Neutron measurement device, neutron measurement system, neutron measurement method, and neutron detector

Abstract

To provide a neutron measurement device, a neutron measurement system, a neutron measurement method, and a neutron detector for easily measuring energy distribution of fast neutrons.SOLUTION: A neutron measurement system 1 performs: calculating a response function for each of a plurality of predetermined thresholds in accordance with a specification of a detector 3 capable of detecting neutrons and the predetermined thresholds, which are different from each other; calculating a discrete value for each threshold using a measurement result by the detector 3; and deriving neutron energy distribution in accordance with the response function and discrete value.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a neutron measurement device, a neutron measurement system, a neutron measurement method, and a neutron detector for measuring, for example, the energy distribution of neutrons. [Background technology]

[0002] Conventional methods for measuring neutron energy distribution include the multi-foil activation method, a method using a neutron detector with a multi-moderator, and a method using an organic liquid scintillator detector. Among these methods, the method using a neutron detector with a multi-moderator is suitable for measuring neutron energy distribution in a neutron field in a high-energy region such as in a particle beam therapy room.

[0003] An example of a multi-moderator neutron detector is the Bonner sphere neutron detector (see Patent Document 1). The Bonner sphere neutron detector is made up of a spherical neutron moderator and a neutron detector inserted in the center of the neutron moderator. 3 It also has a thermal neutron detector such as a He gas counter.

[0004] In a neutron measurement system using a Bonner sphere type neutron detector, multiple Bonner sphere detectors with different specifications (different neutron energy ranges with high sensitivity) such as the sphere diameter of the neutron moderator (thickness of the moderator) are prepared, and the measurement values ​​of each Bonner sphere detector are applied to calculate the energy distribution of neutron rays via the unfolding method.

[0005] Here, in conventional neutron measurement systems, it was necessary to prepare multiple Bonner sphere detectors with different specifications, which not only complicated the configuration of the measurement system, but also complicated the calculation processing because the neutron beam energy distribution was derived using the measurement values ​​for each Bonner sphere detector specification, requiring long measurement times, which was inconvenient. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2018-200235 A Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above-mentioned problems, an object of the present invention is to provide a neutron measurement device, a neutron measurement system, a neutron measurement method, and a neutron detector that can easily measure the energy distribution of neutrons. [Means for solving the problem]

[0008] The present invention is a neutron measurement device that measures neutrons using a detector capable of detecting neutrons, and is characterized in that it comprises a response function calculation unit that calculates a response function for each of a plurality of thresholds based on the specifications of the detector and a plurality of thresholds that are mutually different numerical values, a count value calculation unit that calculates a derivation count value calculated for each of the plurality of thresholds based on a count value in the detector and the plurality of thresholds, and a derivation unit that derives a neutron energy distribution based on the response function and the derivation count value. Effect of the Invention

[0009] According to the present invention, it is possible to provide a neutron measurement device, a neutron measurement system, a neutron measurement method, and a neutron detector that can easily measure the energy distribution of neutrons. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic explanatory diagram showing the overall configuration of a neutron measurement system. [Diagram 2] FIG. 2 is a cross-sectional view illustrating the configuration of a detector according to the present invention. [Diagram 3] FIG. 2 is a flow diagram showing a neutron measurement method of the present invention. [Figure 4] 13 is a graph showing the distribution of energy imparted to a detector by a Monte Carlo simulation. [Diagram 5]13 is a graph showing a response function for each threshold value calculated from the distribution of applied energy. [Figure 6] Graph showing neutron energy distribution derived from count values ​​and response functions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] An embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic explanatory diagram showing the overall configuration of a neutron measurement system 1 according to a first embodiment of the present invention. Note that the neutron measurement system 1 according to an embodiment of the present invention is merely one embodiment, and the present invention is not limited to the neutron measurement system of the embodiment.

[0012] As shown in Fig. 1, the neutron measurement system 1 includes a calculation unit 2 and a detector 3. The neutron measurement system 1 of the present invention is a system for measuring the neutron energy distribution in a neutron field. In particular, the neutron measurement system 1 is a system suitable for measuring the neutron energy distribution in a high energy region such as in a particle beam therapy room (e.g., a carbon beam therapy room).

[0013] The calculation unit 2 has a CPU (Central Processing Unit) and a storage unit (memory) that stores various data. For example, the calculation unit 2 is configured as a part of a general-purpose computer (terminal) such as a tablet PC, a desktop PC, a notebook (laptop) PC, or a smartphone. This calculation unit 2 executes at least a neutron energy distribution derivation process. That is, the calculation unit 2 derives the neutron energy distribution based on a signal output from the detector 3. The calculation unit 2 may function as a main control unit of the neutron measurement system 1.

[0014] The memory unit of the calculation unit 2 stores at least one threshold data (threshold data) for deriving the neutron energy distribution and data on the specifications of the detector 3 (specification data). The specification data includes at least data for identifying the detector 3 and data on a thresholdless response function for the detector 3. In this embodiment, multiple threshold data having mutually different values ​​are stored. The threshold is a value for defining the lower limit of the data to be adopted from the imparted energy distribution.

[0015] The detector 3 has a gas counter 31 and a signal output unit 32. The gas counter 31 is capable of detecting neutrons. It is preferable that the gas counter 31 is capable of detecting high-energy neutrons. High energy means at least 10 -1 This refers to the region above MeV.

[0016] Fig. 2 is a cross-sectional view for explaining the configuration of a gas counter 31 of the present invention. As shown in Fig. 2, the gas counter 31 has at least a gas-filled vessel 31a in which a gas is filled, a first moderator 31b covering the outer periphery of the gas-filled vessel 31a, and a second moderator 31c covering the outer periphery of the gas-filled vessel 31a and the first moderator 31b. In the gas counter shown in Fig. 2, a moderator having a two-layer structure having a first moderator and a second moderator is exemplified, but in addition to this example, it is also possible to apply a gas counter having a moderator having a three-layer structure or more, which further has a third moderator, a fourth moderator, ... in addition to the first moderator and the second moderator.

[0017] The gas filled container 31a is formed in a hollow sphere having a sealed space inside. The gas filled container 31a having such a configuration can detect neutrons incident from 4π directions (output a signal corresponding to the incident neutrons) in combination with the properties of the gas filled inside. The gas filled container 31a is made of metal, for example, stainless steel. The thickness of the gas filled container 31a is, for example, 0.5 mm. A gas for neutron detection is filled in the gas filled container 31a. The gas for neutron detection can be, for example, hydrogen gas or a gas containing hydrogen. In this embodiment, the gas for neutron detection is hydrogen gas.

[0018] The first moderator 31b and the second moderator 31c are intended to reduce the energy of the neutron beam directed toward the center of the gas counter 31 (gas sealed vessel 31a) to an energy level that corresponds to the sensitivity at which the gas sealed vessel 31a can detect it.

[0019] One of the first moderator 31b and the second moderator 31c is made of a high-density material. The other of the first moderator 31b and the second moderator 31c is made of a material with a high hydrogen content. The high-density material is iron, tungsten, lead, etc. The material with a high hydrogen content is polyethylene (high-density polyethylene), etc.

[0020] In this embodiment, the first moderator 31b is made of iron, and the second moderator 31c is made of polyethylene. The thickness of the first moderator 31b and the second moderator 31c can be set within a range of 5 mm to 15 mm, for example. For example, the thickness of the first moderator 31b and the second moderator 31c is 10 mm. The material and thickness of each of the first moderator 31b and the second moderator 31c can be appropriately designed so that neutrons entering the gas-filled vessel 31a are slowed down by interacting with the first moderator 31b and the second moderator 31c so that they can be detected by the gas-filled vessel 31a in the center.

[0021] The specifications of the detector 3 are determined by factors (specification determining factors) such as the type and pressure of the gas for neutron detection, the shape (including the outer diameter or shape) of the gas sealing vessel 31a, the material of the first moderator 31b, the thickness of the first moderator 31b, the material of the second moderator 31c, the thickness of the second moderator 31c, etc. Therefore, when the neutron measurement system 1 includes multiple detectors 3, if the detectors 3 differ from each other in one or more specification determining factors, they can be said to be detectors 3 with different specifications.

[0022] The signal output unit 32 is communicatively connected to the calculation unit 2, extracts an electric signal (output signal) from the gas counter 31, and outputs it to the calculation unit 2. In addition, the detector 3 can generate an output signal corresponding to the energy of the neutrons incident on the gas counter 31, and therefore can be suitably used for measuring the neutron energy spectrum (energy distribution).

[0023] 3 is a flow diagram showing the neutron energy distribution measuring method (neutron measurement method) of the present invention. In the following, in this embodiment, it is assumed that a single detector 3 is used, and each process (each step) in the neutron measurement method is executed in the calculation unit 2.

[0024] 3, in the neutron measurement method of the present invention, first, threshold data is acquired (threshold data acquisition step: step S1). In the threshold data acquisition step S1, the threshold data stored in the memory unit of the calculation unit 2 is read out.

[0025] Next, after the threshold data acquisition step S1, specification data of the detector 3 used in neutron measurement is acquired (specification data acquisition step: step S2). In the specification data acquisition step S2, the specification data stored in the memory unit of the calculation unit 2 is read out. At this time, if the neutron measurement system 1 includes multiple types of detectors 3 with different specifications, the specification data corresponding to each type of detector 3 is read out.

[0026] Subsequently, after the specification data acquisition step S2, a response function (reaction rate in the detector 3) for each threshold is acquired based on the threshold data and the specification data (response function acquisition step: step S3). In the response function acquisition step S3, the distribution of energy imparted to the detector 3 with respect to neutron energy is acquired by a Monte Carlo simulation based on the specifications of the detector 3. Here, the distribution of energy imparted in advance by the Monte Carlo simulation is stored (memorized) in the calculation unit 2, and data based on the specifications is acquired. Then, a response function is derived based on the distribution of energy imparted to the detector 3. Here, a value obtained by integrating the imparted energy in a range exceeding the threshold for each threshold from the distribution of energy imparted to the detector 3 is regarded as a response to neutron energy, and this is performed for all neutron energies to derive a response function (function of neutron energy). Note that the Monte Carlo simulation is a mathematical calculation method that utilizes a computer program or the like to predict the possible outcome of an uncertain event. In the present invention, the interaction between radiation (neutron beam) and a substance is calculated probabilistically using random numbers, and the transport behavior of the radiation is calculated. The deposited energy refers to the energy deposited to the gas in the detector 3 by the charged particles generated by the interaction between the gas in the detector 3 and neutrons. The principle of radiation measurement by the gas counter 31 is to detect electrons produced by the charged particles passing through the gas counter 31 ionizing gas molecules, and the amount of electron ionization in this case is proportional to the deposited energy.

[0027] Fig. 4 is a graph showing the distribution of energy imparted to a single detector 3 derived by a Monte Carlo simulation. Fig. 5 is a graph showing a response function for each threshold calculated from the distribution of energy imparted in Fig. 4. The X-axis in Fig. 4 indicates the neutron energy range, the Y-axis indicates the imparted energy range, and the Z-axis indicates the reaction probability (reaction frequency) in a certain neutron energy range and a certain imparted energy range. The imparted energy distribution shown in Fig. 4 was obtained by simulating a case where the first moderator 31b was made of iron and the second moderator 31c was made of polyethylene, each with a thickness of 10 mm.

[0028] As shown in Figures 4 and 5, multiple response functions corresponding to multiple thresholds (taking into account only response values ​​exceeding the thresholds) can be obtained from the distribution of energy imparted to a single detector 3. The response functions are applied to a detector response function matrix to reconstruct the source spectrum (neutron beam energy distribution) of a radioactive material from the measurement results using an unfolding method.

[0029] In the example shown in Figure 5, eight thresholds were prepared: a first threshold (0.01 MeV), a second threshold (0.02 MeV), a third threshold (0.03 MeV), a fourth threshold (0.04 MeV), a fifth threshold (0.05 MeV), a sixth threshold (0.10 MeV), a seventh threshold (0.20 MeV), and an eighth threshold (0.30 MeV), and response functions corresponding to each of the eight thresholds were obtained.

[0030] The number of types of thresholds and the values ​​of each threshold are not particularly limited, but it is necessary for the response function derived at each threshold to have a different shape in order to derive the neutron energy distribution. Therefore, the thresholds can be appropriately set so that the response functions have various shapes. Assuming the measurement of a neutron field with a high dose rate in a particle beam therapy room, 10 -1 It is preferable that there are multiple response functions with different shapes in the MeV or higher region. In the example shown in FIG. -1 It has been realized that the response function has a different shape in the MeV region and above.

[0031] Subsequently, after the response function acquisition step S3, a count value for each threshold is acquired based on the threshold data and the measurement value (frequency distribution for each applied energy) (count value acquisition step: step S4). In the count value acquisition step S4, a plurality of count values ​​(count values ​​for derivation) corresponding to each response function are obtained using each threshold value applied when calculating the response function in the response function acquisition step S3.

[0032] That is, in the method of the present invention, multiple thresholds are set for the distribution of applied energy (response function) and the measured values ​​obtained by one simulation calculation, and multiple response functions and multiple count values ​​that are equal to or greater than the thresholds are obtained. Here, the thresholds used in the calculation of the count values ​​and the response functions are the same, and each of the multiple response functions and multiple count values ​​is associated with one of the thresholds.

[0033] Subsequently, after the count value acquisition step S4, the neutron energy distribution is derived based on the response function and the count value (derivation step: step S5), and the neutron measurement method is terminated. In the derivation step S5, for example, the neutron energy distribution is derived by an unfolding method using the response function calculated in the response function acquisition step S3 and the count value calculated in the count value acquisition step S4.

[0034] The unfolding method is a method for reproducing the radiation source spectrum of a radioactive material from measurement results (count values), in which the relationship between a monochromatic radiation source of each energy (e.g., neutrons of one energy) and the count values ​​is prepared in advance as a detector response function matrix, and the energy spectrum of the radiation source (corresponding to the energy distribution of neutron beams in this invention) is obtained from the measurement results using the inverse matrix of the response function matrix (by solving the inverse problem).

[0035] In principle, it is possible to derive the neutron energy distribution by the unfolding method if the response functions and count values ​​corresponding to each of the multiple thresholds can be obtained. In addition, we performed verification to confirm the accuracy and validity of the neutron energy distribution calculated by the method of the present invention.

[0036] Fig. 6 is a graph showing a simulated neutron energy distribution created for verification purposes and a neutron energy distribution calculated by the method of the present invention. In the graph of Fig. 6, the horizontal axis indicates the neutron energy range, and the vertical axis indicates the number of particles of the neutron beam.

[0037] To verify the derivation of the neutron energy distribution, the response function R was calculated based on a simulated neutron energy distribution. iUsing the neutron energy distribution φ(E), the simulated count value C corresponding to each response function is calculated by the following formula 1. i The neutron energy distribution φ(E) was calculated based on the energy distribution of neutrons generated by a carbon ion therapy device disclosed in the literature "S. Matsumoto, S. Yonai, "Evaluation of Neutron Ambient Dose Equivalent in Carbon-Ion Radiotherapy with Energy Scanning", Radiation Protection Dosimetry, Vol. 191, No. 3, 310-318 (2020)".

[0038]

number

[0039] As shown in Figure 6, it was found that the neutron energy distribution calculated by the method of the present invention almost coincided with the simulated neutron energy distribution created for verification. In other words, the accuracy and validity of the derivation method of the present invention was confirmed.

[0040] As described above, in the present invention, a response function is calculated for each of a plurality of thresholds based on the detector specifications and a plurality of predetermined thresholds, a count value is calculated for each of the plurality of thresholds based on the count value in the detector and the plurality of thresholds, and the energy distribution of neutrons (particularly in the high energy region) can be derived based on the response function and the count value. This simplifies the calculation process and shortens the measurement time. In other words, according to the present invention, the energy distribution of neutrons can be easily measured.

[0041] According to the present invention, at least 10 -1In the MeV range or higher, multiple response functions with different shapes and thresholds that can derive such response functions are set, making it possible to measure neutrons in high energy regions such as inside a particle beam therapy room, and to easily and accurately evaluate the safety of particle beam therapy. In particular, the neutron energy distribution can be properly measured in the range of around 10 MeV. Also, the neutron energy distribution can be properly measured in the range of 20 MeV or higher.

[0042] Furthermore, according to the present invention, the neutron energy distribution can be measured with only a single detector 3. Therefore, it is possible to reduce the number of detectors in the neutron measurement system 1, and the neutron energy distribution can be measured easily. Therefore, it is not necessary to take a long measurement time to measure spherical moderators with different diameters using multiple detectors and derive the neutron energy distribution from these multiple measurements as in the past, and it is possible to measure the neutron energy distribution in a short time with only a single detector 3. In addition, it is not necessary to perform Monte Carlo simulation and measurement multiple times, and the imparted energy distribution can be obtained with one simulation and measurement, so calculations can be performed easily. Furthermore, it is possible to calculate the dose from the measured energy distribution. Therefore, compared to conventional measurement methods, the present invention is overwhelmingly simple, and since the measurement system is single, it will be possible to apply it to survey meters, which are portable and simple radiation measurement devices, in the future.

[0043] Furthermore, according to the present invention, in addition to being able to reduce the number of detectors, hydrogen gas is used as the gas for neutron detection in the gas counter 31, which is in short supply worldwide at the time of filing this application. 3 By not using He, it is possible to reduce the labor required for quality control, reduce manufacturing costs, improve the stability of material procurement, and ultimately improve the business continuity of the neutron measurement system 1. In addition, by using hydrogen gas as the gas for neutron detection, -1 Neutron energy distribution can be easily measured in the MeV and above range.

[0044] The neutron measurement system of the present invention corresponds to the neutron measurement system 1 of the above embodiment, and similarly, the response function calculation unit corresponds to the response function acquisition step S3 and the calculation unit 2 that executes this, the count value calculation unit corresponds to the count value acquisition step S4 and the calculation unit 2 that executes this, the derivation unit corresponds to the derivation step S5 and the calculation unit 2 that executes this, the calculation device corresponds to the calculation unit 2, the gas-filled vessel corresponds to the gas-filled vessel 31a, the first moderator corresponds to the first moderator 31b, and the second moderator corresponds to the second moderator 31c, but this invention is not limited to this embodiment and can be various other embodiments. Also, the specific configurations and the like given in the above embodiment are merely examples and can be appropriately changed according to the actual product.

[0045] For example, in the above embodiment, the neutron energy distribution is measured using eight types of thresholds, but this is not necessary to be limited to this. The neutron energy distribution may be measured using two types of thresholds, or may be measured using five types of thresholds. For example, although it depends on the distribution of energy imparted to the detector 3 that is the basis of the response function, the neutron energy distribution can be measured with high accuracy by using five or more types of thresholds.

[0046] In the above embodiment, the case where a single detector 3 is used has been described as an example, but this is not limiting, and the neutron measurement method may be performed using multiple detectors 3. In this case, the threshold value (threshold set when multiple threshold values ​​are used) applied to each detector 3 may be the same or different. However, for the same detector 3, the same threshold value is used in the response function acquisition step S3 and the count value acquisition step S4. For example, the neutron measurement method can be performed using multiple detectors 3 with different specifications. In this way, the distribution of energy imparted to each detector 3 changes due to differences in specifications, and the response function changes accordingly. Therefore, the variation of the response function increases, and the neutron energy distribution can be measured more accurately.

[0047] Furthermore, the detector 3 may have a measurement data acquisition unit and a recording unit. The detector 3 and the calculation unit 2 may be configured as separate devices, or may be configured as parts of the same device. [Industrial Applicability]

[0048] The present invention can be used in industries that measure the energy distribution of neutrons. [Explanation of symbols]

[0049] 1...Neutron measurement system 2...Arithmetic section 3. Detector 31…Gas counter 31a...container 31b…1st moderator 31c…Second moderator 32...Signal output section

Claims

1. A neutron measurement device that measures neutrons using a detector capable of detecting neutrons, a response function calculation unit that calculates a response function for each of the plurality of threshold values ​​based on specifications of the detector and a plurality of threshold values ​​that are different from each other; a count value calculation unit that calculates a count value for each of the plurality of threshold values ​​based on the count value of the detector and the plurality of threshold values; and a derivation unit for deriving a neutron energy distribution based on the response function and the derivation count value. Neutron measuring device.

2. Each of the plurality of thresholds is 10 －１ In the energy region of MeV or higher, multiple response functions with different shapes are set to values ​​that can be calculated.

2. The neutron measurement device according to claim 1.

3. The derivation unit is characterized in that it derives a neutron energy distribution using an unfolding method based on the response function and the derivation count value.

3. The neutron measuring device according to claim 2.

4. The threshold value is set to five or more types.

4. The neutron measuring device according to claim 3.

5. A neutron measurement system comprising a detector capable of detecting neutrons and a computing device capable of acquiring an output signal of the detector, The computing device includes: a response function calculation unit that calculates a response function for each of a plurality of threshold values ​​based on a specification of the detector and a plurality of predetermined threshold values ​​different from each other; a count value calculation unit that calculates a count value for each of the plurality of threshold values ​​based on the count value of the detector and the plurality of threshold values; and a derivation unit for deriving a neutron energy distribution based on the response function and the derivation count value. Neutron measurement system.

6. Calculating a response function for each of a plurality of thresholds based on the specifications of a detector capable of detecting neutrons and a plurality of predetermined thresholds; calculating a derivation count value for each of the plurality of threshold values ​​based on the count value at the detector and the plurality of threshold values; Deriving a neutron energy distribution based on the response function and the derivation count value. Neutron measurement method.

7. a gas-filled container formed in a hollow sphere and containing a gas for neutron detection therein; a first moderator covering an outer periphery of the gas container; a second moderator covering an outer periphery of the gas container and the first moderator, One of the first moderator and the second moderator is made of a high density material, and the other of the first moderator and the second moderator is made of a material with a high hydrogen content. Neutron detector.

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