Yttrium tantalate crystal, method for manufacturing the same, scintillator material and application thereof

Abstract

To provide a yttrium tantalate crystal having scintillation characteristics equal to or more than these of cadmium tungstate (CdWO4) without including a harmful substance, such as cadmium, and a method for manufacturing the same.SOLUTION: A yttrium tantalate crystal represented by a general formula Y(Ta1-xNbx) O4-y (where, x and y independently satisfy 0≤x≤1 and 0≤y≤0.5, respectively.) has a transmittance of 70% or more in a wavelength range of 380-780 nm in a transmission spectrum.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to yttrium tantalate crystals, their manufacturing method, scintillator materials, and uses thereof. [Background technology]

[0002] Radiation inspection devices are used in various fields and modes, such as inspection devices for non-destructive inspection, medical diagnostic devices, etc. Exemplary modes include a mode in which a subject is irradiated with radiation (X-rays, neutron rays, etc.) and the transmitted radiation is detected by a radiation detector, and a mode in which a radiation source (radioactive material) is placed inside the subject in advance and radiation (gamma rays, etc.) generated from the radiation source and transmitted through the subject is detected by a radiation detector.

[0003] A radiation detector generally includes a scintillator material (also simply called a scintillator) that converts radiation into light, and a photoelectric converter that detects the light emitted from the scintillator material and converts it into an electrical signal.

[0004] Conventionally, known scintillator materials used in X-ray detectors include single crystals such as cadmium tungstate (CdWO4) and thallium-doped cesium iodide (Tl:CsI), and polycrystalline ceramics such as terbium-doped gadolinium oxysulfide (Tb:Gd2O2S) and praseodymium-doped gadolinium oxysulfide (Pr:Gd2O2S).

[0005] In particular, CdWO4 is widely used as the scintillator material in X-ray detectors used in baggage and cargo inspection at airports, etc. CdWO4 single crystals have practical scintillation properties, such as high luminescence and low afterglow, and can be manufactured at relatively low cost. However, because cadmium is a hazardous substance, CdWO4 single crystals may be harmful to the environment.

[0006] In recent years, there have been research reports on yttrium tantalate (YTaO4) and niobium (Nb)-containing YTaO4 as scintillator materials that do not contain harmful substances (harmful elements) such as cadmium (Non-Patent Documents 1-5). Non-Patent Document 1 describes the scintillation properties of YTaO4 single crystals prepared in a vacuum by vertical directed crystallization, but the analytical value of the scintillation efficiency (%) using NaI (Tl) as a standard sample is only half (20%) of that of CdWO4 (40%). Non-Patent Documents 2-5 describe YTaO4 and Nb-containing YTaO4 ceramics prepared by solid-state methods, some of which are said to have a higher luminescence output than CdWO4. However, due to their opacity, the ceramic materials described in Non-Patent Documents 2-5 are limited to thin-plate applications and can only be used as scintillator materials for X-ray detectors that use low-energy X-rays. Furthermore, ceramic scintillator materials suffer from performance degradation when exposed to high-energy X-rays. In addition, because ceramics are polycrystalline, there are concerns that the scintillation properties may be reduced due to grain boundaries and crystal defects that inevitably occur during manufacturing. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] LI Kazakova, et al., Radiation Measurements, 1995, 24, 359-360. [Non-patent document 2] C. W. E. Eijk, et al., Physics in Medicine and Biology, 2002, 47, R85-R106. [Non-patent document 3] OV Voloshyna, et al., Materials Science and Engineering B, 2013, 178, 1491-1496.

Non-Patent Document 4

Non-Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a yttrium tantalate crystal that does not contain harmful substances such as cadmium and has scintillation characteristics comparable to or superior to those of CdWO4. Another object of the present invention is to provide a method for producing the above-mentioned yttrium tantalate crystal. Another object of the present invention is to provide a scintillator material using the above-mentioned yttrium tantalate crystal and its use.

Means for Solving the Problems

[0009] The yttrium tantalate crystal according to the present invention has the general formula Y(Ta 1-x Nb x )O 4-y (where x and y each independently satisfy 0 ≦ x ≦ 1 and 0 ≦ y ≦ 0.5), and in the transmission spectrum, the transmittance in the wavelength range of 380 nm or more and 780 nm or less is 70% or more, thereby solving the above problems. In the yttrium tantalate crystal of the present invention, x in the above general formula may satisfy the condition 0 < x ≦ 1. The yttrium tantalate crystal of the present invention may have a crystal structure belonging to the I12 / a1 space group, and the crystal phase may be a single phase of monoclinic crystal. In an X-ray resistance test using an X-ray source device, the yttrium tantalate crystal of the present invention may be such that a sample exposed to X-rays generated at a voltage of 45 kV and a current of 300 mA for at least 60 minutes or more exhibits luminescence substantially equivalent to that before the X-ray irradiation, and may have no X-ray irradiation marks.

[0010] The method for producing the above-mentioned yttrium tantalate crystal according to the present invention includes the steps of: preparing a feedstock rod using a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb); fixing the feedstock rod obtained in the preparation step to the lower end of a first support drive shaft extending vertically from above; fixing a seed crystal rod to the upper end of a second support drive shaft extending vertically from below opposite the first support drive shaft; heating a portion of the feedstock rod and the seed crystal rod; forming a molten zone between the feedstock rod and the seed crystal rod; and continuously moving the molten zone to obtain a crystal; the step of obtaining the crystal is carried out under inert gas; and in the step of preparing the feedstock rod, Y, Ta, and Nb satisfy the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1) in atomic ratio, thereby solving the above-mentioned problem. Here, the above manufacturing method may further include a step of heat-treating the obtained crystal. The heat treatment step may involve heat treating the obtained crystal in an inert gas atmosphere or an air atmosphere at a temperature range of 100°C or higher and lower than the melting point of the target yttrium tantalate crystal for 0.5 hours to 150 hours.

[0011] The above-mentioned method for producing yttrium tantalate crystals according to the present invention includes the steps of melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb), and the steps of contacting a seed crystal with the surface of the melt obtained in the melting step and continuously lowering the melt to crystallize the contact area, wherein the crystallization step is carried out under an inert gas, and in the melting step, Y, Ta, and Nb satisfy the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1) in atomic ratio, thereby solving the above-mentioned problem. Here, the above manufacturing method may further include a step of heat-treating the crystals obtained in the crystallization step. The heat treatment step may involve heat treating the obtained crystal in an inert gas atmosphere or an air atmosphere at a temperature range of 100°C or higher and lower than the melting point of the target yttrium tantalate crystal for 0.5 hours to 150 hours.

[0012] The above-mentioned method for producing yttrium tantalate crystals according to the present invention includes the steps of melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb), and the steps of bringing a seed crystal into contact with the liquid surface of the melt obtained in the melting step, solidifying the contact area, and pulling up the seed crystal for crystallization, wherein the melting step and the crystallization step are carried out in an inert gas, and in the melting step, Y, Ta, and Nb satisfy the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1) in terms of atomic ratio, thereby solving the above-mentioned problem. Here, the above manufacturing method may further include a step of heat-treating the crystals obtained in the crystallization step. The heat treatment step may involve heat treating the obtained crystal in an inert gas atmosphere or an air atmosphere at a temperature range of 100°C or higher and lower than the melting point of the target yttrium tantalate crystal for 0.5 hours to 150 hours.

[0013] The scintillator material according to the present invention is a scintillator material made of the above-mentioned yttrium tantalate crystal, thereby solving the above-mentioned problems. A radiation detector according to the present invention comprises the above-described scintillator material and a photoelectric converter that detects light from the scintillator material and converts it into an electrical signal, thereby solving the above-described problems. A radiological inspection apparatus according to the present invention comprises a radiation source that irradiates a subject with radiation, and the above-described radiation detector that detects radiation that passes through the subject, thereby solving the above-described problems. [Effects of the Invention]

[0014] According to the present invention, there are provided yttrium tantalate crystals that do not contain harmful substances such as cadmium and have scintillation properties comparable to or superior to those of CdWO4, and a method for producing the same. The yttrium tantalate crystals of the present invention are suitable for use as scintillator materials for radiation detectors, particularly as scintillator materials for X-ray radiation detectors. Furthermore, radiation detectors that include the yttrium tantalate crystals of the present invention as scintillator materials can be suitably used in radiation inspection devices.

[0015] The method for producing yttrium tantalate crystals of the present invention allows for the growth of chemically stable bulk single crystals, making it suitable for mass production and reducing production costs. In addition, since no harmful substances such as cadmium are used, the material is environmentally friendly (environmentally conscious), and the production process is also environmentally friendly. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view of one embodiment of a manufacturing apparatus used to manufacture an yttrium tantalate crystal according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of another embodiment of a manufacturing apparatus used to manufacture an yttrium tantalate crystal according to one embodiment of the present invention. [Figure 3]1 is a schematic diagram illustrating a configuration of a radiation detector according to an embodiment of the present invention. [Figure 4] 1 is a schematic diagram showing a configuration of a radiological inspection apparatus according to an embodiment of the present invention; [Figure 5] 1A and 1B show photographic images of (a) Sample 1, (b) Sample 13, (c) Sample 8, and (d) Sample 14. [Figure 6] FIG. 1 shows the transmission spectra of Samples 1, 7, 13, and 14. [Figure 7A] 1 shows Laue photographs of (top) Sample 1 and (bottom) Sample 8. [Figure 7B] 1 shows Laue photographs of sample 13 (top) and sample 14 (bottom). [Figure 8] FIG. 1 shows X-ray excited luminescence spectra of Samples 1 to 7 and Samples 9 to 12. [Figure 9] FIG. 1 shows the results of an X-ray resistance test for Samples 1, 8, 13, and 14. [Figure 10] (a) A diagram showing the pulse height spectra of Sample 1, Sample 1A, Sample 7, and Sample 7A. (b) A diagram showing a graph in which the channel numbers of the peak values ​​read from the pulse height spectra of Samples 1 to 7 and Samples 9 to 12, and Samples 1A to 7A and Samples 9A to 12A are plotted against the Nb content in the crystal composition. [Figure 11] FIG. 1 is a graph in which the quantum efficiency values ​​of crystals corresponding to samples 1 to 7 and samples 9 to 12 are plotted against the content of Nb in the crystal composition. [Figure 12] FIG. 1 shows pulse height spectra of Sample 1A, Sample 2A, Comparative Sample Z1 (standard sample), and Comparative Sample Z2. [Figure 13] FIG. 1 is a graph plotting the relative values ​​of the amount of light emitted and the calculated values ​​of the stopping power of X-rays for Samples 1A to 7A and Samples 9A to 12A against the content of Nb in the crystal composition. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described.

[0018] [Yttrium tantalate crystal] <Composition> The yttrium tantalate crystal according to an embodiment of the present invention (hereinafter, also simply referred to as "yttrium tantalate crystal") contains at least yttrium (Y), tantalum (Ta), and oxygen (O) as constituent elements, and further contains niobium (Nb) as an optional element. The yttrium tantalate crystal has a composition represented by the general formula Y(Ta 1-x Nb x )O 4-y (where x and y each independently satisfy 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.5).

[0019] In one aspect, x and y in the above general formula are zero, and the yttrium tantalate crystal has a composition represented by YTaO4. In another aspect, x in the above general formula satisfies the condition 0 < x ≤ 1 and y is zero, and the yttrium tantalate crystal can be referred to as a niobium-substituted yttrium tantalate crystal and can be denoted as Nb:YTaO4. When x is 1, the yttrium tantalate crystal has a composition represented by YNbO4 as a result of all of the constituent element tantalum being substituted with niobium.

[0020] Due to defects in the crystal, the number of oxygen atoms in the yttrium tantalate crystal of the present embodiment may be less than 4. However, by satisfying the condition 0 ≤ y ≤ 0.5 for y in the above general formula, a decrease in the transmittance of the crystal is sufficiently suppressed. In other words, in the above general formula, it is preferable that the value of y is small. Most preferably, y in the above general formula is zero. In this case, the yttrium tantalate crystal has a composition represented by the general formula Y(Ta 1-x Nb x )O4 (where 0 ≤ x ≤ 1).

[0021] <Crystal structure> The yttrium tantalate crystal belongs to the monoclinic crystal system, and belongs to the I12 / a1 (space group 15 in the International Tables for Crystallography (also simply referred to as the International Table(s))) space group, with the crystal parameters and atomic coordinate positions shown in Table 1. Furthermore, the yttrium tantalate crystal has a single crystal phase as identified by powder X-ray diffraction measurement.

[0022] [Table 1]

[0023] The lattice constant of yttrium tantalate crystals can change when its constituent elements are replaced by other elements or when the optional element (additive element) Nb is dissolved in solid solution. However, the atomic positions given by the crystal structure, the sites occupied by the atoms, and their coordinates do not change so much that the chemical bonds between the skeletal atoms are broken.

[0024] <Main characteristics> Yttrium tantalate crystal has the same level of transparency as conventional CdWO4. Specifically, in the transmission spectrum, yttrium tantalate crystal has a transmittance of 70% or more in the wavelength range of 380 nm to 780 nm. This allows the light generated within the scintillator crystal to be transmitted efficiently when used as a scintillator material, preventing a decrease in the amount of light emitted. Furthermore, yttrium tantalate crystal has a density and ρZ comparable to those of CdWO4. 4 where ρZ 4 is the product of density (ρ) and the fourth power of effective atomic number (Z), and is known to be proportional to the radiation stopping power (Reference: T. Yanagida, Proc. Jpn. Acad., Ser., 2018, 94, 223-231.). This means that yttrium tantalate crystals have the stopping power required for scintillator materials, and can have X-ray stopping power comparable to that of CdWO4.

[0025] The yttrium tantalate crystal has the same radiation resistance as conventional CdWO4. Specifically, in an X-ray resistance test using an X-ray source device, a sample of the yttrium tantalate crystal exposed to X-rays generated at a voltage of 45 kV and a current of 300 mA for at least 60 minutes exhibits substantially the same luminescence as before the X-ray irradiation and does not show any X-ray irradiation marks. The yttrium tantalate crystal with such radiation resistance is practically advantageous for use as a scintillator material.

[0026] Examples of indices for evaluating the scintillation characteristics of yttrium tantalate crystal include the amount of light emitted, decay time (decay characteristics), and persistence (afterglow characteristics).

[0027] Yttrium tantalate crystals can exhibit luminescence comparable to or even greater than that of conventional CdWO4. Furthermore, the emission wavelength range of yttrium tantalate crystals can be varied between the ultraviolet and visible light regions (more specifically, the blue light region) depending on whether or not they contain Nb, allowing the crystal composition to be designed in various ways depending on the application. Regarding decay characteristics, yttrium tantalate crystals can exhibit scintillation decay times significantly shorter than those of CdWO4. Additionally, regarding afterglow characteristics, yttrium tantalate crystals can exhibit afterglow comparable to that of CdWO4. Specific examples of methods for analyzing these scintillation characteristics will be described in detail in the Examples section.

[0028] In the above context, it should be noted that it is not an essential requirement for yttrium tantalate crystals to exceed CdWO4 in all properties. That is, depending on various applications exemplified below, if yttrium tantalate crystals satisfy the properties (scintillation properties) required for the application, they can be said to be a practically useful material. In other words, yttrium tantalate crystals can be said to be environmentally friendly both as a material itself and in its manufacturing method in that they do not contain harmful substances such as cadmium, and even if they have properties that are slightly inferior (but not comparable) to those of CdWO4 in comparison with CdWO4, they can be a material that can replace CdWO4.

[0029] [Manufacturing Method of Yttrium Tantalate Crystal] Next, the manufacturing method of the above-described yttrium tantalate crystal will be described.

[0030] [Manufacturing by FZ Method] FIG. 1 is a schematic cross-sectional view of one aspect of a manufacturing apparatus used for manufacturing a yttrium tantalate crystal according to an embodiment of the present invention.

[0031] The manufacturing apparatus 10 according to this aspect is a crystal growth apparatus also referred to as a floating zone (FZ) furnace. The manufacturing apparatus 10 has two symmetrically shaped rotating elliptical mirrors 11 and 12. Each of the rotating elliptical mirrors 11 and 12 has one focus F1, F2 and the other focus F0, and they are oppositely coupled so that the other foci F0 of each coincide to form a heating furnace. Here, the other focus F0 is located at approximately the center of the heated portion 15 described later, and in FIG. 1, for the sake of convenience, the symbol F0 is shown in parentheses below the symbol 15. The inner surfaces of the rotating elliptical mirrors 11 and 12, that is, the reflecting surfaces, are subjected to gold plating treatment in order to reflect infrared rays with a high reflectivity.

[0032] Near the foci F1 and F2 of the rotating elliptical mirrors 11 and 12, infrared lamps 13 and 14 are fixedly arranged. At the coinciding focus F0 of the rotating elliptical mirrors 11 and 12, the heated portion 15 is located, and a quartz tube 16 is installed in the vertical direction so as to include the heated portion 15.

[0033] Quartz tube 16 separates internal space m1 of quartz tube 16 from internal spaces m2 of the other spheroidal mirrors 11 and 12, thereby replacing the internal space m1 of quartz tube 16 with an atmosphere suitable for crystal growth and making it easier to maintain that atmospheric state. In addition, because internal space m1 is separated from internal space m2, infrared lamps 13 and 14 located in internal space m2 can be cooled without affecting heated portion 15 within internal space m1.

[0034] A first support drive shaft 17 extending vertically from above and a second support drive shaft 18 extending vertically from below opposite the first support drive shaft 17 are disposed in the internal space m1 of the quartz tube 16. A feed rod FR can be fixed to the lower end of the first support drive shaft 17, and a seed crystal rod SR can be fixed to the upper end of the second support drive shaft 18. The feed rod FR fixed to the first support drive shaft 17 and the seed crystal rod SR fixed to the second support drive shaft 18 are butted against each other in the heated section 15 located at the focal point F0 of the spheroidal mirrors 11 and 12. The upper part of the first support drive shaft 17 and the lower part of the second support drive shaft 18 are each held by holding members (not shown), and these holding members keep the internal space m1 of the quartz tube 16 airtight. The first support drive shaft 17 and the second support drive shaft 18 are rotatable by a drive means (not shown) such as a servo motor, and are also movable up and down synchronously or at a relative speed.

[0035] Known infrared lamps such as halogen lamps and xenon lamps can be used as the infrared lamps 13 and 14. Examples of halogen lamps include halogen lamps with coiled filaments. Examples of xenon lamps include xenon short arc lamps, xenon long arc lamps, and xenon flash lamps.

[0036] The manufacturing apparatus 10 shown in FIG. 1 is a bi-elliptical type having two spheroidal mirrors 11 and 12, but depending on the type of infrared lamps 13 and 14, a single-elliptical type configuration having one spheroidal mirror can also be adopted.

[0037] Next, a method for producing yttrium tantalate crystal by the FZ method using the production apparatus 10 shown in FIG. 1 will be described.

[0038] <Step S110: Step of Producing Feedstock Rod> A method for producing an yttrium tantalate crystal according to one embodiment of the present invention includes the steps of producing a raw material rod using a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb).

[0039] In step S110, raw materials necessary to obtain a compound satisfying the composition of the target yttrium tantalate crystal are prepared. Specifically, at least a raw material containing Y and a raw material containing Ta are prepared, and one or more of these compounds are converted into oxides. In addition, a raw material containing Nb is prepared as needed. For convenience, in this specification, the raw materials containing each constituent element prepared in step S110 will also be referred to as the raw materials of each constituent element. For example, "a raw material containing Y" and "a raw material of Y" are synonymous.

[0040] As each raw material, a simple substance of a constituent element, an oxide, a hydroxide, a halide, an inorganic salt (sulfate, nitrate, carbonate, etc.), an organic salt (acetate, etc.), etc. can be used. Each compound other than a simple substance may be an anhydride or a hydrate. Non-limiting examples of raw materials that can be used in the manufacturing method of this embodiment are shown below.

[0041] As a raw material for Y, for example, Y2O3 can be used. As a raw material for Ta, for example, Ta2O5 can be used. As a raw material for Nb, for example, Nb2O5 can be used.

[0042] Each raw material is prepared so that the atomic ratio of Y, Ta, and Nb satisfies the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1). For example, if the composition of the target yttrium tantalate crystal is YTaO4, Y2O3 and Ta2O5 are used and weighed out so that the molar ratio satisfies Y2O3:Ta2O5=1:1. Also, if the composition of the target yttrium tantalate crystal is Y(Ta 0.8 Nb 0.2 )O4, Y2O3, Ta2O5, and Nb2O5 may be used and weighed out so that the molar ratio of Y2O3:Ta2O5:Nb2O5=1:0.8:0.2 is satisfied.

[0043] A feedstock rod is fabricated using the prepared raw materials. Specifically, the same procedures and conditions as those for fabricating a feedstock rod used in conventional crystal growth by the FZ method can be used. As an example, the raw materials are filled into a molding member such as a latex tube and isostatically pressed under a predetermined pressure (approximately several hundred MPa) to form a rod. It is preferable to fill the raw materials in the form of a premixed raw material mixture, as this prevents uneven distribution of the components in the resulting feedstock rod. The resulting rod-shaped product is solid-phase sintered at a predetermined temperature (e.g., approximately 1500°C) for a certain time (e.g., approximately 10 hours) to obtain a sintered body. The sintered body obtained in this manner is referred to as the feedstock rod FR.

[0044] <Step S120: Step of Obtaining Crystals> The manufacturing method of yttrium tantalate crystals according to this embodiment includes the steps of fixing the feed rod FR obtained in step S110 to the lower end of the first support drive shaft 17, fixing a seed crystal rod SR to the upper end of the second support drive shaft 18, heating a portion of the feed rod FR and the seed crystal rod SR to form a molten zone between the feed rod FR and the seed crystal rod SR, and continuously moving this molten zone to obtain a crystal.

[0045] In step S120, a crystal is grown by the FZ method using the feed rod FR obtained in step S110 and a seed crystal rod SR. The seed crystal rod SR can be a rod-shaped crystal that has been grown in advance and meets the desired composition of the yttrium tantalate crystal. Alternatively, a sintered body produced using the same procedures and conditions as those for producing the feed rod FR can be used as the seed crystal rod SR.

[0046] In the manufacturing apparatus 10, the raw material rod FR is fixed to the lower end of a first support drive shaft 17, and the seed crystal rod SR is fixed to the upper end of a second support drive shaft 18. As a result, the lower surface of the raw material rod FR and the upper surface of the seed crystal rod SR are butted against each other in the heated section 15 located at the coincident focal point F0 of the spheroidal mirrors 11 and 12.

[0047] Next, after the inside of the quartz tube 16 is replaced with an appropriate atmosphere such as an inert gas (e.g., argon gas), power is applied to the infrared lamps 13 and 14, and the infrared light emitted from the infrared lamps 13 and 14 is reflected by the spheroidal mirrors 11 and 12 and focused on the heated portion 15 located at the focal point F0, thereby heating with the infrared light. As a result, the lower end (the end opposite the end fixed to the first support drive shaft 17) of the feed rod FR and the upper end (the end opposite the end fixed to the second support drive shaft 18) of the seed crystal rod SR located in the heated portion 15 are heated and melted, and the smooth contact between these melted portions forms a molten zone (floating zone) in the heated portion 15 between the feed rod FR and the seed crystal rod SR.

[0048] The first support drive shaft 17, to which the feed rod FR is fixed at its lower end, and the second support drive shaft 18, to which the seed crystal rod SR is fixed at its upper end, are rotated together (e.g., at about 10 to 30 rpm) and moved slowly downward in sync. This causes the molten zone formed in the heated section 15 between the feed rod FR and the seed crystal rod SR to gradually move toward the feed rod FR, cooling the molten zone above the seed crystal rod SR and growing a crystal. The rotational speed of the first support drive shaft 17 (i.e., the rotational speed of the feed rod FR) and the rotational speed of the second support drive shaft 18 (i.e., the rotational speed of the seed crystal rod SR) may be the same or different. The rotational direction of the first support drive shaft 17 (i.e., the rotational direction of the feed rod FR) and the rotational direction of the second support drive shaft 18 (i.e., the rotational direction of the seed crystal rod SR) are typically opposite directions. The speed at which the first support drive shaft 17 and the second support drive shaft 18 are continuously moved in synchronization determines the range over which the molten zone is cooled, and can therefore be referred to as the crystal growth speed. The speed (crystal growth speed) is preferably in the range of 0.5 mm / h to 10 mm / h.

[0049] <Step S130: Heat-treating the crystal> The method for producing an yttrium tantalate crystal according to this embodiment may further include, as necessary, a step of heat-treating (annealing) the crystal obtained in step S120. This makes it possible to control defects (crystal defects (oxygen defects) caused by oxygen deficiency, etc.) that may inevitably occur during the crystal growth process, thereby improving the scintillation properties of the yttrium tantalate crystal.

[0050] In other words, when yttrium tantalate crystals are used as scintillator materials, it is preferable to perform heat treatment, but heat treatment is not an essential step in the manufacturing method of the crystal itself.

[0051] The atmosphere during the heat treatment is an inert gas atmosphere or an air atmosphere. In the latter case, the presence of oxygen in the atmosphere may result in improved scintillation characteristics compared to crystals with a similar composition. Therefore, instead of an air atmosphere, an oxygen-containing atmosphere containing a gas adjusted to a certain oxygen content (oxygen partial pressure) may be used.

[0052] The heat treatment temperature is set to a range of 100°C or higher and lower than the melting point of the target yttrium tantalate crystal. If the heat treatment temperature is lower than 100°C, the desired effect of improving the scintillation characteristics may not be sufficiently obtained. Furthermore, if the heat treatment temperature exceeds the melting point of the crystal, unintended defects may occur in the crystal structure, which may impair the scintillation characteristics. From the viewpoint of more reliably obtaining the effect of improving the scintillation characteristics, the heat treatment temperature is preferably 500°C or higher. Furthermore, the lower limit of the heat treatment temperature may be set to a temperature that is about 50°C±10°C lower than the melting point of the target crystal. In addition, if the upper limit of the heat treatment temperature is set to a temperature that is about 10°C to about 20°C lower than the melting point of the target crystal, the unintended effects on the crystal structure described above can be more reliably avoided.

[0053] The heat treatment time is preferably 0.5 hours or more. If the above-mentioned atmosphere and temperature conditions are met, a heat treatment time of 0.5 hours or more can achieve the desired effect of improving the scintillation characteristics. In particular, from the viewpoint of increasing the amount of light emitted, a heat treatment time of 0.5 hours or more is preferable. Note that, from the viewpoint of more reliably obtaining the effect of improving the scintillation characteristics, the heat treatment time may be 1 hour or more, 5 hours or more, 10 hours or more, or 30 hours or more. On the other hand, even if heat treatment is performed for a longer period than necessary, it is difficult to obtain the effect of improving the scintillation characteristics that is correlated only with the time, so the upper limit of the heat treatment time is set to 150 hours or less as a rough guideline.

[0054] In the yttrium tantalate crystal of the present invention, when comparing crystals having similar compositions obtained in step S120 with crystals subjected to the heat treatment in step S130, the latter crystals tend to have larger channel numbers indicating peak values ​​in the pulse height spectrum than the former crystals.

[0055] (Variation) It will be understood by those skilled in the art that the method for producing yttrium tantalate crystals by the FZ method described above with reference to FIG. 1 is an example, and that the method for producing crystals by the FZ method is not limited to this.

[0056] For example, a method for producing yttrium tantalate crystals according to another embodiment of the present invention includes the steps of melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb), and the steps of contacting a seed crystal with the surface of the melt obtained in the melting step and continuously lowering the melt to crystallize the contact area.

[0057] Here, as the above raw materials, the same ones as those in the above step S110 can be used, and each raw material is prepared such that Y, Ta, and Nb satisfy the relationship Y:Ta:Nb = 1:1−x:x (where 0≦x≦1) in terms of atomic ratio. The melting of the raw materials is performed, for example, by heating the raw materials filled in a predetermined container (such as a member having a desired shape) in a heating furnace. As the seed crystal to be brought into contact with the liquid surface of the obtained melt, a crystal body that has been grown in advance and satisfies the composition of the target yttrium tantalate crystal may be used in the same manner as in the above step S120, or a sintered body produced under the same procedures and conditions as in the production of the raw material rod FR in the above step S110 may be used as the seed crystal. The step of continuously lowering the melt in contact with the seed crystal to crystallize the contact portion is performed under an inert gas in the same manner as in the above step S120, and the crystal is grown by cooling the melt by the lowering. Note that the speed at which the melt is continuously lowered can be rephrased as the crystal growth rate in order to define the range in which the melt is cooled. The speed (crystal growth rate) is preferably in the range of 0.5 mm / h to 10 mm / h.

[0058] In addition, in the above-described modification, a step of heat-treating (annealing) the obtained crystal may be further included as necessary. Specifically, it is the same as that described for step S130 of the above embodiment.

[0059] <Manufacture by CZ method> FIG. 2 is a schematic cross-sectional view of another aspect of a manufacturing apparatus used for manufacturing a yttrium tantalate crystal according to an embodiment of the present invention.

[0060] The manufacturing apparatus 20 according to this embodiment is a crystal growth apparatus also referred to as a pulling apparatus or a pulling furnace. The manufacturing apparatus 20 mainly comprises a crucible 21, a ceramic cylindrical container 22 that houses the crucible 21, and a heating coil 23 wound around the cylindrical container 22. The heating coil 23 is configured to generate an induced current in the crucible 21 housed in the cylindrical container 22, thereby heating the crucible 21. The material of the crucible 21 is selected taking into consideration the melting point of the target yttrium tantalate crystal. However, since the melting point of the yttrium tantalate crystal having the composition represented by the above general formula exceeds 2000°C, it is preferable to use a crucible made of iridium (Ir).

[0061] Next, a method for producing yttrium tantalate crystal by the CZ method using the production apparatus 20 shown in FIG. 2 will be described.

[0062] <Step S210: Melting the Raw Material> A method for producing an yttrium tantalate crystal according to another embodiment of the present invention includes the step of melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and, if necessary, a raw material containing niobium (Nb).

[0063] In step S210, raw materials necessary to obtain a compound satisfying the composition of the target yttrium tantalate crystal are prepared. Specifically, this is the same as that described in step S110 of the above embodiment, and therefore a detailed description thereof will be omitted here.

[0064] The procedures and conditions for melting the prepared raw materials can be the same as those for crystal growth using the conventional CZ method. As an example, the raw materials are loaded into a crucible 21, and induction heating is performed by applying a current at a predetermined frequency to a heating coil 23 wound around a cylindrical container 22, thereby heating the crucible 21 to a temperature at which the raw materials in the crucible 21 can be melted. This melts the raw materials, resulting in a melt 24. It is preferable to load the raw materials in the form of a premixed raw material mixture, since this prevents uneven distribution of the components in the resulting melt 24.

[0065] <Step S220: Step of Melting Raw Materials> The method for producing an yttrium tantalate crystal according to this embodiment includes the steps of bringing a seed crystal into contact with the surface of the melt obtained in step S210 above, solidifying the contact area, and then pulling up the seed crystal to crystallize it.

[0066] In step S220, a rod-shaped crystal having the composition of the target yttrium tantalate crystal, which has been grown and prepared in advance, can be used as the seed crystal 25 to be brought into contact with the liquid surface of the melt 24. Alternatively, a sintered body produced by the same procedure and conditions as those for producing the feed rod FR described in step S110 of the above embodiment may be used as the seed crystal.

[0067] The tip of a seed crystal 25 is brought into contact with the liquid surface of the melt 24, and the contact portion is solidified. Then, the seed crystal 25 is used as a pulling axis, and while rotating at a predetermined rotation speed, the seed crystal 25 is pulled up at a predetermined pulling speed to crystallize. As a result, a bulk grown crystal 26 is obtained at the tip of the seed crystal 25.

[0068] Steps S210 and S220 are performed under an inert gas atmosphere. Examples of inert gases that can be used include nitrogen gas, helium gas, neon gas, and argon gas. The crucible 21 and other components of the manufacturing apparatus 200 may be placed in a sealed housing (not shown) to control the atmosphere. Alternatively, a quartz tube (not shown) may be placed between the cylindrical container 22 and the heating coil 23 to isolate the crystal growth area, and an inert gas may be introduced into the quartz tube to control the atmosphere.

[0069] <Step S230: Heat-treating the crystal> The method for producing an yttrium tantalate crystal according to this embodiment may further include a step of heat-treating (annealing) the crystal obtained in step S220, as necessary. This makes it possible to control defects (crystal defects (oxygen defects) caused by oxygen deficiency, etc.) that may inevitably occur during the crystal growth process, thereby improving the scintillation properties of the yttrium tantalate crystal.

[0070] The specific conditions (atmosphere, temperature, and time) of the heat treatment in step S230, as well as the effects thereof, are the same as those explained for step S130 in the above embodiment, and therefore detailed explanations will be omitted here.

[0071] [Uses of yttrium tantalate crystals] Next, applications of the above-mentioned yttrium tantalate crystal will be described.

[0072] The use of the yttrium tantalate crystal is not particularly limited, but it is suitable for use as a scintillator material.

[0073] The use of the crystalline scintillator material is not particularly limited, but it can preferably be used in a radiation detector. The configuration of the radiation detector is not particularly limited, and a configuration similar to that of a conventional radiation detector can be adopted. Specifically, as schematically shown in FIG. 3 , a radiation detector 30 according to one embodiment of the present invention includes a scintillator material (scintillator) 31 made of the above-mentioned yttrium tantalate crystal, and a photoelectric converter 32 (e.g., a photomultiplier tube (PMT), a silicon photomultiplier (SiPM), a solid-state image sensor (CCD), an avalanche photodiode (APD), a multi-pixel photon counter (MPPC), etc.) that detects light L emitted from the scintillator material 31 and converts it into an electrical signal. By using a scintillator material made of the above-mentioned yttrium tantalate crystal instead of the scintillator used in conventional radiation detectors, improved radiation detection accuracy is expected.

[0074] Furthermore, a radiological inspection apparatus can be configured by combining the above-described radiation detector with a radiation source that irradiates a subject with radiation. For example, as schematically shown in FIG. 4, a radiological inspection apparatus 40 according to one embodiment of the present invention includes a radiation detector 30 (see FIG. 3) and a radiation source 43. The radiation source 43 irradiates a subject Sbj with radiation R, and the radiation detector 30 is configured to detect radiation E that passes through the subject Sbj (i.e., is emitted from the subject Sbj). In the radiation detector 30, a scintillator material (scintillator) 31 is excited by the radiation E to emit (light-emit) light L, and a photoelectric converter 32 detects the light L emitted from the scintillator material 31 and converts it into an electrical signal. Such a radiological inspection apparatus 40 can be used, for example, as an inspection apparatus for nondestructive inspection, such as a detector for nondestructive inspection, a detector for resource exploration, or a detector for high-energy physics, or as a medical diagnostic apparatus for medical image processing (such as an X-ray CT).

[0075] Alternatively, the radiation detector may be applicable to applications for detecting radiation (such as gamma rays) emitted from a subject, such as medical image processing devices such as SPECT (Single Photon Emission Computed Tomography) devices, exploration devices for underground resource exploration, and devices used in the field of space-related research and development, such as cosmic rays and astrophysics.

[0076] The present invention will be explained in more detail below based on examples, but the present invention is not limited to these examples. [Example]

[0077] [Crystal production] <Example 1: YTaO4> YTaO4 was prepared by the FZ method according to the following procedure. Y2O3 and Ta2O5 (both 99.99% pure, manufactured by Furuuchi Chemical Co., Ltd.) were prepared as raw materials for Y and Ta. Each raw material was weighed to satisfy the YTaO4 ratio and mixed to obtain a raw material mixture. The obtained raw material mixture was filled into a latex tube and isostatically pressed under a pressure of 300 MPa to form it into a rod-like shape. The obtained rod-like molded product was subjected to solid-phase sintering at a temperature of 1500°C for 10 hours to obtain a sintered body. The obtained sintered body was used as a raw material rod and fixed to the lower end of the first support drive shaft of the manufacturing apparatus having the configuration shown in Figure 1. A seed crystal rod was fixed to the upper end of the second support drive shaft. Here, a sintered body produced by the same procedure and conditions as described above was used as the seed crystal rod. Next, the inside of the quartz tube was replaced with argon gas, and then an infrared lamp was energized to heat and melt the lower end of the feed rod and the upper end of the seed crystal rod, forming a molten zone between the feed rod and the seed crystal rod. The first support drive shaft (and the feed rod fixed thereto) and the second support drive shaft (and the seed crystal rod fixed thereto) were rotated at speeds of 15 rpm and 10 rpm, respectively, and were continuously moved downward in synchronization at a speed of 5 mm / h, thereby growing a crystal. This resulted in the crystal of Example 1.

[0078] Example 2: Y(Ta 0.995 Nb 0.005 )O4> As raw materials for Y, Ta, and Nb, Y2O3, Ta2O5, and Nb2O5 (all 99.99% purity, manufactured by Furuuchi Chemical Co., Ltd.) were prepared. 0.995 Nb 0.005 The raw material mixture was obtained by weighing and mixing the ingredients so as to satisfy the ratio of 0 to 1. The subsequent procedures and conditions for crystallization were the same as in Example 1, and the crystals of Example 2 were obtained.

[0079] Example 3: Y(Ta 0.99 Nb 0.01 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.99 Nb 0.01The raw material mixture was obtained by weighing and mixing the ingredients so as to satisfy the ratio of 0 to 1. The subsequent procedures and conditions for crystallization were the same as in Example 1, and the crystals of Example 3 were obtained.

[0080] Example 4: Y(Ta 0.98 Nb 0.02 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.98 Nb 0.02 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystals of Example 4 were obtained.

[0081] Example 5: Y(Ta 0.95 Nb 0.05 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.95 Nb 0.05 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystals of Example 5 were obtained.

[0082] Example 6: Y(Ta 0.9 Nb 0.1 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.9 Nb 0.1 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystals of Example 6 were obtained.

[0083] Example 7: Y(Ta 0.8 Nb 0.2 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.8 Nb 0.2 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystals of Example 7 were obtained.

[0084] Example 8: Y(Ta 0.8 Nb 0.2 )O4> The following procedure is used to obtain Y(Ta) by the CZ method. 0.8 Nb 0.2 )O4 was produced. As raw materials for Y, Ta, and Nb, Y2O3, Ta2O5, and Nb2O5 (all 99.99% purity, manufactured by Furuuchi Chemical Co., Ltd.) were prepared. 0.8 Nb 0.2 The raw material mixture was weighed and mixed to satisfy the ratio of 04 to 104, obtaining a raw material mixture. The obtained raw material mixture was filled into an iridium crucible having the configuration shown in Figure 2, and argon gas was introduced as atmospheric gas at a flow rate of 1 L / min into the furnace (more specifically, the interior of the quartz tube located between the cylindrical container 22 and the heating coil 23). Next, the crucible was heated by high-frequency induction heating to a temperature at which the raw material mixture melted. A seed crystal was brought into contact with the surface of the melt, and this contact area was solidified. The seed crystal was then rotated at 10 rpm and pulled upward at a speed of 1 mm / h to crystallize it. This yielded the crystal of Example 8. Here, a sintered body prepared using the same procedures and conditions as those described above for Example 1 was used as the seed crystal.

[0085] Example 9: Y(Ta 0.6 Nb 0.4 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.6 Nb 0.4 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, to obtain the crystals of Example 9.

[0086] Example 10: Y(Ta 0.4 Nb 0.6 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.4 Nb 0.6 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, and a raw material mixture was obtained. The subsequent procedures and conditions for crystallization were the same as in Example 1, and the crystals of Example 10 were obtained.

[0087] Example 11: Y(Ta 0.2 Nb 0.8 )O4> Y2O3, Ta2O5 and Nb2O5 are 0.2 Nb 0.8 The ingredients were weighed and mixed to satisfy the ratio of 0 to 4, to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystals of Example 11 were obtained.

[0088] <Example 12: YNbO4> Y2O3 and Nb2O5 (both 99.99% pure, manufactured by Furuuchi Chemical Co., Ltd.) were prepared as raw materials for Y and Nb. Each raw material was weighed to satisfy the ratio of YNbO4 and mixed to obtain a raw material mixture. The subsequent procedures and conditions for crystal preparation were the same as in Example 1, and the crystal of Example 12 was obtained.

[0089] [Ceramics manufacturing] <Example 13: YTaO4> YTaO4 was fabricated by solid-state sintering according to the following procedure. Y2O3 and Ta2O5 (both 99.99% pure, manufactured by Furuuchi Chemical Co., Ltd.) were prepared as raw materials for Y and Ta. Each raw material was weighed to satisfy the YTaO4 ratio and mixed to obtain a raw material mixture. The obtained raw material mixture was filled into a latex tube and isostatically pressed under a pressure of 300 MPa to form it into a rod-like shape. The obtained rod-like molded product was subjected to solid-phase sintering at a temperature of 1300°C for 10 hours to obtain a sintered body. The obtained sintered body was pulverized, and the pulverized material was packed into a latex tube and isostatically pressed at a pressure of 300 MPa to form a rod. The obtained rod-shaped molded body was subjected to solid-phase sintering at a temperature of 1500°C for 10 hours to obtain a sintered body. This resulted in the ceramic of Example 13.

[0090] Example 14: Y(Ta 0.8 Nb 0.2 )O4> As raw materials for Y, Ta, and Nb, Y2O3, Ta2O5, and Nb2O5 (all 99.99% purity, manufactured by Furuuchi Chemical Co., Ltd.) were prepared. 0.8 Nb 0.2The ingredients were weighed and mixed so as to satisfy the ratio of 0.1 to 1.0, and a raw material mixture was obtained. The subsequent procedures were the same as in Example 13, and the ceramic of Example 14 was obtained.

[0091] [Sample preparation and analysis] From the crystals and ceramics obtained in each of the above examples, thin plate-shaped (approximately disk-shaped or approximately oval-shaped) specimens with a thickness of approximately 1 mm were cut out to prepare samples. Hereinafter, the specimen prepared from the crystal of Example 1 will be referred to as Sample 1, and the same will be applied to Examples 2 to 14. Two types of commercially available CdWO4 crystals were also prepared as comparative samples. Comparative sample Z1 was made in Japan (Nippon Crystal Optics Co., Ltd.), and comparative sample Z2 was made in China (Epic Crystal Co., Ltd.). Both comparative samples were thin (approximately rectangular) specimens with a thickness of approximately 1 mm.

[0092] <Appearance observation> The appearance of each sample was visually observed. Samples 1 to 12 had excellent transparency. Comparative samples Z1 and Z2 also had similar transparency. On the other hand, samples 13 and 14 were non-transparent.

[0093] Figures 5(a), 5(b), 5(c), and 5(d) show photographs of representative samples: Sample 1, Sample 13, Sample 8, and Sample 14, respectively. The photographs in Figures 5(a) and 5(b) were taken with each sample placed on a white sheet of paper. Similar photographs were obtained for Samples 2 to 12, as shown in Figure 5(a). For clarity, the photographs in Figures 5(c) and 5(d) were taken with the four letters "NIMS" printed in black on a white sheet of paper, and a rectangular piece of sample large enough to cover the letter "N" was placed on top. For Sample 8 shown in Figure 5(c), the letter "N" beneath the sample is clearly visible, making the four letters "NIMS" legible. However, for Sample 14 shown in Figure 5(d), the letter "N" beneath the sample is not visible, and only the three letters "IMS" are legible.

[0094] <UV-visible spectroscopy> The transmittance of light for each sample was analyzed by ultraviolet-visible spectroscopy. As the analyzer, an ultraviolet-visible near-infrared spectrophotometer (V-570, manufactured by JASCO Corporation) was used. The irradiation direction of the light was parallel to the thickness direction of the sample, and the optical path length t was set to approximately 1 mm (i.e., the same as the thickness of the sample).

[0095] In FIG. 6, as a representative example, the transmittance spectra of Sample 1, Sample 7, Sample 13, and Sample 14 are shown. Also, in Table 2, the transmittance values at a wavelength of 410 nm are shown.

[0096] According to FIG. 6, it was found that for Sample 1 and Sample 7, the transmittance in the range of wavelengths from 380 nm to 780 nm was 70% or more. Also, although not shown in FIG. 6, the transmittance spectra of Samples 2 to 6 and Samples 8 to 12 were the same as those of Sample 1 and Sample 7, and it was found that the transmittance in the range of wavelengths from 380 nm to 780 nm was 70% or more. In contrast, for Sample 13 and Sample 14, it was found that the transmittance in the range of wavelengths from 200 nm to 800 nm shown in FIG. 6 was zero.

[0097] <X-ray diffraction analysis> Laue photographs of each sample were taken by the Laue method. In FIG. 7A, as a representative example, the Laue photograph images of Sample 1 and Sample 8 are shown. In FIG. 7B, the Laue photograph images of Sample 13 and Sample 14 are shown.

[0098] [[ID=二十二]]According to FIG. 7A, for Sample 1 and Sample 8, X-ray diffraction photographs consisting of many spots were obtained, and it was confirmed that these samples were single crystals. Also, as a result of analysis based on the symmetry of the Laue spots centered on the X-ray incident point (the white point visible in the approximate center of each image), it was found that Sample 1 and Sample 8 had a crystal structure belonging to the I12 / a1 space group. Although not shown in FIG. 7A, the Laue photograph images of Samples 2 to 7 and Samples 9 to 12 were the same as those of Sample 1 and Sample 8, and it was confirmed that these samples were single crystals and had a crystal structure belonging to the I12 / a1 space group.

[0099] On the other hand, as shown in Figure 7B, no spots were observed around the X-ray incidence point in Samples 13 and 14. These results are in good agreement with the fact that Samples 13 and 14 are ceramics.

[0100] In addition, powder samples were prepared from the crystals of Examples 1 to 12 and subjected to powder X-ray diffraction measurement. As a result, it was confirmed that these crystals were single-phase and monoclinic.

[0101] The densities of Samples 1 to 12 shown in Table 2 tend to be slightly smaller than that of the comparative sample CdWO4. However, the lower the Nb content in the crystal composition, the higher the density, and it was confirmed that there is no particular problem when using them as scintillator materials or the like, and that these values ​​are practically acceptable.

[0102] <Light-emitting characteristics> The luminescence characteristics of each sample were analyzed. A fiber multi-channel compact spectrometer (Blue Wave UVN-50, manufactured by Stellarnet Inc.) was used as the analytical device. The analytical conditions were as follows: the emission wavelength (λ ) when excited with X-rays (18kW X-ray generator RA-HF 18RB, manufactured by Rigaku Corporation). em ) and luminescence intensity were measured.

[0103] As representative examples, Figure 8 shows the X-ray excited emission spectra of Samples 1 to 7 and Samples 9 to 12. Note that in Figure 8, the emission intensity on the vertical axis is a normalized value, with the peak value of emission intensity in the emission wavelength range of 300 nm to 400 nm for Sample 1 and the peak value of emission intensity in the emission wavelength range of 350 nm to 500 nm for Samples 2 to 7 and Samples 9 to 12 set to 1.0.

[0104] According to FIG. 8, in Sample 1, luminescence derived from Ta—O bonds was confirmed in the range of 300 nm to 400 nm. In Sample 2, in addition to the luminescence derived from Ta—O bonds, luminescence derived from Nb—O bonds was confirmed in the range of 350 nm to 500 nm, and the luminescence spectrum became bimodal. In Samples 3 and 4, the luminescence derived from Nb—O bonds was dominant, but the luminescence derived from Ta—O bonds also occurred at a certain intensity, so the spectral shape in the range of 300 nm to 400 nm had a slightly gentle waveform gradient. In Samples 5 to 7 and Samples 9 to 12, the spectral shapes were almost the same, and only the luminescence derived from Nb—O bonds was confirmed.

[0105] Also, Table 2 shows the emission wavelength (λ max ) when the emission intensity showed the maximum value. According to Table 2, the emission wavelength of the crystal of YTaO4 (Sample 1) is in the ultraviolet region, whereas the emission wavelengths of the crystals (Samples 2 to 12) having a composition represented by the general formula Y(Ta 1-x Nb x )O4 (where 0 < x ≦ 1) contain Nb and are in the visible light region (more specifically, the blue light region). From this, for example, it can be said that the latter crystals are useful as a scintillator material for a radiation inspection device having a configuration as shown in FIG. 4. In other words, in the aspect of using the yttrium tantalate crystal of the present invention as a scintillator material, according to the configuration of the target radiation inspection device, the composition of the crystal can be variously designed within the range satisfying the general formula Y(Ta 1-x Nb x )O 4-y (where x and y each independently satisfy 0 ≦ x ≦ 1 and 0 ≦ y ≦ 0.5).

[0106] <Durability analysis> An X-ray resistance test was conducted using an X-ray source device (Rigaku Corporation, 18kW X-ray generator RA-HF 18RB) to analyze the X-ray durability of each sample. Specifically, each sample was exposed to X-rays generated at a voltage of 45 kV and a current of 300 mA for a certain period of time, and the emission intensity was measured under the same conditions as above and compared with that of a sample not exposed to X-rays. Seven X-ray irradiation times were set: 0.3 minutes, 3 minutes, 15 minutes, 45 minutes, 75 minutes, 135 minutes, and 255 minutes.

[0107] Figure 9 shows the results obtained for sample 1, sample 8, sample 13, and sample 14 as representative examples. In the graph of Figure 9, the light gray area indicates the range of measurement error. The horizontal axis of the graph of Figure 9 represents the irradiation time, and the above seven conditions are converted into time (h) and plotted.

[0108] According to Figure 9, Sample 1 (plotted with squares) and Sample 8 (plotted with circles) exhibited stable luminescence intensities under all seven of the above conditions, and even after 255 minutes (i.e., more than four hours) of X-ray exposure, they exhibited luminescence intensities equivalent to those observed after 0.3 minutes of exposure. Furthermore, visual inspection of the appearance of each sample after the test revealed that all samples were in the same condition as before exposure, and no damage due to X-ray irradiation was observed. Although not shown in Figure 9, Samples 2 to 7 and Samples 9 to 12 also exhibited results similar to those of Sample 1 and Sample 8.

[0109] On the other hand, sample 13 showed a significant decrease in luminescence intensity after 3 minutes of X-ray irradiation, and after less than 60 minutes of irradiation, the luminescence intensity fell to about 90% compared to 0.3 minutes of irradiation, indicating a significant degradation of performance. Sample 14 showed a similar trend, but the degree of performance degradation was greater than that of sample 13, with the luminescence intensity falling to less than 90% compared to 0.3 minutes of irradiation. Furthermore, visual inspection of the appearance of each sample after the test revealed clear traces of X-ray irradiation in all cases, confirming damage caused by X-ray irradiation.

[0110] From these results, it was confirmed that the crystals of Examples 1 to 12, which are in the form of single crystals, are excellently durable materials suitable for use as scintillator materials in X-ray radiation detectors.

[0111] The results of the above analysis are summarized in Table 2 below.

[0112] [Table 2]

[0113] [Analysis of scintillation characteristics] <Pulse height analysis> Pulse height analysis was performed on the crystals of Examples 1 to 12 and comparative samples Z1 and Z2. 137 Using Cs (662 keV) as the excitation source, the light emitted from each sample upon irradiation with radiation (γ-rays) was received by a photomultiplier tube (PMT), and the signal from the PMT was processed by an analog module to obtain a pulse height spectrum. Here, the PMT used was a Hamamatsu Photonics R7600U-200, and the measurement conditions were an operating voltage of 600 V, a gain value of 20, and a shaping time of 10 μs.

[0114] Here, the samples to be analyzed for the crystals of Examples 1 to 12 were prepared as described above, Samples 1 to 12, and sample pieces cut out in the same manner as above that were subjected to heat treatment (air atmosphere, 1300°C, 6 hours). For convenience, the latter (heat-treated) samples will be referred to below as "Sample 1A," etc., and will be identified by adding the letter A after the number.

[0115] FIG. 10(a) shows the pulse height spectra of Sample 1, Sample 1A, Sample 7, and Sample 7A as representative examples.

[0116] According to Figure 10(a), it was found that the channel number of the peak value in the spectrum of sample 1A was larger than the channel number of the peak value in the spectrum of sample 1, and the channel number of the peak value in the spectrum of sample 7A was larger than the channel number of the peak value in the spectrum of sample 7.

[0117] FIG. 10(b) shows a graph in which the channel numbers of the peak values ​​read from the pulse height spectra of Samples 1 to 7, 9 to 12, 1A to 7A, and 9A to 12A are plotted against the Nb content (Nb concentration) in the crystal composition. Here, the Nb content is expressed by the general formula Y(Ta 1-x Nb x The value of x in )O4 is expressed as a percentage (%). The upper line graph in the figure shows the results for the heat-treated samples, with the leftmost plot being Sample 1A, followed by Sample 2A, Sample 3A, ..., and the rightmost plot being Sample 12A. The lower line graph in the figure shows the results for the non-heat-treated samples, with the leftmost plot being Sample 1, followed by Sample 2, Sample 3, ..., and the rightmost plot being Sample 12.

[0118] 10(b), for crystals with similar compositions (e.g., Sample 1 and Sample 1A), the heat-treated crystals tended to have higher channel numbers for peak values ​​in the pulse height spectrum than the unheat-treated crystals. The degree of increase in channel number varied depending on the crystal composition, but was in the range of approximately 40% to 70%.

[0119] In the following description of the scintillation properties of the crystals, reference will be made mainly to heat-treated samples (e.g., Sample 1A), but since there is no difference in the crystal composition between Sample 1A and Sample 1, the scintillation properties confirmed using Sample 1A should be understood as one of the properties of the crystal of Example 1. In addition, the above properties confirmed using Sample 1 (see Table 2) also apply to Sample 1A.

[0120] <Analysis of luminescence intensity> The light emission from each crystal was analyzed using the pulse height spectrum described above, and the analysis was calibrated by weighting the scintillation spectrum of each sample with the quantum efficiency of the corresponding PMT detector using the comparative sample Z1 as a standard.

[0121] Figure 11 shows a graph in which the quantum efficiency values ​​of crystals corresponding to Samples 1A to 7A and Samples 9A to 12A are plotted against the Nb content in the crystal composition. The "Undoped" mark in the figure indicates that the Nb content is 0%, i.e., the plot shows the quantum efficiency of a YTaO4 crystal. The quantum efficiency value of a standard sample (CdWO4) is also shown in the figure as a reference sample.

[0122] Specifically, the luminescence intensity (LY value) of each sample was calculated using the following formula:

number

[0123] FIG. 12 shows the pulse height spectra of sample 1A, sample 2A, comparative sample Z1 (standard sample), and comparative sample Z2 as representative examples.

[0124] From the pulse height spectra shown in Fig. 12, the light emission amounts of Sample 1A, Sample 2A, Comparative Sample Z1, and Comparative Sample Z2 were calculated to be 15900, 13900, 13900, and 6500, respectively. Here, taking the light emission amount of Comparative Sample Z1, which is a standard sample, as 1 and showing the light emission amounts of the above samples as relative values, for Sample 1A, Sample 2A, and Comparative Sample Z2, they are 1.13, 1, and 0.16, respectively. That is, it can be said that Sample 1A and Sample 2A obtained a high light emission amount comparable to or exceeding that of CdWO4, which is a conventional scintillator material.

[0125] Table 3 shows the values of the light emission amounts of each sample calculated in the same manner as above (see the column of "pulse height value"). In addition, for Samples 1A to 12A and Comparative Sample Z1, the light emission amount was estimated by integrating the scintillation intensity obtained from the above X-ray excitation emission spectrum, and taking the value in Comparative Sample Z1 as 1, the values obtained for each sample were shown as relative values (see the column of "integrated value"). Here, focusing on the latter integrated value (relative value), compared with the crystal of YTaO4 (Sample 1A), the crystals having a composition represented by the general formula Y(Ta 1-x Nb x )O4 (where 0 < x ≦ 1) (Samples 2A to 12A) were found to have a significantly larger light emission amount. From this, it can be said that in the aspect of using the yttrium tantalate crystal of the present invention as a scintillator material, in addition to the configuration of the target radiation inspection device, considering also the magnitude of the light emission amount required by the configuration, within the range satisfying the general formula Y(Ta 1-x Nb x )O 4-y (where x and y each independently satisfy 0 ≦ x ≦ 1 and 0 ≦ y ≦ 0.5), it is possible to variously design the composition of the crystal.

[0126] Figure 13 shows a graph in which the relative light yield and the calculated value of X-ray stopping power for Samples 1A to 7A and Samples 9A to 12A are plotted against the Nb content in the crystal composition. Here, the meaning of "Undoped" in the figure is the same as in Figure 11 described above. The relative light yield is shown as a relative value to the light yield (pulse peak value) of the other samples, with the light yield (15900 ph / MeV) of Sample 1A set to 1. The value of stopping power was calculated based on the above references, using ρZ 4 was calculated using the values ​​of

[0127] Looking at the relative values ​​of the luminescence intensity shown by the pentagonal plots in Figure 13, the YTaO4 crystal (sample 1A) had the largest luminescence intensity, and as the Nb content increased, the luminescence intensity slightly decreased. However, when the Nb content exceeded 60% (when the Nb content became significantly higher than the Ta content in the crystal composition), the luminescence intensity tended to increase. Furthermore, looking at the X-ray stopping power shown by the hexagonal plots, when the Nb content was in the range of more than 0% and less than 10%, values ​​almost the same as those of the YTaO4 crystal (sample 1A) were obtained, and when the Nb content exceeded approximately 10%, a tendency was observed in which the stopping power decreased as the Nb content increased.

[0128] <Evaluation of scintillation decay time and afterglow> The scintillation decay time and afterglow were evaluated from the scintillation spectrum of each sample obtained using pulsed X-rays as an excitation source. The results are shown in Table 3.

[0129] Here, for the scintillation decay times shown in Table 3, the values ​​of τ1 and τ2 are the respective values ​​obtained from the two components, and the figures in parentheses are the percentages (%) of both components. Note that for comparative sample Z2, the values ​​of τ1 and τ2 are unknown, so the published values ​​(primary decay time) from the source of the sample are shown.

[0130] The afterglow was determined by evaluating the emission intensity after 40 ms, based on the emission intensity at the time when the pulsed X-ray irradiation was stopped.

[0131] According to Table 3, it was found that all of Samples 1A to 12A had shorter scintillation decay times than the comparative samples Z1 and Z2. 1-x Nb x )O4 (where 0≦x≦1) showed a significantly shorter scintillation decay time than the existing scintillator material CdWO4.

[0132] Furthermore, Table 3 shows that the YTaO4 crystal (Sample 1A) has afterglow characteristics comparable to those of CdWO4. Looking at the relationship with the Nb content, when the Nb content was 20% to 100% (Samples 7A to 12A), the afterglow value was less than 0.05%, and when the Nb content was 5% or less (Samples 2A to 5A), the afterglow value tended to exceed 0.2%.

[0133] [Table 3]

[0134] Table 4 below shows the analysis results of the scintillation characteristics obtained using Samples 1 to 12.

[0135] [Table 4]

[0136] Comparing Tables 3 and 4, for crystals having similar compositions (for example, Sample 1 and Sample 1A), it was found that crystals that were heat-treated tended to have improved scintillation properties compared to crystals that were not heat-treated. In particular, when focusing on the amount of light emitted, it can be seen that Samples 1A to 12A all achieved higher amounts of light emission than Samples 1 to 12. These results also confirmed that, in relation to the method for producing yttrium tantalate crystals of the present invention, the scintillation properties of the yttrium tantalate crystals can be improved by heat-treating the crystals obtained. [Industrial Applicability]

[0137] As described above, the yttrium tantalate crystal of the present invention does not contain harmful substances such as cadmium, and has scintillation properties comparable to or superior to those of CdWO4. Furthermore, the manufacturing method of the yttrium tantalate crystal of the present invention allows for the growth of chemically stable bulk single crystals, making it suitable for mass production and reducing manufacturing costs. Additionally, since it does not use harmful substances such as cadmium, it is an environmentally friendly material, and the manufacturing process is also environmentally friendly. The yttrium tantalate crystal of the present invention, which has these characteristics, and its manufacturing method are expected to be applied to various applications as a material and manufacturing method to replace conventional CdWO4. [Explanation of symbols]

[0138] 10. Yttrium tantalate crystal manufacturing equipment 11,12 Spheroidal mirror 13,14 Infrared lamp 15 Heated part 16 Quartz tube 17 First support drive shaft 18 Second support drive shaft FR raw material rod SR seed crystal rod F0,F1,F2 focus m1,m2 internal space 20. Yttrium tantalate crystal manufacturing equipment 21 Crucible 22 Cylindrical container 23 Heating coil 24 Melt 25 Seed Crystal 26 Growing crystals 30 Radiation detector 31 Scintillator materials (scintillators) 32 Photoelectric converter 40 Radiation inspection equipment 43 Radiation source L light R Radiation E Radiation emitted from the subject Sbj subject

Claims

1. General formula Y(Ta １－ｘ Nb ｘ ) O ４－ｙ (wherein x and y each independently satisfy 0≦x≦1, 0≦y≦0.5), and in its transmission spectrum, the transmittance in the wavelength range of 380 nm to 780 nm is 70% or more.

2. The crystal according to claim 1, which has a crystal structure belonging to the I12 / a1 space group and is a single monoclinic crystal phase.

3. 3. The crystal according to claim 1 or 2, wherein in an X-ray resistance test using an X-ray source device, a sample exposed to X-rays generated under conditions of a voltage of 45 kV and a current of 300 mA for at least 60 minutes or more exhibits luminescence substantially equivalent to that before the X-ray irradiation and has no X-ray irradiation marks.

4. A step of producing a raw material rod using a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and a raw material containing niobium (Nb) as needed; a step of fixing the raw material rod obtained in the producing step to the lower end of a first support drive shaft extending vertically from above, fixing a seed crystal rod to the upper end of a second support drive shaft extending vertically from below opposite the first support drive shaft, heating a part of the raw material rod and the seed crystal rod to form a molten zone between the raw material rod and the seed crystal rod, and continuously moving the molten zone to obtain a crystal; It encompasses The step of obtaining the crystals is carried out under an inert gas, In the step of producing the raw material rod, Y, Ta, and Nb satisfy the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1) in atomic ratio. A method for producing the yttrium tantalate crystal according to any one of claims 1 to 3.

5. Melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and optionally a raw material containing niobium (Nb); a step of bringing a seed crystal into contact with the surface of the melt obtained in the melting step, and continuously pulling down the melt to crystallize the contact portion; It encompasses The crystallizing step is carried out under an inert gas; In the melting step, the atomic ratio of Y, Ta, and Nb satisfies the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1). A method for producing the yttrium tantalate crystal according to any one of claims 1 to 3.

6. Melting a raw material containing yttrium (Y), a raw material containing tantalum (Ta), and optionally a raw material containing niobium (Nb); a step of bringing a seed crystal into contact with the surface of the melt obtained in the melting step, solidifying the contact portion, and pulling up the seed crystal to crystallize it; It encompasses the melting and crystallizing steps are carried out under an inert gas; In the melting step, the atomic ratio of Y, Ta, and Nb satisfies the relationship Y:Ta:Nb=1:1-x:x (where 0≦x≦1). A method for producing the yttrium tantalate crystal according to any one of claims 1 to 3.

7. 7. The method for producing an yttrium tantalate crystal according to claim 4, further comprising the step of heat treating the obtained crystal.

8. 8. The method for producing yttrium tantalate crystals according to claim 7, wherein the heat-treating step heat-treats the obtained crystals in an inert gas atmosphere or an air atmosphere at a temperature range of 100°C or higher and lower than the melting point of the target yttrium tantalate crystals for 0.5 hours to 150 hours.

9. A scintillator material comprising the yttrium tantalate crystal according to any one of claims 1 to 3.

10. The scintillator material of claim 9; a photoelectric converter that detects light from the scintillator material and converts it into an electrical signal; A radiation detector comprising:

11. a radiation source for irradiating a subject with radiation; The radiation detector of claim 10 , which detects radiation passing through the subject; A radiological inspection device comprising:

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