JP6833611B2 - Plastic wavelength conversion fiber and its manufacturing method - Google Patents

Description

The present invention relates to a plastic wavelength conversion fiber and a method for manufacturing the same.

Scintillators such as inorganic scintillators, organic liquid scintillators, and plastic scintillators have long been widely used in the field of radiation detection such as elementary particle detection, and are important members in radiation measurement. These scintillators contain a fluorescent agent that emits blue light in the vicinity of 430 nm by irradiation according to the maximum sensitivity of the photomultiplier tube. Here, instead of directly detecting the blue light generated from the scintillator by irradiation, a method is known in which the blue light generated by the scintillator is wavelength-converted into green light or the like in an optical fiber and propagated, and indirectly detected. ing.

Such an optical fiber is called a wavelength shifting (WLS: WaveLength Shifting) fiber, and has a lower refractive index than the core on the outer peripheral surface of the core that absorbs blue light generated in a scintillator and converts the wavelength into green light or the like. It is coated with a clad. In particular, plastic wavelength conversion fibers made of plastic are cheaper and easier to process than those made of glass. Therefore, like the plastic scintillation fiber whose optical fiber itself has scintillation property, it is often used for particle physics research.

As the core base material of the plastic wavelength conversion fiber, for example, polystyrene, which is a highly transparent resin material having a relatively high refractive index, is used. By using polystyrene as the core base material, a large difference in refractive index can be obtained, so that the total reflection angle with the clad can be increased. Therefore, the green light generated by the wavelength conversion in the core can be confined in the core at a wider angle and transmitted to the fiber end face. That is, by using polystyrene as the core base material, a wavelength conversion fiber having high light emission can be realized.

FIG. 12 is a diagram showing the principle of green light emission inside the core of the wavelength conversion fiber irradiated with blue light. An organic fluorescent agent that absorbs blue light having a wavelength of about 430 nm, which is the light emitted by a scintillator, and converts it into green light having a wavelength of about 550 nm is dissolved in the core base material. Therefore, a part or all of the blue light generated in the scintillator and crossing the wavelength conversion fiber is absorbed in the core and converted into green light as shown in FIG.

FIG. 13 is a schematic view of a detector using a wavelength conversion fiber. As shown in FIG. 13, blue light having a wavelength of about 430 nm generated from a scintillator (not shown) is absorbed by a fluorescent agent contained in the core 11 when crossing the wavelength conversion fiber 1, and becomes green light having a wavelength of about 550 nm. Will be converted. The green light generated inside the core 11 propagates in the direction of both ends of the wavelength conversion fiber 1 while repeating total reflection at the interface between the core 11 and the clad 12. Then, the green light guided to one end of the wavelength conversion fiber 1 is detected as an electric signal by a photomultiplier tube (PMT).

Instead of the PMT shown in FIG. 13, a semiconductor detector such as a Si-PM (Silicon-PhotoMultiplier) using a silicon avalanche photodiode (Si-APD) may be used. Recently, MPPC (Multi-Pixel Photon Counter) arrays in which Si-PM is segmented into small segments and arranged in large numbers are often used. In the MPPC array, each pixel is connected to each of the wavelength conversion fibers, and the green light generated by each wavelength conversion fiber can be detected at the same time.

By the way, FIG. 14 is a perspective view of wavelength conversion fibers having a circular cross section arranged in parallel on the main surface of a flat plate scintillator. FIG. 15 is a cross-sectional view of a plurality of wavelength conversion fibers having a circular cross section arranged in parallel on the main surface of a flat plate scintillator. As shown in FIGS. 14 and 15, a detector using a plurality of wavelength conversion fibers 1 having a circular cross section arranged in parallel is used, for example, for an opaque scintillator composed of an inorganic polycrystal that detects a neutron beam.

In the examples of FIGS. 14 and 15, a plurality of wavelength conversion fibers 1 extending in the y-axis direction are arranged in the x-axis direction on the main surface of the scintillator 2 parallel to the xy plane. As shown in FIG. 15, when radiation passes through the flat plate scintillator 2, blue light is generated in the scintillator 2. When the blue light crosses the wavelength conversion fiber 1, the wavelength is converted into green light inside the core 11 coated on the clad 12.

FIG. 16 is a cross-sectional view showing how blue light traverses the wavelength conversion fiber 1 having a circular cross section. As shown in FIG. 16, in the case of the wavelength conversion fiber 1 having a circular cross section, the case where blue light crosses the vicinity of the central axis (central portion) of the core 11 and the vicinity of the outer periphery (outer peripheral portion) which is the interface with the clad 12 ), The crossing distance is different.

Therefore, the amount of light emitted in the core 11, that is, the number of photons generated, increases when crossing the central portion and decreases when crossing the outer peripheral portion, depending on the crossing distance of the blue light. That is, since the detection sensitivity is based on the crossing distance of blue light, in some cases, photons are detected only when blue light crosses the central portion, and photons are not detected when blue light crosses the outer peripheral portion. There is a risk.

As a result, in the detector in which the wavelength conversion fibers 1 having a circular cross section as shown in FIGS. 14 and 15 are arranged, there is a possibility that an insensitive dead region may be formed in the intermediate portion between the wavelength conversion fibers 1. (Patent Documents 1 to 3). As described above, it is not desirable even for a single wavelength conversion fiber that the amount of light emitted changes depending on the transverse position of the incident blue light.

In order to suppress such a problem, a wavelength conversion fiber having a rectangular cross section is usually used as shown in FIG. FIG. 17 is a perspective view of wavelength conversion fibers having a rectangular cross section arranged in parallel on the main surface of a flat plate scintillator.

FIG. 18 is a cross-sectional view showing how blue light traverses a wavelength conversion fiber having a rectangular cross section. As shown in FIG. 18, if the wavelength conversion fiber 1 has a rectangular cross section, the cross-sectional distance is the same at both the central portion and the outer peripheral portion of the core 11. Therefore, the dead region is eliminated except for the clad 12, and the amount of light emitted in the core 11 can be made the same regardless of the crossing position of the incident blue light.

For this purpose, many detectors in which wavelength conversion fibers 1 having a rectangular cross section are arranged in a single layer or multiple layers have been put into practical use. When arranging in multiple layers, the wavelength conversion fibers arranged in the x direction and the wavelength conversion fibers arranged in the y direction may be alternately laminated.

The right-handed xyz coordinates shown in FIGS. 14 to 18 correspond to each other in the drawings, but are for convenience to explain the positional relationship of the components.
Further, Patent Document 4 will be referred to in the description of the embodiment of the present invention.

JP-A-63-129304 Japanese Unexamined Patent Publication No. 2000-137122 JP-A-59-210401 International Publication No. 2015/046512

However, a wavelength conversion fiber having a rectangular cross section is more difficult to manufacture than a wavelength conversion fiber having a circular cross section, and is therefore expensive. Further, it is extremely difficult to improve the accuracy of the shape of the four corners and the straightness of the four sides as compared with the improvement of the accuracy of the roundness. Therefore, as an optical fiber that transmits light while repeating total reflection at the core / clad interface, only one that is inferior in optical performance can be obtained. That is, in the wavelength conversion fiber having a rectangular cross section, the deterioration of the transmission loss due to the failure to satisfy the total reflection condition was remarkable as compared with the wavelength conversion fiber having a circular cross section. Therefore, there is a limit to using a wavelength conversion fiber having a rectangular cross section for a long detector or a highly sensitive detector.

An object of the present invention is to provide a plastic wavelength conversion fiber having a circular cross section capable of suppressing a decrease in the amount of light emitted depending on a transverse position of incident light.

The plastic wavelength conversion fiber according to one aspect of the present invention is
A plastic wavelength conversion fiber that converts the first light emitted from a scintillator due to irradiation into second light having a different wavelength and transmits the second light.
A core containing a fluorescent agent that absorbs the first light and emits the second light, and a clad that covers the outer peripheral surface of the core and has a refractive index lower than that of the core are provided.
The cross section is circular,
The concentration of the fluorescent agent in the core has a distribution that increases from the center of the cross section of the core toward the outer periphery.
Since the concentration of the fluorescent agent in the core has a distribution that increases from the center of the cross section of the core toward the outer peripheral direction, the decrease in the amount of light emitted due to the cross-sectional position of the radiation is suppressed even though the cross section is circular. be able to.

It is preferable that the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent increases discontinuously in two or more steps from the center of the cross section of the core toward the outer periphery. Alternatively, it is preferable to have a distribution in which the concentration of the fluorescent agent continuously increases from the center of the cross section of the core toward the outer peripheral direction.

It is preferable that the clad has a multi-clad structure including an inner clad and an outer clad that covers the outer peripheral surface of the inner clad and has a refractive index lower than that of the inner clad. With such a configuration, light at a wider angle can be captured and propagated, and a wavelength conversion fiber having high light emission can be obtained.

The method for manufacturing a plastic wavelength conversion fiber according to one aspect of the present invention is
A method for manufacturing a plastic wavelength conversion fiber, comprising a core containing the fluorescent agent and a clad covering the outer peripheral surface of the core.
A step of producing a preform by inserting a rod-shaped core rod made of a resin containing a fluorescent agent into a cylindrical clad pipe made of a resin having a refractive index lower than that of the core rod.
A step of drawing a line while heating the preform is provided.
The concentration of the fluorescent agent in the core rod has a distribution that increases from the center of the cross section of the core rod toward the outer periphery.
Since the concentration of the fluorescent agent in the core rod has a distribution that increases from the center of the cross section of the core rod toward the outer peripheral direction, the obtained plastic wavelength conversion fiber has a circular cross section. It is possible to suppress a decrease in the amount of light emitted due to the cross-sectional position of the radiation.

It is preferable that the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent increases discontinuously in two or more steps from the center of the cross section of the core rod toward the outer periphery. Alternatively, it is preferable that the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent continuously increases from the center of the cross section of the core rod toward the outer peripheral direction.

The clad pipe may be composed of an inner clad pipe and an outer clad pipe made of a resin having a refractive index lower than that of the inner clad pipe while covering the outer peripheral surface of the inner clad pipe. preferable. With such a configuration, the obtained plastic wavelength conversion fiber can capture and propagate light at a wider angle, and can be a wavelength conversion fiber having high emission.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a plastic wavelength conversion fiber having a circular cross section capable of suppressing a decrease in the amount of light emitted depending on a transverse position of incident light.

It is sectional drawing of an example of the wavelength conversion fiber which concerns on embodiment. It is sectional drawing of an example of the wavelength conversion fiber which concerns on embodiment. It is sectional drawing of an example of the wavelength conversion fiber which concerns on embodiment. It is a schematic diagram of the wavelength conversion fiber which has a multi-clad structure. It is a graph which shows the concentration distribution of the fluorescent agent in the wavelength conversion fiber which concerns on Example 1 and Comparative Example. It is a schematic diagram which shows the measurement system for measuring the change in the amount of light emission depending on the position in the radial direction of a fiber. It is sectional drawing of the movable box body which has a slit. It is a graph which shows the measurement result of the light emission amount change by the fiber radial position. It is a perspective view which shows the other use example of the wavelength conversion fiber which concerns on embodiment. It is a perspective view which shows the other use example of the wavelength conversion fiber which concerns on embodiment. FIG. 10 is a cross-sectional view taken along the line XI-XI of FIG. It is a figure which showed the principle of green light emission inside the core of the wavelength conversion fiber irradiated with blue light. It is a schematic diagram of a detector using a wavelength conversion fiber. It is a perspective view of the wavelength conversion fiber having a circular cross section arranged in parallel on the main surface of a flat plate scintillator. It is sectional drawing of the wavelength conversion fiber of the cross section circular shape arranged in parallel on the main surface of a flat plate scintillator. FIG. 5 is a cross-sectional view showing how blue light traverses a wavelength conversion fiber having a circular cross section. It is a perspective view of the wavelength conversion fiber having a rectangular cross section arranged in parallel on the main surface of a flat plate scintillator. FIG. 5 is a cross-sectional view showing how blue light traverses a wavelength conversion fiber having a rectangular cross section.

The wavelength conversion fiber according to the embodiment of the present invention will be described with reference to FIGS. 1 to 3. 1 to 3 are cross-sectional views of an example of a wavelength conversion fiber according to the embodiment. As shown in FIGS. 1 to 3, the wavelength conversion fiber 1 according to the present embodiment is a plastic wavelength conversion fiber having a circular cross section, and includes a core 11, a clad 12 that covers an outer peripheral surface of the core 11, and a clad 12. It has.

The core 11 is made of a transparent resin containing a fluorescent agent that absorbs the first light emitted from a scintillator (not shown) due to irradiation and emits the second light having a different wavelength. The fluorescent agent is not limited to those that absorb blue light having a wavelength of about 430 nm, but is also suitably applied to and not limited to those that absorb ultraviolet light of about 350 nm, those that absorb green light of about 500 nm, and the like. Hereinafter, the light absorbed by the fluorescent agent will be described as, for example, blue light, and the light emitted will be described as, for example, green light. As the core base material, for example, an inexpensive and easy-to-handle styrene resin is used. Details of the core base material and the fluorescent agent constituting the core 11 will be described later.
The clad 12 that covers the outer peripheral surface of the core 11 is made of a transparent resin having a refractive index lower than that of the core 11. The outer diameter of the clad 12 is, for example, 0.2 to 2.0 mm. The clad base material constituting the clad 12 will be described later.

Here, FIG. 4 is a schematic view of a wavelength conversion fiber having a multi-clad structure.
As shown in FIG. 4, the clad 12 may be used as the inner clad, and the outer clad 13 having a low refractive index may be provided on the outer peripheral surface of the clad 12 to form a multi-clad structure. By adopting a multi-clad structure, light at a wider angle can be captured and propagated, so that the detection sensitivity is improved. The outer clad base material constituting the outer clad 13 will be described later.

<Concentration distribution of fluorescent agent>
At the same time, FIGS. 1 to 3 show the concentration distribution of the fluorescent agent doped in the core 11. In the conventional wavelength conversion fiber, the concentration of the fluorescent agent in the core is uniform, whereas in the wavelength conversion fiber 1 according to the present embodiment, as shown in FIGS. 1 to 3, the fluorescent agent in the core 11 is used. The concentration has a distribution in which the concentration increases from the center of the cross section of the core 11 toward the outer periphery. Each of FIGS. 1 to 3 will be described below.

In the wavelength conversion fiber 1 shown in FIG. 1, the concentration of the fluorescent agent is discontinuously increased from the central portion to the outer peripheral portion in two stages of a low concentration C1 in the central portion and a high concentration C2 in the outer peripheral portion. There is.
In the wavelength conversion fiber 1 shown in FIG. 2, a medium concentration C3 is provided between the low concentration C1 in the central portion and the high concentration C2 in the outer peripheral portion, and the concentration of the fluorescent agent is increased in three stages from the central portion to the outer peripheral portion. It is rising discontinuously.
In the wavelength conversion fiber 1 shown in FIG. 3, the concentration of the fluorescent agent is smoothly and continuously increased from the low concentration C1 in the central portion to the high concentration C2 in the outer peripheral portion.

As described with reference to FIG. 16, in the wavelength conversion fiber, the probability that the core base material reacts with the blue light and emits green light is proportional to the crossing distance of the blue light across the core base material. Therefore, the crossing distance is shortened at the outer peripheral portion of the core, and the amount of light emitted is reduced in the conventional wavelength conversion fiber having a uniform concentration of the fluorescent agent in the core.

On the other hand, the wavelength conversion fiber 1 according to the present embodiment shown in FIGS. 1 to 3 has a concentration distribution such that the concentration of the fluorescent agent is high in the outer peripheral portion where the crossing distance of blue light is short. .. Therefore, it is possible to suppress a decrease in the amount of light emitted from the outer peripheral portion of the wavelength conversion fiber 1. That is, although the wavelength conversion fiber has a circular cross section, it is possible to suppress a decrease in the amount of light emitted due to the transverse position of blue light.
Therefore, in the detector in which the wavelength conversion fibers 1 according to the present embodiment are arranged as shown in FIGS. 14 and 15, the dead region formed in the intermediate portion between the wavelength conversion fibers 1 can be suppressed. ..

This will be described in more detail. The wavelength conversion fiber 1 according to the present embodiment shown in FIGS. 1 to 3 is rotationally symmetric so that the fluorescent agent has a low concentration in the central portion of the core 11 and the fluorescent agent has a high concentration in the outer peripheral portion of the core 11. It has a concentration distribution. Therefore, when the blue light crosses the central portion of the core 11, the total crossing distance is long, but the crossing distance of the outer peripheral portion having a high fluorescent agent concentration is short, and the crossing distance of the central portion having a low fluorescent agent concentration is long. On the other hand, when the blue light crosses the outer peripheral portion of the core 11, the total crossing distance becomes shorter, but the crossing distance of the outer peripheral portion having a high fluorescent agent concentration becomes longer.

Therefore, it is possible to suppress a decrease in the amount of light emitted when the blue light crosses the outer peripheral portion as compared with the case where the blue light crosses the central portion of the core 11. By optimizing the concentration distribution of the fluorescent agent in the core 11, the amount of light emitted when the blue light crosses the central portion and the outer peripheral portion of the core 11 can be made uniform.

Although the amount of light emitted is proportional to the cross-sectional distance, it is not necessarily proportional to the fluorescent agent concentration because it is saturated in the high concentration range. Therefore, in order to improve the uniformity of the amount of light emitted in the core, it is preferable to confirm the dependence of the amount of light emitted on the fluorescent agent concentration by an actual trial production. Qualitatively, if the concentration of the fluorescent agent is too low, it is not possible to efficiently absorb blue light and convert it to a longer wavelength. On the other hand, if the concentration of the fluorescent agent is too high, the wavelength conversion efficiency is lowered due to self-absorption, or the transparency due to self-absorption deteriorates at the emitted wavelength. Here, the self-absorption is due to the overlap of the absorption and light emission characteristics shown in FIG.

For example, regarding the concentration distribution of the fluorescent agent, the concentration of the fluorescent agent on the outer periphery of the core is 1.5 times or more, 2 times or more, 3 times or more, 5 times or more, 10 times or more with respect to the concentration of the fluorescent agent in the center of the cross section of the core. It can be more than doubled.

Further, the fluorescent agent may be composed of a plurality of fluorescent agents, and as a whole, the first light emitted from the scintillator due to irradiation may be absorbed to emit the second light having a different wavelength. For example, in the case of a wavelength conversion fiber using two types of a first fluorescent agent and a second fluorescent agent, both may have a concentration distribution, or only one of them may have a concentration distribution. The role of the fluorescent agent in the wavelength conversion fiber is to sequentially convert the light emitted by the core substrate by blue light irradiation into a long wavelength in multiple stages. Therefore, for example, the effect of the present invention can be obtained even if only the first fluorescent agent or the second fluorescent agent has a concentration distribution.

<Core base material>
As the core base material, it is preferable to use a transparent resin material having a relatively high refractive index. For example, benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, chlorobenzyl methacrylate, 1-phenylethyl methacrylate, 1,2-diphenylethyl methacrylate, diphenylethyl methacrylate, furfuryl methacrylate, 1-phenylcyclohexylmethacrylate, pentachlorophenyl methacrylate, pentabromophenyl. A polymer obtained by polymerizing any one of the monomer group consisting of methacrylate, 1-naphthyl methacrylate, 2-naphthyl methacrylate, styrene, α-methylstyrene and vinyltoluene is preferable.

In addition, methyl methacrylate, benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, chlorobenzyl methacrylate, 1-phenylethyl methacrylate, 1,2-diphenylethyl methacrylate, diphenylethyl methacrylate, flufuryl methacrylate, 1-phenylcyclohexyl methacrylate, pentachlorophenyl methacrylate, A copolymer obtained by copolymerizing any two or more of the monomer group consisting of pentabromophenyl methacrylate, 1-naphthyl methacrylate, 2-naphthyl methacrylate, styrene, α-methylstyrene and vinyltoluene is also suitable. .. At the time of polymerization, a general polymerization initiator and molecular weight modifier may be added.

<Clad base material>
The clad substrate is obtained by polymerizing or copolymerizing one or more of a group of monomers consisting of methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, cyclohexyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The polymer to be used is suitable. Of these, a polymer of methyl methacrylate or a copolymer of methyl methacrylate and another monomer is desirable. Methyl methacrylate has the advantage of being highly transparent and easy to handle because it polymerizes easily. At the time of polymerization, a general polymerization initiator and molecular weight modifier may be added.

<Outer clad base material>
As the outer clad base material, a commercially available one may be used as long as it has a lower refractive index than the above-mentioned clad base material. Specifically, methyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 2,2,3,3-tetrafluoropropyl methacrylate, 2,2,3,3,3-pentafluoropropyl methacrylate, 2,2 , 3,4,4,4-hexafluorobutyl methacrylate, 2,2,3,3,4,5,5-octafluoropentyl methacrylate, methyl α-fluoroacrylate and 2- (trifluoromethyl) propene A polymer obtained by polymerizing or copolymerizing one or more of a group of monomers composed of methyl acrylate, and a mixture of the polymer and polyvinylidene fluoride are suitable. In particular, a mixture of 2,2,2-trifluoroethyl methacrylate polymer and polyvinylidene fluoride is desirable.

<Fluorescent agent>
For example, HOSTASOL YELLOW 3G and HOSTASOL YELLOW 8G are fluorescent agents that absorb blue light and convert the wavelength to green light, and LUMOGEN F ORANGE 240 is a fluorescent agent that absorbs blue light and converts it to orange light. LUMOGEN F RED 300 is used for those that absorb blue light and convert the wavelength to red light.

In selecting the wavelength conversion fluorescent agent, it is preferable that the monomer having high solubility in the monomer as the raw material of the core base material and the polymer thereof has high solubility. Further, in order to obtain a wavelength conversion fiber having high emission, it is preferable that the absorption spectrum of the fluorescent agent and the emission spectrum of blue light by the scintillator have a large overlap. At the same time, it is preferable that the quantum efficiency is high and the absorbed photons are converted into long-wavelength photons without being lost by heat or the like. Further, it is preferable that the overlap between the absorption wavelength spectrum distribution and the emission wavelength spectrum distribution in the fluorescent agent is small. If the overlap is large, the wavelength-converted green light is self-absorbed inside the core, so that the conversion efficiency is deteriorated, the long-distance transmission of the green light in the wavelength conversion fiber is hindered, and the transmission loss is deteriorated.

The concentration of the fluorescent agent is preferably 30 to 10000 mass ppm, more preferably 50 to 1000 mass ppm, and further preferably 100 to 500 mass ppm. A suitable concentration index is the molar extinction coefficient of the fluorescent agent. For example, in the case of a wavelength conversion fiber having a diameter of 1 mm, the concentration is set so that 70 to 99% of blue light from the scintillator can be absorbed at a crossing distance of 1 mm. When this index is used, if the diameter of the wavelength conversion fiber is halved to 0.5 mm, the concentration of the fluorescent agent may be doubled. On the other hand, if the diameter of the wavelength conversion fiber is doubled to 2 mm, the concentration of the fluorescent agent may be doubled.

In order for the wavelength conversion fiber to absorb the blue light generated from the scintillator with high efficiency, the fluorescent agent may be dissolved at a high concentration. However, as described above, since all fluorescent agents for wavelength conversion have some overlap between the absorption spectrum and the emission spectrum, excessive addition of the fluorescent agent increases self-absorption. As a result, the conversion efficiency is deteriorated, the transparency is lowered, and the transmission loss is deteriorated. Therefore, the optimum fluorescent agent concentration is determined by balancing the amount of light emission and the transparency so as to be maximized according to the fluorescent agent type and the fiber diameter.

<Fiber manufacturing method>
There are no particular restrictions on the method for manufacturing the wavelength conversion fiber. For example, a transparent rod-shaped polymer for a core (rod for a core) made of a high-refractive index base material is inserted into a cylindrical transparent polymer for a clad (pipe for a clad) made of a low-refractive index base material to form a preform rod. After the production, a rod drawing method in which the tip is heated to draw a thin line can be used.

<Manufacturing method of core rod>
The core rod can be manufactured by heat polymerization by putting a monomer in a cylindrical polymerization container. As the polymerization method, spontaneous polymerization by heat alone without adding an initiator is preferable, but a minimum amount of thermal rupture type radical initiator may be added. Further, a photo-cleavable radical initiator may be used in combination. Further, if the molecular weight of the core rod is too low, the mechanical strength and reliability of the optical fiber may not be ensured. On the contrary, if the molecular weight is too high, the melt viscosity becomes high, so that it is necessary to raise the heating temperature, which may cause problems such as coloring and thermal decomposition due to thermal deterioration. Therefore, a molecular weight modifier may be added as needed.

For example, in the case of manufacturing a wavelength conversion fiber having a discontinuous fluorescent agent concentration distribution as shown in FIGS. 1 and 2, a rod having a small fluorescent agent concentration and a small diameter and a rod having a high fluorescent agent concentration and a large diameter are produced. After stacking a plurality of different rods and pipes such as pipes, they may be combined to form a core rod.

Further, as disclosed in Patent Document 4, the monomer is continuously injected, and the monomer is polymerized and solidified while forming a hollow portion by pressing the monomer against the side surface by centrifugal force in a rotating cylindrical container. An injection method may be used. When the concentration of the fluorescent agent or the like according to the present invention is changed intermittently or continuously as shown in FIG. 3, this monomer continuous injection method can be preferably used.

<Clad pipe manufacturing method>
The clad pipe can be manufactured by a method in which thermoplastic resin pellets are put into a melt extruder equipped with a circular die and extruded into a pipe shape. Further, a method of polymerizing and solidifying while forming a hollow portion by pressing the monomer against the side surface by centrifugal force in a rotating cylindrical container may be used. Further, a method of forming a hollow portion by making a hole in the shaft center portion of the rod-shaped polymer with a drill or the like may be used.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

<Example 1>
[Core rod]
A rod for the core was produced by a continuous monomer injection method. Specifically, polymerization was carried out while injecting a styrene monomer to which the fluorescent agent HOSTASOL YELLOW 8G was added so that the concentration continuously changed from the outer peripheral portion to the central portion into a rotating cylindrical container having an inner diameter of 70 mm. Here, the concentration of the fluorescent agent was 400 mass ppm (0.04 mass%) at the outer peripheral portion and 50 mass ppm (0.005 mass%) at the central portion. It was taken out from a cylindrical container to obtain a hollow rod for a polystyrene core having an outer diameter of 70 mm and an inner diameter of about 10 mm. The refractive index of this polystyrene core hollow rod was 1.59 at 25 ° C.

[Clad pipe]
Polymerization initiator (Nichiyu Co., Ltd. Perocta O (registered trademark) (PO-O): 0.05% by mass, Nichiyu Co., Ltd. Perhexa V (registered trademark) (PH-V): 0.05% by mass) on methyl methacrylate monomer %) And the chain transfer agent n-octyl mercaptan (n-OM: 0.25% by mass) for adjusting the molecular weight were added. This is filled in a cylindrical container having an inner diameter of 75 mm and heated and polymerized at 70 to 120 ° C. while rotating about the axis in a heat medium to obtain a polymethylmethacrylate (PMMA) clad pipe having an outer diameter of 75 mm and an inner diameter of 71 mm. Obtained. The refractive index of this PMMA clad pipe was 1.49 at 25 ° C.

[Preform preparation and heating delineation]
A preform was prepared by combining a hollow rod for a polystyrene core and a pipe for clad made of PMMA. While depressurizing the gap between the center of the hollow rod for the core and the hollow rod for the core and the clad pipe, this preform was heated and drawn to obtain a wavelength conversion fiber having an outer diameter of 1 mm. The core diameter was 960 μm and the clad thickness was about 30 μm.

<Example 2>
A core rod and a clad pipe were produced in the same manner as in Example 1, and the clad pipe was used as an inner clad pipe.

[Pipe for outer clad]
2,2,2-Trifluoroethyl methacrylate (3FMA) monomer, polymerization initiator (PO-O: 0.05% by mass, PH-V: 0.05% by mass) and chain transfer agent n for adjusting the molecular weight -Octyl mercaptan (n-OM: 0.025% by mass) was added. This was placed in a cylindrical container having an inner diameter of 80 mm and heat-polymerized while rotating about the axis in a heat medium to obtain a polytrifluoroethyl methacrylate outer clad pipe having an outer diameter of 80 mm and an inner diameter of 76 mm. The refractive index of the outer clad pipe made of polytrifluoroethyl methacrylate was 1.42 at 25 ° C.

[Preform preparation and heating delineation]
A preform was prepared by combining a polystyrene core rod, a PMMA inner clad pipe, and a polytrifluoroethyl methacrylate outer clad pipe. While depressurizing the center of the hollow rod for the core, the gap between the hollow rod for the core and the pipe for the inner clad, and the gap between the pipe for the inner clad and the pipe for the outer clad, this preform is heated and drawn to form an outer diameter. A 1 mm wavelength conversion fiber was obtained. The core diameter was 900 μm and the clad thickness was about 50 μm as a whole.

<Comparison example>
[Core rod]
The fluorescent agent HOSTASOL YELLOW 8G was uniformly dissolved in a styrene monomer, placed in a cylindrical container having an inner diameter of 70 mm, and the temperature was adjusted to 70 to 120 ° C. for thermal polymerization. It was taken out from the cylindrical container to obtain a polystyrene core rod containing a fluorescent agent. The refractive index of this polystyrene core rod was 1.59 at 25 ° C.

[Clad pipe]
A PMMA clad pipe having an outer diameter of 75 mm and an inner diameter of 71 mm was obtained in the same manner as in Example 1. The refractive index of this PMMA clad pipe was 1.49 at 25 ° C.

[Preform preparation and heating delineation]
A preform was prepared by combining a polystyrene core rod and a PMMA clad pipe. This preform was heated and drawn to obtain a wavelength conversion fiber having an outer diameter of 1 mm. The core diameter was 960 μm and the clad thickness was about 30 μm.

<Measurement of fluorescent agent concentration distribution>
A part of the core rod used for manufacturing the wavelength conversion fiber according to Example 1 and Comparative Example is cut, sampled in the radial direction, dissolved and diluted in a chloroform solvent, and then the absorbance of the fluorescent agent is measured. The concentration distribution of the fluorescent agent was obtained.

FIG. 5 shows the measurement result of the concentration distribution of the fluorescent agent converted into the diameter of the wavelength conversion fiber after drawing. FIG. 5 is a graph showing the concentration distribution of the fluorescent agent in the wavelength conversion fibers according to Example 1 and Comparative Example. The horizontal axis represents the fiber radial position (μm), and the vertical axis represents the fluorescent agent concentration (mass ppm). The vicinity of the fiber radial position of 500 μm corresponds to the central portion.

As shown in FIG. 5, in the comparative example, the concentration was substantially the same regardless of the position in the radial direction of the fiber, whereas in the first embodiment, the concentration continuously increases from the central portion to the outer peripheral portion. It became the concentration distribution of the fluorescent agent. That is, the wavelength conversion fiber according to the first embodiment corresponds to the wavelength conversion fiber shown in FIG.

<Measurement of change in luminescence>
For the wavelength conversion fiber having an outer diameter of 1 mm according to Example 1 and Comparative Example, the change in the amount of light emitted depending on the position in the radial direction of the fiber was measured. FIG. 6 is a schematic view showing a measurement system for measuring a change in the amount of light emitted depending on the position in the radial direction of the fiber. FIG. 7 is a cross-sectional view of a movable box body having a slit. The right-handed xyz coordinates shown in FIGS. 6 and 7 correspond to each other in the drawings, but are for convenience to explain the positional relationship of the components. The z-axis plus direction is vertically upward, and the xy plane is the horizontal plane.

As shown in FIG. 6, the wavelength conversion fiber 1 extended in the y-axis direction was irradiated with blue light (peak wavelength 450 nm) emitted from the blue LED from above. Here, as shown in FIG. 7, the blue light emitted from the blue LED is parallel to the z-axis via the collimator lens, and then passes through the slit provided in the movable box to the wavelength conversion fiber 1. Be irradiated. A slit having a width of 100 μm is provided at a position corresponding to the top plate and the bottom plate of the movable box body. Therefore, the wavelength conversion fiber 1 is irradiated with pseudo-linear light having a width of 100 μm. By moving the movable box in the x-axis direction, the irradiation position of the pseudo-linear light having a width of 100 μm also moves in the x-axis direction, so that the change in the amount of light emitted depending on the position in the radial direction of the fiber can be measured.

Then, as shown in FIG. 6, the green light generated in the wavelength conversion fiber 1 by the irradiation of blue light was detected by the Si-APD detector installed at one end of the wavelength conversion fiber 1. The electric signal output from the Si-APD detector was measured with an optical power meter. With such a configuration, the light intensity of the green light generated in the wavelength conversion fiber 1 by the irradiation of the blue light was measured.

FIG. 8 is a graph showing the measurement result of the change in the amount of light emitted depending on the position in the radial direction of the fiber. The horizontal axis represents the fiber radial position (μm), and the vertical axis represents the relative emission amount. The vicinity of the fiber radial position of 500 μm corresponds to the central portion.

As shown in FIG. 8, in the comparative example, the higher the light emission was as the blue light passing position was closer to the central portion (near the fiber radial position 500 μm), and the lower the light emission was as the distance from the central portion was closer to the outer peripheral portion. That is, in one wavelength conversion fiber, the amount of light emitted differs depending on whether the blue light passes through the central portion and the outer peripheral portion, and the detection sensitivity becomes lower toward the outer peripheral portion. As described above, it is not preferable that the amount of light emitted changes depending on the crossing position of blue light even in a single wavelength conversion fiber. In particular, in a detector in which wavelength conversion fibers are arranged and used as shown in FIGS. 14 and 15, a dead region may be formed in an intermediate portion between the wavelength conversion fibers, which is not preferable.

On the other hand, as shown in FIG. 8, in Example 1, the amount of light emitted was substantially the same regardless of the position in the radial direction of the fiber, and high light emission was maintained. That is, although the wavelength conversion fiber according to the first embodiment has a circular cross section, a uniform light emission amount can be obtained regardless of the position in the radial direction of the fiber, similarly to the wavelength conversion fiber having a rectangular cross section. As described above, it is preferable that the decrease in the amount of light emitted due to the crossing position of the blue light is suppressed even in one wavelength conversion fiber. In particular, in a detector in which wavelength conversion fibers are arranged and used as shown in FIGS. 14 and 15, it is possible to suppress the formation of a dead region in the intermediate portion between the wavelength conversion fibers, which is preferable.

The multi-clad type wavelength conversion fiber according to Example 2 had the same fluorescent agent concentration distribution as in Example 1 shown in FIG. 5, and showed a change in the amount of light emission similar to that in Example 1 shown in FIG. .. Further, the amount of light emitted from Example 2 was 30% higher on average than that of Example 1 of the single clad type.

As described above, Example 1 is a wavelength conversion fiber having a continuous fluorescent agent concentration distribution shown in FIG. 3, but has a discontinuous fluorescent agent concentration distribution as shown in FIGS. 1 and 2. The same effect can be obtained with.

<Other embodiments>
9 and 10 are perspective views showing another use example of the wavelength conversion fiber according to the embodiment. In the example of FIG. 9, the wavelength conversion fiber 1 is arranged in a groove 21 extending in the longitudinal direction on the upper surface of the square bar-shaped transparent scintillator 2. In the example of FIG. 10, the wavelength conversion fiber 1 is arranged in the groove 21 formed in a U shape on the upper surface of the flat plate-shaped transparent scintillator 2.

FIG. 11 is a cross-sectional view taken along the line XI-XI of FIG. As shown in FIG. 11, when radiation penetrates the scintillator 2, blue light is generated inside the scintillator 2. A part of the blue light crosses the wavelength conversion fiber 1 while repeating total internal reflection inside the scintillator 2. At that time, green light is generated inside the core 11 of the wavelength conversion fiber 1, and the green light is transmitted to the end face of the wavelength conversion fiber 1 and detected by a photodetector such as PMT or Si—PM. The same applies to the example of FIG.

In the examples of FIGS. 9 and 10, a through hole may be provided instead of the groove, and the wavelength conversion fiber 1 may be inserted into the through hole. Alternatively, the wavelength conversion fiber 1 may be arranged on the flat upper surface of the scintillator 2 without providing the groove.

The present invention is not limited to the above-described embodiment, and can be appropriately modified without departing from the spirit.

1 Wavelength conversion fiber 2 Scintillator 11 Core 12 clad (inner clad)
13 Outer clad 21 Groove

Claims (8)

A plastic wavelength conversion fiber that converts the first light emitted from a scintillator due to irradiation into second light having a different wavelength and transmits the second light.
A core containing a fluorescent agent that absorbs the first light and emits the second light, and a clad that covers the outer peripheral surface of the core and has a refractive index lower than that of the core.
The cross section is circular
A plastic wavelength conversion fiber having a distribution in which the concentration of the fluorescent agent in the core increases from the center of the cross section of the core toward the outer periphery. The plastic wavelength conversion fiber according to claim 1, wherein the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent increases discontinuously in two or more steps from the center of the cross section of the core toward the outer periphery. The plastic wavelength conversion fiber according to claim 1, wherein the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent continuously increases from the center of the cross section of the core toward the outer peripheral direction. Any of claims 1 to 3, wherein the clad has a multi-clad structure including an inner clad and an outer clad that covers the outer peripheral surface of the inner clad and has a refractive index lower than that of the inner clad. The plastic wavelength conversion fiber according to claim 1. The method for manufacturing a plastic wavelength conversion fiber according to any one of claims 1 to 4.
A step of producing a preform by inserting a rod-shaped core rod made of a resin containing a fluorescent agent into a cylindrical clad pipe made of a resin having a refractive index lower than that of the core rod.
A step of drawing a line while heating the preform is provided.
A method for manufacturing a plastic wavelength conversion fiber, which has a distribution in which the concentration of the fluorescent agent in the core rod increases from the center of the cross section of the core rod toward the outer periphery. The method for manufacturing a plastic wavelength conversion fiber according to claim 5, wherein the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent increases discontinuously in two or more steps from the center of the cross section of the core rod toward the outer periphery. The method for manufacturing a plastic wavelength conversion fiber according to claim 5, wherein the concentration of the fluorescent agent has a distribution in which the concentration of the fluorescent agent continuously increases from the center of the cross section of the core rod toward the outer peripheral direction. A claim that the clad pipe is composed of an inner clad pipe and an outer clad pipe made of a resin that covers the outer peripheral surface of the inner clad pipe and has a refractive index lower than that of the inner clad pipe. Item 8. The method for manufacturing a plastic wavelength conversion fiber according to any one of Items 5 to 7.

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