Ceramic scintillator, photon counting x-ray detector, and radiation imaging device

Abstract

A ceramic scintillator according to an embodiment is a garnet compound which contains Pr, the ceramic scintillator containing at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu as a composition element in the garnet compound, while additionally containing at least one of Al, Ga, Sc, Si, Ge, and Sn as a composition element in the garnet compound. The ceramic scintillator has no peak in the range of not less than 200°C but less than 400°C in a glow curve that is obtained after irradiation with X-rays or ultraviolet rays at room temperature.

Description

Ceramic scintillator, photon counting type X-ray detector, and radiation imaging device

[0001] Embodiments of the present invention relate to a ceramic scintillator, a photon-counting type X-ray detector, and a radiation imaging apparatus.

[0002] Radiation imaging devices, such as X-ray imaging systems, are widely used in industrial applications like baggage inspection and non-destructive testing, as well as in medical applications such as X-ray diagnostic equipment and X-ray CT (Computed Tomography) devices. The current mainstream detection method in X-ray imaging systems is the energy integration type, which typically combines a light-emitting material with a photodetector.

[0003] However, energy-integrating X-ray detectors have drawbacks, such as the inability to obtain energy information from X-rays and high radiation exposure. In recent years, development of X-ray detectors employing photon counting methods has progressed to address these issues. Photon-counting X-ray detectors are also called photon detectors or photon counting detectors.

[0004] The photon counting method is characterized by processing each incident X-ray photon as a pulse signal. Photon counting methods are further classified into direct types, which use semiconductors such as CdTe to directly convert X-rays into electrical signals, and indirect types, which convert X-rays into light using light-emitting materials and then convert that into an electrical signal using a photodetector.

[0005] In photon counting methods, direct methods can measure the energy of X-ray photons from the number of carriers, while indirect methods can measure it from the number of emitted photons. Furthermore, because noise components can be easily removed, data with a high signal-to-noise ratio can be obtained. Due to these advantages, in medical applications, it is expected to enable new functional diagnostics and the reduction of radiation exposure through low-dose measurement. The mainstream of research and development in photon counting methods is the direct method, which can obtain high energy resolution. However, the high cost of semiconductor materials such as CdTe, the difficulty in obtaining uniform characteristics and manufacturing large-area detectors, and the signal pile-up phenomenon caused by the low response speed of the materials are challenges for the direct method.

[0006] On the other hand, in indirect photon counting methods, photomultiplier tubes with high multipliers are commonly used as photodetectors. However, the large size of photomultiplier tubes and the difficulty in constructing pixels in a narrow gap have been challenges for indirect methods. Recently developed silicon photomultipliers (Si-PMs), which are Si-based photodetectors operating in Geiger mode, have solved the aforementioned problems of photomultiplier tubes, have a multiplier close to that of photomultiplier tubes, and are low-cost. Therefore, it is expected that the use of silicon photomultipliers in indirect methods will expand in the future.

[0007] International Publication No. 2011 / 158580, Japanese Patent Publication No. 2012-1637

[0008] The technical challenge of indirect X-ray CT systems is their inability to handle high counting rates. The counting rate refers to the number of photons of incident X-rays per unit area and unit time, and corresponds to the intensity of the X-rays. The counting rate required for X-ray CT systems is 10 ８ [cps / mm ２ This is considered to be the case.

[0009] Counting rate 10 ８ [cps / mm ２As a ceramic scintillator that satisfies the above requirements, one example is one made of a garnet compound containing Pr (a polycrystalline garnet structure oxide). However, ceramic scintillators made of garnet compounds containing Pr have radiation degradation characteristics, such as poor resistance to X-rays and a tendency to discolor.

[0010] The problem that this invention aims to solve is to provide a ceramic scintillator, a photon-counting type X-ray detector, and a radiation imaging device that are applicable to indirect photon-counting X-ray detectors and have suppressed radiation degradation characteristics.

[0011] The ceramic scintillator according to this embodiment is a garnet compound containing Pr, and the garnet compound contains at least one element from among Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu as a constituent element, and further contains at least one element from among Al, Ga, Sc, Si, Ge, and Sn as a constituent element. Furthermore, the ceramic scintillator does not have a peak in the range of 200°C to less than 400°C in the glow curve obtained after irradiation with X-rays or ultraviolet light at room temperature.

[0012] A schematic diagram showing the configuration of a photon-counting type X-ray detector according to the embodiment. An enlarged cross-sectional view of a partial region shown in Figure 1(B) in the photon-counting type X-ray detector according to the embodiment. A diagram showing an example of an absorption spectrum to explain the coloration of a ceramic scintillator. A diagram showing the principle of coloration of a fluorescent material to explain the ceramic scintillator according to the embodiment. A diagram showing the energy state when the fluorescent material is colored to explain the ceramic scintillator according to the embodiment. A diagram showing the energy state when the coloration of the fluorescent material is reduced to explain the ceramic scintillator according to the embodiment. A diagram showing the glow curve obtained by thermoluminescence measurement to explain the ceramic scintillator according to the embodiment. A diagram showing the glow curve obtained by thermoluminescence measurement to explain the ceramic scintillator according to the embodiment. A diagram showing an example of an absorption spectrum to explain the ceramic scintillator according to the embodiment. A diagram showing the manufacturing method of the ceramic scintillator according to the embodiment as a flowchart. A diagram showing Examples 1 to 21 as a table. A diagram showing Comparative Examples 1 to 4 as a table. Embodiment

[0013] The following describes in detail embodiments of the ceramic scintillator, photon-counting type X-ray detector, and radiation imaging device with reference to the drawings.

[0014] (Photon Counting X-ray Detector) Figures 1(A) to 1(C) are schematic diagrams showing the configuration of a photon counting X-ray detector according to the embodiment. Figure 1(A) is a top view of the photon counting X-ray detector according to the embodiment. Figure 1(B) is a side view showing the channel direction CH of the photon counting X-ray detector according to the embodiment. Figure 1(C) is a side view showing the slice direction SL of the photon counting X-ray detector according to the embodiment.

[0015] Figures 1(A) to 1(C) show a photon-counting type X-ray detector (hereinafter simply referred to as "X-ray detector") 1 according to an embodiment. Figures 1(B) and 1(C) also show a collimator device 3 in addition to the X-ray detector 1. Figure 1(C) shows an X-ray tube 2 in addition to the X-ray detector 1.

[0016] The X-ray detector 1 is provided on the rotating frame of the gantry device. The X-ray detector 1 is provided with n (n: plural) X-ray detection elements 1n. The X-ray detection elements 1n are two-dimensionally arranged in a matrix in the channel direction and the slice direction SL. Here, the channel direction means the spreading direction of the fan beam X-ray irradiated from the X-ray tube 2, and the slice direction is the thickness direction of the fan beam X-ray.

[0017] The X-ray incident surface of the X-ray detector 1 is formed by the X-ray incident surfaces of the X-ray detection elements 1n. For example, about 1000 X-ray detection elements 1n are arranged in the channel direction CH and 64 in the slice direction SL.

[0018] The X-ray tube 2 is provided on the rotating frame of the gantry device so as to face the X-ray detector 1. The X-ray tube 2 is a vacuum tube that generates X-rays by irradiating thermoelectrons from the cathode (filament) toward the anode (target) by applying a high voltage. For example, there is a rotating anode type X-ray tube in the X-ray tube 2 that generates X-rays by irradiating thermoelectrons to the rotating anode.

[0019] The collimator device 3 has a plurality of collimator plates having a function of absorbing scattered X-rays. The plurality of collimator plates are plates extending in the slice direction SL and are composed of plates erected so as to divide the X-ray detection elements 1n in the channel direction CH (one-dimensional collimator). Alternatively, the plurality of collimator plates are composed of a plate extending in the slice direction SL and erected so as to divide the X-ray detection elements 1n in the channel direction CH, and a plate extending in the channel direction CH and erected so as to divide the X-ray detection elements 1n in the slice direction SL (two-dimensional collimator). The plate surface of the collimator plate is tilt-adjusted to be parallel to the X-ray irradiation direction E, which is the direction in which the X-rays from the X-ray focal point F of the X-ray tube 2 are irradiated. FIG. 1(C) shows the case where the collimator device 3 is a one-dimensional collimator. In some cases, the combination of the X-ray detector 1 and the collimator device 3 is referred to as a "photon counting type X-ray detector".

[0020] Incidentally, the X-ray detector 1 may be configured by modularizing a predetermined number of X-ray detection elements among the X-ray detection elements 1n and arranging a plurality of detector modules. Similarly, the collimator device 3 may be configured by modularizing a predetermined number of collimator plates and arranging a plurality of collimator modules.

[0021] FIG. 2 is an enlarged cross-sectional view of a partial region R shown in FIG. 1(B) in the X-ray detector 1.

[0022] The X-ray detection element 1n is provided on the ceramic substrate 4. Each of the X-ray detection elements 1n includes a ceramic scintillator 11 and a photoelectric conversion element 12.

[0023] The ceramic scintillator 11 is an element that converts incident X-rays into photons and emits them. Here, X-rays usually have a predetermined X-ray energy distribution. X-rays having a specific X-ray energy can be considered as a lump of X-ray particles corresponding to the magnitude of the X-ray energy. The ceramic scintillator 11 converts the X-ray particles into photons with a predetermined probability while maintaining the lump of the X-ray particles. That is, when X-rays are incident on the ceramic scintillator 11, photon groups corresponding to the X-ray energy are emitted substantially simultaneously for each X-ray energy.

[0024] The ceramic scintillator 11 is a light conversion element having a substantially rectangular or cubic shape. The X-ray incident surface of the ceramic scintillator 11 is substantially perpendicular to the X-ray irradiation direction, that is, the X-ray incident direction E, and the side surface parallel to the X-ray incident direction E is arranged to be substantially parallel to the channel direction CH and the slice direction SL.

[0025] The photoelectric conversion element 12 has a substantially plate-like rectangular shape, converts incident photons into electrical signals, and outputs an electrical signal. The electrical signal is an electrical pulse signal corresponding to each individual incident photon. When a group of photons is incident on the photoelectric conversion element 12, it outputs a pulse signal with a pulse height corresponding to the number of photons constituting the group. The photoelectric conversion element 12 is a semiconductor device suitable for so-called photon counting, such as a silicon photomultiplier (Si-PM). A silicon photomultiplier is a high-performance semiconductor photodetector that is capable of photon counting measurements and is also applicable to analog measurements such as scintillation detection. A silicon photomultiplier is an element in which a large number of avalanche photodiode (APD) pixels operating in Geiger mode are connected in parallel.

[0026] The photoelectric conversion element 12 receives photons emitted from the ceramic scintillator 11 and outputs an electrical signal in pulse form. When the intensity of the transmitted X-rays from the subject is sufficiently low, groups of photons of different X-ray energies are emitted from the ceramic scintillator 11 in a scattered state along the time axis. At this time, the photoelectric conversion element 12 outputs pulse signals of pulse height corresponding to the magnitude of the X-ray energy, in a scattered state along the time axis, in a number of pulses corresponding to the dose of X-rays with that X-ray energy. Therefore, by counting the pulse signals output within a certain period of time, the dose of transmitted X-rays from the subject can be determined for each X-ray energy. Furthermore, by counting all pulse signals output within a certain period of time, regardless of pulse height, the total dose of transmitted X-rays from the subject can be determined.

[0027] Each photoelectric conversion element 12 is connected to a conductive wire pattern (not shown) formed on the ceramic substrate 4. Electrical signals from the photoelectric conversion elements 12 are output to an external processing device (not shown) through these conductive wire patterns. The electrical signals output from the photoelectric conversion elements 12 are used to collect projection data using a photon counting method.

[0028] As a promising fluorescent material for the ceramic scintillator 11, there is a garnet compound containing Pr (hereinafter simply referred to as "garnet compound"). Condition 0: The garnet compound contains, as composition elements, at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and at least one of Al, Ga, Sc, Si, Ge, and Sn.

[0029] As an example of the case where the above Condition 0 is satisfied, the garnet compound is represented by the following compositional formula (A): (R １－ｘ Pr ｘ ) ａ M ｂ O １２ When it is in the compositional formula (A), 2.8 ≤ a ≤ 3.4, preferably 2.9 ≤ a ≤ 3.3, 4.6 ≤ b ≤ 5.4, preferably 4.7 ≤ b ≤ 5.1, 0.0 < x ≤ 0.06, preferably 0.01 < x ≤ 0.05 are satisfied, and it is preferable that the compositional formula (A) satisfies the following Conditions 1 and 2. Condition 1: The element R in the compositional formula (A) contains at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Condition 2: The element M in the compositional formula (A) contains at least one of Al, Ga, Sc, Si, Ge, and Sn.

[0030] To analyze the content of the compositional elements of the garnet compound, for example, the following methods can be mentioned. Metal elements such as Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, Sc, Si, Ge, and Sn can be analyzed by ICP emission spectrometry using an ICP emission spectrometer such as PS3520VDDII manufactured by Hitachi High-Technologies Corporation, after melting the ceramic scintillator with a mixed flux of sodium carbonate and boric acid and dissolving it with dilute nitric acid. For the non-metal element O, the composition may deviate slightly depending on the synthesis method, analysis method, etc. Even in such cases, as long as the luminescence characteristics are not impaired, the effects of the present invention are sufficiently exhibited.

[0031] Furthermore, garnet compounds may contain small amounts of metallic elements such as Mg, Ca, and Ba, as well as elements such as F and Cl. These elements are used as auxiliary agents during phosphor synthesis or scintillator synthesis, and may inevitably be present in small amounts. Even with small amounts of these elements present, the scintillator will exhibit similar luminescence properties and achieve the desired effect. However, from the viewpoint of the luminescence properties and stability of the ceramics, it is preferable to have a low content of these elements.

[0032] Furthermore, in composition formula (A), it is preferable that element M contains at least Ga. In that case, composition formula (A) is (R １－ｘ Pr ｘ ) ａ (M' １－ｙ Ga ｙ ) ｂ O １２ It is represented by the following compositional formula (B). When it is compositional formula (B), the following conditions are satisfied: 2.8 ≤ a ≤ 3.4 4.6 ≤ b ≤ 5.4 0 < x ≤ 0.06 0 < y ≤ 0.65 and compositional formula (B) preferably satisfies the following conditions 1' and 2'. Condition 1': The element R in compositional formula (B) contains at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Condition 2': The element M' in compositional formula (B) contains at least one of Al, Sc, Si, Ge, and Sn.

[0033] On the other hand, for ceramic scintillators 11 made of garnet compounds containing Pr, satisfying only the above conditions 1 and 2 or conditions 1' and 2' presents a problem with radiation degradation characteristics, such as weak resistance to X-rays and a tendency to discolor.

[0034] Figure 3 shows an example of the absorption spectrum of a fluorescent material that satisfies the above conditions 1 and 2. As shown in Figure 3, when a fluorescent material is exposed to 1 kGy of X-rays, an increase in the absorption spectral intensity occurs. In particular, the increase in absorption spectral intensity in the short wavelength range of 300 nm to 400 nm indicates that the fluorescent material is colored. In other words, an increase in absorption spectral intensity causes the scintillator material to become colored, and therefore the emission output decreases.

[0035] Figure 4 shows the coloring mechanism of a fluorescent material. As shown in Figure 4, when anionic defects such as oxygen vacancies exist in the crystal of a fluorescent material, the vacant space left by the oxygen ions becomes positively charged due to the influence of surrounding cations. When X-rays are shone on this space, the oxygen in the crystal becomes positively charged. ２－ Electrons are released from the material and taken up by oxygen vacancies, causing the fluorescent material to become colored.

[0036] Figure 5 schematically shows the electron energy states when the fluorescent material is not colored (Figure 5(A)) and when it is colored (Figure 5(B)). In Figure 5(A), when exposed to X-rays, electrons in the valence band are excited to the conduction band. The excited electrons move through the crystal and return to the ground state via several Pr levels created by Pr contained in the crystal. In this process, the electrons release some of the excess energy as light during the transition to a lower energy level. On the other hand, in Figure 5(B), when exposed to X-rays, some of the excited electrons in the conduction band are trapped in defect levels created by lattice defects, etc. When electrons in the conduction band are trapped in defect levels, the fluorescent material becomes colored.

[0037] Therefore, if the number of electrons trapped in defect levels can be reduced, the coloration of the fluorescent material can be suppressed. In other words, it was found that the radiation degradation characteristics of the ceramic scintillator 11, which is a fluorescent material, can be improved by satisfying at least one of the following conditions 3 and 4. Specifically, as shown in Figure 6, by providing an energy level shallower than the defect level (referred to simply as a "shallow level" in this specification) on the higher energy side of the defect level, and trapping electrons in the conduction band at the shallow level, the number of electrons trapped in the defect level can be reduced. The term "shallow" in "shallow level" qualitatively describes that it is located closer to the conduction band than the defect level. Figure 6 shows the energy states of electrons when the coloration of the fluorescent material is reduced. Along the thick arrow in Figure 6, electrons excited in the conduction band are trapped at the shallow level, re-excited by thermal energy at room temperature, and return to the ground state via multiple Pr levels created by Pr contained in the crystal.

[0038] Condition 3: When thermoluminescence (TL) intensity is measured after irradiation with 10 mGy of X-rays at room temperature at a heating rate of 1.0 ± 0.5 °C / min, the glow curve obtained does not contain a peak (also referred to as the "TL peak") in the range of 200 °C to less than 400 °C.

[0039] Furthermore, if condition 3 above is met, the glow curve obtained when the thermoluminescence intensity is measured after irradiating with 10 mGy of X-rays at room temperature and then heating at a rate of 1.0 ± 0.5 °C / min should have at least one peak in the range of 50 °C to less than 200 °C. Here, the glow curve is a waveform in which multiple signals overlap. Therefore, in order to determine the presence or absence of a peak in the glow curve, the peak intensity, and to measure the peak, the glow curve is decomposed into a group of independent signals by a glow curve fitting process. A Gaussian function is used for peak identification in the fitting, and the fitting process is performed using the least squares method. Furthermore, since accurate spectral measurement becomes difficult above 400 °C due to the effect of background increase due to blackbody radiation, it is preferable to measure at temperatures below 400 °C. Also, the term "peak" here refers to the local maximum value, not the maximum value of the spectrum. This local maximum value does not include the local maximum value due to noise components. Furthermore, thermoluminescence can be measured using a thermoluminescence measuring device such as the TL-2000 manufactured by NanoGray Co., Ltd.

[0040] Condition 4: When thermoluminescence intensity is measured after irradiation with 10 mGy of X-rays at room temperature, and the resulting glow curve is heated at a rate of 1.0 ± 0.5 °C / min, there must be two or more peaks in the range of 50 °C to less than 200 °C.

[0041] The number of electrons trapped in shallow energy levels and the energy depth of these shallow levels from the conduction band can be estimated using glow curves obtained from thermoluminescence measurements. When the temperature of a ceramic scintillator that has been irradiated with X-rays or ultraviolet light for a certain period of time is gradually increased, electrons trapped in shallow energy levels are re-excited to the conduction band by thermal energy and return to the ground state via multiple Pr energy levels. At this time, by measuring the amount of light emitted when transitioning between Pr energy levels, the energy depth of the shallow energy levels from the conduction band and their density can be estimated. The distribution of the amount of light emitted with respect to temperature when the temperature is raised at a constant rate is called a glow curve. A larger amount of light emitted means that there are more trapped electrons, i.e., a higher density of shallow energy levels. Also, a higher temperature during emission means that electrons are trapped at a deeper position from the conduction band.

[0042] Figures 7 and 8 show glow curves obtained from thermoluminescence measurements that satisfy conditions 1 and 2 above. In Figure 7, peaks exist around 100°C, 280°C, and 330°C, respectively. Figure 7 shows the case where condition 3 is not satisfied. When peaks exist above 200°C, electrons trapped at this energy depth are not re-excited to the conduction band at room temperature, and as a result, the number of electrons trapped in defect levels cannot be reduced. On the other hand, as shown in Figure 8, when there are no peaks in the range of 200°C to less than 400°C in the glow curve obtained after irradiation with X-rays at room temperature, the trapped electrons are re-excited to the conduction band at room temperature, and as a result, the number of electrons trapped in defect levels can be reduced. Furthermore, to obtain such characteristics as those described in condition 3 above, ceramic scintillators can be manufactured by the manufacturing method described later.

[0043] Figure 9 shows the absorption spectrum of a ceramic scintillator 11, manufactured using the method described later, superimposed on the absorption spectrum shown in Figure 3, after exposure to 1 kGy of X-rays. As shown in Figure 9, by creating shallow energy levels and reducing the number of electrons trapped in defect levels, the absorption spectrum intensity, particularly in the 300 nm to 400 nm range, is reduced (indicated by the double-dotted arrow), thereby suppressing radiation degradation characteristics. This means that the coloration when irradiated with the same dose of X-rays is reduced, i.e., radiation degradation characteristics are suppressed.

[0044] The ceramic scintillator of the embodiment of the present invention is manufactured through steps ST1 to ST7 in the flowchart shown in Figure 10.

[0045] First, in the first step, sieving step ST1, the reaction accelerator (flux) powder is sieved using a sieve with a maximum particle size of 30 μm to obtain only the reaction accelerator with a maximum particle size of 30 μm. The reaction accelerator contains at least BaF ２ (Contains barium fluoride), AlF ３ Aluminum fluoride may be added.

[0046] In the second step, mixing step ST2, a powder mixture of an oxide of element R and an oxide of element M (oxide powder), a reaction accelerator powder with a maximum particle size of 30 μm, and a sintering aid (for example, SiO ２ A slurry is prepared by filling an alumina container with at least one of MgO and CaO and mixing it in a pot mill or the like. BaF ２ The addition of garnet phase (the basic structure is R ３ M ５ O １２ It has the effect of stabilizing (element R: rare earth element, element M: Al, Ga). Also, BaF ２ The more Ba is added, the more the garnet phase can be stabilized, but if there is a lot of residual Ba, the scintillator performance (luminescence and decay time constant) may deteriorate. Ba and F react readily with Al, especially Ba(Al,Ga) ０．２～２．５ O ｘ Ya Ba (Al, Ga) ７．５～１０ O ｘThis is because it is easy to generate. Therefore, the residual Ba is reduced by the sieving process ST1 and the firing process ST3 described later. Note that in the firing process ST3, Ba(Al,Ga) ０．２～２．５ O ｘ Ba(Al,Ga) ７．５～１０ O ｘ Control to generate Ba(Al,Ga) in the washing step ST4 ７．５～１０ O ｘ By removing it, it is possible to prevent Ba and F from being incorporated into the scintillator material to a certain extent.

[0047] In mixing step ST2, the slurry is stirred on a pot mill turntable for 30 minutes to over ten hours until the desired viscosity is reached. There are no particular restrictions on the order of filling, however, when slurring a large amount of oxide powder, aggregates such as lumps are likely to form. Therefore, an aqueous solution may be prepared by adding a surfactant and binder to water, and then a predetermined amount of oxide powder may be added in small amounts before being dispersed and slurried on a pot mill turntable. In addition to a pot mill, dispersion equipment such as a bead mill or planetary mixer may also be used for dispersion.

[0048] In the third step, the firing step ST3, after mixing the reaction accelerator in the mixing step ST2, the firing is performed at a temperature of 1200 to 1800°C. Below 1200°C, the reaction may be insufficient, and above 1800°C, the temperature is too high and BaF ２ It vaporizes easily and separate phases are easily formed. Also, the firing atmosphere is N ２ An inert atmosphere such as gas or Ar gas is preferred. In some cases, a vacuum (10 －２ The firing may be carried out in a low-pressure atmosphere (Pa or less) or a reducing atmosphere. The firing time is preferably 1 to 8 hours. For example, firing step ST3 may include only one firing step, but may include a first firing step in which firing is performed in air at a temperature of 1300°C or higher for about 5 hours, and N ２ It is preferable to include a second firing step in which the product is fired at a temperature of 1200°C or higher in a gas atmosphere for about 5 hours.

[0049] After calcination in step ST3, the mixture becomes a garnet compound powder. However, the garnet compound powder after calcination contains BaF2 as a reaction accelerator. ２Ba and F compounds remain due to the process. Therefore, it is important to wash the garnet compound powder after calcination to remove any remaining Ba and F compounds. To efficiently perform the washing, it is also effective to crush and sieve the garnet compound powder after calcination to an average particle size of 0.5 to 20 μm, if necessary.

[0050] In the fourth step, the washing step ST4, the garnet compound powder obtained in the calcination step ST3 is washed. Washing step ST4 involves a combination of washing with pure water (water from which impurities have been removed using an ion exchange resin) and acid washing. By repeating this process several times, any remaining reaction accelerator can be removed.

[0051] Since fluorine (F) is an element that reacts readily with water, it is relatively easy to remove. Furthermore, by repeatedly performing the cleaning process ST4, the amount of F can be reduced to zero (below the detection limit). On the other hand, while a lower amount of residual barium (Ba) is desirable for better ceramic properties, Ba is less reactive with water than fluorine and is easily incorporated into Al, so a considerable number of cleaning cycles are required to reduce its amount to zero (below the detection limit).

[0052] The washed garnet compound powder is dried to become a scintillator material (phosphor powder).

[0053] In the fifth step, the molding step ST5, the product obtained in the washing step ST4 is molded. In the molding step ST5, a molded body may be produced using a mold having the desired shape, or a cylindrical or rectangular prism-shaped molded body may be produced. Furthermore, the molding step ST5 may use mold molding or CIP (cold isostatic pressing), and may include a degreasing step to degrease the molded body.

[0054] In the sixth step, the sintering step ST6, a ceramic scintillator as a sintered body is manufactured by sintering the product obtained in the molding step ST5. Sintering methods include hot pressing, HIP (hot isostatic pressing), vacuum sintering, SPS (discharge plasma sintering), and millimeter-wave heating. The sintering temperature is preferably 1400 to 1700°C, and the sintering time is preferably 1 to 10 hours. The applied pressure is preferably 20 MPa or higher. The sintering atmosphere is an inert atmosphere such as Ar or a vacuum (10 －２ It is preferable that the pressure is Pa or less.

[0055] The fluorescent material shall consist of the elements included in the above composition, and shall not contain any other elements except for unavoidable impurities. This is because the decay time constant of luminescence changes depending on the elements included, and if a large amount of impurities are present, the luminescence characteristics may deteriorate. However, if the required amount of luminescence and / or the required decay time constant of luminescence is met, it may contain impurities of 100 ppm or more.

[0056] In the seventh step, the annealing process ST7, a ceramic scintillator is manufactured by performing an annealing treatment on the sintered body obtained in the sintering process ST6. Here, the purpose of the annealing treatment is to improve the crystallinity of the material by rearranging the crystal structure by applying heat to the sintered body.

[0057] Examples of annealing treatments include the following: Reduction annealing is a treatment in which a ceramic sintered body is subjected to a reducing atmosphere (for example, H ２ / N ２ Atmosphere, N ２ / H ２ This involves heat treatment (reducing heat treatment) in an oxidizing atmosphere (for example, air, O). Oxidizing annealing is performed on a ceramic sintered body in an oxidizing atmosphere (for example, air, O). ２ This involves heat treatment (oxidative heat treatment) in a controlled atmosphere.

[0058] In the annealing process ST7, for example, H ２ / N ２ , or N ２ / H ２In an O atmosphere, it is preferable to heat-treat the ceramic sintered body at a heat treatment temperature (maximum temperature) of 800°C to 1200°C (preferably 900°C to 1100°C), and to hold the heat treatment temperature (maximum temperature) for, for example, 1 hour to 20 hours (preferably 3 hours to 15 hours). If the annealing treatment is performed for more than 20 hours, the ceramic sintered body will be excessively sintered, and the luminescence properties will decrease. Also, H ２ / N ２ When performing annealing in an atmospheric environment, the hydrogen concentration should preferably be 5.0 vol% or less. It is more preferable to perform the annealing treatment at least twice. In this case, the annealing conditions may be changed.

[0059] As described above, in the fluorescent materials of Comparative Examples 1 to 4, a large number of electrons are trapped in the defect levels, resulting in discoloration of the ceramic scintillator, which is the fluorescent material. On the other hand, in the fluorescent materials of Examples 1 to 21, by providing shallow levels and reducing the number of electrons trapped in the defect levels, the discoloration of the ceramic scintillator 11, which is the fluorescent material, can be suppressed.

[0060] Furthermore, in the cases where conditions 1 to 3 above are met, the cases where conditions 1, 2, and 4 above are met, and the cases where conditions 1 to 4 above are met, it is preferable that the following condition 5 is also met. Here, a larger intensity ratio of the two peaks located in the range of 50°C to less than 200°C means an increase in the number of electrons trapped in shallow levels. This can further reduce the number of electrons trapped in defect levels.

[0061] Condition 5: In the glow curve obtained when thermoluminescence intensity is measured after irradiation with 10 mGy of X-rays at room temperature at a heating rate of 1.0 ± 0.5 °C / min, two peaks exist in the range of 50 °C to less than 200 °C, and the intensity ratio (Ph / Pl), which is the ratio of the intensity of the high-temperature peak (Ph) to the intensity of the low-temperature peak (Pl) of the two peaks, is 0.25 or more (preferably 0.25 or more and less than 3.20).

[0062] Although the exact reason why the radiation degradation characteristics improve when the intensity ratio (Ph / Pl), which is the ratio of the peak intensity on the high-temperature side (Ph) to the peak intensity on the low-temperature side (Pl) in the range of 50°C to less than 200°C, is 0.25 or higher, the inventors' studies suggest that the presence of a certain number of electrons trapped at the peaks of the glow curve in the above range, particularly the peaks on the high-temperature side, i.e., at the defect levels below 200°C, is effective in suppressing the coloration of the scintillator, i.e., the radiation degradation characteristics.

[0063] As explained in Figure 9, the ceramic scintillator 11 of the embodiment of the present invention can improve radiation degradation characteristics. A simple method for evaluating radiation degradation characteristics is to measure the absorption spectrum of the ceramic scintillator. Specifically, it is preferable that the ceramic scintillator 11 of the embodiment of the present invention has a ratio of the absorption spectrum intensity at 400 nm to the absorption spectrum intensity at 280 nm when exposed to 1 kGy of X-rays ((400 nm intensity) / (280 nm intensity)) of 0.450 or less.

[0064] As shown in Figure 3, the region in which the absorption spectral intensity increases due to X-ray exposure is on the wavelength side longer than 300 nm. Therefore, the degree of coloration due to X-rays can be evaluated by taking the ratio of the 400 nm intensity to the 280 nm intensity. The larger this value, the greater the coloration due to X-ray exposure. In particular, if (400 nm intensity) / (280 nm intensity) exceeds 0.450, the decrease in the amount of light emitted by the ceramic scintillator due to coloration becomes large, making practical application in radiation imaging devices difficult. For this reason, the upper limit of the above absorption spectral ratio is 0.45. Although the details have not been fully elucidated, the ceramic scintillator 11 of this embodiment can suppress coloration after X-ray exposure, and because the radiation degradation characteristics are suppressed by suppressing coloration, the above evaluation becomes possible.

[0065] Absorption spectra can be measured using a spectrophotometric instrument such as the PMA-12 manufactured by Hamamatsu Photonics K.K. The measurement conditions can be as follows: Wavelength resolution: 10 nm Measurement wavelength range: 250 nm to 750 nm It is preferable to measure the absorption spectrum using a multi-channel spectrometer.

[0066] Figure 11 will now be explained. Figure 11 shows fluorescent materials according to Examples 1 to 21 that satisfy conditions 3 to 5 among the compositions of ceramic scintillators that satisfy conditions 1 and 2 above. In particular, the fluorescent materials according to Examples 1 to 21 contain at least Ga as element M in composition formula (A), and also satisfy conditions 1' and 2' above. On the other hand, Figure 12 shows fluorescent materials according to Comparative Examples 1 and 4, which consist of garnet compounds that do not contain Pr. Furthermore, the fluorescent materials according to Comparative Examples 1 to 4 shown in Figure 12 do not satisfy conditions 3 to 5 above, based on the results of thermoluminescence measurements, among the compositions of ceramic scintillators that satisfy conditions 1 and 2 above.

[0067] Furthermore, as shown in Figure 11, the fluorescent materials according to Examples 1 to 21, when exposed to 1 kGy of X-rays, have a ratio of the absorption spectral intensity at 400 nm to the absorption spectral intensity at 280 nm of the scintillator material ((400 nm intensity) / (280 nm intensity)) of 0.450 or less. On the other hand, the fluorescent materials according to Comparative Examples 1 to 4 have a ratio of absorption spectral intensities exceeding 0.450.

[0068] According to at least one embodiment described above, it is possible to suppress radiation degradation characteristics while accommodating a high counting rate.

[0069] Furthermore, changing thermoluminescence measurement conditions such as the X-ray irradiation dose, the type of electron excitation source, and the heating rate will also change the peak temperature and peak intensity values ​​of the resulting glow curve. Therefore, even if the thermoluminescence measurement conditions are not those described above, ceramic scintillators, photon-counting type X-ray detectors, and radiation imaging devices that are characterized by an inherent increase in the density of shallow energy levels can be considered identical to those of the present invention.

[0070] Furthermore, the ceramic scintillator 11 is not limited to its application to a photon-counting type X-ray detector equipped with a silicon photomultiplier in an X-ray CT scanner. For example, the ceramic scintillator 11 may be applied to an X-ray detector equipped with a photodiode in an X-ray CT scanner. Also, the ceramic scintillator 11 may be applied to a flat panel detector (FPD) equipped with a CMOS (Complementary Metal Oxide Semiconductor) in an X-ray diagnostic device. Moreover, the ceramic scintillator 11 may be applied to a photon-counting type detector equipped with a silicon photomultiplier in a PET (Positron Emission Tomography) scanner. In addition, the ceramic scintillator 11 may be applied to imaging for industrial applications such as baggage inspection and non-destructive testing.

[0071] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

Claims

1. A ceramic scintillator comprising a garnet compound containing Pr, wherein the garnet compound comprises at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu as constituent elements, and further comprises at least one of Al, Ga, Sc, Si, Ge, and Sn as constituent elements, and the glow curve obtained after irradiation with X-rays or ultraviolet light at room temperature does not have a peak in the range of 200°C to less than 400°C.

2. The garnet compound is (R １－ｘ Pr ｘ ) ａ M ｂ O １２ The ceramic scintillator according to claim 1, which is represented by the following compositional formula, wherein in the compositional formula, 2.8 ≤ a ≤ 3.4, 4.6 ≤ b ≤ 5.4, and 0.0 < x ≤ 0.06, the element R in the compositional formula comprises at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the element M in the compositional formula comprises at least one of Al, Ga, Sc, Si, Ge, and Sn.

3. The garnet compound is (R １－ｘ Pr ｘ ) ａ (M′ １－ｙ Ga ｙ ) ｂ O １２ and is represented by the composition formula. In the composition formula, 2.8 ≦ a ≦ 3.4, 4.6 ≦ b ≦ 5.4, 0.0 < x ≦ 0.06, 0 < y ≦ 0.

65. The element R in the composition formula contains at least one of Y, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The element M′ in the composition formula contains at least one of Al, Sc, Si, Ge, and Sn. The ceramic scintillator according to claim 2 4. The ceramic scintillator according to claim 1, wherein the glow curve has two or more peaks in the range of 50°C or more and less than 200°C.

5. The ceramic scintillator according to claim 2, wherein the glow curve has two or more peaks in the range of 50°C or more and less than 200°C.

6. The ceramic scintillator according to any one of claims 1 to 5, wherein in the glow curve, there are two peaks in the range of 50°C or more and less than 200°C, and the ratio of the peak intensity of the high-temperature peak to the peak intensity of the low-temperature peak is 0.25 or more.

7. A ceramic scintillator according to any one of claims 1 to 5, wherein, when exposed to 1 kGy of X-rays, the ratio of the absorption spectral intensity at 400 nm to the absorption spectral intensity at 280 nm of the scintillator material ((400 nm intensity) / (280 nm intensity)) is 0.450 or less.

8. A photon counting type X-ray detector comprising the ceramic scintillator described in claim 1 and a photoelectric conversion element.

9. A radiation imaging apparatus comprising the photon counting type X-ray detector according to claim 8.

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