Neutron guide tube and neutron experimental equipment

Abstract

To provide a technique which enables efficient transportation of a neutron beam at low cost without requiring high installation precision when assembling a device.SOLUTION: A hollow neutron guide tube 1 for transporting a neutron beam includes a plurality of magnets 12 with alternating N-poles and S-poles arranged along a longitudinal direction on at least a portion of an inner wall 10 thereof.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to neutron guides and neutron experimental equipment. [Background technology]

[0002] A neutron guide tube using specular reflection is known as a device for transporting a neutron beam (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] “Neutron Transport”, Kakuyoshi Tamura, Japan Neutron Science Society “Ripple” Vol.28, No.4, 2018 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, in the technology described in Non-Patent Document 1, a neutron guide is formed by assembling flat reflecting mirrors into a tubular shape and connecting and installing them. Because total reflection is used in this process, mirror alignment errors accumulate, reducing transport efficiency. This requires assembly precision and the prevention of alignment and axial misalignment between neutron guides during installation. Furthermore, precision machining and polishing of the inner surface of the guide is required to suppress non-specular reflection caused by phase disturbance of neutron waves during reflection. Thus, with conventional technology, it was difficult to achieve highly efficient neutron beam transport, and the overall equipment was expensive.

[0005] The present disclosure has been made in light of these circumstances, and its purpose is to provide a technology that can transport neutron beams efficiently and at low cost without requiring high installation precision in assembling the device. [Means for solving the problem]

[0006] In order to solve the above problems, a neutron guide according to one embodiment of the present disclosure is a hollow neutron guide for transporting a neutron beam, having an inner wall and a hollow portion, and at least a portion of the inner wall contains a plurality of magnets with alternating north and south poles arranged along the longitudinal direction.

[0007] According to this embodiment, a neutron guide tube that does not require high installation precision in assembling the device and can transport a neutron beam with high efficiency at low cost can be realized.

[0008] Another aspect of the present disclosure is a neutron experimental apparatus, which includes a neutron source that generates a neutron beam, a measuring device that measures the neutron beam, and the above-mentioned neutron guide tube connected at one end to the neutron source and at the other end to the measuring device.

[0009] According to this embodiment, a neutron experiment device can be realized that does not require high installation precision when assembling the device and that can transport a neutron beam highly efficiently at low cost.

[0010] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure. [Effects of the Invention]

[0011] According to the present disclosure, a neutron beam can be transported highly efficiently at low cost without requiring high installation precision in assembling the device. [Brief explanation of the drawings]

[0012] [Figure 1] This is a schematic diagram of a neutron research experimental device seen from above. [Figure 2] 1 is a graph showing ε∞ as a function of Γ. [Figure 3] FIG. 1 is a schematic diagram showing neutrons in a magnetic field when multiple magnets are arranged on a substrate with alternating north and south poles. [Figure 4]FIG. 2 is a schematic diagram showing the magnetic field lines when magnets are arranged so that their north and south poles alternate. [Figure 5] 5 is a graph showing the magnetic field distribution in the magnet arrangement of FIG. 4. [Figure 6] FIG. 1 is a schematic diagram showing a neutron guide tube according to a first embodiment. [Figure 7] FIG. 10 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of a neutron guide tube according to a third embodiment. [Figure 8] FIG. 10 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of a neutron guide tube according to a fourth embodiment. [Figure 9] FIG. 10 is a schematic diagram showing a cross section perpendicular to the longitudinal direction of a neutron guide tube according to a fifth embodiment. [Figure 10] FIG. 13 is a schematic diagram showing the arrangement of magnets in a neutron guide tube according to a sixth embodiment. [Figure 11] FIG. 11 is a schematic diagram showing the state of magnetic field lines when magnets are arranged as shown in FIG. 10. [Figure 12] 11 is a graph showing the magnetic field distribution when magnets are arranged as shown in FIG. 10. DETAILED DESCRIPTION OF THE INVENTION

[0013] Before describing specific embodiments, we will first describe the basic findings. Neutrons have no electric charge and therefore have a high ability to penetrate matter. A neutron beam generated by such neutrons is a quantum beam that can extract information about bulk materials or entire industrial products. Furthermore, because neutrons have masses on the same order as atoms, they are highly sensitive to dynamic information within a sample. Furthermore, because neutrons have a magnetic dipole moment, they can also extract magnetic information within a sample. By utilizing these properties, slow neutron beams are being used in a wide range of fields, from fundamental physics and materials research to industrial applications. For these reasons, there is a desire to expand opportunities for using neutrons.

[0014] In Japan, in addition to large-scale neutron facilities such as J-PARC and JRR3, there has been an increase in small and medium-sized neutron sources and plans to build a new research reactor at the Monju site, and other initiatives to meet the expanding demand for neutrons are gaining momentum. In particular, dramatic advances in fundamental technologies that can achieve sufficient utilization characteristics even with limited neutron production will fundamentally improve the current situation. Noteworthy fundamental technologies are diverse, including neutron moderation systems, neutron beam transport, neutron detection, and data analysis methods, but the following explanation focuses on slow neutron beam transport, which is thought to have the greatest effect.

[0015] Slow neutrons are neutrons in the MeV range generated by nuclear reactions, and are then converted into thermal neutrons and cold neutrons (10 -3 e~10 -2 Due to the low phase space density, slow neutron sources are extended sources, and therefore efficient beam transport optics are essential.

[0016] Neutron beam transport in a research neutron experimental device will be explained using Figure 1. Figure 1 is a schematic top view of a neutron experimental device 100 consisting of a neutron source 30 including a nuclear reactor 40, a neutron guide tube 1, and a measurement device 50. Neutrons are generated by nuclear reactions at the neutron source located in the reactor core. The generated neutrons are repeatedly scattered in the moderator to become low-energy neutrons. These low-energy neutrons are guided as a beam outside the shielding and used as a neutron beam. The neutron beam is transported a distance of several tens of meters using the neutron guide tube before reaching the measurement device. Various experiments and measurements are performed in the measurement device using this neutron beam.

[0017] The neutron guide tubes conventionally used for this purpose are devices that transport neutron beams by total reflection on the inner wall of a hollow tube. Generally, neutrons experience a work function in a material, which is the nuclear potential averaged over the atomic volume. This is called the Fermi potential, and its value is approximately 0.25 μeV. When the neutron energy in the normal direction at the surface of the material becomes less than the Fermi potential, the neutron undergoes specular reflection, known as total reflection. The normal velocity of neutrons that undergo total reflection is approximately 7 m / s. The velocity of neutrons used in measurements is 500 m / s to 1000 m / s, so the maximum expected angle of incidence for total reflection (critical angle of reflection) is φ c The angle is small, about 0 mrad.

[0018] In the example shown in Figure 1, multiple straight neutron guides are connected and bent by about 10 mrad in the shielding area. This allows the neutron beam to be transported a total distance of about 40 m. The diameter of a typical guide is about 2 cm. In this case, when transported with a beam divergence of 10 mrad, the neutron beam is reflected about 20 times horizontally on average. Therefore, if the reflection rate per reflection is R, the overall transport efficiency is R 20 That is, R 20 To achieve ≥ 50%, R ≥ 97% is required.

[0019] Neutron guides are actually installed by assembling flat reflecting mirrors into a tubular shape and connecting them. Because total reflection is used in this process, mirror alignment errors accumulate, reducing transport efficiency. This requires assembly precision and the prevention of alignment and axial misalignment between neutron guides during installation. Furthermore, precision machining and polishing of the inner surface of the guide is required to suppress non-specular reflections caused by phase disturbances in the neutron wave during reflection. As a result, conventional neutron guides have difficulty achieving highly efficient neutron beam transport and have the problem of high equipment cost. For example, manufacturing a typical neutron guide based on flat reflecting mirrors like the one described above and ensuring sufficient alignment requires several million yen per meter.

[0020] In response to these issues, the inventors conducted extensive research and discovered that by forming a gradient magnetic field within the neutron guide tube, it is possible to transport a neutron beam efficiently and at low cost without requiring high installation precision in assembling the device. The principle behind this is explained below.

[0021] Neutrons have a spin of 1 / 2. Although neutrons have no electric charge, they have an anomalous magnetic dipole moment due to their internal structure. The direction of the magnetic dipole moment of a neutron is antiparallel to its spin.

[0022] The equation of motion of a neutron moving through a magnetic field B is expressed as follows:

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[0023] When a magnetic field of a certain magnitude rotates at a certain angular velocity, the polarization of neutrons, p n,0 In the infinite future, the polarization p n,∞ Figure 2 shows the ε ∞ =p n,∞ / p n,0 is calculated as a function of Γ. From this, we can understand the rough relationship between the value of Γ and the rate at which the neutron polarization follows the magnetic field. Since neutrons actually move in a non-uniform magnetic field, the angular velocity of rotation of the magnetic field ω along the neutron trajectory is B and Larmor precession angular velocity ω L Therefore, even if the neutron passes through a region where the Γ value is small, if the time it takes to pass through that region is short enough, it may be possible to consider that the neutron polarization follows the magnetic field.

[0024] If the value of Γ is large enough along the neutron orbit, the neutron spin can be considered to be fixed parallel or antiparallel to the magnetic field. In this case, the equations of motion (1) and (2) become

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[0025] Neutron spin σ n The interaction potential U between the magnetic field B and the magnetic field B is U=|μ n |σ n ·B, where the nuclear magneton is μ N Then, μ n ~-1.91μ N When a neutron moves through a non-uniform magnetic field B, the magnetic field seen by the neutron changes. As long as this change in the magnetic field is not extremely large, the spin follows the change in the magnetic field direction. In this case, the direction of the spin relative to the local magnetic field is preserved, and the neutron is subjected to a force proportional to the gradient of the magnetic field magnitude. As a result, neutrons whose spin is parallel to the magnetic field (positive polarity neutrons) are repelled, and neutrons whose spin is anti-parallel to the magnetic field (negative polarity neutrons) are attracted.

[0026] Consider a configuration in which multiple magnets are arranged on a substrate with alternating north and south poles, and a neutron absorbing film is placed on the opposite side of the substrate from these magnets, as shown in Figure 3. Suppose an unpolarized neutron beam is incident on this configuration. Initially, substantially all negatively polarized neutrons are attracted to the magnets and absorbed by the neutron absorbing film in front of the magnets. Conversely, substantially all positively polarized neutrons are reflected by the magnets. Thereafter, as long as spin transport is adiabatic, the positively polarized neutron beam is reflected with a "reflection rate" of 100%. The inventors realized that by utilizing this phenomenon, a positively polarized neutron beam can be transported without loss.

[0027] Figure 4 shows the magnetic field lines when 1cm high magnets are arranged so that their north and south poles alternate. Figure 5 shows the magnetic field distribution on a path 1mm from the magnet surface with a half period of 1.5cm for the magnet arrangement in Figure 4.

[0028] [First embodiment] 6 shows a schematic diagram of a neutron guide tube 1 according to the first embodiment. The hollow neutron guide tube 1 includes an inner wall 10 and a hollow portion 20 through which a neutron beam passes. At least a portion of the inner wall 10 includes a plurality of magnets 12 whose north and south poles are alternately arranged along the longitudinal direction of the neutron guide tube 1.

[0029] An unpolarized neutron beam is injected into the hollow portion 20 from one end of the neutron guide tube 1 toward the other end. As described above, substantially all of the negative polarity neutrons in the unpolarized neutron beam are initially attracted to the magnet 12. Conversely, substantially all of the positive polarity neutrons are reflected by the magnet 12. In this way, the positive polarity neutron beam is repeatedly reflected by the magnet 12 and transported to the other end of the neutron guide tube 1 with substantially no loss. This can be said to be neutron beam transportation by a magnetic transport optical system.

[0030] The magnet 12 may be, for example, a neodymium magnet. The surface residual magnetic flux density of a neodymium magnet is approximately 1 T, and its potential is approximately ±60 neV for positive and negative neutrons. This is only about one-quarter of the work function of nickel, for example, and at first glance, it may seem impossible to use. However, since the reflectivity is 100% and there is no limit to the number of reflections, the low potential can be offset by transporting the neutrons through multiple reflections. A disadvantage of using multiple reflections is the need to narrow the aperture. However, in practice, the aperture and beam divergence of conventional neutron guides are not always fully utilized. In small-angle neutron scattering and neutron reflectometry, the neutron beam transported through the neutron guide is extracted by a collimator system and input to a measurement device. As a result, the emittance of the magnetic transport optical system according to this embodiment is often sufficient for the measurement device's acceptance.

[0031] There is no particular limit to the spacing between adjacent magnets 12, but it has been found that the shorter the spacing, the better the neutron transport efficiency. In a preferred embodiment, adjacent magnets are closely spaced.

[0032] In this embodiment, the "reflection" phenomenon using a magnetic gradient is the deflection of a neutron beam by a gradually changing potential. This has the distinctive feature that, as long as the system is aligned on average, trajectory disturbances due to local potential disturbances do not accumulate. Furthermore, even if there is a mutual axial misalignment between multiple guide tubes, the magnetic field near their boundaries is continuously connected. Therefore, compared to conventional neutron guide tubes, the installation precision can be significantly reduced. As such, according to this embodiment, high installation precision is not required in assembling the device, and neutron beams can be transported highly efficiently at low cost.

[0033] [Second embodiment] In one embodiment, the neutron guide 1 may be provided with a neutron absorbing film radially inward of the inner wall 10. According to this embodiment, the negative polarity neutrons initially attracted to the magnet 12 can be absorbed by the neutron absorbing film.

[0034] [Third embodiment] In one embodiment, the magnet 12 may be closed along the circumferential direction of the inner wall 10 in a cross section perpendicular to the longitudinal direction of the neutron guide 1 in the portion where the magnet 12 is present. In other words, when viewed in the cross section where the magnet 12 is present, the magnet 12 completely surrounds the inner wall 10. Figure 7 shows a cross section perpendicular to the longitudinal direction of the neutron guide 1 according to this embodiment. In this cross section, the magnet 12 is arranged with its north pole on the inner side in the radial direction of the neutron guide 1 and its south pole on the outer side. When viewed in this cross section, the magnet 12 completely surrounds the inner wall 10 of the neutron guide 1. The shape of the cross section is arbitrary.

[0035] According to this embodiment, the magnet 12 completely surrounds the inner wall 10, thereby enabling more efficient neutron beam transport.

[0036] [Fourth embodiment] In one embodiment, the magnet 12 may form a regular polygon along the circumferential direction of the inner wall 10 in a cross section perpendicular to the longitudinal direction of the neutron guide tube 1 in the portion where the magnet 12 is present. FIG. 8 shows a cross section perpendicular to the longitudinal direction of the neutron guide tube 1 according to this embodiment. In this cross section, the magnet 12 is arranged with its north pole on the inner side in the radial direction of the neutron guide tube 1 and its south pole on the outer side. When viewed in this cross section, the magnet 12 completely surrounds the inner wall 10 of the neutron guide tube 1. The cross section has a square shape. However, the cross section is not limited to a square and may have any regular polygon shape, such as an equilateral triangle, a regular pentagon, or a regular hexagon.

[0037] According to this embodiment, the magnet 12 has rotational symmetry in a cross section perpendicular to the longitudinal direction of the neutron guide tube 1, so that the neutron beam can be transported with higher efficiency.

[0038] [Fifth embodiment] In one embodiment, in the portion where the magnet 12 is present, the magnet 12 may have a circular shape along the circumferential direction of the inner wall 10 in a cross section perpendicular to the longitudinal direction of the neutron guide tube 1. Figure 9 shows a cross section perpendicular to the longitudinal direction of the neutron guide tube 1 according to this embodiment. In this cross section, the magnet 12 is arranged with its north pole on the inner side in the radial direction of the neutron guide tube 1 and its south pole on the outer side. When viewed in this cross section, the magnet 12 completely surrounds the inner wall 10 of the neutron guide tube 1. The cross section has a circular shape.

[0039] According to this embodiment, the magnet 12 is completely isotropic in a cross section perpendicular to the longitudinal direction of the neutron guide tube 1, so that the neutron beam can be transported with even higher efficiency.

[0040] [Sixth embodiment] In one embodiment, each of the magnets 12 constituting the inner wall 10 is a rectangular parallelepiped with north and south poles arranged along the longitudinal direction. These magnets 12 are installed adjacent to each other while being rotated by 90 degrees around the longitudinal axis of the magnet 12, thereby constituting the inner wall 10. Figure 10 shows a schematic diagram of the arrangement of the magnets 12 in the neutron guide tube 1 according to this embodiment. In this case, the north and south poles of the magnets 12 are arranged alternately along the longitudinal direction of the neutron guide tube 1, but return to their original arrangement after four 1 / 4 rotations.

[0041] Figure 11 shows the state of the magnetic field lines when the magnet 12 is placed as shown in Figure 10. Figure 12 shows the magnetic field distribution at this time.

[0042] According to this embodiment, the neutron beam can be transported with higher efficiency, particularly when the magnet 12 is made of a magnet having a relative permeability close to 1, such as a neodymium magnet.

[0043] [Seventh embodiment] In one embodiment, the neutron guide 1 may include a solenoid coil for forming an additional magnetic field along the longitudinal direction inside the hollow portion 20. The spin of the neutron beam may reverse when it passes through an area where no magnetic field exists. Furthermore, even in areas inside the neutron guide 1 where there is no magnetic field due to the magnet 12, an irregular magnetic field due to the earth's magnetic field or the like exists. In such cases, the spin of the neutron beam becomes unstable. However, by providing an additional magnetic field along the longitudinal direction inside the hollow portion 20, the spin of the neutron beam can be kept stable.

[0044] According to this embodiment, the loss of neutrons can be prevented by keeping the spin of the neutron beam transported within the neutron guide tube 1 stable, and therefore the neutron beam can be transported with higher efficiency.

[0045] [Eighth embodiment] FIG. 1 shows a schematic diagram of a neutron experimental apparatus according to an eighth embodiment. This neutron experimental apparatus includes a neutron source 30 that generates a neutron beam, a measuring device 50 that measures the neutron beam, and any of the above-described neutron guide tubes connected at one end to the neutron source 30 and at the other end to the measuring device. While FIG. 1 shows an example in which the neutron source includes a nuclear reactor, the present invention is not limited to this configuration; the neutron source may also include, for example, an accelerator. The measuring device measures various physical properties of the neutron beam after interaction with the sample (e.g., the wavelength of the neutrons and the scattering intensity of the neutrons depending on the scattering angle). This allows obtaining information about the motion of atoms and molecules within the sample and the crystalline structure of the sample. Such measuring devices are not limited to those used for neutron experiments in the narrow sense, such as academic research, but also cover a wide range of applications and industrial uses, such as elastic scattering, inelastic scattering, neutron radiography, prompt gamma-ray analysis, and neutron capture medicine.

[0046] According to this embodiment, a neutron experiment device can be realized that does not require high installation precision in assembling the device and that can transport a neutron beam with high efficiency at low cost.

[0047] [Each aspect of the present disclosure] A neutron guide according to one embodiment of the present disclosure is a hollow neutron guide for transporting a neutron beam, having an inner wall and a hollow portion, wherein at least a portion of the inner wall contains a plurality of magnets with alternating north and south poles arranged along the longitudinal direction.

[0048] According to this aspect, a neutron guide tube can be realized that does not require high installation precision in assembling the device and that can transport a neutron beam with high efficiency at low cost.

[0049] In one embodiment, the neutron conduit includes a neutron absorbing film radially inward from the inner wall.

[0050] According to this aspect, the negative polarity neutrons that are initially attracted to the magnet can be absorbed by the neutron absorbing film.

[0051] In one embodiment, the neutron guide is closed in a cross section perpendicular to the longitudinal direction in the portion where the magnet is present, with the magnet being arranged along the circumferential direction of the inner wall.

[0052] According to this embodiment, the magnet completely surrounds the inner wall, so that the neutron beam can be transported with higher efficiency.

[0053] In one embodiment, in a section where the magnet is present, the magnet forms a regular polygon along the circumferential direction of the inner wall in a cross section perpendicular to the longitudinal direction of the neutron guide tube.

[0054] According to this aspect, the magnet has rotational symmetry in a cross section perpendicular to the longitudinal direction of the neutron guide tube, so that the neutron beam can be transported with higher efficiency.

[0055] In one embodiment, in a section where the magnet is present, the magnet forms a circle along the circumferential direction of the inner wall in a cross section perpendicular to the longitudinal direction of the neutron guide tube.

[0056] According to this aspect, the magnet is completely isotropic in a cross section perpendicular to the longitudinal direction of the neutron guide tube, so that the neutron beam can be transported with even higher efficiency.

[0057] In one aspect, the neutron guide includes a solenoid coil for generating an additional longitudinal magnetic field within the hollow portion.

[0058] According to this aspect, the loss of neutrons can be prevented by keeping the spin of the neutrons transported in the neutron guide stable, and therefore the neutron beam can be transported with high efficiency.

[0059] A neutron experimental apparatus according to an embodiment of the present disclosure includes a neutron source that generates a neutron beam, a measuring device that measures the neutron beam, and any of the neutron guide tubes described above, which are connected at one end to the neutron source and at the other end to the measuring device. In this case, a plurality of neutron guide tubes and measuring devices may be provided for the neutron source.

[0060] According to this aspect, a neutron experimental device can be realized that does not require high installation precision when assembling the device and that can transport a neutron beam with high efficiency at low cost.

[0061] In one aspect, the neutron source comprises a nuclear reactor.

[0062] According to this aspect, a neutron experiment device that generates neutrons using a nuclear reactor can be realized.

[0063] The present disclosure has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and the respective treatment processes, and that such modifications are also within the scope of the present disclosure.

[0064] For example, in the seventh embodiment, the solenoid coil for forming the additional magnetic field may be disposed inside the hollow portion 20 of the neutron guide tube 1, or may be disposed outside the neutron guide tube 1. Such modifications can increase the degree of freedom in the configuration.

[0065] The above describes the embodiments and modifications. When understanding the abstract technical ideas of the embodiments and modifications, the technical ideas should not be interpreted as being limited to the contents of the embodiments and modifications. The above-described embodiments and modifications are merely illustrative examples, and many design modifications, such as changes, additions, and deletions of components, are possible. In the embodiments, the contents in which such design modifications are possible are emphasized by adding the notation "embodiment." However, design modifications are also permitted even in contents not so notated. [Explanation of symbols]

[0066] 1··Neutron guide tube, 10··Inner wall, 12··Magnet, 20··Hollow section, 30··Neutron source, 40··Reactor, 50··Measurement device, 100··Neutron experimental equipment.

Claims

1. A hollow neutron guide tube for transporting a neutron beam, the hollow guide tube having an inner wall and a hollow portion, A neutron guide tube, wherein at least a portion of the inner wall includes a plurality of magnets arranged with alternating north and south poles along the length thereof.

2. 2. The neutron guide tube according to claim 1, further comprising a neutron absorbing film disposed radially inward of the inner wall.

3. 3. The neutron guide tube according to claim 1, wherein the magnet is closed along the circumferential direction of the inner wall in a cross section perpendicular to the longitudinal direction in the portion where the magnet is present.

4. 4. The neutron guide tube according to claim 3, wherein the magnets form a regular polygon along the circumferential direction of the inner wall in a cross section perpendicular to the longitudinal direction in the portion where the magnets are present.

5. 4. The neutron guide tube according to claim 3, wherein the magnet has a circular shape along the circumferential direction of the inner wall in a cross section perpendicular to the longitudinal direction in the portion where the magnet is present.

6. 6. The neutron guide tube according to claim 1, further comprising a solenoid coil for forming an additional magnetic field along the longitudinal direction inside the hollow portion.

7. 7. A neutron experimental device comprising: a neutron source for generating a neutron beam; a measuring device for measuring the neutron beam; and a neutron guide tube according to claim 1, which is connected at one end to the neutron source and at the other end to the measuring device.

8. 8. The neutron experimental apparatus according to claim 7, wherein the neutron source comprises a nuclear reactor.

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