Neutron and gamma ray collimators, radiography equipment

Abstract

To provide a neutron and gamma-ray collimator that enables radiography thermal neutron and γ ray to be guided simultaneously to an irradiation field of view, or any one of the thermal neutron and γ ray to be guided selectively thereto.SOLUTION: A neutron and gamma-ray collimator has: a neutron diaphragm; a gamma-ray diaphragm; a diaphragm clamping plate that clamps the neutron diaphragm and gamma-ray diaphragm; and a flight tube that is made integral with the diaphragm clamp plate, and has a neutron shield body and a gamma-ray shield body lined inside of the neutron shield body, in which the collimator is configured to use the neutron diaphragm, and a gadolinium compound for the neutron shield body of the flight tube, and use a bismuth compound for the gamma-ray shield body of the flight tube.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to neutron and gamma ray collimators, radiography devices. [Background technology]

[0002] Collimators have traditionally been used to shield radiation generated from nuclear reactors, accelerators, and radioisotope sources, particularly neutrons, gamma rays, and X-rays, as well as secondary radiation that is reflected or scattered when these rays react with matter, thereby collimating the beam.

[0003] Radiation, such as X-rays, gamma rays, and neutron rays, is extracted in the form of beams and used in industrial and medical fields. These radiation sources include spontaneously generated radiation sources and neutrons, gamma rays, and X-rays generated by nuclear reactions such as those in atomic reactors (below, X-rays and gamma rays differ in terms of the mechanism of generation, but since they share the same radiation quality, they will be referred to as gamma rays). These are emitted isotropically in a 360-degree sphere.

[0004] In order to use the radiation as a beam at the irradiation site, it is necessary to extract the generated radiation and guide it through a moderator, reflector, diaphragm, conduit (flight tube), etc. In the case of RI (radioisotope) sources and nuclear reactors, especially when the source volume is large, a diaphragm is used as an aperture to extract the radiation, similar to the aperture of a camera for light.

[0005] In order to prevent scattered radiation from around the radiation source or radiation cut off by the diaphragm from being reflected and reaching the irradiation site, a flight tube (mainly intended to prevent scattered and reflected radiation components from the radiation source at the irradiation site) may be installed. Both the diaphragm and the flight tube are made of basic materials, and their ability to block radiation is important. These diaphragms and flight tubes are collectively called a collimator.

[0006] The L / D value, which is the relationship between the diaphragm opening diameter (D) and the flight tube distance (L) from the diaphragm to the point where the beam is irradiated, is used as a performance index for a beam collimator. The larger this L / D value, the higher the performance of the collimator, and when used for radiography imaging, it has a significant impact on the resolution of the image. The larger the L / D value, the better the resolution, but for the same distance L, the smaller the D value (the more narrowed), so the amount of radiation that passes through the diaphragm is reduced, and for an imaging system with the same sensitivity, the exposure time and imaging time will be longer.

[0007] The materials for the diaphragm and flight tube must be able to block radiation, but the material and shape must be optimized depending on the type and energy of the radiation. For example, in the case of a visible light camera, the aperture, which corresponds to the diaphragm, is made of a thin, light-blocking material. Naturally, a transparent material will not function as an aperture. Similarly, a semi-transparent material or a material that allows even a small amount of light to pass through will not function as an aperture. Furthermore, even if the aperture is able to block light, if the thickness of the aperture is too thick, the light will be reflected and scattered by the inner wall of the aperture, degrading the aperture's performance. Therefore, a thin material with high light-blocking properties is suitable for the aperture. The same is true for the diaphragm. The generation of these beams for neutron imaging is described in detail in Non-Patent Document 1.

[0008] Gamma rays exhibit similar properties to light, but the amount of transmission varies in proportion to the fourth power of the atomic number Z of the aperture material, with lower density allowing for easier transmission. Therefore, lead (Pb, Z=82, density=11.34g / cm 3 ) and tungsten (W, Z = 74, density = 19.25 g / cm 3 However, lead and tungsten do not act as shielding materials against neutrons, so they are similar to transparent materials against light.

[0009] In the case of neutrons, it depends on the energy, but in the thermal neutron region of 0.025 eV, gadolinium (Gd, Z=64, density=7.90 g / cm 3) has the highest neutron shielding effect (large reaction cross section) among natural elements. Gd has a smaller atomic number Z than lead, but it has a higher neutron shielding effect than boron (B, Z=5, density=2.08g / cm). 3 ) and lithium (Li, Z=3, density=0.534g / cm 3 Gadolinium has a higher atomic number Z and a higher density than boron and lithium, making it a more effective shield against gamma rays. Therefore, gadolinium is superior to boron, lithium, lead, and tungsten as a material that simultaneously blocks thermal neutrons and gamma rays.

[0010] Gadolinium exists naturally as the isotopes Gd-152, 154, 155, 156, 157, 158, and 160. Gd-158, which has a natural abundance of 24.84%, reacts with thermal neutrons to undergo a (n,γ) reaction, emitting gamma rays. This reaction produces internal conversion electrons and Gd-159 (half-life 18.479 hours), which emits beta rays (mainly 970.6 keV: emission fraction 62%, 912.6 keV: 26%, 607.09 keV: 12%, 622.42 keV: 0.31%) and gamma rays (mainly 363.55 keV: 11.4%, 58 keV: 2.15%, 348.16 keV: 0.234%, 226.01 keV: 0.215%).

[0011] When gadolinium is used as an imaging system in neutron radiography, it is used as a converter, attached to a film or similar, and images are taken using the internal conversion electrons generated during neutron irradiation. In real-time or short-time imaging, the amount of beta and gamma rays produced by Gd-159, which has a long half-life compared to the imaging time, is small, so there is little impact. However, when used as a diaphragm or flight tube collimator, there is an impact from the produced nuclide Gd-159 with long-term irradiation.

[0012] Boron Neutron Capture Therapy (BNCT) is an example of the use of neutrons and gamma ray collimators in the medical field. In recent years, a method has been developed for the neutron source of BNCT that uses an accelerator instead of a nuclear reactor, and the fast neutrons generated are slowed down to the epithermal region before being used.

[0013] BNCT is a method in which a patient is administered a boron compound, and the areas that have accumulated in cancer cells are irradiated with neutrons. The boron isotope B-10 reacts with the neutrons to emit alpha rays, resulting in the (n,α) reaction that kills the cancer cells. The neutron beams used in treatment require high enough energy to penetrate deep into tissue, and are required to have high energy intensity in the epithermal neutron region and low energy intensity in the fast neutron and thermal neutron regions. From the perspectives of efficiently and effectively performing radiation treatment in a short time, avoiding damage to healthy cells, and reducing radiation exposure, shielding materials and collimators have been devised to ensure that the affected area of ​​the patient is appropriately irradiated (see Patent Document 1).

[0014] The characteristics of this collimator differ from the concept of a collimator used in radiography. The basic concept of a collimator is the same as that of a radiation-shielding partition wall with through-holes, but unlike radiography, which aims to remove as much scattered radiation as possible, this collimator is designed to focus neutrons passing through the through-holes and shape the irradiation field. In particular, the aim is to extract as many epithermal neutrons as possible for irradiation. Therefore, the design of the collimator needs to be optimized depending on the energy of the neutrons to be used.

[0015] The materials used for the neutron moderation irradiation device and extended collimator for BNCT are magnesium fluoride for the moderator, lead, graphite, iron, beryllium, bismuth, nickel, etc. for the reflector, boron-polyethylene, lithium fluoride-polyethylene, boron carbide, etc. for the absorber, and the same materials as the reflector are used for the collimator. However, although this collimator is designed to extract as much epithermal neutron energy as possible, it is desirable to shield as much as possible from thermal neutron energy and gamma rays. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-161449 [Non-patent literature]

[0017] [Non-Patent Document 1] Serial lecture "Fundamentals and Applications of Neutron Imaging Technology (Basics Part 6)" by Hisao Kobayashi, RADIOISOTOPES, 56, 573-583 (2007) Summary of the Invention [Problem to be solved by the invention]

[0018] The basic features of neutron and gamma ray collimators are that they have a structure that shields radiation from areas other than the field of view that is to be irradiated, and that they are able to shield other energy components as little as possible depending on the energy of the neutrons or gamma rays being irradiated.

[0019] The structure of the collimator varies greatly depending on the purpose of use. For imaging applications such as radiography, the structure is a hood or tube that blocks the aperture of a camera and scattered radiation, while for BNCT, the structure focuses neutrons, including scattered radiation, in the same way as focusing light with a lens.

[0020] Both collimator structures must shield all energy other than the neutron and gamma ray energy desired in the irradiation field (for BNCT, gamma rays in the irradiation field must also be shielded as much as possible).In addition, the structure must be thin, small, and compact, allowing for shielding without using complex combinations of materials.

[0021] However, when using a neutron source, the energy of the neutrons reacts with the collimator material, generating radiation of various energies such as gamma rays and beta rays. Gadolinium (Gd) is the naturally occurring element that has the largest reaction rate (reaction cross section) with thermal neutrons, and it reacts with neutrons to cause an (n,γ) reaction that emits gamma rays, resulting in the emission of gamma rays and internal conversion electrons. When using equipment that is affected by these gamma rays, electron beams, and beta rays in the irradiation field, there is a challenge of eliminating these rays.

[0022] An object of the present invention is to provide a neutron and gamma ray collimator and a radiography apparatus which can guide thermal neutrons and gamma rays for radiography simultaneously or selectively to an irradiation field. [Means for solving the problem]

[0023] One aspect of the present invention comprises a neutron diaphragm, a gamma ray diaphragm, a diaphragm fixing plate that fixes the neutron diaphragm and the gamma ray diaphragm, and a flight tube that is integrated with the diaphragm fixing plate and has a neutron shield and a gamma ray shield lining the inside of the neutron shield, wherein a gadolinium compound is used for the neutron diaphragm and the neutron shield of the flight tube, and a bismuth compound is used for the gamma ray shield of the flight tube. [Effects of the Invention]

[0024] According to the embodiments of the present invention, it is possible to provide a neutron and gamma ray collimator and a radiography device that can guide thermal neutrons and gamma rays for radiography simultaneously or selectively to an irradiation field. [Brief explanation of the drawings]

[0025] [Figure 1] 1A, 1B, and 1C are diagrams schematically illustrating the configuration of a neutron and gamma ray collimator according to an embodiment, in which FIG. 1A is a side view, FIG. 1B is a rear view, and FIG. 1C is a front view. [Figure 2] 2A and 2B are enlarged views showing the main components of the neutron and gamma-ray collimators of FIG. 1, where FIG. 2A is a schematic diagram showing the configuration of a neutron diaphragm and FIG. 2B is a schematic diagram showing the configuration of a gamma-ray diaphragm. [Figure 3] FIG. 2 is an enlarged cross-sectional view showing the main part of the neutron and gamma ray collimator shown in FIG. 1. [Figure 4] FIG. 2 is a diagram showing a schematic diagram of the neutron source installation configuration of the neutron and gamma ray collimator shown in FIG. 1; [Figure 5] A graph showing the calculation results of the gamma ray transmission rate for a 1 cm thick shielding material. [Figure 6] Graph showing the calculation results of the shielding thickness and transmission rate for gamma rays with an effective energy of 100 keV. [Figure 7] 10 is a graph showing test results of thermal neutron transmission rate versus step wedge thickness. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, neutron and gamma ray collimators and radiography devices according to embodiments will be described with reference to the drawings.

[0027] This embodiment relates to a neutron and gamma ray collimator that shields radiation generated from a nuclear reactor, accelerator, or radioisotope source, particularly neutrons, gamma rays, and X-rays, as well as secondary radiation that is reflected or scattered when these radiations react with a substance, and extracts a uniform beam of radiation, and a radiography device using the same.

[0028] Facilities to which the neutron and gamma ray collimator of this embodiment can be applied include, for example, nuclear reactors, accelerator facilities, and RI neutron source facilities that generate neutrons, as well as BNCT (neutron capture therapy) facilities in the medical field. In these facilities, radiation is extracted and used for non-destructive testing and medical irradiation treatment. In particular, the surrounding area must be sufficiently shielded except for the area (field of view) where radiation is extracted and irradiated.

[0029] Neutron and gamma ray collimators are used to guide and extract the radiation to the desired location. Collimators have different designs depending on the purpose of use. When concentrating neutrons on the affected area of ​​a patient, such as in medical irradiation, the neutrons from the source are reflected as much as possible and collected at the opening of the collimator outlet.

[0030] On the other hand, for imaging in non-destructive testing, a collimator is used in which a diaphragm and a flight tube, which correspond to an aperture and a shielding cylinder (shielding box) like those in an optical camera, narrow the radiation to cut it out, shield radiation from outside, and prevent the narrowed radiation from scattering on the inner surface of the flight tube. This embodiment will particularly explain neutron and gamma ray collimators aimed at the latter type of imaging in non-destructive testing.

[0031] The basic principle for both the collimator diaphragm and the flight tube is that they are made of a material that can block the type of radiation you want to collimate (in the case of light, this corresponds to being able to block light), and that the inner surface does not reflect or scatter these radiations.

[0032] X-rays and gamma rays have the property that they are less likely to penetrate depending on the atomic number Z of the material being irradiated, and the higher the Z, and the higher the density of the material being irradiated. In the case of X-rays and gamma rays, if the density is about the same, the transmittance (shielding effect) will be roughly the same for elements with similar atomic numbers Z. Also, the amount of penetration of X-rays varies at different energies of the absorption edge depending on the orbit of the electron shell, such as the K shell or L shell, of the atom, but there is no distinction between isotopes. Furthermore, the higher the energy of X-rays and gamma rays, the higher the transmittance (the worse the shielding efficiency).

[0033] On the other hand, the mode of interaction between neutrons and matter generally does not depend directly on the atomic number, but changes depending on the isotope of the atom. Therefore, even for the same element, the reaction and absorption rates change depending on the isotope and the neutron energy, and the rate of neutron transmission (shielding effect) differs. This reaction rate is expressed as an index of the interaction between neutrons and matter, using the neutron cross section (unit: barn (b): 1b = 10 -24 cm 2 ) is used. The neutron cross section multiplied by the atomic number density ρ is the macroscopic cross section Σ(cm -1 ) and corresponds to the linear attenuation coefficient of X-rays.

[0034] Neutron scattering has two main characteristics: coherent scattering and incoherent scattering. For many elements, coherent scattering is dominant. For elements with large coherent cross sections, diffraction is the main cause of scattering, and Bragg edges appear depending on the neutron energy. Incoherent scattering occurs due to atomic motion, and the cross section increases in inverse proportion to the neutron velocity V, which is equivalent to the neutron energy. This is known as the "1 / V law." For example, elements such as the boron isotope B-10 and the lithium isotope Li-6 correspond to the 1 / V law, and as the energy increases (the velocity V increases), the reaction becomes more difficult and the shielding efficiency decreases. In addition to these scattering cross sections, there is also an absorption cross section due to resonance capture, which appears in different neutron energy ranges depending on the element isotope.

[0035] Generally, neutron beams emitted from a neutron source contain neutrons of various energies. At energies below 1 MeV, reactions in the resonance region, such as the 1 / V law and resonance absorption, occur (primarily reactions in which gamma rays are emitted in reaction with a neutron; hereafter, these reactions are referred to as (n,γ) reactions). However, at energies above 1 MeV, threshold reactions occur that differ from these reactions (such as the (n,2n) reaction in which two neutrons are emitted in reaction with one neutron, the (n,p) reaction in which a proton is emitted in reaction with a neutron, and the (n,α) reaction in which alpha rays are emitted in reaction with a neutron). In particular, unlike reactions with gamma rays, reactions with neutrons can change elements, such as boron becoming lithium or sulfur becoming phosphorus, or elements that were not originally radioactive can become radioactive. The half-life indicates the time it takes for a substance to become radioactive and the rate at which it emits radiation is halved due to decay. If the half-life is in the millisecond or nanosecond range, it is not a particular problem because the material will no longer be radioactive almost instantly. However, if it is irradiated at a high dose and becomes a material with a long half-life, the shielding itself will become a source of radiation for a long period of time, and will no longer be able to fulfill its role as a shielding material.

[0036] The neutron reactant in boric acid (B2O3) and boron carbide (B4C), which are used as neutron shielding materials, is boron. Natural boron contains B-10 (natural abundance 19.9%: 10 B) and B-11 (natural abundance 80.1%:11 There are isotopes of 1000 uranium and 1000 uranium (same as those described in B). The nuclear data JENDL-4.0, which shows the reaction rate with neutron energy (cross section: barns), can be confirmed by each data research group at the Japan Atomic Energy Agency (https: / / wwwndc.jaea.go.jp / jendl / j40 / J40_J.html♯Reports). [Reference: K. Shibata, O. Iwamoto, T. Nakagawa, N. Iwamoto, A. Ichihara, S. Kunieda, S. Chiba, K. Furutaka, N. Otuka, T. Ohsawa, T. Murata, H. Matsunobu, A. Zukeran, S. Kamada, and J. Katakura: "JENDL-4.0: A New Library for Nuclear Science and Engineering," J. Nucl. Sci. Technol. 48(1), 1-30 (2011)]

[0037] In the thermal neutron region, B-10 has a larger cross section by about six orders of magnitude than B-11. However, in the high energy region above 1 MeV, B-10 and B-11 have almost the same reaction cross section. B-10 reacts with neutrons to emit alpha rays and become Li-7. More precisely, 10 B(n,α) 7* It is described as a Li reaction, 7* Li is given an initial recoil energy of 840 keV by the (n,α) reaction, and then, moving with a short lifespan of 0.105 ps, emits a 478 keV prompt gamma ray and becomes ground-state Li-7. Therefore, when using natural minerals, etc. from the perspective of radiation shielding, it is determined by the product of the number of boron atoms present in the mineral, the proportion of the B-10 isotope, which has a high reaction rate with neutrons, and the cross section, which depends on the neutron energy. Furthermore, since the cross section decreases as the neutron energy increases, it is important to reduce the energy of high-energy neutrons by using moderators such as hydrogen or carbon to increase the number of collisions with the neutrons, thereby allowing for efficient reaction. Furthermore, since it emits a 478 keV prompt gamma ray, shielding against this gamma ray must also be considered. The density of boron-containing compounds is approximately 2.5 g / cm. 3Its mass attenuation coefficient μ / ρ (μ: linear absorption coefficient, ρ: density of material), which is important in interactions with X-rays and gamma rays, is generally said to be proportional to the third or fourth power of Z. Therefore, the shielding effect of compounds containing boron against gamma rays is low.

[0038] In this embodiment, we focus on gadolinium (Gd), which has the largest reaction cross section for thermal neutrons among all elements, and its atomic number Z = 64, which also enhances its shielding effect against gamma rays. Gadolinium is a rare earth element and a rare metal. There are six naturally occurring isotopes: Gd-154 (2.18%), Gd-155 (14.80%), Gd-156 (20.47%), Gd-157 (15.65%), Gd-158 (24.84%), and Gd-160 (21.86%) (the percentages in parentheses indicate the natural abundance). In particular, Gd-157 and Gd-155 have cross sections 66 times and 15.8 times larger than B-10 in the thermal neutron region with a neutron energy of 0.0253 eV. At energies between 100 eV and 1 keV, it has multiple resonance absorption peaks, including other Gd isotopes, and its cross section is larger than that of B-10.

[0039] Unlike boron, the main reaction of gadolinium is the (n,γ) reaction, which reacts with neutrons and emits gamma rays. In the case of boron, it is the (n,α) reaction. 7* Li is produced and emits a prompt gamma ray of 478 keV in the process of becoming Li-7. In the case of Gd, 154 Gd(n,γ) 155 Gd, 155 Gd(n,γ) 156 Gd, 156 Gd(n,γ) 157 Gd, 157 Gd(n,γ) 158 The isotopes produced by Gd are Gd-155, Gd-156, Gd-157, and Gd-158, respectively, and do not emit beta or gamma rays during decay. 158 Gd(n,γ) 159Gd-159, produced by the Gd reaction, undergoes beta decay to become the stable isotope Tb-159. This decay releases beta rays with energies of 970.6 keV (62%), 912.6 keV (26%), 607.09 keV (12%), and 622.42 keV (0.31%), and gamma rays with energies of 363.55 keV (11.4%), 58 keV (2.15%), 348.16 keV (0.234%), and 226.01 keV (0.215%). Gd-157 and Gd-155, which have particularly large reaction cross sections in the thermal neutron region, emit neutron capture gamma rays of 8 MeV in the (n, gamma) reaction.

[0040] The gamma-ray emission modes are broadly divided into two: (1) continuous spectrum (93.8%) and (2) discrete spectrum (6.2%). Most are (1) continuous spectrum, which means that the emission proportion increases from high to low energies, from unstable compound nuclei to the stable ground level, rather than a single energy of 8 MeV. (2) discrete spectrum includes 5.62 MeV + 2.25 MeV (1.3%), 5.88 MeV + 1.99 MeV (1.6%), 6.74 MeV + 1.11 MeV (3.2%), and 7.87 MeV (0.02%). The energies of the discrete spectrum are higher, but their proportion is low.

[0041] What is important about radiation shielding technology for collimators is that their collimating ability differs depending on the type of radiation. Another important factor is the sensitivity of the detector to radiation. High-energy gamma rays that pass through materials with a high atomic number Z pass through thin-film detector elements made of light elements in the same way as shielding materials. In other words, if the linear energy transfer (LET: Linear Energy Transfer, which indicates how much energy radiation transfers to a material as it passes through it) by the radiation in the material to be protected is low, little energy is transferred to the material it passes through, and it will not be detected. Conversely, the higher the LET, the more energy is transferred to the material, and the greater the detection effect.

[0042] Depending on the degree, considering the shielding purpose of such collimator materials, even if high-energy gamma rays pass through the gamma-ray shielding material, the shielding purpose can be achieved. For example, even though 1 MeV and 50 keV gamma rays are monochromatic (strictly speaking, they have a range of energies) at the time of generation, their energy is attenuated by the photoelectric effect and Compton scattering due to the interaction between the gamma rays and the material, and they spread to the lower energy range. In particular, in the low-energy range, 50 keV gamma rays react more with and are attenuated by gamma-ray shielding materials than 1 MeV gamma rays. Furthermore, the lower the energy of gamma rays below several tens of keV, the greater the LET on sensors and other devices, resulting in imaging fogging and reduced image sharpness.

[0043] The gamma-ray transmission rate of the collimator shielding material at a thickness of 1 cm was calculated. In particular, lithium fluoride (LiF) and boron carbide (B4C), which are used as thermal neutron shielding materials, lead (Pb) and iron (Fe), which have high gamma-ray absorption, and gadolinium oxysulfide (Gd2O2S) (hereinafter referred to as GOS), which is used in this embodiment, were compared. The results are shown in the graph in Figure 5. As shown in the graph in Figure 5, at 0.5 MeV (500 keV) or less, GOS has approximately twice the shielding effect of lithium fluoride and boron carbide, which are neutron shielding materials. Although not as effective as lead, GOS still has a higher shielding effect than iron.

[0044] Taking into account the sensitivity characteristics of the film and imaging plates actually used in imaging, we compared the shielding thickness and gamma ray transmission rate for an effective energy of 100 keV. The results are shown in the graph in Figure 6. As shown in the graph in Figure 6, it can be seen that GOS has a higher shielding effect than iron.

[0045] Next, to examine the thermal neutron transmission rate, a resin-molded sample of GOS for use as a shielding material and a concrete sample were used to create step wedges of different thicknesses, and a comparative test was conducted with a step wedge of the same shape made of heavy concrete with lead (Pb) and iron as aggregate. The test utilized the thermal neutron irradiation port of Hokkaido University's accelerator-driven pulsed neutron source "HUNS." Dysprosium foil was used as a converter to activate the step wedge at the transmission rate, and the activated dysprosium foil was transferred onto an imaging plate (IP), and the transmission rate was digitally calculated from the brightness data (PSL value) of the image obtained. This transfer method is not affected by gamma rays, so it is possible to determine the transmission rate purely due to thermal neutrons.

[0046] Figure 7 shows the relative thermal neutron transmission rate versus step wedge thickness, similar to the gamma-ray transmission rate. As with heavy concrete containing lead or iron as aggregate, the transmission rate is exponential and decays linearly with thickness, as shown in Figure 7. However, in the case of GOS, whether resin or concrete, the transmission rate rapidly decays to 2 / 100–3 / 100 at a thickness of 5 mm, after which it tends to saturate in measurements. While the calculated rate decays linearly, similar to lead and heavy concrete, due to the large thermal neutron absorption cross section of gadolinium, it is believed that the IP also exhibits a base brightness effect in measurements. These results indicate that the same shielding capacity is achieved at a thickness of 1 / 50 or less compared to heavy concrete. Compared to lead, which has a high gamma-ray absorption rate, the shielding capacity is approximately 30 times higher. These results demonstrate that using GOS as a shielding material for collimators, including thermal neutrons, is effective in shielding against combined gamma rays.

[0047] However, there are concerns about gadolinium and thermal neutrons. 158 Gd(n,γ) 159The Gd-159 produced by the Gd reaction undergoes beta decay to become the stable isotope Tb-159. This decay releases beta rays with energies of 970.6 keV (62%), 912.6 keV (26%), 607.09 keV (12%), and 622.42 keV (0.31%), and gamma rays with energies of 363.55 keV (11.4%), 58 keV (2.15%), 348.16 keV (0.234%), and 226.01 keV (0.215%). A thickness of 5 mm provides sufficient shielding for thermal neutron penetration, but when considering reflections within the collimator, it is necessary to shield beta rays and low-energy gamma rays, for which the detector is highly efficient.

[0048] Therefore, focusing on bismuth, which does not emit gamma rays in particular when reacting with neutrons, the inside of the GOS shielding material (GOS concrete or GOS resin) (inside the flight tube 4 described below) is lined with a bismuth compound [e.g., bismuth oxide (Bi2O3), bismuth subgallate (C7H5BiO6), bismuth oxychloride (BiOCl), bismuth subnitrate (Bi5O(OH)9(NO3)4)] to a thickness of several millimeters to several centimeters. Bismuth (Bi) is the Bi-209 isotope, which is 100% abundant in nature, and when reacting with neutrons, Bi-210 is produced primarily in the (n,γ) reaction. Its half-life is 5.013 days, and it does not emit gamma rays, but does emit 1.162 MeV beta rays. Because the thermal neutron cross section of bismuth is relatively small, thermal neutrons from the inside penetrate the bismuth lining and react with the surface layer of the GOS shielding material. The beta rays and low-energy gamma rays emitted in the reaction are absorbed by the bismuth material, reducing their chances of reaching the inner surface of the collimator, enabling the detector to provide images with good sharpness with less fogging and noise.

[0049] Gadolinium compounds have a density of 6.5 g / cm 3 It is preferable to use a ceramic or sintered body that is insoluble in water. Such a gadolinium compound has a density of 7.4 g / cm. 3 Gadolinium oxide (Gd2O3), density 7.09 g / cm 3 Gadolinium gallium garnet (Gd3Ga5O 12 ), density 7.3g / cm 3Gadolinium oxysulfide (Gd2O2S), density 6.7 g / cm 3 Examples include gadolinium silicate (Gd2SiO5). Furthermore, phosphor materials can be used that use these as base materials and mix them with activators such as praseodymium (Pr), terbium (Tb), europium (Eu), and cerium (Ce). These gadolinium compounds can be used as aggregates to mix with cement to form concrete shielding materials, or they can be molded with resin instead of cement to form shielding walls.

[0050] As mentioned above, gadolinium emits internal conversion electrons and beta and gamma rays from the Gd-159 produced by the (n,γ) reaction with neutrons. However, a few millimeters of gadolinium provides shielding against thermal neutrons by more than two orders of magnitude, and the internal conversion electrons and beta rays generated within these few millimeters are also shielded by a few millimeters. Furthermore, if a gadolinium compound is present in a few centimeters, the gadolinium itself will self-shield against gamma rays. This is particularly applicable to the construction of collimator diaphragms.

[0051] For the collimator diaphragm, GOS shielding material for thermal neutrons and gamma-ray shielding material such as tungsten, tungsten compounds, or tungsten compounds such as heavy alloys or bismuth compounds can be used, and these can be used in combination. For gamma rays in particular, a thin diaphragm is preferable, just like an optical aperture. A thicker diaphragm results in gamma rays being reflected by the inner surface of the hole, resulting in more scattered rays. Therefore, a material with a high atomic number Z and high density (specific gravity) is preferable. Furthermore, to shield gamma rays generated by reaction with neutrons, it is preferable to install a diaphragm made of GOS shielding material on the radiation source side and a diaphragm made of a tungsten compound or the like on the exit side.

[0052] As described above, the neutron and gamma ray collimator of this embodiment uses a gadolinium compound, for example, a GOS shielding material, for the neutron diaphragm, and uses a shielding material such as tungsten or a tungsten compound or a bismuth compound for the gamma ray diaphragm, and the flight tube has a structure in which the inner surface of the GOS shielding material is lined with a bismuth compound, thereby shielding beta rays and low-energy gamma rays from Gd-159 generated by the reaction of thermal neutrons with gadolinium, and providing images of thermal neutrons and gamma rays with a good S / N ratio.

[0053] The configurations of neutron and gamma ray collimators and radiography devices according to embodiments will be described below with reference to Figures 1 to 4. Note that the configurations described below are examples and do not limit the scope of the present invention in any way. In addition, in the description of the drawings, identical elements are given the same reference numerals, and duplicated descriptions will be omitted as appropriate. In addition, in the drawings referred to in the following description, the size and thickness of each component are for the sake of convenience and do not necessarily represent the actual dimensions or ratios.

[0054] 1 is a diagram schematically showing the configuration of a neutron and gamma ray collimator 1 according to an embodiment, where (a) is a side view, (b) is a rear view (the side view in FIG. 1 seen from the left side), and (c) is a front view (the side view in FIG. 1 seen from the right side). The neutron and gamma ray collimator 1 includes a diaphragm 2, a diaphragm fixing plate 3 for fixing the diaphragm 2, and a flight tube 4 formed integrally with the diaphragm fixing plate 3.

[0055] As shown in Fig. 2, the diaphragm 2 is composed of a neutron diaphragm 21 which serves as a neutron aperture, and a gamma ray diaphragm 22 which serves as a gamma ray aperture. A through hole 21a is provided in the center of the neutron diaphragm 21, and a through hole 22a is provided in the center of the gamma ray diaphragm 22. The diameter D4 of the through hole 22a is smaller than the diameter D5 of the through hole 21a. Furthermore, the neutron diaphragm 21 and the gamma ray diaphragm 22 are attached to the diaphragm fixing plate 3 so that the gamma ray diaphragm 22 is located on the diaphragm fixing plate 3 side, as shown in Fig. 3.

[0056] The size of the neutron diaphragm 21 and the gamma ray diaphragm 22 is, for example, the same, with a length A1 and a width A2. A1 and A2 may be the same. The dimensions of the neutron and gamma ray collimator 1 vary depending on the neutron source to be installed. Therefore, in the drawings, the dimensions are shown by symbols taking into account the scale rather than specific dimensions.

[0057] The neutron diaphragm 21 and the gamma ray diaphragm 22 are secured to the diaphragm fixing plate 3 by, for example, screws at the four corners. The gamma ray diaphragm 22 is made of a material with a high atomic number Z and high density (e.g., tungsten, tungsten compounds, tungsten compounds such as heavy alloy, or the aforementioned bismuth compounds) that provides high gamma ray shielding. While this depends on the gamma ray energy, these high-Z, high-density materials can shield up to relatively high energy levels, improving the gamma ray diaphragm's performance. Furthermore, the thickness t3 can be reduced. The aperture diameter D4, which is the diameter of the aperture, improves resolution, just as with light. However, the gamma ray dose also decreases, so the intensity of the radiation source and the imaging time must be increased (longer).

[0058] In this embodiment, the neutron diaphragm 21 is made of a GOS shielding material (GOS plate, GOS resin, or GOS concrete). Gadolinium in GOS has a high shielding effect against thermal neutrons even when it is thin. Note that t4 shown in FIG. 3 is the thickness of the neutron diaphragm 21. The index representing the resolution of medium thermal neutrons is expressed as L / D=L1 / D5, where D (D5 shown in FIG. 2(b)) is the diameter of the diaphragm (diameter of the through hole 21a) and L (L1 shown in FIG. 1) is the length L of the collimator; the larger this value, the higher the resolution.

[0059] As mentioned above, the diameter D5 of the neutron diaphragm 21 is set larger than the diameter D4 of the gamma ray diaphragm 22. By setting the diameters D4 and D5 in this way, the gamma ray diaphragm 22 can block gamma rays generated by the reaction of neutrons with the gadolinium in the neutron diaphragm 21, thereby reducing the influence of low-energy gamma rays, which are particularly sensitive to detectors and imaging elements, and obtaining a gamma ray image with a good S / N ratio. On the other hand, neutrons are not affected by the aperture of the gamma ray diaphragm 22 because the gamma ray diaphragm 22 is thin and has a small cross-sectional area.

[0060] As shown in Fig. 3, the diaphragm fixing plate 3 is configured as an integral structure of a neutron shielding plate 31 and a gamma ray shielding plate 32. The diaphragm fixing plate 3 not only fixes the neutron diaphragm 21 and the gamma ray diaphragm 22, but also shields thermal neutrons emitted from a neutron source 51 shown in Fig. 4 through a moderator (moderator) 52, the moderator around and behind the neutron source, and a shielding body 53 with a neutron shielding plate (made of a GOS shielding material in this embodiment) 31 (thickness t2 shown in Fig. 3), and shields gamma rays emitted when neutrons react with the GOS of this neutron shielding plate 31 with a gamma ray shielding plate 32 (thickness t1 shown in Fig. 3).

[0061] The gamma-ray shielding material constituting the gamma-ray shielding plate 32 is preferably a bismuth compound, which is resistant to neutrons and does not emit gamma rays. However, other materials such as tungsten, tungsten compounds, and heavy alloys (a sintered body of tungsten containing mainly tungsten, nickel, and copper) can also be used. Furthermore, the diaphragm fixing plate 3 is integrally structured with the flight tube 4 to support the collimator. Therefore, the dimensions of the diaphragm fixing plate 3 are set to a height A3 and width A4 shown in Figure 1(b) taking into account the size of the diaphragm fixing plate 3 when installed in the neutron source. Depending on the shape of the installation location, a circular or other shape may also be used. Note that W1 and W2 in Figure 1(b) represent the height and width of the flight tube 4, respectively.

[0062] Furthermore, although the flight tube 4 shown in Fig. 1 is a quadrangular pyramid, the flight tube 4 may also be conical. For the purpose of shielding against neutrons and gamma rays from the moderator and shielding body 53 shown in Fig. 4, this flight tube 4 is integrally configured with a neutron shield (GOS shielding in this embodiment) 41 with a thickness of ((D3-D2) / 2) on the outside and a gamma ray shield (bismuth compound in this embodiment) 42 with a thickness of ((D2-D1) / 2) on the inside, as shown in Fig. 3. The inner gamma ray shielding (bismuth compound) 42 is provided to prevent gamma rays emitted when the outer neutron shielding (GOS shielding) 41 reacts with neutrons from entering the collimator.

[0063] FIG. 4 is a diagram showing the schematic configuration of a radiography apparatus in which a neutron and gamma-ray collimator 1 is installed at the neutron source, and is a simplified diagram assuming that the neutron source is an accelerator neutron. An electron beam or proton beam passes through a pneumatic tube 54 and reacts with a target in the neutron source 51, emitting neutrons, gamma rays, or bremsstrahlung X-rays. If the neutron source is a nuclear reactor, there is no pneumatic tube 54, and the neutron source is a fuel rod. Also, in the case of an RI neutron source, there is no pneumatic tube 54, as in a nuclear reactor, and the neutron source is an RI neutron source. In FIG. 4, an object to be imaged (not shown) is placed at the right end of the neutron and gamma-ray collimator 1.

[0064] The installation of the neutron and gamma ray collimator 1 in Figure 4 is configured so that the neutron source 51 is located at the center of the collimator. In other words, when looking at the tip of the collimator from the collimator outlet, the neutron source 51 is located on an extension line. In this case, high-energy neutrons and high-energy gamma rays from the neutron source 51 are emitted from the neutron and gamma ray collimator 1. To prevent this and extract only low-energy neutrons, the entire neutron and gamma ray collimator 1 can be installed by shifting it above or below the center line in Figure 4, or tilting it diagonally or vertically so that the neutron source 51 is not visible from the collimator outlet. In other words, the installation configuration can be changed depending on the energy of the neutrons and gamma rays that are to be extracted from the neutron and gamma ray collimator 1.

[0065] As described above, the neutron and gamma ray collimator of this embodiment makes it possible to irradiate neutron rays and gamma rays (including bremsstrahlung) emitted from a neutron source with aligned beams and minimal scattered rays. In particular, the inclusion of a thermal neutron component minimizes the influence of gamma rays emitted in reaction with thermal neutrons, enabling gamma ray imaging with minimal noise, which was a problem with a gamma ray-only collimator. Furthermore, imaging with thermal neutrons also has minimal noise due to scattered rays, making it possible to obtain clear images with minimal scattered ray fogging, enabling simultaneous neutron and gamma ray radiography in non-destructive testing. The collimator can also be selectively used as a thermal neutron-only collimator or a gamma ray-only collimator.

[0066] 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 novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0067] 1...Neutron and gamma ray collimator, 2...Diaphragm, 3...Diaphragm fixing plate, 4...Flight tube, 21...Neutron diaphragm, 21a...Through hole, 22...Gamma ray diaphragm, 22a...Through hole, 31...Neutron shielding plate, 32...Gamma ray shielding plate, 41...Neutron shielding body, 42...Gamma ray shielding body, 51...Neutron source, 52...Moderator, 53...Moderator and shielding body, 54...Pneumatic tube.

Claims

1. a neutron diaphragm; a gamma ray diaphragm; a diaphragm fixing plate that fixes the neutron diaphragm and the gamma ray diaphragm; a flight tube that is integrated with the diaphragm fixing plate and has a neutron shield and a gamma ray shield that is lined inside the neutron shield; and a gadolinium compound is used for the neutron diaphragm and the neutron shield of the flight tube; The gamma ray shield of the flight tube uses a bismuth compound.

1. A neutron and gamma ray collimator comprising:

2. 2. The neutron and gamma ray collimator of claim 1, The gamma ray diaphragm is made of tungsten, a tungsten compound, or a bismuth compound.

1. A neutron and gamma ray collimator comprising:

3. 3. A neutron and gamma ray collimator according to claim 1 or 2, The gadolinium compound has a density of 6.5 g / cm ３ These are ceramics or sintered bodies that do not dissolve in water. The gadolinium compound has a density of 7.4 g / cm ３ Gadolinium oxide (Gd ２ O ３ ), density 7.09g / cm ３ Gadolinium gallium garnet (Gd ３ Ga ５ O １２ ), density 7.3g / cm ３ Gadolinium oxysulfide (Gd ２ O ２ S), density 6.7g / cm ３ Gadolinium silicate (Gd ２ SiO ５ ) as the base material, A phosphor material containing at least one of praseodymium (Pr), terbium (Tb), europium (Eu), and cerium (Ce) as an activator is used.

1. A neutron and gamma ray collimator comprising:

4. 3. A neutron and gamma ray collimator according to claim 1 or 2, The bismuth compound contains bismuth oxide (Bi ２ O ３ ), bismuth subgallate (C ７ H ５ Bio ６ ), bismuth oxide chloride (BiOCl), bismuth subnitrate (Bi ５ O(OH) ９ (NO ３ ) ４ ) 1. A neutron and gamma ray collimator comprising:

5. 3. A neutron and gamma ray collimator according to claim 1 or 2, The diaphragm fixing plate includes a neutron shielding plate and a gamma ray shielding plate.

1. A neutron and gamma ray collimator comprising:

6. 6. A neutron and gamma ray collimator according to claim 5, The neutron shielding plate uses a gadolinium compound.

1. A neutron and gamma ray collimator comprising:

7. 3. A radiography apparatus for obtaining an image by irradiating an object with neutrons and gamma rays from a radiation source, wherein the neutrons and gamma rays from the radiation source are collimated by a neutron and gamma ray collimator according to claim 1 or 2.

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