JP7494040B2 - Scintillator unit and radiation detector - Google Patents

Description

The present invention relates to a scintillator unit and a radiation detector.

Radiation detectors are used in medical settings and other settings to take radiographic images of subjects. In particular, radiation detectors used in CT (Computed Tomography) are composed of a scintillator that generates light in response to radiation (e.g., X-rays), and a detection section that includes multiple light-receiving elements arranged two-dimensionally. When X-rays are irradiated onto a subject, light is generated from the scintillator by the X-rays that pass through the subject, and the light generated is detected by the light-receiving elements corresponding to each scintillator, resulting in a two-dimensional image of the transmitted X-rays.

However, the light generated from the scintillator is emitted in all directions. In order to guide most of the light generated from the scintillator to the detection section, a reflective layer may be attached via an adhesive layer to the surface of the scintillator opposite the surface on which the detection section is located. For example, Patent Document 1 describes a flat panel detector (FPD) that includes, in order from the direction in which radiation is incident, an air layer, a dielectric multilayer reflective film, an adhesive layer, a phosphor layer, and a photodetector. Patent Document 2 also describes a configuration that has, in order from the scintillator side, an adhesive layer and a refractive coating with a refractive index smaller than 1.3 between the scintillator and the silver coating in order from the scintillator side in order to improve the efficiency of light collection to the detector.

International Publication No. 2016/167334 JP 2001-516888 A

The inventors of the present invention conducted an investigation by applying the configuration of the FPD described in Patent Document 1 to a detector equipped with multiple scintillators and multiple light receiving elements corresponding to each scintillator, such as those used in CT. As a result, it was found that light generated from a certain scintillator does not enter the corresponding light receiving element, but enters the light receiving element adjacent to the corresponding light receiving element, resulting in crosstalk.

In addition, in Patent Document 2, a detector having multiple scintillators and multiple light receiving elements corresponding to each scintillator does not have a configuration having an adhesive layer and a refractive coating between the scintillator and the silver coating, in that order from the scintillator.

Therefore, an object of the present invention is to provide a scintillator unit that can reduce crosstalk when using a scintillator unit that includes multiple scintillators. Another object of the present invention is to provide a radiation detector that uses the scintillator unit.

The present invention relates to a scintillator unit having a plurality of scintillators and a reflective layer between the plurality of scintillators, characterized in that, between the scintillators and the reflective layer, there are, in order from the scintillators, an adhesive layer and a low refractive index layer having a refractive index lower than that of the adhesive layer.

The present invention also relates to a radiation detector comprising a scintillator unit having a plurality of scintillators and a reflective layer between the plurality of scintillators, and a detection unit that detects light generated from the scintillator, characterized in that the scintillator unit has, between the scintillator and the reflective layer, an adhesive layer and a low refractive index layer having a refractive index lower than that of the adhesive layer, in that order from the scintillator.

The present invention provides a scintillator unit and a radiation detector that can reduce crosstalk when using a scintillator unit that includes multiple scintillators.

FIG. 2 is a schematic cross-sectional view showing an example of a low refractive index layer in an embodiment of the present invention. 1 is a schematic bird's-eye view illustrating an example of a radiation detector according to an embodiment of the present invention. 1 is a schematic bird's-eye view showing a sample used in reflectance measurement according to an embodiment of the present invention. 1A is a schematic cross-sectional view showing a sample used for reflectance measurement in an embodiment of the present invention, FIG. 1A is a schematic cross-sectional view of a sample in Example 1, FIG. 1B is a schematic cross-sectional view of a scintillator/air layer/reflective layer, and FIG. 1C is a schematic cross-sectional view of Comparative Example 1. 1 shows an example of the measurement results of the reflectance spectrum in the explanation of the embodiments of the present invention, where (a) shows the measurement results of the reflectance spectrum of Example 1 and Comparative Example 1 at an incident angle of 70 degrees, and (b) shows the measurement results of the reflectance spectrum of Example 1 and Comparative Example 1 at an incident angle of 74 degrees.

The scintillator unit of the present invention has a reflective layer between multiple scintillators. In addition, between the scintillators and the reflective layer, there is an adhesive layer and a low refractive index layer with a lower refractive index than the adhesive layer, in that order from the scintillator. Of the light generated by a scintillator, the light directed toward another adjacent scintillator is reflected by the interface between the adhesive layer and the low refractive index layer and by the reflective layer, since the low refractive index layer is disposed between the scintillator and the reflective layer. This phenomenon occurs by utilizing the physical phenomenon of "total reflection" caused by the difference in refractive index between two media. This makes it possible to reduce crosstalk caused by light generated from a scintillator entering a light receiving element adjacent to the corresponding light receiving element, rather than entering the corresponding light receiving element.

Embodiments of the present invention are described in detail below. All physical properties are measured at 25°C unless otherwise specified.

Figure 2 is a schematic bird's-eye view showing one embodiment of the radiation detector of the present invention. In Figure 2, the scintillator unit 205 has a configuration in which a plurality of scintillators 201 are arranged two-dimensionally, and a reflective layer 204 is disposed between two scintillators 201. Between the scintillator 201 and the reflective layer 204, an adhesive layer 202 and a low refractive index layer 104 are provided in this order from the scintillator 201, so that the respective members are in close contact with each other.

In addition, between multiple scintillators, if there are, in order from the scintillator, an adhesive layer, a low refractive index layer having a lower refractive index than the adhesive layer, and a reflective layer, the scintillator and the reflective layer may be in contact with each other.

In addition, as long as the effects of the present invention can be obtained, other layers may be included between the scintillator and each layer.

Examples of layers constituting a scintillator unit include a configuration including, in order, a scintillator, an adhesive layer, a low refractive index layer, and a reflective layer, and a configuration including, in order, a scintillator, an adhesive layer, a low refractive index layer, a reflective layer, a low refractive index layer, an adhesive layer, and a scintillator.

Other examples include a configuration including a scintillator, an adhesive layer, a low refractive index layer, a substrate, a low refractive index layer, an adhesive layer, and a scintillator, in that order, and a configuration including a scintillator, a reflective layer, a low refractive index layer, an adhesive layer, a low refractive index layer, a reflective layer, and a scintillator, in that order. Materials described below can be used as the substrate.

The scintillator unit 205 has a structure in which a reflective layer 204 is sandwiched between two low-refractive index layers 104, and low-refractive index layers 104 can be arranged on both sides of one reflective layer 204. This allows light traveling from one scintillator (e.g., the upper part of the paper in FIG. 2) to the other scintillator (e.g., the lower part of the paper in FIG. 2) to be reflected at the interface between the reflective layer 204 and the low-refractive index layer 104 (the lower part of the paper in FIG. 2; the other scintillator side) with a high probability. As a result, it is possible to reduce crosstalk even if a small number of reflective layers are used. Therefore, multiple scintillators can be arranged at a high density, improving sensitivity to radiation and the resolution of the resulting image.

In the case of the FPD configuration described in Patent Document 1, an air layer is formed by the uneven prism sheet and the dielectric multilayer reflective film. If a prism sheet with such an uneven shape is inserted between multiple scintillators, sufficient strength cannot be obtained as a scintillator unit. In addition, since an air layer can only be formed on one side of the dielectric multilayer reflective film, multiple dielectric multilayer reflective films are required to reduce crosstalk, making it difficult to arrange multiple scintillators at high density.

Therefore, the scintillator unit of this embodiment is particularly useful for applications such as CT, which uses a radiation detector configured with multiple scintillators arranged two-dimensionally. Furthermore, the low refractive index layer 104 is in surface contact with adjacent members (in the case of FIG. 2, the reflective layer 204 and the adhesive layer 202). Therefore, the scintillator unit of this embodiment has sufficient strength for fabrication into a device.

[Scintillator 201]
The scintillator 201 used in the scintillator unit 205 of this embodiment preferably contains a material that emits light when exposed to radiation (X-rays, gamma rays, charged particles, etc.).

Examples of materials that can be used as the scintillator 201 include halides of alkali metals, alkaline earth metals, transition metals, typical elements, and rare earth metals, as well as oxides, nitrides, chalcogenides, and Group 13-14 compounds. Specifically, _{examples of} the doped _oxide include Tl _- doped NaI, Tl _- doped CsI, Na-doped CsI, Ce-doped _Lu2SiO5 (LSO ₎ , Ce _- doped _Lu2Y2SiO5 (LYSO), _{Gd2SiO5 ,} Bi4Ge3O12, _ZnWO4 , _{CdWO4 ,} PbWO4, _LuAlO3 , Ce-doped _Y3Al5O12 , Ce-doped _YAlO3 (YAP), Ce-doped _GdAlO3 (GAP) _, Ce-doped _LuAlO3 ( _LuAP ), Ce _{-doped Lu3Al5O12 ,} Pr _{-doped Lu3Al5O12 , and CeF3} . In particular, it is preferable that the scintillator 201 contains at least one of Ce-doped Lu ₂ Y ₂ SiO ₅ and Ce-doped Lu ₂ SiO ₅ .

[Adhesive layer 202]
The adhesive layer 202 can be made of at least one optical adhesive selected from the group consisting of epoxy resin, acrylic resin, and vinyl resin.

The refractive index of the adhesive layer 202 is preferably, for example, greater than 1.30 and less than 1.70, and more preferably greater than 1.30 and less than 1.50. By making the refractive index of the adhesive layer 202 smaller than 1.70, the probability that light reflected at the interface between the reflective layer 204 and the low refractive index layer 104 will be reflected at the interface between the low refractive index layer 104 and the adhesive layer 202 and head toward another adjacent scintillator is reduced, and the light can return to the scintillator 201, which is the original light source.

The thickness of the adhesive layer 202 is preferably 5 μm or less, more preferably 1 μm to 5 μm, and even more preferably 2 μm to 5 μm. When the adhesive layer 202 is 5 μm or less, thickness variation is suppressed, so that assembly errors of the scintillator unit can be reduced. When the adhesive layer 202 is 2 μm or more in thickness, sufficient adhesive strength is obtained.

[Reflective layer 204]
For example, a metal material such as aluminum or silver can be used as the reflective layer 204. A PET (polyethylene terephthalate) film on which aluminum is vapor-deposited (Alpet (registered trademark)) may also be used as the reflective layer.

When a metal material is used for the reflective layer, the reflectance of light having a wavelength of 400 nm to 700 nm is 80% or more and 100% or less. Here, the reflectance of light is a value measured in air.

(Dielectric multilayer film)
It is preferable to use a dielectric multilayer film with high reflectance as the reflective layer 204. When a dielectric multilayer film is used as the reflective layer, the reflectance of light having a wavelength of 400 nm to 700 nm is 95% or more and 100% or less. Here, the reflectance of light is a value measured in air. In addition, it is sufficient that the reflectance is high in the emission wavelength band of the scintillator used, and it is not necessarily required that the reflectance is high in wavelength ranges other than the emission wavelength. Compared to the case where a metal material such as aluminum or silver is used, the reflectance of light is high when a dielectric multilayer film is used. This makes it possible to further reduce crosstalk that occurs as a result of light generated from a scintillator leaking to an adjacent scintillator.

The dielectrics constituting the dielectric multilayer film may be inorganic or organic materials, or a combination of these. Organic materials are more preferable because they are lightweight and flexible. Examples of organic materials include polyester resins, urethane resins, and acrylic resins.

The dielectric multilayer film is preferably in contact with a low refractive index layer. This increases the difference in refractive index between the dielectric multilayer film and the adjacent layer, improving the reflectance of light with wavelengths of 400 nm to 700 nm. When the dielectric multilayer film is in contact with the low refractive index layer, the refractive index of the low refractive index layer is preferably 1.05 or more and 1.30 or less, more preferably 1.10 or more and 1.20 or less, and particularly preferably 1.15 or less.

(Low Refractive Index Layer)
<Composition and structure>
In FIG. 1, when a solid substance having a refractive index of 1.65 or less is used as the skeleton, the porosity can be appropriately set to lower the refractive index, and the strength of the low refractive index layer 104 can be improved.

The solid substance may be either crystalline or amorphous. The solid substance may be particles. There are no particular limitations on the type of particle, and examples include spherical particles, particles with irregular shapes, particles in which the spherical or irregular particles are linked in a beaded or branched chain shape, hollow particles having a cavity inside, and particles in which the hollow particles are linked in a beaded or branched chain shape.

Examples of solid substances include resins such as fluoropolymers and acrylic resins, fluorides such as magnesium fluoride and calcium fluoride, carbonates such as calcium carbonate and potassium carbonate, sulfates such as barium sulfate, and oxides such as silicon dioxide (hereinafter also referred to as silica) and aluminum oxide.

Examples of solid materials with a low refractive index include organic materials such as fluorine-based polymers, and inorganic materials such as magnesium fluoride and silicon dioxide.

However, the refractive index of fluoropolymers is as low as 1.30, while the refractive indexes of magnesium fluoride and silicon dioxide (quartz) are 1.38 and 1.46, respectively. Materials with a refractive index significantly below 1.30 as a single substance are mainly gases such as nitrogen and oxygen.

From the viewpoints of refractive index, cost, and chemical stability, it is preferable that the solid material contains silicon dioxide. That is, it is preferable that the main component of the solid material is silicon dioxide. Here, "the main component of the solid material is silicon dioxide" means that silicon dioxide is 50 mass% or more in the solid material. Typically, silicon dioxide is 90 mass% or more in the solid material.

Specific examples of silicon dioxide particles include the Snowtex series and Organosilicasol manufactured by Nissan Chemical Industries, Ltd., the Sururia series manufactured by JGC Catalysts and Chemicals, and the Aerosil series manufactured by EVONIK and sold by Nippon Aerosil Co., Ltd.

In general, the refractive index n _c of a composite material C composed of a material A having a refractive index n _a and a material B having a refractive index n _b is approximately represented by the following formula (1).
Formula (1)
n _c = [n _a × v _a / 100] + [n _b × v _b / 100] (1)
Here, v _a and v _b are the volume fractions of material A and material B constituting the composite material, respectively (v _a +v _b =100).

According to formula (1), by using a composite material of a solid material and air, that is, a porous film having a solid material as a skeleton, as the low refractive index layer 104, the refractive index can be made lower than that of the original solid material. In this case, the lower the refractive index of the solid material that forms the skeleton, and the higher the porosity of the low refractive index layer 104, the lower the refractive index of the low refractive index layer 104. In order to increase the porosity of the low refractive index layer 104, the low refractive index layer 104 may have a porous structure. From this perspective, the low refractive index layer 104 can be called a porous film.

In the above formula (1), if material A is air and material B is silicon dioxide, the refractive index of air n _a =1.00, the refractive index of silicon dioxide n _b =1.46, and the volume fraction of silicon dioxide v _b =100- _va . In other words, v _a is a function of the refractive index n _c of the low refractive index layer 104, and v _a can be obtained. This v _a is the porosity.

When the low refractive index layer 104 has voids, the porosity of the low refractive index layer is preferably 60.0% or more and 95.0% or less, and more preferably 65.0% or more and 90.0% or less.

For example, according to formula (1), if the porosity of the low refractive index layer 104, which has a skeleton of silicon dioxide (refractive index 1.46), is less than 60.0%, the refractive index may exceed 1.15.

On the other hand, if the porosity exceeds 95.0%, the refractive index of the low refractive index layer 104 will be excessively low, less than 1.05, and the strength may decrease due to the small amount of skeleton that constitutes the low refractive index layer 104.

It is preferable that the silicon dioxide has at least one of an organic group and a hydroxyl group on its surface. Since silicon dioxide having hydroxyl groups on its surface is highly hydrophilic, a highly hydrophilic low refractive index layer 104 can be formed by using such silicon dioxide particles as the skeleton.

In addition, functionality can be imparted to the low refractive index layer 104 by modifying the surface of the silicon dioxide with a silane coupling agent, for example. When the silicon dioxide surface has hydroxyl groups, a dehydration condensation reaction between the hydroxyl groups and the hydrolysis product of the silane coupling agent can be utilized.

Examples of organic groups include alkyl groups having 1 to 4 carbon atoms, such as methyl, ethyl, propyl, and butyl groups; hydrocarbon groups having a polymerizable site, such as vinyl, acrylic, and methacrylic groups; and aromatic hydrocarbon groups, such as phenyl groups.

When the silicon dioxide surface has organic groups, various functions can be imparted to the low refractive index layer 104, such as water repellency, oil repellency, biocompatibility, electron transport properties, and polymerizability.

In addition, it is not necessary for all of the functional groups present on the surface of the silicon dioxide to be substituted with organic groups, and both organic groups and hydroxyl groups may be present in any ratio.

[Hollow Silica Particles]
A further description will be given of the case where the low refractive index layer 104 contains hollow particles, but is not limited thereto. A hollow particle is a particle having an outer shell formed of a solid substance and having a cavity (void) inside the outer shell.

The low refractive index layer 104 preferably contains a plurality of hollow particles. The low refractive index layer 104 containing a plurality of hollow particles may contain solid particles or a binder in addition to the hollow particles.

Figure 1 shows an example of the structure of the low refractive index layer 104 when it contains hollow particles as primary particles made of a solid substance.

The low refractive index layer 104 contains a plurality of hollow particles 301, and gaps 302 exist between the plurality of hollow particles 301. Furthermore, gaps 304 exist inside the hollow particles. In FIG. 1, reference numeral 303 denotes the outer shell, and reference numeral 305 denotes the substrate.

When the ratio of the total volume of voids in a plurality of hollow particles to a unit volume of the low refractive index layer 104 is the porosity X (%), and the ratio of the total volume of voids between hollow particles to a unit volume of the low refractive index layer 104 is the porosity Y (%), it is preferable that the relationship X<Y is satisfied. Here, (X+Y) means the porosity of the low refractive index layer 104.

The refractive index n of the low refractive index layer 104 is expressed by the following formula (2).
Equation (2)
n = [n _a × (X + Y) / 100] + [n _s × (100 - X - Y) / 100]
(2) Here, n _a is the refractive index of air (n _a =1), and n _s is the refractive index of the outer shell of the hollow particle (n _s >1). According to formula (2), the larger X+Y is and the lower n _s is, the lower n is.

The refractive index n of the low refractive index layer 104 is also expressed by the following formula (3).
Equation (3)
n = [n _a × Y / 100] + [n _p × (100 - Y) / 100]
(3) Here, _np is the refractive index of one hollow particle ( _np >1). The refractive index _np is an apparent refractive index calculated from the ratio of the volume and refractive index of the shell to the volume and refractive index of the void in one hollow particle. That is, in formula (1), if n _a =1, n _b =n _s , v _a is the volume of the void of the hollow particle, and v _b is the volume of the shell, then n _c =n _p . According to formula (3), the larger Y is and the lower n _p is, the lower n is.

It is also possible to estimate X and Y by determining the refractive index n of the low refractive index layer 104 by optical measurement and substituting the known values n _a , n _s , and n _p into equations (2) and (3).

When the hollow particles are densely arranged, the volume fraction of the gaps between the hollow particles decreases and the volume fraction of the outer shell, which is a component with a higher refractive index than air, increases, resulting in a higher refractive index of the low refractive index layer 104. On the other hand, when the hollow particles are sparsely arranged, the volume fraction of the gaps between the hollow particles increases and the volume fraction of the outer shell decreases, resulting in a lower refractive index of the low refractive index layer 104. In other words, to further lower the refractive index of the low refractive index layer 104, it is preferable to increase Y/X. Specifically, it is preferable that the relationship Y/X>1, that is, X<Y, is satisfied.

It is also preferable that X and Y satisfy the relationship X<(100-X-Y)<Y.

The low refractive index layer 104 may contain particles made of a solid material and a binder that binds the particles together to increase strength. When a binder is used, the solids contained in the low refractive index layer 104 are the outer shells of the hollow particles and the binder, and the volume fraction of the solids relative to the unit volume of the low refractive index layer 104 is expressed as (100-X-Y) (%).

When the relationship X<(100-X-Y) is satisfied, the strength of the low refractive index layer 104 is further improved. Also, when the relationship (100-X-Y)<Y is satisfied, the refractive index of the low refractive index layer 104 is further reduced.

The total value of X and Y (X+Y) is preferably 60.0% or more and 95.0% or less, and more preferably 65.0% or more and 90.0% or less. By setting (X+Y) within the above range, it becomes easy to adjust the strength of the low refractive index layer 104 and the refractive index of the low refractive index layer 104 to the desired range.

In the low refractive index layer 104, X is preferably 8.0% or more and 32.0% or less, more preferably 10.0% or more and 28.0% or less, and even more preferably 12.0% or more and 24.0% or less.

On the other hand, Y is preferably 30.0% or more and 80.0% or less, more preferably 35.0% or more and 75.0% or less, and even more preferably 40.0% or more and 70.0% or less.

By setting X and Y within the above ranges, it becomes easy to adjust the strength of the low refractive index layer 104 and the refractive index of the low refractive index layer 104 to the desired range.

In the example of FIG. 1, the hollow particles are substantially spherical in shape, but are not limited to this. The hollow particles have an outer shell 303 and a void 304 that is surrounded by the outer shell and formed inside the hollow particle. In this case, the hollow particles can be considered as a type of core-shell particle with air as the core.

The refractive index _np of one hollow particle is expressed by formula (4).
Equation (4)
n _p = [n _s × (100 - V _a ) / 100] + [n _a × V _a / 100] (4)
Here, _Va is the volume fraction of the internal void with respect to the total volume of the hollow particle. In other words, the refractive index _np of one hollow particle is determined by the refractive index _ns of the shell material and the porosity _Va of one hollow particle.

The porosity _np of one hollow particle is preferably 30.0% or more and 70.0% or less, and more preferably 35.0% or more and 65.0% or less.

When the porosity _np is within the above range, the refractive index of the low refractive index layer 104 can be easily reduced, and the strength of the hollow particle outer shell and the strength of the low refractive index layer 104 can be stabilized.

The refractive index of the outer shell n _s of the hollow particles is preferably 1.10 or more, 1.20 or more, 1.25 or more, 1.30 or more, or 1.35 or more, and preferably 1.65 or less, or 1.60 or less, similar to the refractive index of the solid substance. These numerical ranges can be combined in any manner. The refractive index of the outer shell n _s of the hollow particles is more preferably 1.35 or more and 1.60 or less.

When the refractive index n _s of the outer shell of the hollow particle is within the above range, the ease of production of the low refractive index layer 104, the strength of the hollow particle, and the strength of the low refractive index layer 104 are excellent, and the refractive index of the low refractive index layer 104 can be adjusted to a low value.

The shell of the hollow particles can be made of the same material as the solid substances mentioned above.

The outer shell of the hollow particles may also have micropores. Forming micropores in the outer shell can further reduce the refractive index of the outer shell.

The number average particle size of the primary particles of the hollow particles is preferably 1 nm or more and 200 nm or less, more preferably 5 nm or more and 100 nm or less, even more preferably 10 nm or more and 100 nm or less, and particularly preferably 20 nm or more and 100 nm or less.

When the number-average particle size is within the above range, hollow particles are easy to produce, light scattering is easily reduced, and the transmittance of the low refractive index layer 104 can be further improved.

[Fumed silica particles, chain silica particles]
The low refractive index layer 104 preferably contains at least one type of particle selected from the group consisting of secondary particles in which primary particles made of a solid substance form a three-dimensional structure, chain-like secondary particles in which primary particles made of a solid substance are linked in a chain, and branched-chain secondary particles in which primary particles made of a solid substance are linked in a branched chain. Here, when particles made of a solid substance form aggregates, the aggregates are also included in the secondary particles in which primary particles made of a solid substance form a three-dimensional structure.

Secondary particles in which primary particles made of solid substances form a three-dimensional structure, chain-like secondary particles in which primary particles made of solid substances are linked in a chain, and branched-chain secondary particles in which primary particles made of solid substances are linked in a branched chain reduce the volume fraction of solid substances in the low refractive index layer 104. The volume fraction of voids can then be increased. In other words, the refractive index of the low refractive index layer 104 can be reduced.

The number-average particle size of the primary particles made of solid substances is preferably 1 nm or more and 200 nm or less, and more preferably 5 nm or more and 100 nm or less. The number-average particle size of the primary particles made of solid substances is further preferably 10 nm or more and 100 nm or less, and particularly preferably 20 nm or more and 100 nm or less.

When the number-average particle size of the primary particles is within the above range, it is possible to appropriately control particle aggregation, and the dispersibility in the coating liquid can be improved. In addition, the primary particles are less likely to become light scatterers in the wavelength range of 400 nm to 700 nm, and the transmittance of the low refractive index layer 104 can be further improved.

Here, we will explain an example of using fumed silica particles as a secondary particle in which primary particles made of a solid substance form a three-dimensional structure, but this is not limited to this.

Fumed silica particles can be produced by high-temperature hydrolysis of silicon tetrachloride in oxygen and hydrogen flames. The fumed silica particles produced by the above process are made up of primary particles of several tens of nanometers fused together to form secondary particles with a three-dimensional structure. The secondary particles may also aggregate to form complex higher-order structures.

Due to their characteristic structure, fumed silica particles are very bulky particles with an apparent specific gravity of 0.01 g/cm ³ to 0.1 g/cm ^3. Therefore, the low refractive index layer 104 containing fumed silica particles has a large porosity and can significantly reduce the refractive index.

The number-average particle size of the secondary particles is preferably 10 nm or more and 1000 nm or less, and more preferably 50 nm or more and 500 nm or less.

When the number-average particle size of the secondary particles is within the above range, for example, the secondary particles are made up of primary particles of silicon dioxide that form a three-dimensional structure, and do not have a simple structure consisting of an aggregate of several primary particles.

When the secondary particles have the above structure, it becomes easier to control the porosity of the low refractive index layer 104 within the above range, and it becomes easier to control the refractive index of the low refractive index layer 104 within the above range. In addition, large gaps that can scatter light in the wavelength range of 400 nm to 700 nm are less likely to form between the secondary particles, making it easier to control the light transmittance of the low refractive index layer 104 within the above range.

The number-average particle size of the primary particles and secondary particles can be measured using a transmission electron microscope (TEM) (calculated from the arithmetic mean of the maximum diameter). The number-average particle size of the primary particles and secondary particles can be controlled, for example, by adjusting the conditions for high-temperature hydrolysis of silicon tetrachloride in oxygen and hydrogen flame, as described above.

<Film formation method>
[Adjustment of Coating Fluid]
A method for preparing a coating liquid for forming the low refractive index layer 104 will be described. An example will be described in which fumed silica particles in which primary particles of silicon dioxide form a three-dimensional structure are used, but the present invention is not limited thereto.

The fumed silica particles are dispersed in a solvent. As a solvent for dispersing the fumed silica particles, a solvent having a high affinity with the fumed silica particles is preferable, and one type of solvent or a mixed solvent of two or more types may be used depending on the type of functional group on the surface of the fumed silica particles.

As the solvent, organic solvents are preferred, and examples of the solvent that can be used include alcohol-based solvents such as methanol, ethanol, propanol, and isopropanol; glycol-based solvents such as ethylene glycol and propylene glycol; ether-based solvents such as dimethyl ether, diethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether; acetate-based solvents such as ethyl acetate, propyl acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; and ketone-based solvents such as acetone and methyl ethyl ketone.

Water can be used as a solvent, but because water has a high surface tension, a large capillary force is generated when it dries, which may cause the gaps between the fumed silica particles to shrink. This may lead to a decrease in the porosity of the low refractive index layer 104, and an increase in the refractive index.

In addition, when an alkali metal, particularly CsI, is used as the material for the scintillator 105, it is not appropriate to use water as a solvent due to its strong deliquescence. Particles made of solid substances may be used alone or in combination of two or more types.

The content of particles composed of solid substances in the coating liquid is preferably 1.0 mass% or more, more preferably 2.0 mass% or more, even more preferably 3.0 mass% or more, and particularly preferably 7.0 mass% or more. The content of silica particles in the coating liquid is preferably 50.0 mass% or less, more preferably 30.0 mass% or less, and even more preferably 20.0 mass% or less. The numerical ranges can be combined in any way.

For example, the concentration (solids concentration) of fumed silica particles in the coating liquid is preferably 1.0% by mass or more and 30.0% by mass or less, and more preferably 2.0% by mass or more and 20.0% by mass or less.

When the content of particles composed of solid substances in the coating liquid, for example the content (concentration) of fumed silica particles, is within the above range, it becomes easy to adjust the film thickness of the low refractive index layer 104 to 500 nm or more. In addition, the uniform dispersion of the fumed silica particles in the solvent can be improved, and it becomes easy to adjust the transmittance of the obtained low refractive index layer 104 to the above range.

After adding the fumed silica particles to the above solvent, a dispersion treatment is performed. When a coating liquid in which the fumed silica particles are dispersed in the solution while maintaining their complex higher-order structure is formed, the fumed silica particles and the gaps between the fumed silica particles are of a size that scatters visible light, and this may result in a decrease in the transmittance of the low refractive index layer 104. When the fumed silica particles are subjected to a dispersion treatment, the transparency of the coating liquid increases with the duration of the dispersion treatment.

When a coating liquid in which fumed silica particles are appropriately dispersed is formed, the skeleton of the fumed silica particles and the voids between the fumed silica particles are of a size that does not scatter visible light, forming a low refractive index layer 104 with high transmittance.

If further dispersion treatment is performed, the bulky higher-order structure of the fumed silica particles may be easily destroyed down to the primary particles, the porosity may decrease, and the refractive index of the resulting low refractive index layer 104 tends to increase.

Moreover, if the dispersion process is excessive, the fumed silica particles may become overdispersed, and the particles may easily re-aggregate, resulting in a decrease in the transmittance of the low refractive index layer 104 after film formation.

Therefore, it is preferable to achieve a moderately dispersed state. Dispersion processing can be performed using a stirrer, ultrasonic wave, planetary centrifugal mixer, ball mill, bead mill, homogenizer, etc.

An example will be described in which hollow particles with an outer shell of silicon dioxide are used as the solid substance, but the present invention is not limited to this.

A hollow particle dispersion can be used. There are no particular limitations on the hollow particle dispersion, so long as it satisfies the porosity of the hollow particles, the refractive index of the outer shell of the hollow particles, and the number-average particle size of the primary particles of the hollow particles.

For example, the Surulia series manufactured by JGC Catalysts and Chemicals, which is a dispersion in isopropanol (hereinafter also referred to as IPA) of hollow particles whose outer shell is silicon dioxide (hereinafter also referred to as hollow silica particles), is preferably used. In addition to commercially available products such as the Surulia series, hollow silica particles may be dispersed in a solvent in the same manner as the above-mentioned solvent dispersion of fumed silica particles.

The hollow particle concentration in the solvent should be in the same range as the fumed silica particle concentration (solids concentration) in the coating liquid.

The surface of the hollow silica particles has hydroxyl groups and is hydrophilic, so a solvent with strong hydrophobicity is not suitable. Specifically, it is preferable to use an organic solvent with an octanol/water partition coefficient logP _ow of 2 or less. Examples of the organic solvent include alcohol-based solvents such as methanol, ethanol, propanol, and isopropanol, glycol-based solvents such as ethylene glycol and propylene glycol, ether-based solvents such as dimethyl ether, diethyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether, acetate-based solvents such as ethyl acetate, propyl acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate, and ketone-based solvents such as acetone and methyl ethyl ketone.

As mentioned above, to lower the refractive index of the film, it is necessary to increase Y and X+Y. One method for doing this is to randomly arrange hollow silica particles.

Here, we will explain how to randomly arrange hollow silica particles. By forming loose aggregates of hollow silica particles in a dispersion liquid, the hollow silica particles can be arranged randomly.

Agglomerates are also included in the category of secondary particles, which are formed by primary particles made of solid substances forming a three-dimensional structure.

As a method for aggregating hollow silica particles well dispersed in a dispersion liquid, a method of adding a solvent (hereinafter, aggregating agent) having a relatively larger log _Pow than the dispersion medium can be mentioned. However, the method is not limited to this, and a method capable of controlling the aggregating state of hollow silica particles is preferable.

The surfaces of the hollow silica particles have hydroxyl groups and are hydrophilic, so that the log _Pow is greater than that of the dispersion medium. In other words, when a flocculant that is relatively more hydrophobic than the dispersion medium is added, the hollow silica particles are caused to aggregate.

The log _Pow and the amount of flocculant added to flocculate hollow silica particles well dispersed in a dispersion liquid are described below. If the difference between the log _Pow of the dispersion medium and the flocculant is too small, the hollow silica particles do not aggregate, and if the difference is too large, the hollow silica particles aggregate violently even if the amount of the flocculant added is small. If the hollow silica particles form large aggregates, the aggregates themselves may become light scatterers.

On the other hand, the huge gaps formed between the large aggregates of hollow silica particles also tend to scatter light, which can cause the low refractive index layer 104 to become cloudy after application, resulting in a decrease in transmittance.

Furthermore, if severe aggregation occurs, the storage stability of the coating liquid is likely to decrease. Therefore, in order to form a low refractive index layer 104 with a low refractive index and high transmittance, it is advisable to control the aggregation state of the hollow silica particles by the type and amount of aggregating agent added. By utilizing this property, the refractive index of the low refractive index layer 104 can be controlled by controlling the aggregation state of the hollow silica particles by the type and amount of aggregating agent added.

In addition, if the gaps between the hollow silica particles are filled with some substance, Y and X+Y become smaller, and the refractive index of the low refractive index layer 104 may become higher. Therefore, it is preferable that the aggregating agent be removable in a later process, and it is even more preferable that it volatilizes by heating.

As an aggregating agent, for example, silicone oil such as X-22-164 (manufactured by Shin-Etsu Chemical Co., Ltd.) can be used, but is not limited to this.

The method for forming the low refractive index layer 104 is described below.

A film is formed using the above coating liquid. Examples of film formation methods include bar coating, doctor blade, squeegee, spray, spin coating, dip coating, and screen printing. Of these, it is preferable to use the spin coating method from the viewpoint of making the film thickness of the low refractive index layer 104 uniform. In addition, it is preferable to use the spray method when forming a film on a large-area sensor panel on which a scintillator is formed.

In addition, to obtain a low refractive index layer 104 having a desired thickness and a flat upper surface, it is advisable to appropriately adjust the rotation speed in the spin coating method, for example.

The film formed by the above method is preferably dried at a temperature between 20°C and 100°C.

The resulting film may be further subjected to a heat treatment. The heat treatment is preferably performed at a temperature of 100°C or higher and 200°C or lower, and more preferably at a temperature of 120°C or higher and 180°C or lower.

By setting the heating temperature to 100°C or higher, for example, it is possible to reduce the possibility of the solvent remaining in the voids inside the hollow particles. On the other hand, by setting the heating temperature to 200°C or lower, it is possible to reduce the possibility of the performance of the sensor panel corresponding to the light detection unit 113 being degraded.

In addition, when the film contains a binder and a polymerization initiator, it is preferable to include a thermal curing or photocuring process. In the case of thermal curing, the binder can also be thermally cured at the same time as the solvent is evaporated in the drying or heating process.

Generally, films formed from microparticles maintain their shape through intermolecular forces. In addition, hydrophobic interactions occur when the microparticle surface is hydrophobic, and liquid bridging also occurs when the microparticle surface is hydrophilic; both are physical interactions. When the film is heat-treated, for example, the hydroxyl groups present on the surface of the fumed silica particles chemically bond with each other through a dehydration reaction, which is expected to improve the strength of the film.

When a film containing particles composed of the above solid substances is formed to achieve a low refractive index, the structure and film shape are maintained by the van der Waals forces and liquid bridges acting between the particles.

As one method for improving the strength of a film having the above structure, a binder that bonds the particles together may be used. From the viewpoint of improving the strength of the film, the low refractive index layer 104 may further contain a binder.

The low refractive index layer 104 preferably contains a bond in which solid substances are bonded with a binder. Specifically, the low refractive index layer 104 preferably contains a bond in which particles made of solid substances are bonded with a binder. The bonding of solid substances with a binder is a concept that includes any bonding between primary particles made of solid substances, between secondary particles formed by primary particles made of solid substances, between primary particles and secondary particles, etc. The bonding may be a chemical bond such as an ionic bond or a covalent bond, or a bond such as mechanical adhesion.

As binders, resins such as acrylic resin, fluororesin, styrene resin, imide resin, urethane resin, and phenolic resin can be used.

Organosilicon compounds obtained by polymerizing silicone oils having polymerizable groups or by hydrolyzing silicon alkoxides and subjecting them to condensation polymerization can also be used.

As long as it has a low refractive index, is colorless and transparent, and can bond particles together, it is not limited to these.

An example of a method for producing a film containing a binder is a method that includes a step of preparing a mixed liquid containing a solid substance, a solvent, and a binder, a step of dispersing the mixed liquid to obtain a coating liquid, and a step of forming the coating liquid into a film, drying it, and, if necessary, heating or irradiating it with high-energy rays to obtain a film.

The binder preferably contains siloxane, and more preferably contains silsesquioxane.

Silsesquioxane is a compound having a T3 unit structure represented by the composition formula [R ¹ (SiO _1.5 ) _n ] (R ¹ is a reactive functional group, e.g., at least one selected from the group consisting of a polymerizable group, a hydroxyl group, a chlorine atom, an alkyl group having 1 to 6 carbon atoms, and an alkoxy group having 1 to 6 carbon atoms), and is a hybrid material of silicon oxide and an organic substance.

Silsesquioxane (hereinafter sometimes abbreviated as SQ) is a siloxane-based compound whose main chain skeleton is composed of Si—O bonds, and is represented by the composition formula [R ¹ (SiO _1.5 ) _n ]. R ¹ is preferably at least one polymerizable group selected from the group consisting of an acryloyl group, a methacryloyl group, an oxetanyl group, and an epoxy group.

When silsesquioxane acts to bond particles made up of multiple solid substances, it becomes possible to achieve superior film strength while maintaining a high porosity.

There are no particular limitations on the form of the polymer for silsesquioxane, and examples include known linear polysiloxanes, cage polysiloxanes, and ladder polysiloxanes. A silsesquioxane structure is a structure in which each silicon atom is bonded to three oxygen atoms, and each oxygen atom is bonded to two silicon atoms (the number of oxygen atoms relative to the number of silicon atoms is 1.5). From the viewpoint of cost, linear polysiloxanes, cage polysiloxanes, and ladder polysiloxanes may be mixed.

The silsesquioxane is preferably a compound that has a polymerizable group (R ¹ in the above formula) in the molecule and is cured by radical polymerization or cationic polymerization.

Examples of silsesquioxanes that cure by radical polymerization include silsesquioxanes that have an acryloyl group or a methacryloyl group as R. On the other hand, examples of silsesquioxanes that cure by cationic polymerization include silsesquioxanes that have an oxetanyl group or an epoxy group as R.

Specific examples include the SQ series of silsesquioxane derivatives (AC-SQ, MAC-SQ, OX-SQ) manufactured by Toagosei.

Since silsesquioxane is a highly viscous liquid, it is recommended to add it to the coating liquid before use. If necessary, a polymerization initiator may also be added.

The content of the binder in the functional film is preferably 3.0 parts by mass or more and 60.0 parts by mass or less, and more preferably 7.0 parts by mass or more and 30.0 parts by mass or less, per 100 parts by mass of particles composed of solid substances. The content of the binder in the functional film is further preferably 7.0 parts by mass or more and 25.0 parts by mass or less, and particularly preferably 10.0 parts by mass or more and 25.0 parts by mass or less, per 100 parts by mass of particles composed of solid substances.

This coating liquid is applied onto a substrate and the silsesquioxane can be cured by heating or irradiating it with light.

This process forms a film containing particles of solid substances bound together by silsesquioxane. The strength of the film can be increased by hardening the silsesquioxane.

Examples of polymerization initiators include photoradical polymerization initiators, photocationic polymerization initiators, thermal radical polymerization initiators, and thermal cationic polymerization initiators. These polymerization initiators may consist of one type of polymerization initiator, or may consist of multiple types of polymerization initiators.

Examples of photoradical polymerization initiators include 2,4,5-triaryl imidazole dimers that may have a substituent, such as 2-(o-chlorophenyl)-4,5-diphenylimidazole dimer, 2-(o-chlorophenyl)-4,5-di(methoxyphenyl)imidazole dimer, 2-(o-fluorophenyl)-4,5-diphenylimidazole dimer, and 2-(o- or p-methoxyphenyl)-4,5-diphenylimidazole dimer; benzophenone derivatives such as benzophenone, N,N'-tetramethyl-4,4'-diaminobenzophenone (Michler's ketone), N,N'-tetraethyl-4,4'-diaminobenzophenone, 4-methoxy-4'-dimethylaminobenzophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, and 4,4'-diaminobenzophenone. Conductors; α-amino aromatic ketone derivatives such as 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one; quinones such as 2-ethylanthraquinone, phenanthrenequinone, 2-t-butylanthraquinone, octamethylanthraquinone, 1,2-benzanthraquinone, 2,3-benzanthraquinone, 2-phenylanthraquinone, 2,3-diphenylanthraquinone, 1-chloroanthraquinone, 2-methylanthraquinone, 1,4-naphthoquinone, 9,10-phenanthranequinone, 2-methyl-1,4-naphthoquinone, and 2,3-dimethylanthraquinone; benzoins such as benzoin methyl ether, benzoin ethyl ether, and benzoin phenyl ether. benzyl derivatives such as benzyl dimethyl ketal; acridine derivatives such as 9-phenylacridine and 1,7-bis(9,9'-acridinyl)heptane; N-phenylglycine derivatives such as N-phenylglycine; acetophenone derivatives such as acetophenone, 3-methylacetophenone, acetophenone benzyl ketal, 1-hydroxycyclohexyl phenyl ketone, and 2,2-dimethoxy-2-phenylacetophenone; thioxanthone derivatives such as thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, and 2-chlorothioxanthone; 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis( ... Acylphosphine oxide derivatives such as bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide; oxime ester derivatives such as 1,2-octanedione, 1-[4-(phenylthio)-, 2-(O-benzoyloxime)], ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime); xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, carbazole, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 2-hydroxy-2-methyl-1-phenylpropan-1-one, etc., but are not limited to these.

Commercially available photoradical polymerization initiators include, but are not limited to, Irgacure 184, 369, 651, 500, 819, 907, 784, 2959, CGI-1700, -1750, -1850, CG24-61, Darocur 1173, Lucirin TPO, LR8893, LR8970 (all manufactured by BASF, "Darocur" and "Lucirin" are registered trademarks), and Ubecryl P36 (manufactured by UCB).

As the photocationic polymerization initiator, onium salts, aromatic onium salts, arylsulfonium salts, aryliodonium salts, etc. are preferred. Specific examples of anions include tetrafluoroborate ions, hexafluorophosphate ions, hexafluoroantimonate ions, perchlorate ions, trifluoromethanesulfonate ions, fluorosulfonate ions, etc.

Commercially available photocationic polymerization initiators include San-Apro CPI-210S (San-Apro), UVI-6950 (Union Carbide), and Adeka Optomer SP-150 (ADEKA).

The content of the polymerization initiator in the coating liquid is preferably 0.01 parts by mass or more and 1.5 parts by mass or less, and more preferably 0.03 parts by mass or more and 1.0 parts by mass or less, per 100 parts by mass of the silsesquioxane solid content.

The coating liquid may be prepared by mixing particles composed of solid substances, a solvent, a binder, and, if necessary, a polymerization initiator. It is preferable to use an organic solvent as the solvent. The organic solvent is not particularly limited, but may be alcohol, carboxylic acid, aliphatic or alicyclic hydrocarbons, aromatic hydrocarbons, esters, ketones, ethers, or a mixture of two or more of these.

Examples of alcohols include methanol, ethanol, 2-propanol, butanol, 2-methoxyethanol, 2-ethoxyethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 1-propoxy-2-propanol, 4-methyl-2-pentanol, 2-ethylbutanol, 3-methoxy-3-methylbutanol, ethylene glycol, diethylene glycol, and glycerin.

Specific examples of carboxylic acids include n-butyric acid, α-methylbutyric acid, i-valeric acid, 2-ethylbutyric acid, 2,2-dimethylbutyric acid, 3,3-dimethylbutyric acid, 2,3-dimethylbutyric acid, 3-methylpentanoic acid, 4-methylpentanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, 2,2-dimethylpentanoic acid, 3,3-dimethylpentanoic acid, 2,3-dimethylpentanoic acid, 2-ethylhexanoic acid, and 3-ethylhexanoic acid.

Specific examples of aliphatic or alicyclic hydrocarbons include n-hexane, n-octane, cyclohexane, cyclopentane, and cyclooctane.

Preferred aromatic hydrocarbons include toluene, xylene, and ethylbenzene.

Examples of esters include ethyl formate, ethyl acetate, n-butyl acetate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, and gamma-butyrolactone.

Ketones include acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, etc.

Ethers include dimethoxyethane, tetrahydrofuran, dioxane, and diisopropyl ether.

When preparing the coating solution, it is preferable to use alcohol among the various solvents mentioned above in terms of solution stability.

The coating liquid can be prepared by adding a predetermined amount of binder and, if necessary, a polymerization initiator to a liquid in which particles composed of solid substances are dispersed in a solvent. The liquid in which particles are dispersed in an organic solvent may be prepared by dispersing particle powder in a solvent using a method similar to the above-mentioned dispersion treatment (e.g., a ball mill, etc.), or a commercially available dispersion may be used.

When forming a film using a coating liquid, it is preferable to perform the coating in an inert gas atmosphere such as dry air or dry nitrogen. The relative humidity of the dry atmosphere is preferably 30% or less.

Furthermore, as a solution application method for forming a film, known application means such as dipping, spin coating, spraying, printing, flow coating, and combinations of these can be appropriately adopted. The film thickness can be controlled by changing the pulling speed in the dipping method or the substrate rotation speed in the spin coating method, and by changing the concentration of the coating liquid.

The resulting film may be cured by exposure to high-energy rays such as light or radiation, or by heating. Curing may also be achieved by a combination of high-energy ray irradiation and heating.

When curing by irradiation with high energy rays, examples of the high energy rays include, but are not limited to, electron beams, X-rays, and ultraviolet rays. When ultraviolet rays are used as the high energy rays, the irradiation wavelength region is preferably 160 nm to 400 nm, and the output is preferably 0.1 mW/cm ² or more and 2000 mW/cm ² or less. From the viewpoint of preventing oxidation of silsesquioxane, it is preferable to use an inert atmosphere such as nitrogen as the curing atmosphere. When curing by heating, it is preferable to carry out the curing at a temperature of 50° C. to 250° C., preferably 80° C. to 200° C., for 1 minute to 20 minutes.

<Method for evaluating film porosity and refractive index>
The porosity X (%) and the porosity Y (%) of the film can be calculated as follows.

First, the film formed on the substrate is coated with a carbon film using a Model 681 Ion Beam Coater IBC (manufactured by Gatan), then cross-section processing (30 kV-0.1 nA) is performed using an ion beam in a focused ion beam processing device (FIB-SEM, manufactured by FEI, Nova 600), and an SEM image is then obtained using a scanning electron microscope (hereinafter referred to as SEM) at an acceleration voltage of 2 kV.

The observation magnification of the SEM image is set to a magnification that covers the entire film at least in the thickness direction and allows, for example, the shape of each hollow particle to be distinguished. Specifically, it is set to about 50,000 to 200,000 times.

The unit volume of the film is 1000 nm x 1000 nm x 100 nm (thickness direction). To calculate the porosity in the acquired cross-sectional SEM image, the hollow particles and the gaps between the hollow particles are separated by binarizing the grayscale image, and the area of each region is calculated. For image processing, the image analysis software Image J (NIH Image, available from https://imagej.nih.gov/ij/) is used.

Specifically, the porosity X (%) is calculated by multiplying the determined area A (%) of the hollow particle by the volume fraction _Va of the internal voids relative to the total volume of the hollow particle, where X = A × _Va . The porosity Y (%) is calculated by Y = 100 - A.

The refractive index n of the low refractive index layer 104 can be calculated from the X and Y determined above and formula (2).

<Film thickness>
The thickness of the low refractive index layer 104 can be, for example, 100 nm or more and 3.5 μm or less, and can be 150 nm or more and 3 μm or less. The thickness of the low refractive index layer 104 is preferably 300 nm or more and 2 μm or less. The preferred lower limit of the thickness is determined because it is preferable to be equal to or greater than the emission wavelength of the scintillator in order to sufficiently increase the total reflection efficiency. Since a scintillator that emits light in the visible light range is used, the low refractive index layer preferably has a thickness of 300 nm or more. Furthermore, if the thickness of the low refractive index layer is 2 μm or less, cracks are unlikely to occur, and a low refractive index layer with sufficient strength can be obtained. The thickness of the low refractive index layer 104 can be measured from a cross-sectional SEM image as described in the evaluation method of the porosity and refractive index of the film.

[Radiation detector]
2, a radiation detector 207 can be formed by combining a scintillator unit 205 with a detection section 206 that detects light from the scintillator unit 205. As the detection section 206, a silicon photomultiplier array (SiPMA), a complementary metal oxide semiconductor sensor (CMOS), a flat panel detector (FPD), or the like can be used.

The detection unit 206 is preferably arranged so as to be in contact with the scintillator 201. This allows the detection unit 206 to efficiently detect the light generated by the scintillator 201. In addition, the detection unit 206 is preferably paired with the scintillator 201. This allows the detection unit 206 to individually detect the light generated by each scintillator 201.

The light extraction surface 208 shown in FIG. 2 is the surface in the scintillator unit 205 from which the light generated in the scintillator 201 is emitted. In FIG. 2, the reflective layer 204 and the low refractive index layer 104 are arranged substantially perpendicular to the light extraction surface 208. That is, multiple scintillators and multiple detection units paired with the multiple scintillators are arranged via the reflective layer 204 and the low refractive index layer 104. This allows the light generated by a scintillator to be detected by the paired detection unit but is difficult to detect by the unpaired detection unit, making it possible to further reduce crosstalk.

(Method of measuring reflectance)
There are two types of reflectance: absolute reflectance and relative reflectance. Absolute reflectance is the intensity ratio of reflected light to incident light of a certain wavelength on the measurement sample. The angle of incidence is the angle between the incident light and the normal to the reflection surface of the measurement sample. The angle of reflection is the angle between the reflected light and the normal to the reflection surface. In measuring absolute reflectance, the intensity ratio is measured when the angle of incidence and the angle of reflection are equal.

Relative reflectance is the reflectance relative to the absolute reflectance of a measurement sample. For example, the relative reflectance of two measurement samples is the ratio of the absolute reflectance of one measurement sample to the absolute reflectance of the other.

The scintillator unit according to the present invention will be described in detail below with reference to examples, but the present invention is not limited in any way to the following examples.

Example 1
The improvement in reflectance due to the low refractive index layer of the scintillator unit of this example was evaluated as follows. Fig. 3 is a schematic bird's-eye view showing a sample used in the reflectance measurement of the present invention. As shown in Fig. 3, the scintillator was processed so that θ ₁ was 40°. The adhesive layer 202, the low refractive index layer 104, and the reflective layer 204 were arranged in this order so as to be in contact with the processed scintillator 201.

The low refractive index layer was made using hollow silica particles. To prepare the coating liquid for film formation, JGC Catalysts and Chemicals' Sururia 4110 (dispersion medium: IPA, silica solid content concentration: 20.5 mass%, number average particle size per hollow particle: 60 nm, porosity per hollow particle: 45%, refractive index per hollow particle: 1.25) was used. The coating liquid was prepared by adjusting the silica solid content concentration to 6.0 mass%. This coating liquid was applied to the reflective layer by spin coating, and rotated for 10 seconds at a rotation speed of 1000 rpm to form a low refractive index layer.

The fabricated low-refractive index layer was subjected to cross-section processing (30 kV-0.1 nA) using an ion beam in a focused ion beam processing device (FIB-SEM, FEI, Nova 600). After that, a SEM image was obtained using a scanning electron microscope (hereinafter referred to as SEM) at an acceleration voltage of 2 kV. The low-refractive index layer was bonded to the reflective layer without any gaps, and the film thickness was 0.5 μm. The cross-sectional SEM image of the low-refractive index layer was divided into hollow particles and voids between the hollow particles by binarizing the grayscale image, and the area of each region was calculated. The porosity of the low-refractive index layer was calculated to be 67% by taking into account the calculated area of the hollow particles and the volume fraction of the internal voids relative to the total volume of the hollow particles. The refractive index of the low-refractive index layer was calculated to be 1.15 when ns = 1.46 from formula (2).

A scintillator (LSO) was attached to the reflective layer with the low refractive index layer formed using an epoxy-based optical adhesive (refractive index 1.43). The amount of adhesive and the pressure applied during bonding were adjusted so that the adhesive layer was 2 μm thick. A dielectric multilayer film was used for the reflective layer.

Absolute reflectance and relative reflectance were measured using JASCO's VR670 and ARMN-735 as follows:

The cross section A-A' of FIG. 3 is shown in FIG. 4(a). In FIG. 4(a), incident light 401 is incident on reflective layer 204, and the intensity of reflected light 402 reflected by reflective layer 204 is measured. The measured reflection angle 404 of reflected light 402 is set to be the same as incident angle 403. The incident angle in the following measurement results refers to incident angle 403.

In the following measurement results, relative reflectance is used. As a reference for relative reflectance, the absolute reflectance when a spacer (not shown) is inserted as shown in Figure 4(b) and there is air between the reflective layer and the scintillator is used. The absolute reflectance measured under each condition was divided by the absolute reflectance in Figure 4(b) to calculate the relative reflectance. Figure 5 shows an example of a measured relative reflectance spectrum.

(Comparative Example 1)
A sample was prepared in the same manner as in Example 1, except that no low refractive index layer was used. That is, a sample was prepared in which a scintillator 201, an adhesive layer 202, and a reflective layer 204 were laminated in this order, as shown in Fig. 4(c) . The relative reflectance was measured in the same manner as in Example 1. The spectrum of the measured relative reflectance is shown in Fig. 5.

From the relative reflectance spectrum in Figure 5, it can be seen that Example 1 has a higher reflectance than Comparative Example 1 at all wavelengths. In other words, it can be seen that light is easily reflected by arranging, in order from the scintillator, an adhesive layer and a low refractive index layer between the scintillator and the reflective layer.

Furthermore, Figures 5(a) and (b) show the relative reflectance spectra when the angles of incidence are 70° and 74°, respectively. Comparing Figures 5(a) and (b), it can be seen that the difference between the relative reflectance of Example 1 and the relative reflectance of Comparative Example 1 increases as the angle of incidence increases. This shows that when the light extraction surface 208 and the reflective layer 204 are approximately perpendicular as shown in Figure 2, Example 1 has a higher relative reflectance than Comparative Example 1. In other words, it can be seen that light is easily reflected by arranging an adhesive layer and a low refractive index layer, in that order from the scintillator, between the scintillator and the reflective layer.

According to the present invention, it is possible to provide a radiation detector having a scintillator unit capable of reducing crosstalk when using a scintillator unit having multiple scintillators.

REFERENCE SIGNS 104 Low refractive index layer 201 Scintillator 202 Adhesive layer 204 Reflective layer 205 Scintillator unit 206 Detection section 207 Radiation detector 208 Light exit surface 301 Hollow particle 302 Void between hollow particles 303 Outer shell of hollow particle 304 Void formed inside hollow particle 305 Substrate 401 Incident light 402 Reflected light 403 Incident angle 404 Reflection angle

Claims (16)

A scintillator unit including a plurality of scintillators and a reflective layer between the plurality of scintillators,
The scintillator unit has, between the scintillator and the reflective layer, an adhesive layer and a low refractive index layer having a refractive index lower than that of the adhesive layer, in that order from the scintillator. The scintillator unit according to claim 1, wherein the scintillator unit is configured to include, in order, the scintillator, the adhesive layer, the low refractive index layer, the reflective layer, the low refractive index layer, the adhesive layer, and the scintillator. The scintillator unit according to claim 1 or 2, wherein the reflective layer includes a dielectric multilayer film. The scintillator unit according to any one of claims 1 to 3, wherein the low refractive index layer is disposed substantially perpendicular to the light extraction surface of the scintillator unit. The scintillator unit according to any one of claims 1 to 4, wherein the adhesive layer contains at least one of an epoxy resin, an acrylic resin, and a vinyl resin. The scintillator unit according to any one of claims 1 to 5, wherein the thickness of the adhesive layer is 2 μm or more and 5 μm or less. The scintillator unit according to any one of claims 1 to 6, wherein the refractive index of the adhesive layer is greater than 1.30 and less than 1.70. The scintillator unit according to any one of claims 1 to 7, wherein the low refractive index layer contains silicon oxide. The scintillator unit according to any one of claims 1 to 8, wherein the refractive index of the low refractive index layer is 1.10 or more and 1.20 or less. The scintillator unit according to any one of claims 1 to 9, wherein the low refractive index layer has voids, and the porosity of the low refractive index layer is 60.0% or more and 95.0% or less. A scintillator unit according to any one of claims 1 to 10, wherein the thickness of the low refractive index layer is 300 nm or more and 2 μm or less. The scintillator unit according to any one of claims 1 to 11, wherein the low refractive index layer contains hollow particles. The scintillator unit according to claim 12, wherein the refractive index of the outer shell of the hollow particle is 1.35 or more and 1.60 or less. The scintillator unit according to claim 12 or 13, wherein the hollow particles are hollow silica particles. The scintillator unit according to any one of claims 1 to 11, wherein the low refractive index layer contains fumed silica particles. A radiation detector comprising: a scintillator unit including a plurality of scintillators and a reflective layer between the plurality of scintillators; and a detection unit that detects light generated from the scintillators,
A radiation detector comprising: the scintillator unit having, between the scintillator and the reflective layer, an adhesive layer and a low refractive index layer having a refractive index lower than that of the adhesive layer, in that order from the scintillator.

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