JP7611547B2 - Plastic scintillator and its manufacturing method - Google Patents

Description

The present invention relates to a plastic scintillator used for radiation detection, in particular a plastic scintillator containing a metal oxide, and a method for producing the same.

Radiation, which is generally a charged particle such as alpha rays or beta rays, ionizes, excites or dissociates atoms or molecules in a material when it passes through that material, losing energy. The energy transferred to the material is then released as thermal kinetic energy or electromagnetic waves. If the material is a fluorescent material, a large portion of the energy is released as light in the visible range; this phenomenon is called scintillation, and the light that is released is called scintillation light.

In the case of radiation that does not have an electric charge, such as X-rays, gamma rays, and neutron rays, similar scintillation occurs due to secondary charged particles that are released when these radiations interact with matter, and this can be used to detect radiation.

Substances that cause scintillation as described above are generally known as scintillators, and are used in radiation detectors in a wide range of fields, including academic fields such as particle physics and material and life sciences, as well as medicine and industry. Scintillators used in radiation detectors used in positron emission tomography (PET) scanners and positron annihilation lifetime measurement devices are required to have high time resolution and detection efficiency in order to accurately determine the location where annihilation gamma rays are generated and obtain highly accurate images.

Scintillators are classified into inorganic scintillators and organic scintillators.
Inorganic scintillators have the advantage of generally having high detection efficiency and large light emission due to their large effective atomic number, but many inorganic scintillators have problems with low achievable time resolution and counting rate due to the long decay time of their light emission. In addition, they have problems such as a low ratio of light-emitting components with short decay times, deliquescence, and increased background due to the radioactive nuclides contained in the scintillators themselves.

Examples of such inorganic scintillators include NaI (Tl) scintillators, BaF2 scintillators, and LaBr3 (Ce). Although NaI (Tl) scintillators emit a relatively large amount of light, their time resolution is poor due to the long decay time of the emitted light of about 230 ns. Although BaF2 scintillators contain short-lived components with a decay time of 0.6 ns, they also contain a large amount of long-lived components with a decay time of 620 ns, which accounts for about 75% of the total emitted light, making them difficult to use for high count rate measurements. LaBr3 (Ce) scintillators have a relatively short decay time of about 20 ns, but the crystal contains the radioactive nuclide 138La and 227Ac series nuclides mixed in as impurities, and there is a disadvantage in that there is a background caused by the self-radioactivity of these.

On the other hand, a representative example of an organic scintillator is a so-called plastic scintillator, which is obtained by dissolving an organic light-emitting compound in a high molecular weight polymer such as polystyrene or polyvinyl toluene.
For example, Non-Patent Document 1 describes several plastic scintillators. These plastic scintillators have the advantage that they can achieve good time resolution and a high counting rate because the decay time of luminescence is short compared to the inorganic scintillators, but they have a problem in that the detection efficiency is low because the atomic numbers of the atoms (C, H, O, N) constituting the scintillator are small and the probability of electromagnetic interaction between X-rays or gamma rays and electrons in the scintillator is low.

To improve the detection efficiency for X-rays and gamma rays, a plastic scintillator (EJ-256, Eljen Technogy) is commercially available, in which polyvinyl toluene is filled with 1.5 to 5 mass% lead, a heavy metal. However, because the amount of fill is small, no significant increase in detection efficiency can be expected, and because lead is a harmful metal, it is desirable to avoid using it.

Several studies have been conducted to improve these problems. For example, the plastic scintillator in Patent Document 1 contains zirconium oxide nanoparticles or hafnium oxide nanoparticles that have been surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid, and these can be dispersed at high concentrations in the plastic base material of the plastic scintillator, so that it is said that high time resolution, high counting rate, and high detection efficiency can be achieved.

Incidentally, the electromagnetic interaction that occurs when detecting X-rays or gamma rays is a reaction with electrons, and so depends on the atomic number Z, which is equal to the number of electrons in one atom. In particular, in the case of the photoelectric effect, which is the main reaction in the X-ray region, the reaction cross section τ has the relationship τ ∝ Z ^5. Therefore, adding heavy elements with large atomic numbers to plastic scintillators is one effective way to increase the detection efficiency of plastic scintillators.

Therefore, plastic scintillators have been proposed that use oxides of bismuth, an atom with a larger atomic number, such as bismuth oxide particles that have been surface-treated with phenylpropionic acid (Non-Patent Document 2) and plastic scintillators that contain bismuth oxide particles produced by a solvent evaporation method (Non-Patent Document 3).

Radiation Measurement Using Scintillators Masaaki Kobayashi October 2014 Japanese Journal of Applied Physics, Vol. 57, 052203, 2018 Radiation Measurements, Vol. 135, 106361, 2020

Patent No. 6810941

The plastic scintillators in Non-Patent Documents 2 and 3 claim that the detection efficiency can be increased by increasing the bismuth oxide content, but the content concentration is at most 10 to 20 mass %, and there are problems with dispersibility and transparency, so the thickness of the scintillator must be thinned to 1 mm or less, and as the bismuth oxide content increases, the amount of light emitted also drops significantly compared to EJ-256, so it cannot be said that practical detection efficiency and light emission have been achieved.

The present invention aims to provide a plastic scintillator with high time resolution, high counting rate and high detection efficiency.

After extensive research, the inventors discovered that bismuth oxide nanoparticles that have been surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid can be dispersed at high concentrations in the plastic substrate of a plastic scintillator, and that as a result, a high count rate and high detection efficiency can be achieved with high time resolution, leading to the completion of the present invention.

That is, the present invention provides
(1) a first step of mixing a polymerizable monomer , an organic fluorescent compound, and bismuth oxide nanoparticles that have been surface-treated with at least one non- hydroxyl group- containing aliphatic monocarboxylic acid selected from propionic acid, butyric acid, and valeric acid and a hydroxyl group-containing carboxylic acid under conditions of 80 to 140° C. in a nitrogen atmosphere to obtain a dispersion;
and a second step of polymerizing the polymerizable monomer in the resulting dispersion.
A method for producing a plastic scintillator,
(2) The method for producing a plastic scintillator according to (1), wherein the polymerizable monomer is a polymer of at least one aromatic vinyl and at least one (meth)acrylate.
(3) The method for producing a plastic scintillator according to (1) or (2), wherein the hydroxyl-containing carboxylic acid is a hydroxyl-containing aliphatic monocarboxylic acid having 6 to 22 carbon atoms.
(4) A method for producing a plastic scintillator according to any one of (1) to (3), in which the content of the bismuth oxide nanoparticles as a metal oxide is 5% by mass or more and 70% by mass or less with respect to the total amount of the plastic scintillator.
(5) The method for producing a plastic scintillator according to any one of (1) to (4), wherein the organic fluorescent compound is at least one selected from 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (Bu-PBD), p-terphenyl (P-TP), 2,5-diphenyloxazole (DPO), 1,4-bis[2-(5-phenyloxazolyl)]benzene (POPOP), 1,4-bis[2-(4-methyl-5-phenyloxazolyl)]benzene (DMPOPOP), 1,4-bis(2-methylstyryl)benzene (bis-MSB), benzophenone, and 4,4'''- bis (2-butyloctyloxy)-p-quaterphenyl (BIBUQ);
It is.

The plastic scintillator of the present invention uses bismuth oxide nanoparticles that have been surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid, which have excellent dispersibility, as metal oxide particles, and the plastic substrate is also transparent and has excellent dispersibility with these metal oxide particles and organic fluorescent compounds, so that these metal oxide particles can be contained in high concentrations and have excellent transparency even at high concentrations. Therefore, it is possible to increase the detection efficiency for X-rays while maintaining the amount of light emission. By using a pulse signal with sufficiently large and fast light emission, a high counting rate and high time resolution can be obtained.

The following is a detailed description of an embodiment of the present invention. Note that this embodiment is merely one embodiment of the present invention, and the present invention is not limited to this embodiment. Various modifications and embodiments are possible without departing from the gist of the present invention.

In this specification, "~" indicates that the upper and lower limits are included unless otherwise specified.

The plastic scintillator of the present invention is a plastic scintillator containing plastic, an organic fluorescent compound, and metal oxide particles, and the metal oxide particles are bismuth oxide nanoparticles that have been surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid.

The plastic is a polymer of at least one aromatic vinyl and at least one (meth)acrylate.

Examples of the aromatic vinyl include aromatic compounds having a vinyl group, such as styrene, α-methylstyrene, and vinyltoluene.

The (meth)acrylate may be either a monofunctional (meth)acrylate or a polyfunctional (meth)acrylate, and is not particularly selected, but at least one of a (meth)acrylate having an aromatic ring such as a benzene ring and a (meth)acrylate having a carboxylic acid is preferably used.

The (meth)acrylates having aromatic rings such as the benzene ring mentioned above are thought to contribute to improving compatibility with the aromatic vinyl-derived portion of the plastic. Examples of such compounds include 3-phenoxybenzyl (meth)acrylate, o-phenylphenoxyethyl (meth)acrylate, and benzyl (meth)acrylate.

In addition, (meth)acrylates having the above-mentioned carboxylic acid are thought to contribute to improving the dispersibility of metal oxide particles, and examples thereof include 2-(meth)acryloyloxyethyl succinate and 2-(meth)acryloyloxyethyl-phthalate.

The blending ratio of the aromatic vinyl to the (meth)acrylate in the polymer depends on the blending amount of the surface-treated bismuth oxide nanoparticles, but it is preferable that the (meth)acrylate is 2 to 20 parts by mass per 100 parts by mass of the aromatic vinyl. If the (meth)acrylate is less than 2 parts by mass, the dispersibility of the surface-treated bismuth oxide nanoparticles decreases, and if it exceeds 20 parts by mass, the amount of light emitted decreases, and does not reach a level that can be used as a scintillator.

In this specification, "(meth)acrylate" is used to refer to both acrylate and methacrylate.

Examples of the organic fluorescent compound include p-terphenyl (P-TP), 2,5-diphenyloxazole (DPO), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (Bu-PBD), 1,4-bis[2-(5-phenyloxazolyl)]benzene (POPOP), 1,4-bis[2-(4-methyl-5-phenyloxazolyl)]benzene (DMPOPOP), 1,4-bis(2-methylstyryl)benzene (bis-MSB), benzophenone, and 4,4'''-bis(2-butyloctyloxy)-p-quaterphenyl (BIBUQ), and at least one of these can be used.
The content of the organic fluorescent compound is preferably 0.05% by mass or more and 10% by mass or less with respect to the obtained plastic scintillator. If the content of the organic fluorescent compound is less than 0.05% by mass, a sufficient amount of light emission cannot be obtained, whereas if the content of the organic fluorescent compound exceeds 10% by mass, the amount of light emission decreases due to concentration quenching or the like.

The metal oxide particles used in the present invention are bismuth oxide nanoparticles that have been surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid.

The hydroxyl group-free carboxylic acid includes aliphatic and aromatic monocarboxylic acids, and if it is aliphatic, it is a monocarboxylic acid having 3 to 22 carbon atoms, whether saturated or unsaturated, which may have a branched or aromatic substituent such as a phenyl group, and preferably 3 to 8. If it is less than 3, sufficient hydrophobicity cannot be imparted to the metal oxide particle surface, so dispersibility with plastics and monomers decreases, and if it exceeds 22, the surface tends to be sticky, and the molecular weight is large, so the amount of surface treatment increases, resulting in a decrease in the effective component of the bismuth oxide nanoparticles.
Examples of aliphatic monocarboxylic acids include saturated monocarboxylic acids such as propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, octylic acid, pelargonic acid, capric acid, neodecanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearic acid, nonadecanoic acid, eicosanoic acid, heneicosanoic acid, and docosanoic acid, unsaturated fatty acids such as oleic acid, linoleic acid, linoleic acid, and fatty acids obtained by saponification and decomposition of fish oil, and geometric isomers thereof, as well as 3-phenylpropionic acid, cinnamic acid, etc. Hydroxyl-free aromatic monocarboxylic acids are monocarboxylic acids in which a carboxylic acid residue is directly bonded to an aromatic ring, and examples thereof include benzoic acid, toluic acid, etc.

The hydroxyl-containing carboxylic acid is preferably a hydroxyl-containing aliphatic monocarboxylic acid having 6 to 22 carbon atoms, whether saturated or unsaturated, which may be branched or may have an aromatic substituent such as a phenyl group. Specific examples include mevalonic acid, pantoic acid, 2-hydroxydecanoic acid, 3-hydroxyhexanoic acid, 2-hydroxystearic acid, 12-hydroxystearic acid, and ricinoleic acid.

The non-hydroxyl-containing carboxylic acid is believed to contribute to dispersion stability in the monomer or its polymer by imparting hydrophobicity to the surface of the bismuth oxide nanoparticles. In addition, although details of the hydroxyl-containing aliphatic carboxylic acid are unknown, it is believed that the hydroxyl group contributes to dispersion stabilization by hydrogen bonding with the carbonyl group or carboxyl group between the bismuth oxide nanoparticles and the monomer or its polymer.

The bismuth oxide nanoparticles used in the present invention can be of any crystal form, but it is preferable to obtain them as a white powder, since yellow coloring of the bismuth oxide nanoparticle powder will significantly reduce the amount of light emitted by the plastic scintillator.

The bismuth oxide nanoparticles of the present invention can be produced by any method as long as they are surface-treated with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid, respectively. For example, they can be produced by mixing triphenylbismuthine with a hydroxyl-free carboxylic acid and a hydroxyl-containing carboxylic acid, and reacting the resulting mixture at 80 to 140°C.

When the plastic is thermoplastic, the plastic scintillator of the present invention can be produced by kneading the plastic with the organic fluorescent compound and the bismuth oxide nanoparticles that have been surface-treated with the hydroxyl-free carboxylic acid and the hydroxyl-containing carboxylic acid described above. However, to obtain a plastic scintillator that has high transparency even when it contains a high concentration of bismuth oxide nanoparticles, it is preferable to mix and disperse a polymerizable monomer, the organic fluorescent compound, and the bismuth oxide nanoparticles that have been surface-treated with the hydroxyl-free carboxylic acid and the hydroxyl-containing carboxylic acid described above, and then polymerize the polymerizable monomer in the resulting dispersion.

The polymerizable monomer includes at least one aromatic vinyl and at least one (meth)acrylate.

Examples of the aromatic vinyl include aromatic compounds having a vinyl group, such as styrene, α-methylstyrene, and vinyltoluene.

The (meth)acrylate may be either a monofunctional (meth)acrylate or a polyfunctional (meth)acrylate, and is not particularly selected, but at least one of a (meth)acrylate having an aromatic ring such as a benzene ring and a (meth)acrylate having a carboxylic acid is preferably used.

Examples of the (meth)acrylates having an aromatic ring such as a benzene ring include 3-phenoxybenzyl (meth)acrylate, o-phenylphenoxyethyl (meth)acrylate, and benzyl (meth)acrylate.

Examples of (meth)acrylates having the above-mentioned carboxylic acid include 2-(meth)acryloyloxyethyl succinate and 2-(meth)acryloyloxyethyl-phthalate.

The plastic scintillator of the present invention can be produced by mixing and dispersing the polymerizable monomer, the organic fluorescent compound, and the bismuth oxide nanoparticles that have been surface-treated with the hydroxyl-free carboxylic acid and the hydroxyl-containing carboxylic acid, and then polymerizing the mixture by heating in the presence of a radical generator or by irradiating the mixture with active energy rays in the presence of a polymerization initiator.

The plastic scintillator obtained in this way is highly transparent even when it contains a high concentration of bismuth oxide nanoparticles, and can achieve high detection efficiency for X-rays while maintaining a high level of luminescence.

In the present invention, high transparency means a level at which a printed matter can be recognized by placing a test piece of plastic scintillator on the printed matter; specifically, an average transmittance of 60% or more for visible light (380-780 nm).

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto. In the examples and comparative examples, "parts" means "parts by mass" and "%" means "% by mass."

(Synthesis Example 1: Synthesis of bismuth oxide nanoparticles)
46.3 g of triphenylbismuthine was added to a mixed solution of 26.9 g of propionic acid and 26.9 g of ricinoleic acid, and the mixture was stirred at 100 ° C. for 5 hours under a nitrogen atmosphere to obtain a transparent solution. After cooling to room temperature, the obtained transparent solution was added to 3 L of acetone, filtered with a pore size 0.2 μm filter, and the obtained white matter was vacuum dried at 60 ° C. for one day to obtain 33.6 g of bismuth oxide powder surface-treated with carboxylic acid. The surface treatment amount of carboxylic acid was 35.48% from the mass reduction rate when heated to 800 ° C. at a rate of 40 ° C. / min under a nitrogen atmosphere by a thermogravimetric measuring device TGA8000 manufactured by PerkinElmer, and the crystallite diameter was 5.4 nm by XRD.

In the present invention, the average particle diameter of bismuth oxide nanoparticles was measured using an X-ray diffractometer (Rigaku Corporation, fully automated multipurpose X-ray diffractometer SmartLab) under measurement conditions of an X-ray tube voltage of 40 kV, an X-ray tube current of 30 mA, and a scanning range of 2θ of 10.0-65.0°. The half-width β was calculated from the diffraction intensity from the (200) plane near 2θ = 32.67° in the X-ray diffraction measurement, and the crystallite diameter D was calculated using the Scherrer formula in the following equation 1, with the Scherrer constant K set to 0.9 and the wavelength λ of the X-ray tube set to 1.54059.

(Equation 1)
D=K・λ/(β・cosθ)

The plastic scintillators of Examples 1 to 8 below were manufactured using the bismuth oxide nanoparticles of Synthesis Example 1.

Example 1
1.1 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 9.1 parts of styrene monomer, 0.2 parts of 3-phenoxybenzyl acrylate, 0.2 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 10% bismuth oxide.

Example 2
A plastic scintillator containing 20% bismuth oxide was obtained in the same manner as in Example 1, except that the amount of bismuth oxide nanoparticles was changed to 2.5 parts.

Example 3
4.3 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 8.6 parts of styrene monomer, 0.5 parts of 3-phenoxybenzyl acrylate, 0.5 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 30% bismuth oxide.

Example 4
6.7 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 8.1 parts of styrene monomer, 1.4 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 40% bismuth oxide.

Example 5
1.1 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 9.1 parts of vinyltoluene monomer, 0.2 parts of 3-phenoxybenzyl acrylate, 0.2 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 10% bismuth oxide.

Example 6
A plastic scintillator containing 20% bismuth oxide was obtained in the same manner as in Example 5, except that the amount of bismuth oxide nanoparticles was changed to 2.5 parts.

(Example 7)
4.3 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 8.6 parts of vinyltoluene monomer, 0.5 parts of 3-phenoxybenzyl acrylate, 0.5 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 30% bismuth oxide.

(Example 8)
6.7 parts of the bismuth oxide nanoparticles of Synthesis Example 1, 8.1 parts of vinyltoluene monomer, 1.4 parts of 2-acryloyloxyethyl succinate, and 0.3 parts of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole were added to a vial and ultrasonically dispersed. After the mixture became transparent, 0.08 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) was added and the mixture was allowed to stand in an oven at 65°C for 24 hours under a nitrogen atmosphere, to obtain a plastic scintillator containing 40% bismuth oxide.

The plastic scintillators obtained in each example were molded into cylinders with a diameter of 8 mm and a height of 3 mm to prepare evaluation samples.

(Comparative Example 1)
A plastic scintillator (EJ-256, ELJEN TECHNOEOGY) containing 5% lead and molded to a diameter of 8 mm and a height of 3 mm was used as Comparative Example 1 (standard for evaluating the amount of light emitted).

For the plastic scintillators obtained in each Example and Comparative Example, the average light transmittance, light emission amount, and detection efficiency for visible light (380 to 780 nm) were determined using the following methods, and the results are summarized in Table 1.

(Average Light Transmittance)
For the evaluation samples of Examples 1 to 8, the light transmittance of 380 to 780 nm, which corresponds to visible light, was measured at 0.5 nm intervals using an ultraviolet-visible-near infrared spectrophotometer (UH4150, manufactured by Hitachi High-Tech Science Corporation), and the arithmetic average was calculated.

(Light output)
The evaluation sample of each Example was irradiated with 67.4 keV X-rays, and the scintillation light was detected by a photomultiplier tube (product name: R7400P, manufacturer: Hamamatsu Photonics). The detected signal pulse height spectrum was amplified by a charge-sensitive preamplifier (product name: 2005, manufacturer: Canberra), and the channel number of the peak position (peak channel number) for each Example was determined from the spectrum.
The peak channel number was determined for the scintillator of Comparative Example 1 in the same manner as above, and this was used as the reference peak channel number.
The ratio of the peak channel number of each example to this reference peak channel number was determined, and this was taken as the amount of light emitted per energy of the incident X-ray (photons/MeV).

(Detection efficiency)
A 5 mm thick NaI(Tl) scintillator (product name: SP-10, manufacturer: Oyo Koken Kogyo Co., Ltd.) was measured under the same conditions as in the measurement of the amount of luminescence, and the area of the detection signal pulse-height spectrum of each Example was calculated when the area of the obtained detection signal pulse-height spectrum was set to 100. This value corresponds to the total number of detected events, and this value was taken as the detection efficiency (%).

Claims (5)

A polymerizable monomer ;
An organic fluorescent compound;
Bismuth oxide nanoparticles that have been surface-treated with at least one hydroxyl-free aliphatic monocarboxylic acid and a hydroxyl-containing carboxylic acid selected from propionic acid, butyric acid, and valeric acid under conditions of 80 to 140° C. in a nitrogen atmosphere ;
A first step of mixing the above to obtain a dispersion;
and a second step of polymerizing the polymerizable monomer in the resulting dispersion.
A method for producing a plastic scintillator, comprising : 2. The method for producing a plastic scintillator according to claim 1, wherein the polymerizable monomer is a polymer of at least one aromatic vinyl and at least one (meth)acrylate. 3. The method for producing a plastic scintillator according to claim 1, wherein the hydroxyl-containing carboxylic acid is a hydroxyl-containing aliphatic monocarboxylic acid having 6 to 22 carbon atoms. 4. A method for producing a plastic scintillator according to claim 1, wherein the content of the bismuth oxide nanoparticles as a metal oxide is 5% by mass or more and 70% by mass or less relative to the total amount of the plastic scintillator. The method for producing a plastic scintillator according to any one of claims 1 to 4, wherein the organic fluorescent compound is at least one selected from 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (Bu-PBD), p-terphenyl (P-TP), 2,5-diphenyloxazole (DPO), 1,4-bis[2-(5-phenyloxazolyl)]benzene (POPOP), 1,4-bis[2-(4-methyl-5-phenyloxazolyl)]benzene (DMPOPOP), 1,4-bis(2-methylstyryl)benzene (bis-MSB), benzophenone, and 4,4'''-bis(2-butyloctyloxy) -p -quaterphenyl (BIBUQ).