the minimum energy at which the muon μ radiates Cherenkov light, and equal to or lower than the refractive index value corresponding to an energy in the vicinity of the minimum energy at which the radiation radiates Cherenkov light.

[0042] The refractive index n of the emitter 21A will be described in detail. If the refractive index n of the emitter 21A is a value of the refractive index corresponding to an energy in the vicinity of the minimum energy at which the radiation can emit Cherenkov light, the amount of emitted Cherenkov light is reduced, and the Cherenkov detector 21 can sufficiently reduce the sensitivity of the Cherenkov light radiation to the radiation. Here, the relationship between the energy of the electron and the number of photons generated by the Cherenkov light is shown in FIG. 6 for each medium of the emitter 21A. Considering that charged particles including muons μ and radiation lose energy in the emitter 21A, if the refractive index n of the emitter 21A has a value of the refractive index corresponding to an energy in the vicinity of the minimum energy at which the radiation can emit Cherenkov light, the emitter 21A can sufficiently reduce the number of photons generated by the Cherenkov light to the radiation.

[0043] Therefore, the refractive index n of the emitter 21A is set to a refractive index value equivalent to the minimum energy at which the radiation can emit Cherenkov light, and instead of separating the radiation from the muons μ by preventing the radiation from emitting Cherenkov light, the magnitude of the emitted Cherenkov light is made different, thereby distinguishing between Cherenkov light originating from the radiation and that originating from the muons μ, and separating events caused by radiation from events caused by muons μ.

[0044] As configured as above, the second embodiment also provides the same effect as the effect (1) of the first embodiment.

[0045] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, changes, and combinations can be made without departing from the spirit of the invention. Furthermore, these substitutions, changes, and combinations are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0046] 1... object, 10... muon measuring device, 11... muon trajectory detector, 12... Cherenkov detector, 12A... emitter, 12B... optical sensor, 14... trajectory determination unit, 15... weight determination unit, 16... shielding body, 20... muon measuring device, 21... Cherenkov detector, 21A... emitter, n... refractive index, μ... muon

Claims

1. A muon measurement device that operates in a radiation environment, a muon trajectory detector that is provided at opposing positions across a measurement target and measures muons passing through the target to detect a muon trajectory; a Cherenkov detector that emits and measures Cherenkov light, and that sets a sensitivity to the muons for the emission of the Cherenkov light higher than the sensitivity to the radiation, thereby eliminating events caused by the radiation and selecting events caused by the muons; a trajectory determination unit that determines an event caused by the muon based on the measurement result of the Cherenkov detector, and reconstructs the muon trajectory for the determined event caused by the muon based on the measurement value of the muon trajectory detector to analyze muon scattering; a weight determination unit that determines a weight of a specific substance contained in the object based on the muon scattering.

2. 2. The muon measurement device according to claim 1, wherein the muon trajectory detector is configured to include a shield for blocking radiation emitted from an object.

3. The Cherenkov detector includes a radiator having a refractive index n and radiating Cherenkov light, and an optical sensor that measures the Cherenkov light radiated from the radiator, 2. The muon measurement device according to claim 1, wherein the refractive index n is set to a value equal to or greater than a refractive index value corresponding to a minimum energy at which a muon can radiate the Cherenkov light and less than a refractive index value corresponding to a minimum energy at which radiation can radiate the Cherenkov light.

4. The Cherenkov detector includes a radiator having a refractive index n and radiating Cherenkov light, and an optical sensor that measures the Cherenkov light radiated from the radiator, 2. The muon measurement device according to claim 1, wherein the refractive index n is set to a value equal to or greater than a refractive index value corresponding to a minimum energy at which a muon can radiate the Cherenkov light and equal to or less than a refractive index value corresponding to an energy in the vicinity of the minimum energy at which radiation can radiate Cherenkov light.

5. 2. The muon measurement device according to claim 1, wherein the trajectory determination unit is configured to determine an event caused by a muon based on a measurement result from a Cherenkov detector, to reconstruct a muon trajectory from a measurement value of a muon trajectory detector to obtain a change in the muon trajectory, and to analyze muon scattering from the change in the muon trajectory.

6. 2. The muon measurement device according to claim 1, wherein the weight determination unit is configured to derive the weight of the specific substance contained in the object from a relationship between the weight of the specific substance and the muon scattering, based on the muon scattering analyzed by the trajectory determination unit.

7. A muon measurement method performed in a radiation environment, comprising: A step in which muon trajectory detectors provided at opposing positions across a measurement target measure muons passing through the target to detect a muon trajectory; a step of emitting and measuring Cherenkov light by a Cherenkov detector that is set to have a higher sensitivity to the muons than to the radiation for emitting Cherenkov light, thereby eliminating events caused by the radiation and selecting events caused by the muons; a trajectory determination unit determining an event caused by the muon based on the measurement result of the Cherenkov detector, and reconstructing the muon trajectory for the determined event caused by the muon based on the measurement value of the muon trajectory detector to analyze muon scattering; a weight determining unit determining a weight of the specific substance contained in the object based on the muon scattering.

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