Plastic scintillating optical fiber and method of manufacturing the same

Abstract

A plastic scintillation optical fiber capable of detecting a neutron beam and excellent in productivity. A plastic scintillation optical fiber of one embodiment of the present invention includes: an outermost peripheral layer (1) containing a plastic material containing an organic compound containing an element having a larger neutron reaction cross section than hydrogen and emitting scintillation light by irradiation of a neutron beam; a high-refractive-index core (2) provided inside the outermost peripheral layer (1) and containing at least one phosphor that absorbs scintillation light and wavelength-converts to a long wavelength; and a cladding layer (3) that covers the outer peripheral surface of the core (2) and has a lower refractive index than the core (2). A wavelength conversion optical fiber containing the core (2) and the cladding layer (3) is integrated with the outermost peripheral layer (1) that covers the outer peripheral surface of the wavelength conversion optical fiber.

Description

TECHNICAL FIELD

[0001] The present application relates to a plastic scintillating optical fiber and a manufacturing method thereof. BACKGROUND

[0002] A conventional plastic scintillating optical fiber (PSF) is a plastic optical fiber in which a cladding having a lower refractive index than a core is covered on the outer periphery of the core as a scintillator, and is mainly used for radiation detection. Generally, the core is composed of a high molecular material obtained by adding an organic fluorescent material to a base material having an aromatic ring such as polystyrene, polyvinyltoluene, or the like. The cladding is composed of a low refractive index high molecular material such as polymethyl methacrylate, fluorine-containing polymethyl methacrylate, or the like.

[0003] The principle of radiation detection using a plastic scintillating optical fiber will be described. The core base material of the scintillating optical fiber has an aromatic ring, and has a characteristic that, when radiation irradiated thereon passes through the scintillating optical fiber, part of the energy is absorbed by the secondary particles in the core through re-emission or the like, and is emitted as ultraviolet rays. If no fluorescent material is added to the core base material, the ultraviolet rays are self-absorbed by the core base material itself, and do not propagate but disappear in the core.

[0004] In the plastic scintillating optical fiber, the ultraviolet rays are absorbed by the fluorescent material added to the core base material, and are re-emitted as light having a longer wavelength. Therefore, by selecting an appropriate fluorescent material, light having a long wavelength such as blue light is converted and propagated in the optical fiber. The light propagated in the optical fiber is detected in a detector connected to one end or both ends.

[0005] In this way, the scintillating optical fiber has both functions of light emission based on radiation detection and light propagation, and is used for purposes such as calculation of the passing position and amount of radiation. For such a scintillating optical fiber, it is important how to efficiently convert the ultraviolet light emitted from the core to a long wavelength and propagate it over a long distance.

[0006] On the other hand, a plastic wavelength shifting fiber (WLSF) is generally used together with the scintillating optical fiber. The wavelength shifting fiber is used in combination with, for example, a plastic scintillator that emits blue light. A groove or a hole is provided in a plate-like or rod-like plastic scintillator, and a wavelength shifting fiber that absorbs blue light and shifts it to green light is embedded therein.

[0007] For a large and large-area detector, there is a case where it is difficult to propagate light from each scintillator to an external photodetector (e.g., a photomultiplier tube or the like) far away due to light attenuation and spatial constraints. In this case, it is preferable to use a thin, easily bendable, and long-distance propagatable wavelength shifting fiber. A plurality of wavelength shifting fibers can be freely arranged up to the external photodetector.

[0008] The core of the wavelength conversion optical fiber is made of polystyrene resin, polymethyl methacrylate resin, and has wavelength conversion phosphor dissolved therein. The wavelength conversion optical fiber absorbs scintillation light incident from an external scintillator by the phosphor in the core and efficiently performs wavelength conversion while transmitting in the optical fiber. As the scintillator combined with the wavelength conversion optical fiber, not limited to a plastic scintillator, an inorganic scintillator highly sensitive to neutrons or the like can be used.

[0009] Thus, the wavelength conversion optical fiber can easily collect scintillation light emitted from a scintillator of a large area or a long size, or a special scintillator for neutron detection or the like. In addition, with the wavelength conversion optical fiber, light subjected to wavelength conversion in the core can be transmitted and freely connected to a photodetector.

[0010] Here, detection of a neutron beam is difficult to perform with low sensitivity in a plastic scintillator composed only of plastic, and thus, for example, an inorganic scintillator is used. As the inorganic scintillator, there are known inorganic scintillators disclosed in Patent Literature 1, LiF / ZnS:Ag, LiI:Eu 2+ , LiBaF3:Ce 3+ , LiCaAlF6:Ce 3+ , Li2B4O7:Cu + , and the like.

[0011] However, the inorganic scintillator has an attenuation length of several millimeters and is not highly transparent, and thus cannot transmit emitted light (i.e., scintillation light) over a long distance. In addition, since there is a constraint on the crystal size, it is difficult to transmit light to a photodetector with the inorganic scintillator.

[0012] In addition, as disclosed in Patent Literatures 2, 3, and the like, a sheet in which fine particles obtained by crushing an inorganic scintillator are dispersed in a transparent resin is developed for detection of neutrons. In such a sheet, the refractive index difference between the inorganic scintillator and the transparent resin is large, and transparency cannot be ensured, and thus light cannot be efficiently transmitted to a photodetector by the sheet itself.

[0013] Therefore, in Patent Literatures 2, 3, 4, and the like, a wavelength conversion optical fiber is made to follow the end face or the surface of a scintillator, and light is transmitted to a photodetector through the wavelength conversion optical fiber. By using the wavelength conversion optical fiber, detection light can be transmitted over a longer distance.

[0014] Here, in the detection that values spatial resolution, such as image detection, disclosed in Patent Literatures 2, 3, 4, and particularly Patent Literature 3, post-processing in which a scintillator is combined with a wavelength conversion optical fiber is required in many cases.

[0015] Prior Art Documents

[0016] Patent Literatures

[0017] Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-126854

[0018] Patent Literature 2: International Publication No. 2015 / 064588

[0019] Patent Literature 3: Japanese Patent Application Laid-Open No. 2011-141239

[0020] Patent Literature 4: Japanese Patent Application Laid-Open No. 2015-72227 SUMMARY

[0021] PROBLEMS TO BE SOLVED BY THE INVENTION

[0022] For the existing plastic scintillation fiber, high transparency is required for the core in order to make the core itself scintillate and emit light to a photodetector. Therefore, by making the core contain a material that emits scintillation light by irradiation of a neutron beam, a plastic scintillation fiber for detecting a neutron beam cannot be obtained.

[0023] Likewise, for the wavelength conversion fiber, high transparency is required for the core in order to perform wavelength conversion of scintillation light irradiated from an external scintillator in the core and perform light transmission. Therefore, by making the core contain a material that emits scintillation light by irradiation of a neutron beam, a plastic scintillation fiber for detecting a neutron beam cannot be obtained.

[0024] On the other hand, for the existing scintillation detector using the wavelength conversion fiber, post-processing of combining the scintillator with the wavelength conversion fiber is required. Further, in the case of performing image detection, the scintillators need to be separated one by one and combined with each of a plurality of wavelength conversion fibers, and the processing becomes obviously difficult.

[0025] The present application was accomplished in view of such circumstances, and an object thereof is to provide a plastic scintillation fiber capable of detecting a neutron beam, which is excellent in productivity.

[0026] METHOD FOR SOLVING PROBLEM

[0027] The plastic scintillation fiber of one embodiment of the present application includes an outermost peripheral layer containing a plastic material containing an organic compound including an element having a larger neutron reaction cross section than hydrogen and emitting scintillation light by irradiation of a neutron beam; a core having a high refractive index and containing at least one phosphor that absorbs the scintillation light and wavelength-converts to a long wavelength, which is provided inside the outermost peripheral layer; and a cladding layer that covers a peripheral surface of the core and has a lower refractive index than the core, a wavelength conversion fiber containing the core and the cladding layer is formed integrally with the outermost peripheral layer that covers the peripheral surface of the wavelength conversion fiber.

[0028] In the plastic scintillation fiber of the present application, the outermost peripheral layer contains a plastic material containing an organic compound containing an element having a neutron reaction cross-section larger than that of hydrogen and emitting scintillation light by irradiation of a neutron beam. Therefore, the sensitivity to neutrons is improved compared to a conventional plastic scintillation fiber.

[0029] When neutrons are irradiated, elements such as lithium 6, boron 10, gadolinium, and the like, which have a neutron reaction cross-section larger than that of hydrogen, emit radiation much more than elements such as carbon, oxygen, hydrogen, and the like, which are normally contained in a plastic scintillator. In the plastic scintillation fiber of the present application, such radiation is generated in the plastic material constituting the outermost peripheral layer, and scintillation light is generated by the radiation. By this mechanism, a neutron beam can be detected with high sensitivity. In addition, the outermost peripheral layer does not need the high transparency necessary for transmitting light as in a core, and therefore the above-described organic compound can be added at a high concentration.

[0030] In addition, in the plastic scintillation fiber of one embodiment of the present application, scintillation light emitted from the outermost peripheral layer is absorbed by the core inside, wavelength-converted, and transmitted as light. Therefore, a neutron beam, which is difficult to detect with low sensitivity in the case of a conventional plastic scintillation fiber, can be detected. In addition, since the wavelength conversion fiber containing the core and the cladding is formed integrally with the outermost peripheral layer covering the outer peripheral surface of the wavelength conversion fiber, the post-processing of combining a scintillator with a wavelength conversion fiber, which has been necessary in the past, is not needed. That is, a plastic scintillation fiber capable of detecting a neutron beam and excellent in productivity can be provided.

[0031] The above-described outermost peripheral layer can contain at least one phosphor that absorbs the above-described scintillation light and wavelength-converts it to long-wavelength light.

[0032] The above-described organic compound can contain lithium 6.

[0033] Alternatively, the above-described organic compound can contain boron 10. Here, the above-described organic compound can be a carborane compound. In a carborane compound, the proportion of boron in the molecular weight is high, and boron 10 atoms can be added efficiently. Further, the proportion of boron in the molecular weight of the above-described organic compound can be 50% or more. According to such a configuration, boron 10 can be contained at a high concentration, and the neutron reactivity is improved.

[0034] Alternatively, the above-described organic compound can contain gadolinium.

[0035] In addition, the above-described wavelength conversion fiber and the above-described outermost peripheral layer can be formed integrally by a drawing process. The productivity is further improved.

[0036] A protective layer protecting the above-described outermost peripheral layer can be formed integrally on the outside of the above-described outermost peripheral layer. Thereby, durability and the like are improved.

[0037] Further, the above-mentioned cladding layer can have a multi-cladding structure including an inner cladding layer and an outer cladding layer that covers the outer periphery of the inner cladding layer and has a lower refractive index than the inner cladding layer. The total reflection angle widens, and higher light emission is achieved.

[0038] A reflective layer can be provided outside the above-mentioned outermost periphery layer or the above-mentioned protective layer. Scintillating light emitted from the outermost periphery layer, light that has been wavelength-converted in the core, is reflected by the reflective layer, and thus is less likely to leak to the outside from the side surface of the optical fiber, and high light emission is achieved.

[0039] The above-mentioned reflective film can be a metal film. According to this configuration, a high reflectance can be obtained with a thin thickness.

[0040] A manufacturing method of a plastic scintillating optical fiber of one embodiment of the present application is a manufacturing method of a plastic scintillating optical fiber including an outermost periphery layer containing a plastic material, the plastic material containing an organic compound containing an element having a larger neutron reaction cross section than hydrogen and emitting scintillating light by irradiation with a neutron beam; a high-refractive core provided inside the outermost periphery layer and containing at least one phosphor that absorbs the scintillating light and wavelength-converts it to a long wavelength; and a cladding layer that covers the outer periphery of the core and has a lower refractive index than the core, and includes a step of inserting a second cylindrical body for the cladding layer into the inside of a first cylindrical body for the outermost periphery layer, and inserting a rod for the core into the inside of the second cylindrical body to produce a preform rod; and a step of performing drawing processing on the preform rod while heating.

[0041] A manufacturing method of a plastic scintillating optical fiber of one embodiment of the present application is a manufacturing method of a plastic scintillating optical fiber including an outermost periphery layer containing a plastic material, the plastic material containing an organic compound containing an element having a larger neutron reaction cross section than hydrogen and emitting scintillating light by irradiation with a neutron beam; a high-refractive core provided inside the outermost periphery layer and containing at least one phosphor that absorbs the scintillating light and wavelength-converts it to a long wavelength; and a cladding layer that covers the outer periphery of the core and has a lower refractive index than the core, and includes a step of coating the outermost periphery layer on the surface of a wavelength conversion optical fiber containing the core and the cladding layer.

[0042] Effects of Invention

[0043] According to the present application, a plastic scintillating optical fiber that can detect a neutron beam with higher sensitivity than a conventional scintillating optical fiber and has high productivity can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 A cross-sectional view of a plastic scintillating optical fiber of Embodiment 1.

[0045] Figure 2 A cross-sectional view of a plastic scintillating optical fiber of a modification of Embodiment 1.

[0046] Figure 3 A cross-sectional view of a plastic scintillating optical fiber of another modification of Embodiment 1.

[0047] Figure 4 A perspective view for showing a manufacturing method of the plastic scintillating optical fiber of Embodiment 1.

[0048] Figure 5 A perspective view for showing an application example of the scintillating optical fiber of Embodiment 1.

[0049] Figure 6 Emission spectrum of p-terphenyl and absorption and emission spectrum of wavelength conversion phosphor BBOT added in the outermost peripheral layer. DETAILED DESCRIPTION

[0050] A specific embodiment to which the present application is applied will be explained in detail below with reference to the accompanying drawings. However, the present application is not limited to the following embodiment. In addition, the following description and drawings are appropriately simplified for the sake of clarity of explanation.

[0051] (Embodiment 1)

[0052] <Configuration of Plastic Scintillating Optical Fiber>

[0053] Reference Figure 1 The plastic scintillating optical fiber of Embodiment 1 of the present application will be explained. Figure 1 A cross-sectional view of a plastic scintillating optical fiber of another modification of Embodiment 1.

[0054] As Figure 1 shown, the plastic scintillating optical fiber of Embodiment 1 has an outermost peripheral layer 1, a core 2, and a cladding 3.

[0055] The outermost peripheral layer 1 is configured of a plastic material that emits scintillating light by irradiation of a neutron beam. Furthermore, the plastic material contains an organic compound containing an element having a neutron reaction cross-section larger than that of hydrogen. In addition, the plastic material contains, in addition to the organic compound containing an element having a neutron reaction cross-section larger than that of hydrogen, a phosphor that absorbs the scintillating light emitted by the plastic material and converts it to long wavelength.

[0056] The outermost peripheral layer 1 is required to be transparent to the extent that the scintillating light penetrates the cladding 3 and is incident to the core 2 of the central portion, although high transparency is not required, and is desirably as transparent and thin as possible. The thickness of the outermost peripheral layer can be a thickness that gives the required sensitivity to neutrons. Even if the outermost peripheral layer 1 as the scintillator layer has low transparency, if the core 2 of the central portion responsible for light transmission has high transparency, long distance transmission is possible.

[0057] The core 2 is provided inside the outermost peripheral layer 1 and is composed of a transparent resin having a high refractive index containing at least one phosphor that absorbs the scintillation light generated in the outermost peripheral layer 1 and converts to a longer wavelength. The refractive index of the transparent resin constituting the core 2 is preferably 1.5 or more.

[0058] The cladding layer 3 coats the outer peripheral surface of the core 2 and is composed of a transparent resin having a lower refractive index than the core 2. Here, the wavelength conversion optical fiber composed of the core 2 and the cladding layer 3 is integrally formed with the outermost peripheral layer 1 that coats the outer peripheral surface of the wavelength conversion optical fiber.

[0059] For long distance transmission in the form of an optical fiber, the transparency of the cladding layer 3 is also as important as the transparency of the core 2. For long distance transmission, the transparency of the outermost peripheral layer 1 is less important.

[0060] For long distance transmission in the form of an optical fiber, the thickness of the cladding layer 3 is preferably thick enough to be 3 μm to 100 μm compared to the depth of the evanescent wave at which the transmission light leaks from the core 2 to the cladding layer 3. If the thickness of the cladding layer 3 is thick enough compared to the depth of the evanescent wave that leaks to the cladding layer 3, the refractive indexes of the cladding layer 3 and the outermost peripheral layer 1 can be the same, and further, can be the same transparent resin.

[0061] As the phosphor contained in the core 2, a phosphor that absorbs the wavelength of the scintillation light generated in the outermost peripheral layer 1 and has a fluorescent spectrum that is as far as possible from the absorption spectrum for wavelength conversion is desirable. Further, for matching the wavelength sensitivity of a photodetector such as a photomultiplier tube (PMT), an avalanche photodiode (APD), or the like, the core 2 can further contain a second phosphor that performs wavelength conversion. Note that the details of the phosphor will be described later.

[0062] In the plastic scintillation optical fiber of Embodiment 1, scintillation light is generated in the outermost peripheral layer 1 by irradiation of a neutron beam, the scintillation light is absorbed by the core 2 inside, wavelength conversion is performed, and optical transmission is performed. Thus, a neutron beam that is low in sensitivity and difficult to detect in the case of the existing plastic scintillation optical fiber can be detected with high sensitivity. That is, the plastic scintillation optical fiber of Embodiment 1 is a composite plastic optical fiber that has both a scintillation function for a neutron beam and a wavelength conversion function.

[0063] In addition, the wavelength conversion optical fiber composed of the core 2 and the cladding layer 3 is integrally formed with the outermost peripheral layer 1 that coats the outer peripheral surface of the wavelength conversion optical fiber. Thus, the post-processing of combining a scintillator with a wavelength conversion optical fiber, which has been necessary in the past, is not required, productivity can be dramatically improved compared to the past, and cost reduction is also possible.

[0064] In addition, a protective layer (not shown) that protects the outermost peripheral layer can be integrally formed on the outer side of the outermost peripheral layer 1. Due to the protective layer, durability and the like of the plastic scintillation optical fiber are improved.

[0065] <Plastic scintillation fiber of a modification example>

[0066] Figure 2 A cross-sectional view of a plastic scintillation fiber of a modification example of Embodiment 1. As shown in the figure, a surface of the outermost peripheral layer 1 or the protective layer can be provided with a reflective layer 5. Scintillating light emitted in the outermost peripheral layer 1, light which has been subjected to wavelength conversion in the core 2, is reflected by the reflective layer 5, so as not to easily leak to the outside from the side surface of the fiber, achieving high light emission. Here, by providing the reflective layer 5 as a metal film, high reflectivity can be obtained with a thin thickness, so it is preferable. Figure 2

[0067] Figure 3 A cross-sectional view of a plastic scintillation fiber of another modification example of Embodiment 1. The plastic scintillation fiber of the other modification example is provided with the cladding layer 3 as an inner cladding layer, and further provided with the cladding layer 4 as an outer cladding layer. That is, the cladding layer has a multi-cladding structure including the inner cladding layer (cladding layer 3) and the outer cladding layer (cladding layer 4). The cladding layer 4 covers the outer peripheral surface of the cladding layer 3 and is composed of a transparent resin having a lower refractive index than the cladding layer 3.

[0068] Here, the re-emitted light which has been subjected to wavelength conversion in the core 2 is isotropically radiated in a solid angle within the core 2. Therefore, only light within the total reflection angle based on the refractive index difference between the core 2 and the cladding layer 3 or the cladding layer 4 can be transmitted in the fiber direction. The plastic scintillation fiber of the other modification example is provided with the cladding layer 4 having a low refractive index in addition to the cladding layer 3, so the total reflection angle is wider (numerical aperture NA is larger) compared to the plastic scintillation fiber of Embodiment 1, achieving higher light emission. Figure 1

[0069] <Material of the outermost peripheral layer 1>

[0070] The outermost peripheral layer 1 as a scintillator layer is composed of a transparent resin, i.e., a plastic material, which emits scintillating light by irradiation of a neutron beam. Here, the transparent resin contains an organic compound including an element having a larger neutron reaction cross section than hydrogen.

[0071] The transparent resin constituting the outermost peripheral layer 1 is preferably thermoplastic so as to be able to be finely drawn by heating. As such a transparent resin, a homopolymer or copolymer formed of any one of a methacrylate monomer group represented by methyl methacrylate, a methacrylate monomer group represented by methyl acrylate, and an aromatic monomer group having a vinyl group represented by styrene is preferable.

[0072] Here, the transparent resin can contain a phosphor which emits scintillating light by radiation, and the transparent resin itself can also emit scintillating light by radiation. As the transparent resin which emits scintillating light by radiation, a homopolymer or copolymer formed of any one of an aromatic monomer group having a vinyl group represented by styrene is preferable.

[0073] ​​The organic compound containing an element having a neutron reaction cross-section larger than hydrogen is preferably stable and has high solubility in an aromatic monomer having a vinyl group, typified by styrene, i.e., a monomer that is a raw material of a transparent resin. The higher the solubility, the higher the transparency of a polymer obtained by dissolving the organic compound in the aromatic monomer and polymerizing the same. When the solubility is low, the organic compound cannot be uniformly dispersed in the transparent resin, and adverse effects such as a deviation in sensitivity to neutrons can occur. In addition, when dispersed in a powder form, drawing by heating can become difficult.

[0074] As the element having a neutron reaction cross-section larger than hydrogen, lithium 6, boron 10, gadolinium can be listed.

[0075] As the organic compound containing lithium 6, lithium carboxylates such as lithium methacrylate, lithium phenylsalicylate, lithium tert-pentanoate, and the like are preferably used.

[0076] As the organic compound containing boron 10, carboranes such as o-carborane, m-carborane, p-carborane, and derivatives thereof, and the like are preferably used. Among the carborane compounds (B 10 C2H 12 ), the ratio of boron in the molecular weight is high, and boron 10 atoms can be efficiently added. Here, when the total atomic weight of boron accounts for 50% by mass or more of the molecular weight, boron 10 can be contained at a high concentration, and the neutron reactivity is increased.

[0077] As the organic compound containing gadolinium, alkoxygadolinium such as isopropoxygadolinium, gadolinium complexes such as tris(2,2,6,6-tetramethyl-3,5-heptanedionate)gadolinium, and the like are preferably used.

[0078] The wavelength conversion phosphor contained in the outermost peripheral layer 1 is an organic phosphor having an aromatic ring and having a resonable structure, and is preferably dissolved as a single molecule in the core 2. As representative phosphors, 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4- oxadiazole (b-PBD), 2-(4-biphenyl)-5-phenyl-1,3,4- oxadiazole (PBD), p-terphenyl (PTP), p-quaterphenyl (PQP), 2,5-diphenyl oxazole (PPO), 1-phenyl-3-(2,4,6-trimethylphenyl)-2-pyrazoline (PMP), 3-hydroxyflavone (3HF), and the like can be listed.

[0079] In addition, 4,4'-bis(2,5-dimethylstyryl)-biphenyl (BDB), 2,5-bis(5-tert-butyl-2-oxazolyl)thiophene (BBOT), 1,4-bis(2-(5-phenyl oxazolyl)phenyl)benzene (BPO), and the like that absorb 350 to 400 nm are also preferably used. 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene (POPOP), 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene (DMPOPOP), 1,4-diphenyl-1,3-butadiene (DPB), 1,6-diphenyl-1,3,5-hexatriene (DPH), and the like.

[0080] As an example of the outermost layer 1, polystyrene containing lithium pivalate as the organic compound containing an element having a neutron reaction cross section larger than that of hydrogen, and a phosphor that converts scintillating light emitted from the polystyrene to long wavelengths can be given. Of lithium contained in the lithium pivalate, about 7.5% is lithium 6. Lithium 6 generates α-rays by a neutron beam. The polystyrene scintillates and emits light due to the α-rays, and the scintillating light is absorbed by the phosphor and emitted as light of long wavelengths. The outermost layer 1 emits visible light of 400 to 500 nm by a neutron beam.

[0081] As another example of the outermost layer 1, polystyrene containing a carborane compound containing boron at a high ratio as the organic compound containing an element having a neutron reaction cross section larger than that of hydrogen, and a phosphor that converts scintillating light emitted from the polystyrene to long wavelengths can be given. Of boron contained in the carborane compound, about 19.8% is boron 10 having a neutron reaction cross section much larger than that of hydrogen. Boron 10 generates α-rays by a neutron beam. The polystyrene scintillates and emits light due to the α-rays, and the scintillating light is absorbed by the phosphor and emitted as light of long wavelengths. The outermost layer 1 scintillates and emits ultraviolet light of 350 to 400 nm by a neutron beam.

[0082] Note that the kinds of the organic compound having a neutron reaction cross section larger than that of hydrogen, the plastic material (transparent resin), and the phosphor are not limited to the above. In addition, the mixing ratio, concentration, and the like of the above materials can be appropriately selected according to the degree of difficulty of the manufacturing and the like, and are not limited to the above.

[0083] <Material of the Core 2>

[0084] The material used for the core 2 is not limited as long as it is a transparent resin. Among them, a homopolymer or a copolymer formed of any one of a methacrylate monomer group represented by methyl methacrylate, an acrylate monomer group represented by methyl acrylate, and an aromatic monomer group having a vinyl group represented by styrene is preferable.

[0085] Among them, a polymer formed of an aromatic monomer having a vinyl group has a high refractive index, and is thus preferable. The difference in refractive index between the core 2 and the cladding layer 3 becomes large, and the total reflection angle becomes wide. That is, a wider angle of light among the light that has been subjected to wavelength conversion within the core 2 can be transmitted, and thus a scintillating fiber that emits light at a higher luminance can be obtained.

[0086] ​The wavelength conversion phosphor contained in the core 2 is an organic phosphor having an aromatic ring and having a resonant structure, and is preferably dissolved as a single molecule in the core 2. As representative phosphors, 2-(4-tert-butylphenyl)-5-(4-biphenyl)-l,3,4-oxadiazole (b-PBD), 2-(4-biphenyl)-5-phenyl-l,3,4-oxadiazole (PBD), p-terphenyl (PTP), p-quaterphenyl (PQP), 2,5-diphenyl-l,3,4-oxadiazole (PPO), l-phenyl-3-(2,4,6-trimethylphenyl)-2-pyrazoline (PMP), 3-hydroxyflavone (3HF), and the like can be listed.

[0087] Further, 4,4'-bis(2,5-dimethylstyryl)-biphenyl (BDB), 2,5-bis(5-tert-butyl- benzoxy)thiophene (BBOT), l,4-bis(2-(5-phenyl- lH-pyrazol-l-yl))benzene (POPOP), l,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene (DMPOPOP), l,6-diphenyl-l,3,5-hexatriene (DPH), and the like, which absorb 350 to 400 nm, are also preferable.

[0088] In order to obtain high luminescence, it is preferable that the wavelength conversion phosphor contained in the core 2 has a large overlap of the absorption spectrum with the emission spectrum of the phosphor contained in the outermost layer 1.

[0089] The wavelength conversion phosphor described above can be used alone or in combination with a plurality of wavelength conversion phosphors. Each wavelength conversion phosphor is preferably high in quantum yield and has a small overlap of the absorption spectrum with the emission spectrum (large Stokes shift). As a characteristic of the plastic optical fiber, the longer the wavelength, the smaller the transmission loss of visible light, and therefore a wavelength conversion phosphor that emits light of a longer wavelength is preferable, and two or more wavelength conversion phosphors can be appropriately combined. The wavelength conversion phosphor is preferably soluble in the transparent resin that constitutes the core 2.

[0090] The concentration of the wavelength conversion phosphor is preferably 50 to 10,000 ppm, and further preferably 100 to 1,000 ppm, in terms of mass concentration, whether alone or in combination. When the concentration is too low, the core 2 cannot efficiently absorb the scintillation light from the outermost layer 1. In contrast, when the concentration is too high, the influence of self-absorption of the phosphor itself becomes large, the wavelength conversion efficiency decreases, or the transmission of the converted light decreases and the attenuation length deteriorates.

[0091] <Cladding layer 3>

[0092] ​​​​​​The material used for the cladding layer 3 is not limited as long as it is a transparent resin having a lower refractive index than the core 2. Among them, a homopolymer or copolymer using any one of a methacrylate monomer group represented by methyl methacrylate, a fluorinated monomer group such as a perfluoroalkyl methacrylate, an acrylate monomer group represented by methyl acrylate, and a fluorinated monomer group such as a perfluoroalkyl acrylate as a raw material is preferable.

[0093] <Material of Cladding Layer 4>

[0094] The material used for the cladding layer 4 is a transparent resin having a lower refractive index than the cladding layer 3. It can be selected from the monomer group or the like of the cladding layer 3. It is particularly preferable to be selected from a fluorine-containing monomer group having a low refractive index.

[0095] These monomer groups can easily obtain a polymer or a copolymer by heat or light irradiation, and thus have the advantages of being able to form a precise composition distribution and being easy to handle. At the time of polymerization, an organic peroxide or an azo compound can be added as a polymerization initiator. As a representative organic peroxide, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, n-butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)cyclohexane, and the like can be exemplified, but there is no particular limitation as long as it is a substance that generates radicals by heat or light irradiation.

[0096] In addition, in order to adjust the molecular weight, a mercaptan can be added as a chain transfer agent. As a representative mercaptan, octyl mercaptan is exemplified, but there is no particular limitation as long as it is a substance having a structure of R-SH (here, R represents an organic group).

[0097] <Material of Reflective Layer>

[0098] The material constituting the reflective layer 5 is not limited as long as it is a material capable of reflecting light emitted from the side surface of the optical fiber with high reflectance. Among them, a metal film can obtain a high reflectance with a thinner thickness than, for example, a white reflective paint, and is particularly preferable when the diameter of the optical fiber is required to be small.

[0099] The metal film is not particularly limited as long as it is a metal having a high reflectance in the required wavelength range, such as aluminum, gold, silver, nickel, and the like. From the viewpoint of a high reflectance in the visible light region, aluminum and silver are preferable. Furthermore, from the viewpoint of cost, aluminum is preferable.

[0100] Note that the thickness of the metal film is not particularly limited, and it is preferable to obtain a high reflectance with the thinnest possible thickness in the visible light region. For example, in the case of aluminum, 10 to 100 nm is preferable, and further 20 to 70 nm is preferable. In the case of silver, 35 to 150 nm is preferable, and further 50 to 100 nm is preferable. In addition, the film formation method is not particularly limited, and it is a vapor deposition method, a sputtering method, or the like.

[0101] Manufacturing method of plastic scintillation optical fiber

[0102] Figure 4 A perspective view showing the manufacturing method of the plastic scintillation optical fiber of Embodiment 1. Figure 4 A perspective view showing the manufacturing method of the plastic scintillation optical fiber of Embodiment 1. Figure 1 A mother material (preform) of the plastic scintillation optical fiber shown.

[0103] The first cylindrical body 11 is a cylindrical body formed of a thermoplastic resin that emits scintillating light by a neutron beam. The thermoplastic resin contains an organic compound containing an element having a larger neutron reaction cross section than hydrogen. The first cylindrical body 11 constitutes the outermost peripheral layer 1 after drawing processing. An example of the manufacturing method of the first cylindrical body 11 will be described later by way of an example.

[0104] The rod 12 is a cylindrical body formed of a transparent thermoplastic resin in which at least one kind of phosphor that absorbs scintillating light and wavelength-converts to a long wavelength is dissolved. The rod 12 constitutes the core 2 after drawing processing.

[0105] The second cylindrical body 13 is a cylindrical body formed of a thermoplastic resin that is transparent and has a lower refractive index than the rod 12. The second cylindrical body 13 constitutes the cladding layer 3 after drawing processing.

[0106] As shown in FIG. 1, the second cylindrical body 13 is inserted into the inside of the first cylindrical body 11, and the rod 12 is inserted into the inside of the second cylindrical body 13, thereby producing a preform. Figure 4 A situation in the process of inserting the rod 12 into the inside of the second cylindrical body 13 is shown. By drawing the front end of the produced preform while heating it to, for example, an outer diameter of 1 mm, a plastic scintillation optical fiber of Embodiment 1 can be obtained. Figure 4

[0107] Note that, as shown in FIG. 1, although a gap is formed between the first cylindrical body 11, the second cylindrical body 13, and the rod 12, since drawing processing is performed under reduced pressure, the core 2, the cladding layer 3, and the outermost peripheral layer 1 are integrally formed in close contact. Figure 4

[0108] Figure 3 The plastic scintillation optical fiber of the modified example shown can also be produced by the same manufacturing method.

[0109] In the manufacturing method of the plastic scintillation optical fiber of Embodiment 1, a scintillator layer (the outermost peripheral layer 1) that emits light by a neutron beam is integrally formed on the outer peripheral surface of a wavelength conversion optical fiber (the core 2 and the cladding layer 3). Therefore, the plastic scintillation optical fiber can detect a neutron beam and can perform optical transmission. That is, the plastic scintillation optical fiber alone has both the functions of a conventional scintillator and a wavelength conversion optical fiber.

[0110] ​​Therefore, it is not necessary to combine the scintillator with the wavelength conversion optical fiber after processing as required in the past, and productivity can be improved compared with the past, and cost reduction is also possible. In particular, in the case of image detection, it is not necessary to separate the scintillators one by one and combine them with each of the wavelength conversion optical fibers, and the plastic scintillation optical fibers can be simply arranged side by side. Therefore, productivity can be dramatically improved compared with the past, and cost reduction is also possible.

[0111] Note that, after the wavelength conversion optical fiber (core 2 and cladding 3) is manufactured, the scintillator layer (outermost peripheral layer 1) can be integrally formed on the surface of the wavelength conversion optical fiber by coating (including painting). However, as described above, when the scintillator layer (outermost peripheral layer 1) and the wavelength conversion optical fiber (core 2 and cladding 3) are manufactured at the same time by drawing processing, productivity is further improved.

[0112] <Example of application of plastic scintillation optical fiber>

[0113] Next, referring to Figure 5 An example of application of the plastic scintillation optical fiber of Embodiment 1 will be described. Figure 5 A perspective view showing an example of application of the plastic scintillation optical fiber of Embodiment 1 is shown. In this example, the plastic scintillation optical fibers PSF of Embodiment 1 are arranged in an array on a substrate.

[0114] Note that, Figure 5 The right-hand xyz orthogonal coordinate system shown is of course for the convenience of explanation of the positional relationship of the constituent elements. Normally, the z-axis positive direction is the vertical upward direction, and the xy plane is the horizontal plane.

[0115] A photomultiplier tube or the like photodetector (not shown) is connected to each of the plastic scintillation optical fibers PSF, and can detect the transmitted light. With such a configuration, for example, one-dimensional image detection (position detection) can be performed with a resolution of 1 mm. Here, the resolution is equal to the diameter of the plastic scintillation optical fiber PSF. Furthermore, if two such arrays of plastic scintillation optical fibers PSF are provided and arranged in a mutually orthogonal manner so as to be stacked one on top of the other, two-dimensional image detection can also be achieved.

[0116] Thus, by using the plastic scintillation optical fiber of the present embodiment, image detection of a neutron beam with high spatial resolution can be achieved simply and at low cost.

[0117] Example

[0118] The present application will be described in further detail by examples below, but the present application is not limited in any way by the examples.

[0119] <Example 1>

[0120] Adding 5% by mass of m-carborane, an organic compound containing boron with a neutron reaction cross-section greater than that of hydrogen, and the fluorescent 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4- to the styrene monomer as an organic compound... Diazole (b-PBD) 1% by mass was polymerized, and the polymer was shaped into an outermost cylindrical body with an outer diameter of 50 mm and an inner diameter of 40 mm. Figure 4 The first cylindrical body 11).

[0121] Prepare a core rod with a diameter of 32 mm made of polystyrene (refractive index 1.59). Figure 4 The rod 12) and a cladding cylinder with an outer diameter of 38 mm and an inner diameter of 34 mm, made of polymethyl methacrylate (refractive index 1.49). Figure 4 The second cylindrical body 13). In the core rod, 2,5-bis(5-tert-butyl-benzo[2]) is dissolved at a concentration of 200 ppm by mass as a wavelength-converting phosphor. BBOT (azolyl)thiophene.

[0122] like Figure 4 As shown, a cladding cylinder is inserted into the inner part of the outermost cylindrical body, and a core rod is inserted into the inner part of the cladding cylinder to fabricate a preform. While heating the tip of the preform, it is integrally drawn under reduced pressure to achieve an outer diameter of 1 mm, thereby obtaining the plastic scintillating fiber of Example 1. This plastic scintillating fiber has… Figure 1 The cross-section shown is as follows. The outer diameter is 1000 μm, the diameter of cladding 3 is 770 μm, the diameter of core 2 is 680 μm, the thickness of outermost layer 1 is 115 μm, and the thickness of cladding 3 is 45 μm.

[0123] Figure 6 To illustrate the phosphor 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4- added to the outermost peripheral layer 1 The emission spectrum of diazole (b-PBD) and the absorption and emission spectra of BBOT, a wavelength-converting phosphor added to core 2, are shown in the figure. Figure 6 As shown, the emission spectrum of b-PBD added to the outermost layer 1 overlaps significantly with the absorption spectrum of BBOT added to the core 2. The plastic scintillation fiber of Example 1 contains organic compounds with a neutron reaction cross-sectional area larger than that of hydrogen, thus exhibiting improved neutron sensitivity compared to conventional plastic scintillation fibers.

[0124] <Example 2>

[0125] A cylindrical outermost layer with an outer diameter of 50 mm and an inner diameter of 40 mm is formed in the same manner as in Example 1. Figure 4The first cylindrical body 11). Additionally, a core rod with a diameter of 28 mm made of polystyrene (refractive index 1.59) was prepared in the same manner as in Example 1. Figure 4 The rod 12) and an inner cladding cylinder with an outer diameter of 33 mm and an inner diameter of 30 mm, made of polymethyl methacrylate (refractive index 1.49). Figure 4 The second cylindrical body 13). In the core rod, BBOT, which is a wavelength conversion phosphor, is dissolved at a concentration of 300 ppm by mass.

[0126] Furthermore, in Example 2, an outer cladding cylinder (not shown) with an outer diameter of 38 mm and an inner diameter of 35 mm was prepared, consisting of a copolymer of fluorinated monomers such as perfluoroalkyl acrylate (refractive index 1.42). The outer cladding cylinder was constructed after wire drawing. Figure 3 The cladding shown is layer 4.

[0127] Then, an outer cladding cylinder is inserted into the inner cladding cylinder, and a core rod is inserted into the inner cladding cylinder, thereby producing a preform.

[0128] While heating the tip of the preform, the fiber was integrally drawn under reduced pressure to achieve an outer diameter of 1 mm, thereby obtaining the plastic scintillation fiber of Example 2. This plastic scintillation fiber has... Figure 3 The cross-section shown is as follows. The outer diameter is 1000 μm, the outer diameter of cladding 4 is 754 μm, the outer diameter of cladding 3 is 682 μm, the diameter of core 2 is 612 μm, the thickness of outermost layer 1 is 123 μm, the thickness of cladding 4 is 36 μm, and the thickness of cladding 3 is 35 μm.

[0129] When the neutron beam is incident on the plastic scintillation fiber of Example 2, approximately 30% more light intensity can be observed at the front end at a distance of 10m compared to Example 1. It can be considered that although the diameter of the fiber core 2 is smaller than that of Example 1, the cladding 4 with lower refraction results in a wider total internal reflection angle, thus achieving higher light emission.

[0130] <Example 3>

[0131] Adding 1.9% by mass of lithium pivalate, an organic compound containing lithium 6 with a neutron reaction cross-section greater than that of hydrogen, to styrene monomer, along with the fluorescent 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4- Diazole (b-PBD) by mass % was polymerized and shaped into an outermost cylindrical body with an outer diameter of 50 mm and an inner diameter of 40 mm. Figure 4 The first cylindrical body 11).

[0132] Prepare a core rod with a diameter of 32 mm made of polystyrene (refractive index 1.59). Figure 4The rod 12) and a cladding cylinder with an outer diameter of 38 mm and an inner diameter of 34 mm, made of polymethyl methacrylate (refractive index 1.49). Figure 4 The second cylindrical body 13). In the core rod, 2,5-bis-(5-tert-butyl-benzo[a]) is dissolved at a concentration of 200 ppm by mass as a wavelength-converting phosphor. BBOT (azolyl)thiophene.

[0133] like Figure 4 As shown, a cladding cylinder is inserted into the inner part of the outermost cylindrical body, and a core rod is inserted into the inner part of the cladding cylinder to fabricate a preform. While heating the tip of the preform, it is integrally drawn under reduced pressure to achieve an outer diameter of 1 mm, thereby obtaining the plastic scintillating fiber of Example 1. This plastic scintillating fiber has… Figure 1 The cross-sectional structure is shown. The outer diameter is 1000 μm, the diameter of cladding 3 is 770 μm, the diameter of core 2 is 680 μm, the thickness of outermost layer 1 is 115 μm, and the thickness of cladding 3 is 45 μm. The plastic scintillation fiber of Example 3 contains an organic compound with a neutron reaction cross-sectional area larger than that of hydrogen, thus improving neutron sensitivity compared to conventional plastic scintillation fibers.

[0134] <Example 4>

[0135] An aluminum film with a thickness of about 50 nm was formed on the surface of the plastic scintillation fiber in Example 1 by vapor deposition.

[0136] The scintillation light emitted by the outermost peripheral layer 1 and the light that has undergone wavelength conversion in the fiber core 2 are reflected by the reflective layer 5, thus making it less likely to leak from the side of the optical fiber to the outside, achieving high luminescence.

[0137] This invention is not limited to the above-described embodiments, and can be appropriately modified without departing from its spirit.

[0138] This application claims priority based on Japanese Patent Application No. 2020-155285, filed on September 16, 2020, the entire contents of which are incorporated herein by reference.

[0139] Symbol Explanation

[0140] 1. Outermost layer

[0141] 2 Fiber Core

[0142] 3. Cladding (inner cladding)

[0143] 4. Cladding (outer cladding)

[0144] 5. Reflective layer

[0145] 11 First cylindrical body

[0146] 12 rod

[0147] 13 second cylindrical body

[0148] PSF plastic scintillating fiber

Claims

1. A plastic scintillating optical fiber, which comprises: an outermost peripheral layer containing a plastic material containing an organic compound containing an element having a neutron reaction cross-section larger than that of hydrogen and emitting scintillating light by irradiation of a neutron beam, and at least one first fluorescent substance absorbing the scintillating light and wavelength-converting to a long wavelength; a high-refractive core provided inside the outermost peripheral layer and containing at least one second fluorescent substance absorbing light emitted from the outermost peripheral layer and wavelength-converting to a long wavelength; and a cladding layer covering the peripheral surface of the core and having a lower refractive index than the core, a wavelength-conversion optical fiber containing the core and the cladding layer is integrally formed with the outermost peripheral layer covering the peripheral surface of the wavelength-conversion optical fiber, the scintillating light absorbed by the first fluorescent substance is ultraviolet light having a wavelength of 250 to 350 nm, the second fluorescent substance absorbs light having a wavelength of 350 to 400 nm emitted from the outermost peripheral layer and further converts it to light on the long wavelength side.

2. The plastic scintillating optical fiber of claim 1, wherein, The organic compound contains lithium 6.

3. The plastic scintillating optical fiber of claim 1, wherein, The organic compound contains boron 10.

4. The plastic scintillating optical fiber of claim 3, wherein, The organic compound is a carborane compound.

5. The plastic scintillating optical fiber of claim 3, wherein, The ratio of boron in the molecular weight of the organic compound is 50% or more.

6. The plastic scintillating optical fiber of claim 1, wherein, The organic compound contains gadolinium.

7. The plastic scintillating optical fiber of claim 1, wherein, The wavelength-conversion optical fiber is integrally formed with the outermost peripheral layer by drawing processing.

8. The plastic scintillating optical fiber of claim 1, wherein, On the outer side of the outermost peripheral layer, a protective layer that protects the outermost peripheral layer is integrally formed.

9. The plastic scintillating optical fiber of claim 1, wherein, The cladding layer has a multi-cladding structure containing an inner cladding layer and an outer cladding layer covering the peripheral surface of the inner cladding layer and having a lower refractive index than the inner cladding layer.

10. The plastic scintillating optical fiber of claim 1, wherein, There is a reflective layer on the outer side of the outermost peripheral layer.

11. The plastic scintillating optical fiber of claim 10, wherein, The reflective layer is a metal film.

12. A manufacturing method of a plastic scintillating optical fiber, which comprises: an outermost peripheral layer containing a plastic material containing an organic compound containing an element having a neutron reaction cross-section larger than that of hydrogen and emitting scintillating light by irradiation of a neutron beam, and at least one first fluorescent substance absorbing the scintillating light and wavelength-converting to a long wavelength; a high-refractive core provided inside the outermost peripheral layer and containing at least one second fluorescent substance absorbing light emitted from the outermost peripheral layer and wavelength-converting to a long wavelength; and a cladding layer covering the peripheral surface of the core and having a lower refractive index than the core, the manufacturing method comprises: a step of inserting a second cylinder for the cladding layer into the inside of a first cylinder for the outermost peripheral layer, inserting a rod for the core into the inside of the second cylinder, and producing a preform rod; and a step of performing drawing processing while heating the preform rod, the scintillating light absorbed by the first fluorescent substance is ultraviolet light having a wavelength of 250 to 350 nm, the second fluorescent substance absorbs light having a wavelength of 350 to 400 nm emitted from the outermost peripheral layer and further converts it to light on the long wavelength side.

13. A manufacturing method of a plastic scintillating optical fiber, The plastic scintillating optical fiber has: an outermost peripheral layer containing a plastic material and at least one first fluorescent substance, the plastic material containing an organic compound containing an element having a neutron reaction cross section larger than hydrogen and emitting scintillating light by irradiation of a neutron beam, the first fluorescent substance absorbing the scintillating light and wavelength-converting to a long wavelength; a high-refractive core provided inside the outermost peripheral layer and containing at least one second fluorescent substance absorbing light emitted from the outermost peripheral layer and wavelength-converting to a long wavelength; and a cladding layer coating the peripheral surface of the core and having a lower refractive index than the core, In the manufacturing method, The outermost peripheral layer is coated on the surface of a wavelength-converting optical fiber containing the core and the cladding layer, The scintillating light absorbed by the first fluorescent substance is ultraviolet light having a wavelength of 250 to 350 nm, The second fluorescent substance absorbs light having a wavelength of 350 to 400 nm emitted from the outermost peripheral layer and further converts it to light on the long wavelength side.

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