JP7788951B2 - Neutron and gamma ray collimators, radiography equipment - Google Patents

Description

Embodiments of the present invention relate to neutron and gamma ray collimators and radiography devices.

Collimators have traditionally been used to shield radiation generated by 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 of radiation.

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 nuclear reactors (below, X-rays and gamma rays differ in terms of their generation mechanism, 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.

In order to use the generated radiation as a beam at the irradiation site, it is necessary to extract the radiation and guide it through a moderator, reflector, diaphragm, and conduit (flight tube). In the case of RI (radioisotope) sources and nuclear reactors, particularly 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.

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 from reaching the irradiation site) may be installed. Both the diaphragm and flight tube are made of basic materials, and their ability to block radiation is important. These diaphragms and flight tubes are collectively referred to as a collimator.

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 indicator of the beam collimator. The larger the L/D value, the higher the performance of the collimator, and when used for radiography imaging, it has a significant impact on the image resolution. 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 decreases, and for an imaging system with the same sensitivity, the exposure time and imaging time will be longer.

The materials used 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. Materials that are translucent or allow even a small amount of light to pass through will also not function as an aperture. Furthermore, even if the aperture is able to block light, if the thickness of the aperture is thick, the light will be reflected and scattered by the inner walls of the aperture, reducing 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.

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

In the case of neutrons, although it depends on the energy, in the thermal neutron region of 0.025 eV, gadolinium (Gd, Z = 64, density = 7.90 g/cm ³ ) has the highest shielding effect (largest reaction cross section) among natural elements. Although Gd has a smaller atomic number Z than lead, it has a larger atomic number Z and a higher density than other elements with high neutron shielding effects, such as boron (B, Z = 5, density = 2.08 g/cm ³ ) and lithium (Li, Z = 3, density = 0.534 g/cm ³ ), and therefore has a higher gamma-ray shielding effect than boron or lithium. Therefore, gadolinium is superior to boron, lithium, lead, and tungsten as a material that can simultaneously shield thermal neutrons and gamma rays.

Gadolinium naturally exists 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 in an (n,γ) reaction, emitting gamma rays. This reaction produces internal conversion electrons and the resulting Gd-159 (half-life 18.479 hours), which emits beta rays (mainly 970.6 keV: 62% emission rate, 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%).

When gadolinium is used as an imaging system in neutron radiography, it is attached to a film or other material as a converter, and images are taken using the internal conversion electrons generated during neutron irradiation. When imaging is done in real time or for short periods of time, 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 nuclide Gd-159 produced during long-term irradiation.

Boron Neutron Capture Therapy (BNCT) is a medical application of neutrons and gamma-ray collimators. In recent years, a method has been developed for using an accelerator instead of a nuclear reactor as a neutron source for BNCT, and the high-speed neutrons generated are slowed down to the epithermal region for use.

BNCT is a method in which a boron compound is administered to a patient, and neutrons are irradiated onto the cancer cells where the compound has accumulated. 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. Shielding materials and collimators have been devised to ensure that the patient's affected area is appropriately irradiated, from the perspectives of efficiently and effectively performing radiation treatment in a short time, avoiding damage to healthy cells, and reducing radiation exposure (see Patent Document 1).

The characteristics of this collimator differ from the concept of collimators used in radiography. The basic principle of the collimator is the same: a through-hole is provided in the partition that blocks radiation. However, unlike radiography, which aims to remove as much scattered radiation as possible, this collimator is designed to focus neutrons that pass through the through-hole 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 being used.

The materials used for the neutron moderation irradiation device and extended collimator used in 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, while 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.

JP 2018-161449 A

Serial Lecture "Fundamentals and Applications of Neutron Imaging Technology (Basics Part 6)" by Hisao Kobayashi, RADIOISOTOPES, 56, 573-583 (2007)

The basic features of neutron and gamma ray collimators are that they are designed to shield 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 irradiating neutrons or gamma rays.

The structure of a collimator varies greatly depending on its intended use. For imaging applications such as radiography, it has a hood or cylindrical structure that blocks the camera aperture and scattered radiation, while for BNCT, it has a structure that focuses neutrons, including scattered radiation, in the same way that a lens focuses light.

Both have a collimator structure that shields all energy other than the neutron and gamma ray energy desired in the irradiation field (for BNCT, gamma rays in the irradiation field are also shielded as much as possible). Furthermore, the structure is required to be thin, small, and compact, allowing for shielding without using complex combinations of materials.

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 with the highest reaction rate (reaction cross section) with thermal neutrons. It reacts with neutrons in an (n, gamma) reaction, emitting gamma rays, resulting in the release of gamma rays and internal conversion electrons. When using equipment that is affected by gamma rays, electron beams, or beta rays within the irradiation field, eliminating these rays presents a challenge.

The object of the present invention is to provide a neutron and gamma ray collimator and radiography device that can guide thermal neutrons and gamma rays for radiography simultaneously or selectively into the irradiation field.

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 lined inside 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.

Embodiments of the present invention provide a neutron and gamma ray collimator and radiography device that can guide thermal neutrons and gamma rays for radiography simultaneously or selectively into an irradiation field.

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. 2A and 2B are enlarged views showing the main configuration of the neutron and gamma ray collimator of FIG. 1, in which FIG. 2A is a schematic view showing the configuration of a neutron diaphragm and FIG. 2B is a schematic view showing the configuration of a gamma ray diaphragm. 2 is an enlarged cross-sectional view showing the main part of the neutron and gamma ray collimator shown in FIG. 1; FIG. 2 is a diagram showing a schematic diagram of a neutron source installation configuration of the neutron and gamma ray collimator of FIG. 1; Graph showing the calculation results of the gamma ray transmission rate for a shielding material 1 cm thick. 10 is a graph showing calculation results of the thickness of a shield and the transmission rate for gamma rays with an effective energy of 100 keV. 10 is a graph showing test results of thermal neutron transmission rate versus step wedge thickness.

The neutron and gamma ray collimators and radiography devices according to the embodiments will be described below with reference to the drawings.

This embodiment relates to a neutron and gamma ray collimator that shields 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 radiations react with matter, and extracts a uniform beam of radiation, and to a radiography device using the same.

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 adequately shielded, except for the area (field of view) where radiation is extracted and irradiated.

Neutron and gamma ray collimators are used to guide and extract the radiation to the desired location. Collimators are designed differently depending on the intended use. When concentrating neutrons on the patient's affected area, such as in medical irradiation, the neutrons from the source are reflected as much as possible and collected at the opening at the collimator exit.

On the other hand, for imaging in non-destructive testing, a collimator is used in which a diaphragm and flight tube, which correspond to the aperture and shielding cylinder (shielding box) of an optical camera, narrow the radiation to cut it out, shield external radiation, 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.

Whether it's the collimator diaphragm or the flight tube, the basic requirement is that they be 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.

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 Z and the density of the material being irradiated. In the case of X-rays and gamma rays, if the density is about the same, elements with similar atomic numbers Z will have roughly the same transmittance (shielding effect). Furthermore, the amount of X-rays that penetrate varies at different energies at the absorption edge depending on the orbital 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 (poorer the shielding efficiency).

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 neutron energy and the isotope, resulting in different rates of neutron beam transmittance (shielding effect). The neutron cross section (unit: barn (b): 1b = 10 ^-24 cm ² ) is used as an index of this reaction rate to represent the interaction between neutrons and matter. The neutron cross section multiplied by the atomic number density ρ is called the macroscopic cross section Σ (cm ^-1 ), and corresponds to the linear attenuation coefficient of X-rays.

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, scattering is primarily due to diffraction, and Bragg edges appear depending on the neutron energy. Incoherent scattering occurs with 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; 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.

Generally, neutron beams emitted from a neutron source contain neutrons of various energies. At energies below 1 MeV, reactions occur in the resonance region, such as the 1/V law and resonance absorption (primarily reactions in which gamma rays are emitted in response to a neutron; hereafter, these reactions will be referred to as (n,γ) reactions). However, at energies above 1 MeV, threshold reactions different from these reactions occur (such as the (n,2n) reaction in which two neutrons are emitted in response to a single neutron, the (n,p) reaction in which a proton is emitted in response to a neutron, and the (n,α) reaction in which alpha rays are emitted in response to a neutron). In particular, unlike reactions with gamma rays, reactions with neutrons can change elements, such as boron becoming lithium or sulfur becoming phosphorus, and 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 will no longer be a radioactive substance almost instantly, so this is not a particular problem, but if it is irradiated at a high dose and becomes a substance 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.

The neutron reactant of boric acid (B ₂ O ₃ ) and boron carbide (B ₄ C), which are used as neutron shielding materials, is boron. Natural boron exists as the isotopes B-10 (natural abundance 19.9%: same as ¹⁰ B) and B-11 (natural abundance 80.1%: same as ¹¹ B). The nuclear data JENDL-4.0, which indicates the reaction rate with neutron energy (cross section: Cross Section unit barns), can be confirmed from each data research group of the Japan Atomic Energy Agency (https://wwwndc.jaea.go.jp/jendl/j40/J40_J.html#Reports). [References: 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)]

In the thermal neutron region, the cross section of B-10 is approximately six orders of magnitude larger than that of B-11. However, in the high-energy region above 1 MeV, B-10 and B-11 have roughly the same reaction cross section. The B-10 reaction reacts with a neutron, emitting an α-ray to form Li-7. More precisely, this is described as ^{the 10} B(n,α) ^7* Li reaction. ^7* Li is given an initial recoil energy of 840 keV in the (n,α) reaction, and then, moving with a short lifetime of 0.105 ps, emits a prompt γ-ray of 478 keV, becoming ground-state Li-7. Therefore, when using natural minerals, etc., from the perspective of radiation shielding, the cross section is determined by the product of the number of boron atoms present in the mineral, the proportion of B-10 isotopes that react frequently with neutrons, and the cross section, which depends on the neutron energy. Furthermore, since the cross section of neutrons decreases as their 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 neutrons, thereby enabling efficient reactions. Furthermore, since boron emits prompt gamma rays of 478 keV, shielding against these rays must also be considered. The density of boron-containing compounds is approximately 2.5 g/ ^cm³ , about one-third that of iron, and their atomic number Z is also small, at Z = 5. The 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 boron-containing compounds against gamma rays is low.

This embodiment focuses 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 at neutron energy 0.0253 eV. At energies between 100 eV and 1 keV, it has multiple resonance absorption peaks, including other Gd isotopes, and has a larger cross section than B-10.

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, ^7* Li is produced in the (n,α) reaction, and in the process of becoming Li-7, a prompt gamma ray of 478 keV is emitted. In the case of Gd, however, ^the reactions ¹⁵⁴ Gd(n,γ) ¹⁵⁵ Gd, ¹⁵⁵ Gd(n,γ) ¹⁵⁶ Gd, ¹⁵⁶ Gd(n,γ) ¹⁵⁷ Gd, and 157 Gd(n,γ) ¹⁵⁸ Gd produce stable isotopes, Gd-155, Gd-156, Gd-157, and Gd-158, respectively, and do not emit beta or gamma rays during decay. Gd-159, produced by the ¹⁵⁸ Gd(n,γ) ¹⁵⁹ Gd reaction, undergoes β decay to become the stable isotope Tb-159. This decay releases β-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 cross sections in the thermal neutron region, emit neutron capture gamma rays of 8 MeV in the (n,γ) reaction.

These gamma-ray emission modes can be 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 across a range of energies from unstable compound nuclei to the stable ground level, rather than a single energy of 8 MeV. (2) Discrete spectrum modes are 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 discrete spectrum modes are higher, but their proportion is low.

Here, what is important about radiation shielding technology for collimators is that their collimating ability differs depending on the type of radiation. Another factor is the sensitivity of the detector used to radiation. High-energy gamma rays that pass through materials with a high atomic number Z will 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 represents the amount of energy radiation imparts to a material as it passes through it) by the radiation in the material to be protected is low, little energy will be imparted to the material it passes through, and it will not be detected. Conversely, the higher the LET, the more energy is imparted to the material, and the greater the detection effect.

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

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 of Figure 5. As shown in the graph of 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, and although it is not as effective as lead, it still has a higher shielding effect than iron.

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.

Next, to specifically examine the thermal neutron transmission rate, step wedges of different thicknesses were created using GOS as a resin-molded shielding sample and concrete samples, and a comparative test was conducted with a heavy concrete step wedge of the same shape made with lead (Pb) and iron 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 to an imaging plate (IP), where 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.

The graph in Figure 7 shows the relative thermal neutron transmission rate versus step wedge thickness, calculated in the same way as for gamma ray transmission. Like heavy concrete made with Pb or iron 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 rate rapidly decays to 2/100 to 3/100 at a thickness of 5 mm, after which it tends to saturate in measurements. While the calculations show a linear decay, 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 base brightness effects in measurements. These results show that the same shielding ability is achieved at a thickness of 1/50 or less compared to heavy concrete. Compared to lead, which has high gamma ray absorption, the shielding ability is approximately 30 times greater. These results demonstrate that using GOS as a shielding material for collimators, including thermal neutrons, is effective in shielding against combined gamma rays.

However, there are concerns regarding gadolinium and thermal neutrons. The Gd-159 produced by the ¹⁵⁸ Gd(n,γ) ¹⁵⁹ Gd reaction undergoes β decay to form the stable isotope Tb-159. This decay releases β 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%). While a 5 mm thickness provides sufficient shielding for thermal neutron penetration, considering reflections within the collimator, it is necessary to shield β rays and low-energy gamma rays, which are more efficient for detectors.

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) (the inside of the flight tube 4 described below) is lined with a bismuth compound [for example, bismuth oxide (Bi ₂ O ₃ ), bismuth subgallate (C ₇ H ₅ BiO ₆ ), bismuth oxychloride (BiOCl), or bismuth subnitrate (Bi ₅ O(OH) ₉ (NO ₃ ) ₄ )] to a thickness of several millimeters to several centimeters. Bismuth (Bi) is the Bi-209 isotope, which has a natural abundance of 100%, and when it reacts with neutrons, Bi-210 is produced mainly in the (n,γ) reaction. Its half-life is 5.013 days, and it does not emit gamma rays, but it emits 1.162 MeV beta rays. Because the thermal neutron cross section of bismuth is relatively small, thermal neutrons from the inside pass through the bismuth lining and react with the surface of the GOS shielding material. The beta rays and low-energy gamma rays emitted as a result of the reaction are absorbed by the bismuth material, and few of them reach the inner surface of the collimator, allowing the detector to provide a sharp image with less fogging and noise.

The gadolinium compound is preferably a water-insoluble ceramic or sintered body having a density of 6.5 g/ ^cm or more. Examples of such gadolinium compounds include gadolinium oxide ( _Gd2O3 ) with a density of 7.4 g/ ^cm3 , gadolinium gallium garnet ( _Gd3Ga5O12 ) with _a density of 7.09 g/ ^cm3 , gadolinium oxysulfide ( _Gd2O2S ) with a density of 7.3 _g / ^cm3 , and gadolinium silicate ( _Gd2SiO5 ) with a density of 6.7 _g / _cm3 ^. Furthermore, phosphor materials can be used that use these compounds as base materials and incorporate praseodymium (Pr), terbium (Tb), europium (Eu), cerium (Ce) _, or the like as activators. These gadolinium compounds can be used as aggregates to mix with cement to form concrete shielding materials, or can be molded with resin instead of cement to form shielding walls.

As mentioned above, gadolinium emits internal conversion electrons and beta and gamma rays from the Gd-159 produced by its (n, gamma) reaction with neutrons. However, a few millimeters 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 makes it particularly applicable to the component construction of collimator diaphragms.

Collimator diaphragms can be made of 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, or a combination of these can be used. For gamma rays in particular, a thinner diaphragm is preferable, just like an optical aperture. A thicker diaphragm results in more 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 be able 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 similar on the exit side.

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

The configuration of a neutron and gamma ray collimator and radiography device according to an embodiment will be described below with reference to Figures 1 to 4. Note that the configuration described below is an example and does not limit the scope of the present invention in any way. In addition, identical elements in the description of the drawings will be given the same reference numerals, and duplicate explanations will be omitted where appropriate. In addition, in the drawings referenced in the following description, the size and thickness of each component is for the sake of convenience and does not necessarily represent the actual dimensions or proportions.

Figure 1 is a diagram showing a schematic configuration of a neutron and gamma ray collimator 1 according to an embodiment, with (a) being a side view, (b) being a rear view (view of the side view in Figure 1 from the left side), and (c) being a front view (view of the side view in Figure 1 from the right side). The neutron and gamma ray collimator 1 comprises 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.

As shown in Figure 2, the diaphragm 2 is composed of a neutron diaphragm 21, which acts as a neutron aperture, and a gamma ray diaphragm 22, which acts 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 positioned on the diaphragm fixing plate 3 side, as shown in Figure 3.

The size of the neutron diaphragm 21 and the gamma ray diaphragm 22 is, for example, the same, with a vertical dimension A1 and a horizontal dimension 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 diagram, the dimensions are shown using symbols to take into account the scale rather than specific dimensions.

The neutron diaphragm 21 and gamma ray diaphragm 22 are secured to the diaphragm fixing plate 3, for example, by screws at the four corners. The material selected for the gamma ray diaphragm 22 is a high-atomic-number-Z, high-density material with high gamma-ray shielding capability (e.g., tungsten, tungsten compounds, tungsten compounds such as heavy alloy, or the aforementioned bismuth compounds). While this depends on the energy of the gamma rays, these high-Z, high-density materials can shield up to relatively high-energy regions, improving the diaphragm's performance for gamma rays. It also allows for a thinner thickness t3. As with light, the smaller the aperture diameter D4, the better the resolution; however, the lower the gamma-ray dose, so the radiation source intensity and imaging time must be increased (longer).

In this embodiment, the neutron diaphragm 21 is made of 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 in Figure 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 in Figure 2(b)) is the diameter of the diaphragm (diameter of the through hole 21a) and L (L1 in Figure 1) is the length of the collimator; the larger this value, the higher the resolution.

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 when neutrons react with the gadolinium in the neutron diaphragm 21. This reduces the impact of low-energy gamma rays, which are particularly sensitive to detectors and imaging elements, and allows for the production of gamma ray images 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.

As shown in Figure 3, the diaphragm fixing plate 3 is an integral structure consisting 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 the neutron source 51 (shown in Figure 4) from the moderator (moderator) 52, the moderator around and behind the neutron source, and the shielding 53 with the neutron shielding plate (made of GOS shielding in this embodiment) 31 (thickness t2 shown in Figure 3), and shields gamma rays emitted when neutrons react with the GOS in the neutron shielding plate 31 with the gamma ray shielding plate 32 (thickness t1 shown in Figure 3).

The gamma ray shielding material that makes up the gamma ray shielding plate 32 is preferably a bismuth compound, which does not react easily with neutrons and does not emit gamma rays. However, other materials that can be used include tungsten, tungsten compounds, and the tungsten compound heavy alloy (a sintered compact composed primarily of tungsten with nickel and copper). Furthermore, the diaphragm fixing plate 3 is integral with the flight tube 4 and is designed 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 required for installation in the neutron source. Depending on the shape of the installation location, the plate may also have a circular or other external shape. Note that W1 and W2 shown in Figure 1(b) indicate the height and width of the flight tube 4, respectively.

Furthermore, while the flight tube 4 shown in Figure 1 is pyramidal, it may also be conical. For the purpose of shielding against neutrons and gamma rays from the moderator and shielding body 53 shown in Figure 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 Figure 3. The inner gamma ray shield (bismuth compound) 42 is provided to prevent gamma rays emitted when the outer neutron shield (GOS shielding) 41 reacts with neutrons from entering the collimator.

Figure 4 shows the schematic configuration of a radiography device in which a neutron and gamma ray collimator 1 is installed in the neutron source. The neutron source is a simplified, simulated representation of accelerator neutrons. An electron beam or proton beam passes through the pneumatic tube 54 and reacts with the target of 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. Similarly, in the case of an RI neutron source, there is no pneumatic tube 54, and the neutron source is an RI neutron source. In Figure 4, an object to be imaged (not shown) is placed at the right end of the neutron and gamma ray collimator 1.

The neutron and gamma ray collimator 1 in Figure 4 is installed with the neutron source 51 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 the 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 shifted above or below the center line in Figure 4, or tilted 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 you want to extract from the neutron and gamma ray collimator 1.

As described above, the neutron and gamma ray collimator of this embodiment makes it possible to irradiate neutron rays and gamma rays (including bremsstrahlung X-rays) 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 gamma ray-only collimators. Furthermore, thermal neutron imaging also has minimal noise components due to scattered rays, enabling the acquisition of clear images with minimal scattered ray fogging, making simultaneous neutron and gamma ray radiography possible in non-destructive testing. The collimator can also be selected for use as a thermal neutron-only collimator or a gamma ray-only collimator.

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 may be embodied in a variety of other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. These embodiments and their variations are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as set forth in the claims.

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 (7)

a neutron diaphragm;
a gamma ray diaphragm;
a diaphragm fixing plate that fixes the neutron diaphragm and the gamma ray diaphragm;
a flight tube integrated with the diaphragm fixing plate, the flight tube having a neutron shield and a gamma ray shield lined inside the neutron shield;
a gadolinium compound is used for the neutron diaphragm and the neutron shield of the flight tube;
A neutron and gamma ray collimator, characterized in that a bismuth compound is used for the gamma ray shielding of the flight tube. 2. The neutron and gamma ray collimator of claim 1,
A neutron and gamma ray collimator, characterized in that the gamma ray diaphragm is made of tungsten, a tungsten compound, or a bismuth compound. 3. A neutron and gamma ray collimator according to claim 1 or 2,
the gadolinium compound is a ceramic or sintered body that is insoluble in water and has a density of 6.5 g/cm or ^more ;
The gadolinium compound is a compound having as a base material any ^one of gadolinium oxide (Gd ₂ O ₃ ) having a density of 7.4 g/cm ³ , gadolinium gallium garnet (Gd ₃ Ga ₅ O ₁₂ ) having a density of 7.09 g/cm ³ , gadolinium oxysulfide (Gd ₂ O ₂ S) having a density of 7.3 g/cm 3 , and gadolinium silicate (Gd ₂ SiO ₅ ) having a density of 6.7 g/cm ³ .
A neutron and gamma ray collimator using a phosphor material containing at least one of praseodymium (Pr), terbium (Tb), europium (Eu) and cerium (Ce) as an activator. 3. A neutron and gamma ray collimator according to claim 1 or 2,
_A neutron and gamma _ray collimator, characterized in that the bismuth compound is any one of bismuth _oxide ( _Bi2O3 ), bismuth subgallate ( _C7H5BiO6 ), bismuth oxychloride (BiOCl), and bismuth subnitrate ( _Bi5O (OH) ₉ ( _NO3 ) ₄ ). 3. A neutron and gamma ray collimator according to claim 1 or 2,
A neutron and gamma ray collimator, characterized in that the diaphragm fixing plate comprises a neutron shielding plate and a gamma ray shielding plate. 6. A neutron and gamma ray collimator according to claim 5,
A neutron and gamma ray collimator, characterized in that a gadolinium compound is used for the neutron shielding plate. A radiography apparatus that obtains an image by irradiating an object with neutrons and gamma rays from a radiation source, characterized in that the neutrons and gamma rays from the radiation source are collimated by the neutron and gamma ray collimator described in claim 1 or 2.