Plastic wavelength conversion fiber and its manufacturing method

Abstract

A plastic wavelength conversion fiber according to one mode of the present invention comprises a core (11) containing a fluorescent agent having a peak of a fluorescence spectrum in the wavelength range of 430-550 nm, and cladding (12) that coats an outer circumferential surface of the core (11) and that has a lower refractive index than the core (11). The number of carbonyl bonds in the fluorescent agent is one or less.

Description

[Technical Field]

[0001] The present invention relates to a plastic wavelength conversion fiber and a method for manufacturing the same. [Background technology]

[0002] Scintillators, such as organic liquid scintillators, solid (plate) inorganic scintillators, and plastic scintillators, have long been widely used in the field of radiation detection, including elementary particle detection, and are important components in radiation measurement. These scintillators contain a fluorescent agent that emits blue light around 430 nm when exposed to radiation, matching the maximum sensitivity of photomultiplier tubes. Here, rather than directly detecting the blue light emitted from the scintillator when exposed to radiation, a known method is to convert the wavelength of the blue light emitted by the scintillator to green light or other light and transmit it through an optical fiber, thereby indirectly detecting it.

[0003] Such optical fibers are called wavelength-shifting (WLS) fibers, and have a core that absorbs blue light generated in the scintillator and converts it to green light or other wavelengths, and the outer surface of the core is covered with a cladding that has a lower refractive index than the core. Plastic wavelength-shifting fibers, in particular, are less expensive and easier to process than glass fibers. For this reason, they are widely used in particle physics research, just like plastic scintillation fibers, which have scintillation properties themselves.

[0004] The core substrate of a plastic wavelength conversion fiber is typically made of polystyrene, a highly transparent resin material with a relatively high refractive index. By using polystyrene as the core substrate, a large refractive index difference can be achieved, thereby increasing the total reflection angle with the cladding. This allows the green light generated by wavelength conversion within the core to be trapped within the core at a wider angle and transmitted to the fiber end face. In other words, by using polystyrene as the core substrate, a wavelength conversion fiber with high luminescence can be realized.

[0005] Here, Figure 4 is a diagram showing the principle of green light emission inside the core of a wavelength-converting fiber irradiated with blue light. An organic fluorescent agent is dissolved in the core substrate, which absorbs blue light with a wavelength of around 430 nm, which is emitted by the scintillator, and converts it into green light with a wavelength of around 500 nm. Therefore, part or all of the blue light generated in the scintillator and traversing the wavelength-converting fiber is absorbed in the core and converted into green light, as shown in Figure 4.

[0006] 5 is a schematic diagram of a detector using a wavelength conversion fiber. As shown in Fig. 5, blue light having a wavelength of about 430 nm generated from the scintillator traverses the plastic wavelength conversion fiber 1, where it is absorbed by the fluorescent agent contained in the core 11 and converted into green light having a wavelength of about 500 nm. The green light generated inside the core 11 propagates toward both ends of the plastic wavelength conversion fiber 1 while repeatedly undergoing total reflection at the interface between the core 11 and the cladding 12. The green light guided to one end of the plastic wavelength conversion fiber 1 is then detected as an electrical signal by a photomultiplier tube (PMT).

[0007] Instead of the PMT shown in Figure 5, a semiconductor detector such as a silicon photomultiplier (Si-PM) using a silicon avalanche photodiode (Si-APD) may be used. Recently, multi-pixel photon counter (MPPC) arrays, in which Si-PMs are segmented into small segments and arranged in large numbers, have come into widespread use. In an MPPC array, each pixel is connected to a wavelength conversion fiber, allowing the green light generated in each wavelength conversion fiber to be detected simultaneously.

[0008] For detectors used in particle physics research, it is important to be able to distinguish between events of interest and noise events (background events), and there has long been a demand for improved time and position resolution in detectors. Patent Document 1 discloses a single-crystal solid scintillator with a shortened fluorescence lifetime in order to improve the detector's time resolution. Patent Documents 2 and 3 disclose inorganic scintillators containing inorganic solid crystals and having high time resolution. Patent Documents 4 and 5 will be mentioned in the embodiments of the present invention. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-248061 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-002084 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-013216 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-116327 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-104208 Summary of the Invention [Problem to be solved by the invention]

[0010] It is desirable to further improve the time resolution of detectors by shortening the fluorescence lifetime in plastic wavelength conversion fibers. An object of the present invention is to provide a plastic wavelength conversion fiber with a short fluorescence lifetime. [Means for solving the problem]

[0011] A plastic wavelength conversion fiber according to one aspect of the present invention comprises: a core containing a fluorescent agent having a fluorescence spectrum peak in the wavelength range of 430 to 550 nm; The plastic wavelength conversion fiber includes a cladding that covers the outer peripheral surface of the core and has a refractive index lower than that of the core. The fluorescent agent has one or less carbonyl bonds. This makes it possible to provide a plastic wavelength conversion fiber with a short fluorescence lifetime.

[0012] A method for producing a plastic wavelength conversion fiber according to one aspect of the present invention includes the steps of: a core containing a fluorescent agent having a fluorescence spectrum peak in the wavelength range of 430 to 550 nm; The method for manufacturing a plastic wavelength conversion fiber includes a cladding that covers the outer peripheral surface of the core and has a refractive index lower than that of the core. a step of inserting the core rod into the cladding cylinder to prepare a preform; and a step of drawing the preform while heating it, The fluorescent agent has one or less carbonyl bonds. This makes it possible to provide a plastic wavelength conversion fiber with a short fluorescence lifetime.

[0013] The fluorescent spectrum of the fluorescent agent may have a peak wavelength range of 450 to 550 nm.

[0014] The fluorescent agent may be a bisstyrylbenzene compound, a bisvinylcarbazolebenzene compound, or a coumarin compound.

[0015] The outer diameter of the plastic wavelength conversion fiber may be 0.1 to 3.0 mm. [Effects of the Invention]

[0016] The present invention can provide a plastic wavelength conversion fiber with a short fluorescence lifetime. [Brief explanation of the drawings]

[0017] [Figure 1]1 is a cross-sectional view of an example of a plastic wavelength conversion fiber according to a first embodiment. [Figure 2] 1 is a graph showing the relationship between the number of carbonyl bonds in a fluorescent agent and fluorescence lifetime. [Figure 3] FIG. 1 is a schematic diagram of an attenuation length measuring device. [Figure 4] 1A and 1B are diagrams illustrating the principle of green light emission inside the core of a wavelength conversion fiber irradiated with blue light. [Figure 5] FIG. 1 is a schematic diagram of a detector using a wavelength conversion fiber. DETAILED DESCRIPTION OF THE INVENTION

[0018] A plastic wavelength conversion fiber according to a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view of an example of a plastic wavelength conversion fiber according to the embodiment. As shown in Fig. 1, the plastic wavelength conversion fiber 1 according to the present embodiment has a circular cross section and includes a core 11 and a clad 12 that covers the outer peripheral surface of the core 11. The clad 12 may be used as an inner clad, and an outer clad (not shown) having a lower refractive index may be provided on the outer peripheral surface of the clad 12, forming a multi-clad structure. The multi-clad structure allows light with a wider angle to be captured and propagated, thereby improving detection sensitivity.

[0019] The core 11 is made of a transparent resin containing a fluorescent agent that absorbs blue light and emits green light. The core base material is, for example, a styrene-based resin, which is inexpensive and easy to handle. The core base material and fluorescent agent that make up the core 11 will be described in detail later. The clad 12 that covers the outer peripheral surface of the core 11 is made of a transparent resin having a lower refractive index than the core 11. The clad base material that forms the clad 12 will be described later. The outer diameter of the plastic wavelength conversion fiber is, for example, 3.0 mm or less, preferably 2.0 mm or less, and the outer diameter of the plastic wavelength conversion fiber is, for example, 0.1 mm or more, preferably 0.2 mm or more.

[0020] In the plastic wavelength conversion fiber according to the present embodiment, the number of carbonyl bonds contained in the fluorescent agent molecule is controlled, and therefore the plastic wavelength conversion fiber according to the present embodiment can shorten the fluorescence lifetime. The principle will be explained below.

[0021] A more specific description will be given with reference to FIG. 4 mentioned above. Fluorescence is a phenomenon in which fluorescent agent molecules absorb energy, become excited, and then emit light with a longer wavelength than the absorbed light. This phenomenon is commonly seen in π-conjugated organic compounds, which have a structure consisting of repeated double and single bonds.

[0022] The inventors investigated various π-conjugated organic compounds that emit green fluorescence in the wavelength range of 430 to 550 nm, preferably 450 to 550 nm, and found that the number of carbonyl bonds in the chemical structure of the fluorescent agent molecule correlates with the lifetime of the fluorescence emitted from the fluorescent agent. This is thought to be because double bonds contribute significantly to the necessary π-conjugated structure. As a result of experiments described in detail below, it was found that a fluorescent agent with one or fewer carbonyl bonds has a short lifetime. The fluorescent lifetime is preferably 3 ns or less. In this specification, the term "carbonyl bond" refers to a double bond between carbon and oxygen (C=O), and functional groups having a carbonyl bond naturally include amide groups, aldehyde groups, ketone groups, and the like.

[0023] <Core substrate> The core substrate is preferably a transparent resin material having a relatively high refractive index, such as a polymer obtained by polymerizing any one selected from the group consisting of benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, chlorobenzyl methacrylate, 1-phenylethyl methacrylate, 1,2-diphenylethyl methacrylate, diphenylethyl methacrylate, furfuryl methacrylate, 1-phenylcyclohexyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, 1-naphthyl methacrylate, 2-naphthyl methacrylate, styrene, α-methylstyrene, and vinyltoluene.

[0024] Also suitable are copolymers obtained by copolymerizing two or more monomers selected from the group consisting of methyl methacrylate, benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, chlorobenzyl methacrylate, 1-phenylethyl methacrylate, 1,2-diphenylethyl methacrylate, diphenylethyl methacrylate, furfuryl methacrylate, 1-phenylcyclohexyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, 1-naphthyl methacrylate, 2-naphthyl methacrylate, styrene, α-methylstyrene, and vinyltoluene. A typical polymerization initiator and molecular weight modifier may be added during polymerization.

[0025] <Clad substrate> Suitable clad substrates are polymers obtained by polymerizing or copolymerizing one or more monomers selected from the group consisting of methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, cyclohexyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate. Among these, polymers of methyl methacrylate or copolymers of methyl methacrylate with other monomers are desirable. Methyl methacrylate has the advantages of being highly transparent and easily polymerized, making it easy to handle. During polymerization, a general polymerization initiator and molecular weight modifier may be added.

[0026] The outer clad substrate may be a commercially available material as long as it has a lower refractive index than the clad substrate. Specifically, polymers obtained by polymerizing or copolymerizing one or more monomers selected from the group consisting of 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, 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate, methyl α-fluoroacrylate, and methyl 2-(trifluoromethyl)propenoate, as well as mixtures of such polymers with polyvinylidene fluoride, are suitable. A mixture of 2,2,2-trifluoroethyl methacrylate polymer and polyvinylidene fluoride is particularly desirable.

[0027] <Fluorescent agent> The fluorescent agent has a fluorescence spectrum peak in the wavelength range of 430 to 550 nm, preferably 450 to 550 nm, and absorbs blue light and converts the wavelength to green light. There are no limitations on such fluorescent agents, as long as they have one or less carbonyl bonds. Preferred are condensed polycyclic hydrocarbons, hydrocarbon ring assemblies, and condensed heterocyclic systems. From the viewpoint of solubility in the plastic material that constitutes the core, the molecular weight of the fluorescent agent is preferably 200 to 1,000, more preferably 300 to 700. For example, bisstyrylbenzene compounds, bisvinylcarbazolebenzene compounds, and coumarin compounds are preferred.

[0028] Examples of bisvinylcarbazole benzene compounds include styrylbenzene compounds such as 1,4-bis(2-(9-ethylcarbazole-3-yl)vinyl)benzene. An example of a bisstyrylbenzene compound is 1,4-bis(4-(di-p-tolylamino)styrylbenzene). An example of the coumarin compound is 3-(2-benzimidazolyl)-7-(diethylamino)coumarin.

[0029] When selecting a wavelength-converting fluorescent agent, it is preferable that it has high solubility in the monomer, which is the raw material of the core substrate, and further in its polymer. The concentration of the fluorescent agent is preferably 30 to 10,000 mass ppm, more preferably 50 to 1,000 mass ppm, and even more preferably 100 to 500 mass ppm. The molar extinction coefficient of the fluorescent agent is used as an indicator of the preferred concentration. For example, in the case of a wavelength-converting fiber with a diameter of 1 mm, the concentration is set so that 70 to 99% of the blue light from the scintillator can be absorbed over a 1 mm crossing distance.

[0030] <Fiber manufacturing method> There are no particular limitations on the manufacturing method of the wavelength conversion fiber. For example, a preform is made by inserting a transparent rod-shaped polymer core (core rod) made of a high refractive index base material into a cylindrical transparent polymer cladding (cladding cylinder) made of a low refractive index base material. Then, the tip of the preform is heated and drawn into a thin fiber. The outer diameter and length of the fiber are selected appropriately according to the conditions of use.

[0031] <Method of manufacturing core rods> The core rod can be produced by thermal polymerization of a monomer placed in a cylindrical polymerization vessel. The polymerization method is preferably spontaneous polymerization using heat alone without the addition of an initiator, but a minimal amount of a thermally cleavable radical initiator may also be added. Furthermore, a photocleavable radical initiator may also be used. Furthermore, if the molecular weight of the core rod is too low, the mechanical strength and reliability required for optical fiber may not be ensured. Conversely, if the molecular weight is too high, the melt viscosity increases, requiring a higher heating temperature, which may lead to problems such as coloration due to thermal degradation and thermal decomposition. Therefore, a molecular weight modifier may be added as needed.

[0032] <Method for manufacturing cladding cylinders> The cladding cylinder can be produced by a method in which thermoplastic resin pellets are fed into a melt extruder equipped with a circular die and extruded into a cylindrical shape. Alternatively, a method in which the monomer is pressed against the side of a rotating cylindrical container by centrifugal force to form a hollow portion while polymerizing and solidifying the monomer. Furthermore, a method in which a hole is drilled or the like in the axial center of a cylindrical polymer to form a hollow portion can also be used.

[0033] <Fiber performance> As mentioned above, the concentration of the fluorescent agent is set so that 70 to 99% of the blue light is absorbed at the outer diameter of the fiber. In the case of a single clad structure with an outer diameter of 1 mm, the optical performance of the fiber should preferably be such that the attenuation length AL is greater than 300 cm. The attenuation length AL can be adjusted by the clad structure, the type of fluorescent agent, the concentration of the fluorescent agent, and the outer diameter of the fiber. [Example]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. Table 1 shows the molecular structure, molecular weight, classification, molecular formula, number of carbonyl bonds, and fluorescence lifetime (ns) of the fluorescent agents according to Examples 1 to 3.

[0035] The fluorescence lifetime was measured using a compact fluorescence lifetime measurement system, Quantaurus-Tau, manufactured by Hamamatsu Photonics, in which a dilute concentration of approximately 10 ppm of the fluorescent agent was dissolved in styrene monomer. The excitation wavelength was selected to match the absorption spectrum of the fluorescent agent solvent and to be close to the peak wavelength.

[0036] [Table 1]

[0037] [Example 1] The styrylbenzene-based fluorescent agent used was listed in Table 1. The number of carbonyl bonds was 0, and the fluorescence lifetime was 0.9 ns.

[0038] [Example 2] The styrylbenzene fluorescent agent used was listed in Table 1. The number of carbonyl bonds was 0, and the fluorescence lifetime was 1.1 ns.

[0039] [Example 3] The coumarin-based fluorescent agent used was listed in Table 1. It had one carbonyl bond and a fluorescence lifetime of 2.4 ns.

[0040] Table 2 shows the molecular structure, molecular weight, classification, molecular formula, number of carbonyl bonds, and fluorescence lifetime (ns) of the fluorescent agents according to Comparative Examples 1 and 2.

[0041] [Table 2]

[0042] [Comparative Example 1] The perylene-based fluorescent agent used was listed in Table 1. It had two carbonyl bonds and a fluorescence lifetime of 3.8 ns.

[0043] Comparative Example 2 The thioxanthene fluorescent agent used was listed in Table 1. It had two carbonyl bonds and a fluorescence lifetime of 7.0 ns.

[0044] Here, Figure 2 is a graph showing the relationship between the number of carbonyl bonds in a fluorescent agent and the fluorescence lifetime. As shown in Tables 1 and 2 and Figure 2, it was found that if the number of carbonyl bonds is two or more, the fluorescence lifetime exceeds 3 ns, which is undesirable.

[0045] Fibers using the fluorescent agents according to Examples 1 to 3 and Comparative Examples 1 and 2 were manufactured, and the attenuation length AL was evaluated.

[0046] [Evaluation of fluorescent agents in monomer solvents] [Absorption spectrum measurement] The absorption spectrum was measured using a Shimadzu UV-3600 ultraviolet-visible-infrared spectrophotometer, using a solvent prepared by dissolving a fluorescent agent at a dilute concentration of approximately 10 ppm in styrene monomer. The spectrum measurement range was 300 to 700 nm.

[0047] [Fluorescence spectrum measurement] Fluorescence spectra were measured using a FluoroMax-4 spectrophotometer manufactured by Horiba, Ltd., using a solution of a dilute concentration of approximately 10 ppm of fluorescent agent dissolved in styrene monomer. The excitation wavelength was the maximum absorption wavelength of each fluorescent agent. The spectral measurement range was 300 to 700 nm. When the concentration of the fluorescent agent becomes high, the fluorescent spectrum of the fluorescent agent itself cannot be measured properly due to reabsorption by the fluorescent agent, so the limit is set at 10 ppm.

[0048] [Quantum yield QE measurement] The quantum yield (QE) was measured using a Hamamatsu Photonics absolute PL quantum yield measurement system, Quantaurus-QY, in which a dilute concentration of approximately 10 ppm of fluorescent agent was dissolved in styrene monomer. The excitation wavelength was the maximum absorption wavelength of each fluorescent agent.

[0049] [Performance evaluation of fibers using fluorescent agents] [Fiber manufacturing method] Fibers containing fluorescent agents according to Examples 1 to 3 and Comparative Examples 1 and 2 were manufactured according to the methods described in Patent Documents 4 and 5. The fibers had an outer diameter of 1 mm and a single clad structure. The concentration of the fluorescent agent was set so that 70 to 99% of blue light was absorbed over a 1 mm crossing distance.

[0050] [Measurement of attenuation length AL] The attenuation length AL of the fiber was measured using the device shown in Figure 3. Figure 3 is a schematic diagram of the attenuation length measurement device. With the attenuation length measurement device shown in Figure 3, blue light was incident on the sample fiber at different distances from the photomultiplier tube (PMT), and the intensity of the light guided through the sample fiber was observed with the PMT. A blue LED with a peak wavelength at 470 nm was used as the light source. A photomultiplier tube R647 manufactured by Hamamatsu Photonics was used for the PMT. Using data from light guide distances of 100 to 285 cm, the attenuation length AL was calculated from the relationship between the light guide distance and the detected light intensity.

[0051] As shown in Figure 3, the attenuation length measurement device is covered with a black box to block external light. The blue LED is mounted on a movable linear stage that can move in the longitudinal direction of the sample fiber. The movable linear stage is driven by a stepping motor and moves on a linear guide that extends parallel to the sample fiber. Blue light from the blue LED is incident on the sample fiber through an electromagnetic shutter and a pinhole installed on the movable linear stage.

[0052] [Evaluation results] Table 3 summarizes the evaluation results of the fluorescent agents and fibers using them according to Examples 1 to 3 and Comparative Examples 1 and 2. From the top, Table 3 lists the fluorescent agent concentration (ppm), the fluorescent agent lifetime (ns), the quantum yield QE, the fluorescent agent absorption peak wavelength (nm), the emission peak wavelength (nm), and the fiber attenuation length AL (cm).

[0053] [Table 3]

[0054] [Example 1] The quantum yield QE was 0.93, the absorption peak wavelength was 396 nm, and the fluorescence peak wavelength was 438 nm. The attenuation length AL of the fiber was 393 cm at a fluorescent agent concentration of 150 ppm, which exceeded the target of 300 cm and was extremely good.

[0055] [Example 2] The quantum yield QE was 0.89, the absorption peak wavelength was 420 nm, and the fluorescence peak wavelength was 470 nm. The attenuation length AL of the fiber was 416 cm at a fluorescent agent concentration of 200 ppm, which exceeded the target of 300 cm and was extremely good.

[0056] [Example 3] The quantum yield QE was 0.79, the absorption peak wavelength was 436 nm, and the fluorescence peak wavelength was 484 nm. The attenuation length AL of the fiber was 170 cm at a fluorescent agent concentration of 100 ppm, which was shorter than the target value of 300 cm.

[0057] [Comparative Example 1] The quantum yield QE was 0.77, the absorption peak wavelength was 470 nm, and the fluorescence peak wavelength was 512 nm. The fiber attenuation length AL was 205 cm at a fluorescent agent concentration of 250 ppm, which was shorter than the target value of 300 cm.

[0058] Comparative Example 2 The quantum yield QE was 0.81, the absorption peak wavelength was 456 nm, and the fluorescence peak wavelength was 508 nm. The fiber attenuation length AL was 287 cm at a fluorescent agent concentration of 100 ppm, which was shorter than the target value of 300 cm.

[0059] The present invention is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the invention.

[0060] This application claims priority based on Japanese Patent Application No. 2021-004762, filed on January 15, 2021, the disclosure of which is incorporated herein in its entirety. [Explanation of symbols]

[0061] 1. Plastic wavelength conversion fiber 11 cores 12 Clad

Claims

1. a core containing a fluorescent agent having a fluorescence spectrum peak in the wavelength range of 430 to 550 nm; A plastic wavelength conversion fiber comprising: a cladding that covers an outer peripheral surface of the core and has a refractive index lower than that of the core; the number of carbonyl bonds in the fluorescent agent is 0, The fluorescent agent is a bisstyrylbenzene compound or a bisvinylcarbazolebenzene compound. Plastic wavelength conversion fiber.

2. The peak wavelength range of the fluorescence spectrum of the fluorescent agent is 450 to 550 nm; 2. The plastic wavelength conversion fiber according to claim 1.

3. The outer diameter is 0.1 to 3.0 mm.

3. The plastic wavelength conversion fiber according to claim 1 or 2.

4. a core containing a fluorescent agent having a fluorescence spectrum peak in the wavelength range of 430 to 550 nm; a cladding covering the outer peripheral surface of the core and having a refractive index lower than that of the core, a step of inserting the core rod into the cladding cylinder to prepare a preform; and a step of drawing the preform while heating it, the number of carbonyl bonds in the fluorescent agent is 0, The fluorescent agent is a bisstyrylbenzene compound or a bisvinylcarbazolebenzene compound. Manufacturing method of plastic wavelength conversion fiber.

5. The peak wavelength range of the fluorescence spectrum of the fluorescent agent is 450 to 550 nm; The method for producing the plastic wavelength conversion fiber according to claim 4.

6. The outer diameter of the plastic wavelength conversion fiber is 0.1 to 3.0 mm. The method for producing the plastic wavelength conversion fiber according to claim 4 or 5.

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