Secondary gamma ray measuring device and secondary gamma ray measuring method

Abstract

An object of the present invention is to solve the problems of the prior art, that is, to provide a secondary gamma ray measuring device and a secondary gamma ray measuring method capable of discriminating and measuring a secondary gamma ray generated by a reaction with a measurement sample.SOLUTION: A secondary gamma ray measuring device according to the present invention is a device for measuring a dose of secondary gamma rays generated by a reaction between neutrons and a measurement sample, and includes neutron emitting means for emitting neutrons, an intermediate shield for shielding gamma rays, a sample container for containing the measurement sample, a detector for measuring the dose of the secondary gamma rays, and a detector container for containing the detector. The dose of the secondary gamma rays associated with the measurement sample can be obtained from the difference between the dose measured in a state where the partial shield is not disposed in the shielding chamber and the dose measured in a state where the partial shield is disposed in the shielding chamber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technology for measuring secondary gamma rays in neutron irradiation sites, such as spent fuel storage facilities and casks, operators and medical institutions that perform neutron capture therapy (BNCT), and more specifically to a secondary gamma ray measurement device that can distinguish only secondary gamma rays generated by the reaction between neutrons and a measurement sample and measure their dose, and a method for measuring secondary gamma rays using the same. [Background technology]

[0002] Neutron capture therapy is a treatment in which cancer cells are injected with boron compounds, and the boron is destroyed by a nuclear reaction with neutrons. Boron (especially 10B) has the property of reacting strongly with low-energy neutrons, including thermal neutrons, and the boron and neutrons in the cancer cells undergo a nuclear fission reaction, generating particle beams (alpha and Li beams), which destroy the cancer cells.

[0003] The range of particle beams generated by nuclear fission reactions is about the diameter of a cancer cell (approximately 10-14 μm), and does not affect normal cells other than cancer cells. Because conventional treatments using X-rays and gamma rays cause almost the same physical damage to normal cells as they do to cancer cells, neutron capture therapy is also known as "cancer cell selective treatment," and is currently considered to be the most ideal treatment, particularly for treating malignant brain tumors and melanoma.

[0004] Neutron capture therapy involves irradiating neutrons, and there are many facilities (hereinafter simply referred to as "neutron irradiation facilities") that irradiate neutrons, including such medical facilities as well as facilities that handle nuclear reactors and nuclear fuel, and nuclear research facilities. Naturally, there is a risk of employees working at neutron irradiation facilities being exposed to radiation, and for this reason, particular attention is paid to protecting employees from excessive radiation exposure. Furthermore, exposure to radiation can cause malfunctions or accidents in the equipment and facilities installed at neutron irradiation facilities. Secondary gamma rays are generated by nuclear reactions between neutrons and objects within the facility, and it is known that these secondary gamma rays have adverse effects primarily on employees and equipment.

[0005] The dose of secondary gamma rays generated by nuclear reactions with neutrons varies depending on the type of material, and therefore it is extremely useful to understand the "characteristics of generating secondary gamma rays (hereinafter simply referred to as "secondary gamma ray characteristics")" of each material. One way to understand the secondary gamma ray characteristics is to conduct tests using a measurement sample. The secondary gamma ray characteristics of the material related to the measurement sample can be investigated by irradiating the measurement sample with neutrons and measuring the secondary gamma rays generated as a result.

[0006] However, although tests have been conducted to measure gamma rays that are the sum of primary gamma rays emitted directly from the radiation source and secondary gamma rays generated by nuclear reactions between neutrons and objects within the facility, no tests have been conducted to measure secondary gamma rays. For example, Patent Document 1 proposes a method of measuring neutron rays and gamma rays after distinguishing them using a thermoluminescent material, but this is merely a method of measuring gamma rays, not secondary gamma rays. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-24863 Summary of the Invention [Problem to be solved by the invention]

[0008] The reason why no measurement tests targeting secondary gamma rays have been conducted to date is that various gamma rays are generated when a measurement sample is irradiated with neutrons. For example, to investigate the secondary gamma ray characteristics of a measurement sample, a measurement test such as the one shown in Figure 8 can be considered. In the measurement test shown in this figure, a detector such as a NaI (sodium iodide) scintillator is placed above the measurement sample, and the measurement sample is irradiated with neutrons from a neutron source, and the secondary gamma rays generated by the nuclear reaction between the measurement sample and the neutrons are measured with the detector.

[0009] However, in the test shown in this figure, in addition to secondary gamma rays that react with the measurement sample and reach the detector (hereinafter referred to as "measurement sample secondary gamma rays A" for convenience), the detector measures primary gamma rays that reach the detector directly from the neutron source (hereinafter referred to as "direct primary gamma rays B" for convenience), secondary gamma rays that reach the detector after reacting with the concrete wall in the room (hereinafter referred to as "interference secondary gamma rays C" for convenience), and secondary gamma rays that do not react with the measurement sample but are generated by reacting with NaI crystals contained in the detector (hereinafter referred to as "detector secondary gamma rays D" for convenience). In other words, a typical measurement test cannot distinguish and measure only "measurement sample secondary gamma rays A" generated by reaction with the measurement sample, and instead measures gamma rays that act as noise (direct primary gamma rays B, interfering secondary gamma rays C, and detector secondary gamma rays D), making it impossible to properly investigate the secondary gamma ray characteristics of the material.

[0010] The object of the present invention is to solve the problems of the prior art, that is, to provide a secondary gamma ray measurement device and a secondary gamma ray measurement method that can discriminate and measure secondary gamma rays generated in reaction with a measurement sample. [Means for solving the problem]

[0011] The present invention is based on the unprecedented idea of ​​shielding unwanted gamma rays as much as possible, measuring the dose of "measurement sample secondary gamma rays A (secondary gamma rays resulting from the nuclear reaction between neutrons and the measurement sample)" and "detector secondary gamma rays D (secondary gamma rays resulting from the nuclear reaction between neutrons and the detector)", and then measuring and subtracting the dose of detector secondary gamma rays D alone to determine the dose from measurement sample secondary gamma rays A.

[0012] The secondary gamma ray measurement device of the present invention is a device for measuring the dose of secondary gamma rays generated by the reaction between neutrons and a measurement sample, and includes neutron irradiation means for irradiating neutrons, an intermediate shield for shielding gamma rays, a sample container for accommodating the measurement sample, a detector for measuring the dose of secondary gamma rays, and a detector container for accommodating the detector. The sample container has a space surrounded by a sample wall for shielding gamma rays, and the detector container has a space surrounded by a detector wall for shielding gamma rays and a shielding chamber in which a partial shield for shielding gamma rays can be placed. When the neutron irradiation means irradiates neutrons with the partial shield placed in the shielding chamber, the neutrons pass through the intermediate shield, the measurement sample contained in the sample container, and the partial shield, and the detector measures the dose of "detector secondary gamma rays D (secondary gamma rays generated by the reaction between neutrons and the detector)." Furthermore, when the neutron irradiation means irradiates neutrons without placing a partial shield in the shielding chamber, the neutrons pass through the intermediate shield and the measurement sample contained in the sample container, and the detector measures the dose of "measurement sample secondary gamma rays A (secondary gamma rays generated by the reaction between neutrons and the measurement sample)" and "detector secondary gamma rays D." This makes it possible to determine the dose of measurement sample secondary gamma rays A by subtracting the dose measured by the detector with a partial shield in the shielding chamber from the dose measured by the detector with no partial shield in the shielding chamber.

[0013] The secondary gamma ray measuring device of the present invention may also be provided with an intermediate shield made of a heavy metal (for example, lead), a sample wall, a detector wall, and a partial shield.

[0014] The secondary gamma ray measuring device of the present invention may also be configured such that, from bottom to top, a neutron irradiation means, an intermediate shielding body, a sample holder, and a detector holder are arranged. An intermediate plate is arranged in the detector holder, and a shielding chamber is formed below the intermediate plate, and the detector is placed on the upper surface of the intermediate plate.

[0015] The secondary gamma ray measuring device of the present invention may further include a neutron absorber that moderates and absorbs neutrons that do not enter the intermediate shield. This neutron absorber is arranged around the intermediate shield.

[0016] The secondary gamma ray measurement method of the present invention is a method for measuring the dose of secondary gamma rays generated by the reaction between neutrons and a measurement sample using the secondary gamma ray measurement device of the present invention, and includes a partial dose measurement step, a total dose measurement step, and a target dose calculation step. In the partial dose measurement step, a detector is used to measure the dose of "detector secondary gamma ray D," and in the total dose measurement step, a detector is used to measure the doses of "measurement sample secondary gamma ray A" and "detector secondary gamma ray D." In the target dose calculation step, the dose of "measurement sample secondary gamma ray A" is calculated by subtracting the dose measured in the partial dose measurement step from the dose measured in the total dose measurement step. In the partial dose measurement step, the neutron irradiation means irradiates neutrons with a partial shield disposed in the shielded chamber, while in the total dose measurement step, the neutron irradiation means irradiates neutrons with no partial shield disposed in the shielded chamber. Furthermore, in the partial dose measurement process, neutrons irradiated from the neutron irradiation means pass through the intermediate shielding, the measurement sample contained in the sample container, and the partial shielding, thereby generating "measurement sample secondary gamma rays A" and "detector secondary gamma rays D." However, because "measurement sample secondary gamma rays A" are shielded by the partial shielding, only a negligible amount reaches the detector. Therefore, the detector measures only "detector secondary gamma rays D." In contrast, in the total dose measurement process, neutrons irradiated from the neutron irradiation means pass through the intermediate shielding and the measurement sample contained in the sample container, thereby generating "measurement sample secondary gamma rays A" and "detector secondary gamma rays D," and the detector measures "measurement sample secondary gamma rays A" and "detector secondary gamma rays D." [Effects of the Invention]

[0017] The secondary gamma ray measuring device and the secondary gamma ray measuring method of the present invention have the following effects. (1) The dose of secondary gamma rays generated by the reaction between neutrons and the measurement sample can be measured, i.e., the secondary gamma ray characteristics of the material related to the measurement sample can be understood, which makes it possible to identify the main cause of excessive exposure in neutron irradiation facilities. As a result, it is possible to protect employees from excessive exposure and to prevent malfunctions and accidents in the equipment and facilities within the facility due to exposure. (2) Currently, attempts are being made to estimate the dose of secondary gamma rays through analysis, but the reality is that it is difficult to verify the validity of the results. The present invention makes it possible to obtain actual measured values, so that the validity of the analytical method can be easily determined. (3) In recent years, there has been an increase in cases where government agencies, including the national government, require various requirements when granting licenses for the construction of neutron irradiation facilities. As described above, the present invention makes it possible to easily determine the validity of an analytical method, thereby enabling appropriate estimation of the dose of secondary gamma rays using an appropriate analytical method, thereby meeting the government's requests. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view showing the main configuration of a secondary gamma ray measuring device according to the present invention. [Figure 2] FIG. 2A is a plan view schematically showing a sample holder, and FIG. 2B is a vertical cross-sectional view schematically showing a sample holder. [Figure 3] (a) is a plan view showing a schematic diagram of a detector housing, (b) is a vertical cross-sectional view showing a schematic diagram of a detector housing in a partially open state, and (c) is a vertical cross-sectional view showing a schematic diagram of a detector housing in a partially blocked state. [Figure 4] FIG. 10 is a vertical cross-sectional view schematically showing a state in which neutrons are irradiated from the neutron irradiation means in the "partially open state." [Figure 5] FIG. 10 is a vertical cross-sectional view schematically showing a state in which neutrons are irradiated from the neutron irradiation means in the "partially shielded state." [Figure 6] FIG. 10 is a vertical cross-sectional view illustrating that the neutron beam irradiated from the neutron irradiation means reaches the detector even in the "partially shielded state." [Figure 7] 1 is a flow chart showing the main steps of the secondary gamma ray measurement method of the present invention. [Figure 8] FIG. 1 is a vertical cross-sectional view schematically illustrating a test method in which a measurement sample is irradiated with neutrons from a neutron source and secondary gamma rays are measured with a detector. DETAILED DESCRIPTION OF THE INVENTION

[0019] An example of an embodiment of the secondary gamma ray measuring device and the secondary gamma ray measuring method of the present invention will be described with reference to the drawings.

[0020] 1.Overview One of the technical features of the present invention is that a measurement sample is irradiated with neutrons and secondary gamma rays generated by a nuclear reaction between the measurement sample and the neutrons are measured. However, as mentioned above, in a test in which a measurement sample is irradiated with neutrons, in addition to secondary gamma rays that react with the measurement sample and reach the detector (measurement sample secondary gamma rays A), primary gamma rays that reach the detector directly from the neutron source (direct primary gamma rays B), secondary gamma rays that reach the detector after reacting with the concrete wall in the room (interference secondary gamma rays C), and secondary gamma rays that reach the detector after reacting with NaI crystals contained in the detector without reacting with the measurement sample (detector secondary gamma rays D) are measured by the detector.

[0021] Therefore, the present invention first shields the direct primary gamma rays B and the disturbing secondary gamma rays C from reaching the detector, while deliberately using the detector to measure the detector secondary gamma rays D. More specifically, the dose of only the detector secondary gamma rays D (hereinafter referred to as the "partial dose") is measured, and the dose of the measurement sample secondary gamma rays A and the detector secondary gamma rays D (hereinafter referred to as the "total dose") is measured, and the dose of only the measurement sample secondary gamma rays A (hereinafter referred to as the "target dose") is obtained by subtracting the partial dose from the total dose.

[0022] 2. Secondary gamma ray measuring device First, the secondary gamma ray measurement device of the present invention will be described in detail with reference to the drawings. Note that the secondary gamma ray measurement method of the present invention is a method for measuring the dose of secondary gamma rays using the secondary gamma ray measurement device of the present invention. Therefore, the secondary gamma ray measurement device of the present invention will be described first, and then the secondary gamma ray measurement method of the present invention will be described.

[0023] 1 is a cross-sectional view showing the main components of a secondary gamma ray measurement apparatus 100 of the present invention. As shown in this figure, the secondary gamma ray measurement apparatus 100 of the present invention is configured to include neutron irradiation means 110, intermediate shielding 120, sample holder 130, detector 140, and detector holder 150, and may further include a neutron absorber 170, a support stand 180, etc.

[0024] The support base 180 is a box that supports the main components such as the neutron irradiation means 110, the sample container 130, and the detector container 150, and can be made of concrete, polyethylene, carbon steel, or a combination of these materials. When the secondary gamma ray measurement device 100 is installed with the support base 180 facing downward as shown in Fig. 1, the neutron irradiation means 110, the intermediate shield 120, and the sample container 130 are arranged in this order from below. Note that an "irradiation space 181" through which the neutron beam irradiated from the neutron irradiation means 110 passes can also be formed between the neutron irradiation means 110 and the intermediate shield 120.

[0025] Hereinafter, each of the main elements constituting the secondary gamma ray measuring device 100 of the present invention will be described in detail.

[0026] (Neutron irradiation means and detector) The neutron irradiation means 110 constituting the secondary gamma ray measurement device 100 is a device that generates and irradiates neutron rays from a neutron source, and can utilize conventional devices such as particle accelerators, or radioactive isotopes that generate neutrons, spent nuclear fuel, etc. The detector 140 is a device that measures the dose of gamma rays, and can measure the dose of "primary gamma rays" directly irradiated from the neutron irradiation means 110, and "secondary gamma rays" generated when neutron rays react with other substances. Conventional devices such as a NaI scintillation detector, a CsI (cesium iodide) scintillation detector, a cerium bromide scintillation detector, or a lanthanum bromide scintillation detector can be used as this detector 140.

[0027] (Intermediate shield) The intermediate shield 120 is a box-shaped or plate-shaped component that can shield gamma rays but has a low shielding effect against neutrons. It can be made of heavy metals such as lead (Pb), tantalum (Ta), or tungsten (W). As shown in FIG. 1, the intermediate shield 120 is positioned closer to the neutron irradiation means 110 than the measurement sample. Therefore, the neutrons irradiated from the neutron irradiation means 110 first strike the intermediate shield 120. Therefore, the "direct primary gamma rays B" irradiated from the neutron irradiation means 110 are blocked by the intermediate shield 120 and do not reach the measurement sample or the detector 140. A neutron absorber 170 that moderates and absorbs neutrons can also be placed around the intermediate shield 120. The neutron absorber 170 can be made of polyethylene containing boron (B).

[0028] (partial shield) The partial shielding body 160 is a box-shaped or plate-shaped member that can shield gamma rays but has a small shielding effect against neutrons, and can be made of a so-called heavy metal such as lead, tantalum, or tungsten. This partial shielding body 160 is disposed in a "shielded chamber 152L" of the detector housing 150, which will be described later, and can also be removed from this shielded chamber 152L. That is, the secondary gamma ray measurement device 100 can be in a state in which the partial shielding body 160 is disposed in the shielded chamber 152L (hereinafter, for convenience, referred to as a "partially shielded state"), or in a state in which the partial shielding body 160 is not disposed in the shielded chamber 152L (hereinafter, for convenience, referred to as a "partially open state").

[0029] The shielding chamber 152L of the detector housing 150 is disposed above the measurement sample and below the detector 140, i.e., between the measurement sample and the detector 140. Therefore, when the partial shielding body 160 is disposed in the shielding chamber 152L, the "measurement sample secondary gamma rays A" generated by the reaction between the neutron beam and the measurement sample are shielded by the partial shielding body 160 and do not reach the detector 140. On the other hand, when the partial shielding body 160 is removed from the shielding chamber 152L, the measurement sample secondary gamma rays A will reach the detector 140.

[0030] (sample container) 2A and 2B are diagrams schematically illustrating the sample holder 130, with (a) being a plan view seen from above and (b) being a cross-sectional view cut along a vertical plane. As shown in FIG. 2A, the sample holder 130 includes a sample wall 131, and a space surrounded by the sample wall 131 (hereinafter referred to as a "sample holder space 132") is formed inside the sample holder 130. The sample wall 131 is a wall member capable of blocking gamma rays and can be formed from a heavy metal such as lead, tantalum, or tungsten. As a result, for example, "interference secondary gamma rays C" generated when neutrons irradiated from the neutron irradiation means 110 react with a concrete wall inside the room are blocked by the sample wall 131 and do not pass through the measurement sample and reach the detector 140.

[0031] 2(b), a measurement sample is accommodated in the sample accommodation space 132 of the sample accommodation body 130. This measurement sample is made of a substance whose "secondary gamma ray characteristics (characteristics for generating secondary gamma rays)" are to be understood; for example, when the secondary gamma ray characteristics of carbon steel are to be understood, a measurement sample made of carbon steel is accommodated in the sample accommodation space 132. Note that one type of measurement sample can be placed in the sample accommodation space 132, or two or more types of measurement samples (two types in the figure) can be placed in two or more layers (two layers in the figure).

[0032] (detector housing) 3A and 3B are diagrams schematically illustrating the detector housing 150, in which (a) is a plan view seen from above, (b) is a vertical cross-sectional view of the detector housing 150 in a "partially open state (a state in which the partial shielding body 160 is not disposed in the shielding chamber 152L)," and (c) is a vertical cross-sectional view of the detector housing 150 in a "partially shielded state (a state in which the partial shielding body 160 is disposed in the shielding chamber 152L)." As shown in FIG. 3A, the detector housing 150 includes a detector wall 151, and a space surrounded by the detector wall 151 (hereinafter referred to as a "detector housing space 152") is formed inside the detector housing 150. Like the specimen wall 131, the detector wall 151 is a wall member capable of shielding against gamma rays, and can be formed of a heavy metal such as lead, tantalum, or tungsten. As a result, for example, "disturbance secondary gamma rays C" generated when neutron rays irradiated from the neutron irradiation means 110 react with the concrete wall inside the room are blocked by the detector wall 151 and do not reach the detector 140.

[0033] 3(b), an intermediate plate 153 can also be placed in the detector housing space 152 of the detector housing body 150. This intermediate plate 153 is a plate-like member made of aluminum or the like, and divides the detector housing space 152 into an upper space (hereinafter referred to as the "detector chamber 152U") and a lower space (hereinafter referred to as the "shielded chamber 152L"). The detector 140 is placed in this detector chamber 152U so as to be placed on the intermediate plate 153, and a partial shielding body 160 is placed in one of the shielded chambers 152L, as shown in FIG. 3(c). However, as described above, the partial shielding body 160 can be removed from the shielded chamber 152L, i.e., it can be placed in a "partially open state" as shown in FIG. 3(b). In the case where the intermediate plate 153 is not provided in the detector accommodating space 152, the detector 140 may be placed directly on the upper surface of the measurement sample when in the partially open state (FIG. 3(b)), and the detector 140 may be placed directly on the upper surface of the partial shielding body 160 when in the partially shielded state (FIG. 3(c)). In the example of FIG. 3(c), the partial shielding body 160 is placed directly on the upper surface of the measurement sample, but the partial shielding body 160 may also be placed on the upper surface of the measurement sample with a thin member (sheet material or plate material) interposed therebetween.

[0034] (Example of use) An example of measuring a target dose of a measurement sample using the secondary gamma ray measurement device 100 of the present invention will be described with reference to Figures 4 to 6. Figures 4 and 5 are diagrams schematically showing a state in which a neutron beam is irradiated from the neutron irradiation means 110, with Figure 4 showing a "partially open state" in which the partial shield 160 is not arranged in the shielding chamber 152L, and Figure 5 showing a "partially shielded state" in which the partial shield 160 is arranged in the shielding chamber 152L. Figure 6 is a vertical cross-sectional view illustrating that the neutron beam irradiated from the neutron irradiation means 110 reaches the detector 140 even in the "partially shielded state."

[0035] As described above, when measuring the target dose of a measurement sample using the secondary gamma ray measurement device 100 of the present invention, the "partial dose (the dose of only the detector secondary gamma rays D)" is measured, and the "total dose (the dose of the measurement sample secondary gamma rays A and the detector secondary gamma rays D)" is also measured. That is, the "partial dose" is measured by the detector 140 in a partially open state as shown in Fig. 4, and the "total dose" is measured by the detector 140 in a partially shielded state as shown in Fig. 5. The procedure for measuring the target dose using the secondary gamma ray measurement device 100 of the present invention will be described in detail below.

[0036] First, as shown in FIG. 4, the secondary gamma ray measurement device 100 is placed in a partially open state. For example, when a partial shield 160 is placed in the shielding chamber 152L, the partial shield 160 is removed from the shielding chamber 152L. When the partial shield 160 is not placed in the shielding chamber 152L, the state is maintained as is. When the secondary gamma ray measurement device 100 is placed in the partially open state, neutrons are irradiated by the neutron irradiation means 110. At this time, "direct primary gamma rays B" are irradiated from the neutron irradiation means 110, but these direct primary gamma rays B are blocked by the intermediate shield 120 and do not reach the detector 140. Furthermore, "interference secondary gamma rays C" generated by reaction with a concrete wall, for example, are blocked by the sample wall 131 and the detector wall 151 and do not reach the detector 140 either. Furthermore, the neutron absorber 170 moderates and absorbs neutrons generated from the neutron irradiation means 110 that do not enter the intermediate shield 120, and also prevents neutrons scattered by the wall or the like from entering the measurement sample. In other words, only the neutron beam that passes through the intermediate shield 120 enters the measurement sample.

[0037] Neutron rays that pass through the intermediate shield 120 and enter the measurement sample cause a nuclear reaction with the measurement sample, generating "measurement sample secondary gamma rays A." These measurement sample secondary gamma rays A then reach the detector 140. On the other hand, neutron rays from the neutron irradiation means 110 may reach the detector 140 directly without reacting with the measurement sample. In this case, they react with a substance contained in the detector 140 (e.g., NaI crystals) to generate detector secondary gamma rays D, which then reach the detector 140. In other words, when the detector 140 is in a partially open state, it measures the "total dose (the dose of measurement sample secondary gamma rays A and detector secondary gamma rays D)."

[0038] 5, the secondary gamma ray measurement device 100 is placed in a partially shielded state by placing a partial shield 160 in the shielding chamber 152L. Then, neutron rays are irradiated by the neutron irradiation means 110. At this time, as in the partially open state, direct primary gamma rays B are shielded by the intermediate shield 120, and disturbing secondary gamma rays C are shielded by the sample wall 131 and the detector wall 151, so that only neutron rays that have passed through the intermediate shield 120 enter the measurement sample.

[0039] Neutron rays that enter the measurement sample undergo a nuclear reaction with the measurement sample, thereby generating measurement sample secondary gamma rays A, but due to the effect of the partial shielding state, i.e., due to shielding by the partial shielding body 160, these measurement sample secondary gamma rays A do not reach the detector 140. On the other hand, as shown in Figure 6, even in the partial shielding state, neutron rays from the neutron irradiation means 110 reach the detector 140. In other words, neutron rays that reach the detector 140 directly without reacting with the measurement sample react with a substance contained in the detector 140 to generate detector secondary gamma rays D, and these detector secondary gamma rays D reach the detector 140. In other words, when the detector 140 is in the partial shielding state, it measures only the "partial dose (the dose of only the detector secondary gamma rays D)."

[0040] When the total dose is obtained in the measurement test for the partially open state and the partial dose is obtained in the measurement test for the partially shielded state, the dose of secondary gamma rays A of the measurement sample, i.e., the "target dose," can be obtained by subtracting the partial dose from the total dose. In the example described above, the measurement test for the partially shielded state is performed after the measurement test for the partially open state, but it is of course possible to perform the measurement test for the partially open state after the measurement test for the partially shielded state, or to perform the measurement test for the partially shielded state and the measurement test for the partially open state in parallel using two secondary gamma ray measurement devices 100.

[0041] 2. Secondary gamma ray measurement method Next, the secondary gamma ray measurement method of the present invention will be described with reference to the drawings. The secondary gamma ray measurement method of the present invention is a method for measuring the dose of secondary gamma rays using the secondary gamma ray measurement device 100 described so far. Therefore, we will avoid explanations that overlap with the contents described for the secondary gamma ray measurement device 100, and will mainly describe contents unique to the secondary gamma ray measurement method of the present invention. In other words, contents not described here are the same as those described in "2. Secondary gamma ray measurement device."

[0042] 7 is a flow chart showing the main steps of the secondary gamma ray measurement method of the present invention. To measure the dose of secondary gamma rays using the secondary gamma ray measurement device 100, first, the secondary gamma ray measurement device 100 is placed in a partially open state (Step 201 in FIG. 7), as shown in FIG. 7. For example, if a partial shield 160 is placed in the shielded chamber 152L, the partial shield 160 is removed from the shielded chamber 152L, and if a partial shield 160 is not placed in the shielded chamber 152L, the state is maintained as is. When the secondary gamma ray measurement device 100 is placed in the partially open state, neutrons are irradiated by the neutron irradiation means 110, and the "total dose" is measured by the detector 140 (Step 202 in FIG. 7).

[0043] Once the total dose is obtained, the secondary gamma ray measurement device 100 is placed in a partially shielded state by placing the partial shielding body 160 in the shielded room 152L (Step 203 in FIG. 7). Once the secondary gamma ray measurement device 100 is placed in the partially shielded state, neutrons are irradiated by the neutron irradiation means 110, and the "partial total dose" is measured by the detector 140 (Step 204 in FIG. 7). Then, the dose of the secondary gamma ray A of the measurement sample, i.e., the "target dose" is calculated by subtracting the partial dose from the total dose (Step 205 in FIG. 7). Note that Figure 7 shows an example in which a measurement test for a partially open state is performed after a measurement test for a partially shielded state, but it is of course possible to perform a measurement test for a partially shielded state (Step 203 to Step 204) and then a measurement test for a partially open state (Step 201 to Step 202), or to use two secondary gamma ray measurement devices 100 to perform a measurement test for a partially shielded state and a measurement test for a partially open state in parallel. [Industrial Applicability]

[0044] The secondary gamma ray measurement device and secondary gamma ray measurement method of the present invention can be particularly effectively used in neutron irradiation facilities such as medical facilities where neutrons are generated, such as those used in proton therapy, heavy ion therapy, and neutron capture therapy, as well as research facilities, testing facilities, industrial facilities, spent nuclear fuel facilities, and spent nuclear fuel casks. The present invention solves the current problems faced by neutron irradiation facilities, thereby promoting the spread of particle beam cancer therapy and the nuclear fuel cycle, as well as reducing unnecessary exposure of radiation workers and the generation of radioactive waste. Considering this, the present invention is not only applicable industrially but is also expected to make a significant contribution to society. [Explanation of symbols]

[0045] 100 Secondary gamma ray measuring device of the present invention 110 Neutron irradiation means (for secondary gamma ray measuring devices) 120 Intermediate shielding (for secondary gamma ray measurement equipment) 130 (Secondary gamma ray measuring device) sample container 131 (Sample container) sample wall 132 (sample container) sample storage space 140 (Secondary gamma ray detector) 150 Detector housing (for secondary gamma ray measurement device) 151 (detector housing) detector wall 152 (of detector housing space) detector housing space 152U Detector room (detector housing space) 152L (detector housing) shielded room 153 Intermediate plate (for secondary gamma ray measuring device) 160 Partial shielding (for secondary gamma ray measurement equipment) 170 Neutron absorber (for secondary gamma ray measurement devices) 180 Support stand (for secondary gamma ray measuring device) 181 (Secondary gamma ray measuring device) irradiation space

Claims

1. An apparatus for measuring the dose of secondary gamma rays generated by the reaction between neutrons and a measurement sample, neutron irradiation means for irradiating neutrons; an intermediate shield that blocks gamma rays; a sample container that contains the measurement sample; a detector for measuring the dose of secondary gamma rays; a detector housing that houses the detector, The sample container has a space surrounded by a sample wall that shields gamma rays, In the detector housing, a space surrounded by a detector wall that shields against gamma rays is formed, and a shielding chamber in which a partial shield that shields against gamma rays can be placed is also formed; When the neutron irradiation means irradiates neutrons with the partial shield disposed in the shielding chamber, the neutrons pass through the intermediate shield, the measurement sample contained in the sample container, and the partial shield, and the detector measures the dose of secondary gamma rays generated by a reaction between the neutrons and the detector; When the neutron irradiation means irradiates neutrons in a state in which the partial shield is not disposed in the shielding chamber, the neutrons pass through the intermediate shield and the measurement sample contained in the sample container, and the detector measures the dose of secondary gamma rays generated by a reaction between the neutrons and the measurement sample and the dose of secondary gamma rays generated by a reaction between the neutrons and the detector, The dose of secondary gamma rays generated by the reaction between neutrons and the measurement sample can be determined by subtracting the dose measured by the detector when the partial shield is placed in the shielded room from the dose measured by the detector when the partial shield is not placed in the shielded room. A secondary gamma ray measuring device characterized by:

2. the intermediate shield, the sample wall, the detector wall, and the partial shield are made of heavy metal; 2. The secondary gamma ray measuring device according to claim 1.

3. the neutron irradiation means, the intermediate shield, the sample holder, and the detector holder are arranged in this order from the bottom, an intermediate plate is disposed in the detector housing, and the shielding chamber is formed below the intermediate plate; The detector is mounted on the upper surface of the intermediate plate.

2. The secondary gamma ray measuring device according to claim 1.

4. Further provided is a neutron absorber that moderates and absorbs neutrons that do not enter the intermediate shield, The neutron absorber is disposed around the intermediate shield.

2. The secondary gamma ray measuring device according to claim 1.

5. A method for measuring the dose of secondary gamma rays generated by a reaction between neutrons and a measurement sample using a secondary gamma ray measurement device, comprising: the secondary gamma ray measurement device includes a neutron irradiation means for irradiating neutrons, an intermediate shield for shielding gamma rays, a sample container for accommodating the measurement sample, a detector for measuring the dose of secondary gamma rays, and a detector container for accommodating the detector; The sample container has a space surrounded by a sample wall that shields gamma rays, The detector housing has a shielding chamber formed therein, which is a space surrounded by a detector wall that shields against gamma rays, and in which a partial shield that shields against gamma rays can be placed, a partial dosimetry step of measuring, with the detector, the dose of secondary gamma rays generated by the reaction between the neutrons and the detector; a total dose measurement step of measuring, with the detector, the dose of secondary gamma rays generated by the reaction between neutrons and the measurement sample and the dose of secondary gamma rays generated by the reaction between neutrons and the detector; a target dose calculation step of calculating a dose of secondary gamma rays generated by a reaction between neutrons and the measurement sample by subtracting the dose measured in the partial dose measurement step from the dose measured in the total dose measurement step, In the partial dose measurement step, the neutron irradiation means irradiates neutrons with the partial shield placed in the shielded room, In the total dose measurement step, the neutron irradiation means irradiates neutrons in a state where the partial shield is not placed in the shielding room, In the partial dose measurement step, neutrons irradiated from the neutron irradiation means pass through the intermediate shield, pass through the measurement sample contained in the sample container, and pass through the partial shield, and secondary gamma rays generated by a reaction between the neutrons and the detector are measured by the detector; In the total dose measurement step, neutrons irradiated from the neutron irradiation means pass through the intermediate shield and the measurement sample contained in the sample container, and secondary gamma rays generated by a reaction between the neutrons and the measurement sample and secondary gamma rays generated by a reaction between the neutrons and the detector are measured by the detector. A method for measuring secondary gamma rays.

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