Ceramic scintillator, photon counting-type x-ray detector, radiation imaging apparatus, and method for producing ceramic scintillator

Abstract

A ceramic scintillator according to an embodiment comprises a garnet compound containing Pr. The ceramic scintillator contains 250 ppm by mass or less of Ba, and the compositional formula of the Ba-containing Ba compound is given by Bax(Al, Ga)yOx + 1.5y wherein the ratio y / x of y to x is 2.5 or less. The compositional formula of the garnet compound in the ceramic scintillator is given by R3M5O12 wherein the element R in the composition of the garnet compound includes Pr and at least one or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the element M in the composition preferably includes Al and Ga.

Description

Ceramic scintillator, photon-counting X-ray detector, radiation imaging device, and method for manufacturing ceramic scintillator

[0001] FIELD Embodiments of the present invention relate to a ceramic scintillator, a photon-counting X-ray detector, a radiation imaging device, and a method for manufacturing a ceramic scintillator.

[0002] Radiation imaging devices using radiation, for example, imaging systems using X-rays, are widely used in industrial applications such as baggage inspection and non-destructive testing, and in medical applications such as X-ray diagnostic devices and X-ray CT (Computed Tomography) devices. The current mainstream detection method in imaging systems using X-rays is the energy integration type, which generally uses a configuration combining a luminescent material and a photodetector.

[0003] However, energy-integrating detectors have the drawbacks of being unable to obtain X-ray energy information and of resulting in a large amount of radiation exposure. In recent years, development of X-ray detectors that employ the photon-counting method has been progressing to solve these problems. Photon-counting X-ray detectors are also called photon detectors or photon-counting detectors.

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

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

[0006] On the other hand, photomultiplier tubes with high multiplication factors are generally used as photodetectors in indirect photon counting systems. However, the large size of photomultiplier tubes and the difficulty of configuring pixels in narrow gaps have been issues with indirect systems. Silicon photomultipliers (Si-PMs), which are Si-based photodetectors that operate in Geiger mode and have been developed in recent years, have solved the issues mentioned above with photomultiplier tubes, have a multiplication factor close to that of photomultiplier tubes, and are low-cost. For this reason, it is expected that the use of silicon photomultipliers in indirect systems will expand in the future.

[0007] JP 2018-2974 A JP 2010-235388 A

[0008] A common technical issue for both direct and indirect detectors is their inability to handle high counting rates. The counting rate indicates the number of X-ray photons incident on a unit area and unit time, and corresponds to the intensity of the X-rays.

[0009] When a large amount of imaging is performed in a short time using photon counting X-ray measurement, a detector capable of high counting rates is required. This situation is necessary for X-ray CT scanners, and the required counting rate is 10 ８ [cps / mm ２ This means that the X-ray photon is 1 [mm ２] at an average interval of 10 [nsec]. If the counting rate exceeds the capacity of the detector, overlapping of pulse signals occurs, making it difficult to measure the correct X-ray photon energy. Note that an X-ray CT device is an example of a radiation imaging device.

[0010] On the other hand, long-term measurements at a reduced count rate result in longer imaging times, which can lead to problems such as reduced throughput and reduced time resolution.Since the count rate that a detector can handle is primarily determined by the response speed of the material, improvements in response speed are desired.

[0011] The problem to be solved by the present invention is to provide a ceramic scintillator, a photon-counting X-ray detector, and a method for manufacturing a ceramic scintillator that can stabilize a garnet phase (garnet-structure oxide) without degrading performance.

[0012] The ceramic scintillator according to the embodiment is made of a garnet compound (a polycrystalline substance of a garnet-structured oxide) containing Pr. The ceramic scintillator contains 250 mass ppm or less of Ba. The composition formula of the Ba-containing Ba compound in the ceramic scintillator is Ba ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ It is preferable that the ratio of y to x, y / x, is 2.5 or less.

[0013] 1A is a schematic diagram showing the configuration of a photon-counting X-ray detector according to an embodiment; FIG. 1B is an enlarged cross-sectional view of a partial region shown in FIG. 1A in a photon-counting X-ray detector according to an embodiment; FIG. 1C is a flowchart showing a method for manufacturing a ceramic scintillator according to an embodiment; FIG. 1D is a table showing the relationship between residual Ba and the relative light emission amount (relative light emission amount / relative decay time constant) with respect to the relative decay time constant in Examples 1 to 20; FIG. 1F is a table showing the relationship between residual Ba and the relative light emission amount (relative light emission amount / relative decay time constant) with respect to the relative decay time constant in Examples 21 to 29; FIG. 1G is a table showing the relationship between residual Ba and the relative light emission amount (relative light emission amount / relative decay time constant) with respect to the relative decay time constant in Comparative Examples 1 to 20; FIG. 1H is a table showing the relationship between residual Ba and the relative light emission amount (relative light emission amount / relative decay time constant) with respect to the relative decay time constant in Comparative Examples 21 to 33; and FIG. 1H is a table showing the relationship between Examples 1 to 12 and Comparative Examples 1 to 18. Embodiment

[0014] Hereinafter, embodiments of a ceramic scintillator, a photon-counting X-ray detector, a radiation imaging device, and a method for manufacturing a ceramic scintillator will be described in detail with reference to the drawings.

[0015] (Photon-counting X-ray detector) Figures 1A to 1C are schematic diagrams showing the configuration of a photon-counting X-ray detector according to an embodiment. Figure 1A is a top view of the photon-counting X-ray detector according to an embodiment. Figure 1B is a side view showing the channel direction CH of the photon-counting X-ray detector according to an embodiment. Figure 1C is a side view showing the slice direction SL of the photon-counting X-ray detector according to an embodiment.

[0016] 1A to 1C show a photon-counting X-ray detector (hereinafter simply referred to as an "X-ray detector") 1 according to an embodiment. Also, 1B and 1C show a collimator device 3 in addition to the X-ray detector 1. 1C shows an X-ray tube 2 in addition to the X-ray detector 1.

[0017] The X-ray detector 1 is provided on a rotating frame of a gantry. The X-ray detector 1 has n (n: plural) X-ray detection elements 1n. The X-ray detection elements 1n are arranged two-dimensionally in a matrix in a channel direction and a slice direction SL. The channel direction refers to the direction in which the fan beam X-rays emitted from the X-ray tube 2 spreads, and the slice direction refers to the thickness direction of the fan beam X-rays.

[0018] 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.

[0019] The X-ray tube 2 is provided on a 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 applying a high voltage and irradiating thermions from a cathode (filament) to an anode (target). For example, the X-ray tube 2 may be a rotating anode type X-ray tube that generates X-rays by irradiating a rotating anode with thermions.

[0020] The collimator device 3 has multiple collimator plates that absorb scattered X-rays. The multiple collimator plates are made up of plates extending in the slice direction SL and erected to divide the X-ray detection elements 1n in the channel direction CH (a one-dimensional collimator). Alternatively, the multiple collimator plates are made up of plates extending in the slice direction SL and erected to divide the X-ray detection elements 1n in the channel direction CH, and plates extending in the channel direction CH and erected to divide the X-ray detection elements 1n in the slice direction SL (a two-dimensional collimator). The inclination of the collimator plate surfaces is adjusted so that they are parallel to the X-ray irradiation direction E, which is the direction in which X-rays from the X-ray focal point F of the X-ray tube 2 are irradiated. FIG. 1C shows a case in which the collimator device 3 is a one-dimensional collimator. Note that a combination of the X-ray detector 1 and the collimator device 3 is sometimes called a "photon-counting X-ray detector."

[0021] 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.

[0022] FIG. 2 is an enlarged cross-sectional view of a partial region R shown in FIG. 1B in the X-ray detector 1. As shown in FIG.

[0023] The X-ray detection elements 1n are provided on a ceramic substrate 4. Each of the X-ray detection elements 1n includes a ceramic scintillator 11 and a photoelectric conversion element 12.

[0024] The ceramic scintillator 11 is an element that converts incident X-rays into photons and emits them. X-rays typically have a predetermined X-ray energy distribution. X-rays with a specific X-ray energy can be considered as a cluster of X-ray particles, the number of which corresponds 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 cluster of X-ray particles. In other words, when X-rays are incident on the ceramic scintillator 11, the ceramic scintillator 11 approximately simultaneously emits a number of photon groups corresponding to the X-ray energy, depending on the X-ray energy.

[0025] The ceramic scintillator 11 is a light conversion element having a substantially rectangular or cubic shape. The ceramic scintillator 11 is arranged so that its X-ray incident surface is approximately perpendicular to the X-ray irradiation direction, i.e., the X-ray incident direction E, and so that its side surface parallel to the X-ray incident direction E is approximately parallel to the channel direction CH and the slice direction SL.

[0026] The photoelectric conversion element 12 has a substantially rectangular plate-like shape, converts incident photons into an electrical signal, and outputs the electrical signal. The electrical signal is an electrical pulse signal corresponding to each incident photon. When a group of photons is simultaneously incident on the photoelectric conversion element 12, the photoelectric conversion element 12 outputs a pulse signal with a pulse height corresponding to the number of photons constituting the photon 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 capable of photon counting measurement and also applicable to analog measurement such as scintillation detection. A silicon photomultiplier is an element in which multiple avalanche photodiode (APD) pixels operating in Geiger mode are connected in parallel.

[0027] The photoelectric conversion element 12 receives photons emitted from the ceramic scintillator 11 and outputs a pulsed electrical signal. When the intensity of X-rays transmitted through the subject is sufficiently low, photon groups for each X-ray energy are emitted from the ceramic scintillator 11 in a state scattered along the time axis. At this time, the photoelectric conversion element 12 outputs pulse signals with pulse heights corresponding to the magnitude of the X-ray energy for each X-ray energy, the number of which corresponds to the dose of X-rays having that X-ray energy, scattered along the time axis. Therefore, by counting the pulse signals output within a certain period of time by pulse height, it is possible to determine the dose of X-rays transmitted through the subject for each X-ray energy. Furthermore, by counting all pulse signals output within a certain period of time regardless of pulse height, it is possible to determine the total dose of X-rays transmitted through the subject.

[0028] Conductor patterns (not shown) formed on the ceramic substrate 4 are connected to the photoelectric conversion elements 12. Electrical signals from the photoelectric conversion elements 12 are output to an external processing device (not shown) through these conductor patterns. The electrical signals output from the photoelectric conversion elements 12 are used to collect projection data using a photon counting method.

[0029] Detectors used in photon counting generally have very high X-ray detection sensitivity. High sensitivity means that a detector can obtain a signal with a sufficient S / N ratio even when the X-ray dose is low and the number of photons emitted from the scintillator is small. However, when the X-ray dose is high, pulse signals overlap, causing a phenomenon known as pile-up, which makes it impossible to resolve the signal in the time axis direction.

[0030] Indirect photon counting systems have the same problem of counting rate as direct systems. Counting rate refers to the number of X-ray photons incident on a unit area and unit time. To achieve imaging using a photon counting X-ray detector, a detector capable of a high counting rate is required. For example, the counting rate required for an X-ray CT device is 10 ８ [cps / mm ２ This means that the X-ray photon is 1 [mm ２ This means that the photons are incident on an area of ​​1000 nm at intervals of 10 nsec on average. Currently, there are no practical scintillators that can handle such high counting rates. If the counting rate exceeds the capacity of the scintillator, pile-up occurs, making it difficult to measure the correct photon energy. On the other hand, long-term measurements with a suppressed counting rate cause problems such as reduced throughput and reduced time resolution.

[0031] The counting rate of a scintillator is primarily determined by the response speed of the material, so there is an urgent need to develop scintillator materials with improved response speeds to avoid long measurement times and pile-ups.

[0032] (Ceramic scintillator and its manufacturing method) The main factor that determines the response speed of the indirect type is the decay time constant of the scintillator's luminescence. ８ [cps / mm ２In order to achieve this, it is particularly important to control the decay time constant of the scintillator's luminescence, and it is preferable that the decay time constant of the luminescence is approximately 17 nsec or less. By setting the decay time constant of the luminescence to 17 nsec or less, pileup can be prevented and practical imaging can be achieved, as described above. Therefore, experiments were conducted with the aim of setting the decay time constant of the scintillator's luminescence to approximately 17 nsec or less. As a result, it was found that the luminescence dose should be 10,000 ph / MeV or more. This is because an luminescence dose of 10,000 ph / MeV or more provides no problems with the S / N ratio or energy resolution, and a detector with higher accuracy can be obtained.

[0033] That is, it is desirable that the decay time constant is 17 nsec or less and the light emission amount is 10,000 ph / MeV or more. To achieve this, the fluorescent material of the ceramic scintillator 11 used as the light conversion element is made of a garnet compound (a polycrystalline body of a garnet-structured oxide) containing Pr. The fluorescent material of the ceramic scintillator 11 satisfies the following condition 1. Condition 1: Contains 250 mass ppm or less of Ba, and the composition formula of the Ba compound containing Ba is Ba ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ where the ratio y / x of y to x is 2.5 or less. Here, the decay time constant is defined as the time it takes for the emission intensity generated by irradiation with a short-time pulse of radiation to decrease to 1 / e (=0.3679) of the maximum emission intensity, assuming that the time at which the emission intensity reaches its maximum is zero. For example, time-correlated single photon counting (TCSPC) using a pulsed X-ray tube can be performed to record the change in emission intensity over time, and the decay time constant can be calculated from the recorded value.

[0034] It is more preferable that the ceramic scintillator 11 satisfies at least one of the following conditions 2 and 3.

[0035] Condition 2: The composition formula of the garnet compound is R ３ M ５ O １２wherein the element R in the composition includes Pr and at least one of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the element M in the composition includes Al and Ga.

[0036] Condition 3: The composition formula of the garnet compound is R ３ M ５ O １２ The element R in the composition contains Pr, and the element M in the composition contains Al, Ga, and at least one of Sc, Si, Ge, and Sn.

[0037] Next, we will explain the results of investigating the characteristics of the luminescence amount and luminescence decay time constant of fluorescent materials according to examples, which have compositions that satisfy the above-mentioned condition 1, and fluorescent materials according to comparative examples, which do not satisfy the above-mentioned condition 1. The fluorescent materials according to the examples were produced through steps ST1 to ST6 in the flowchart shown in Figure 3. On the other hand, the fluorescent material according to the comparative example was produced by performing the mixing step ST2 using a reaction accelerator containing particles of 30 μm or more in the flowchart shown in Figure 3, then performing the baking step ST3 at a relatively low temperature, and then proceeding to the cleaning step ST4 (step ST1 was not performed, and step ST3 was performed at a low temperature).

[0038] First, in the first sieving step ST1, the reaction accelerator (flux) powder is sieved through a sieve with a maximum particle size of 30 μm to obtain only the reaction accelerator having a maximum particle size of 30 μm. ２ (barium fluoride), and AlF ３ (Aluminum fluoride) may also be added.

[0039] In the second mixing step ST2, a powder of a mixture of an oxide of element R and an oxide of element M (oxide powder), a powder of a reaction accelerator having a maximum particle size of 30 μm, and a sintering aid (e.g., SiO ２ , MgO, and CaO) are filled into an alumina container and mixed in a pot mill or the like to prepare a slurry. ２ The addition of garnet phase (basic structure is R ３ M ５ O １２(Element R: rare earth element, element M: Al, Ga)) has the effect of stabilizing the BaF ２ The more the amount of Ba added, the more the garnet phase can be stabilized. However, if there is a large amount of residual Ba, the performance as a scintillator (emission amount and decay time constant) may deteriorate. Ba and F easily react with Al, and Ba(Al,Ga) ０．２～２．５ O ｘ and Ba(Al,Ga) ７．５～１０ O ｘ Therefore, the residual Ba is reduced by the sieving step ST1 and the firing step ST3 described later. Note that, since Al and Ga are used in combination under the above conditions 2 and 3, Ba(Al,Ga) is easily generated in the firing step ST3. ０．２～２．５ O ｘ From Ba(Al,Ga) ７．５～１０ O ｘ In the cleaning step ST4, Ba(Al,Ga) ７．５～１０ O ｘ By removing the above, it is possible to prevent Ba and F from being incorporated into the scintillator material to a certain extent.

[0040] In the mixing step ST2, the slurry is stirred on a pot mill turntable for 30 minutes to a dozen hours until the desired viscosity is achieved. The order of filling is not particularly limited, but since agglomerates such as lumps are likely to form when a large amount of oxide powder is slurried, an aqueous solution may be prepared by adding a surfactant and a binder to water, and then a predetermined amount of oxide powder may be added little by little to disperse and slurried on the pot mill turntable. For dispersion, a dispersing device such as a bead mill or a planetary mixer may be used in addition to a pot mill.

[0041] In the third step, the firing step ST3, after mixing the reaction accelerator in the mixing step ST2, firing is performed at a temperature of 1200 to 1800° C. If the temperature is less than 1200° C., the reaction may be insufficient, and if the temperature exceeds 1800° C., the temperature is too high and BaF ２ The firing atmosphere is N ２ In some cases, an inert atmosphere such as gas or Ar gas is preferable. －２The firing may be performed in a reducing atmosphere (<100 Pa or less). The firing time is preferably 1 to 8 hours. For example, the 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 a second firing step in which firing is performed in N ２ It is preferable to include a second firing step of firing in a gas atmosphere at a temperature of 1200° C. or higher for about 5 hours.

[0042] After firing in the firing step ST3, the garnet compound powder is obtained. However, the fired garnet compound powder contains BaF as a reaction accelerator. ２ Ba and F resulting from Ba(Al,Ga) ７．５～１０ O ｘ Therefore, when the garnet compound powder after firing is washed, the remaining Ba(Al,Ga) ７．５～１０ O ｘ In order to efficiently carry out the washing, it is also effective to crush and sieve the fired garnet compound powder as necessary to make the average particle size 0.5 to 20 μm.

[0043] In the fourth step, the cleaning step ST4, the garnet compound powder obtained in the firing step ST3 is cleaned. ７．５～１０ O ｘ is removed in the cleaning step ST4. In the cleaning step ST4, it is preferable to perform cleaning using a combination of pure water (water from which impurities have been removed using an ion exchange resin) and acid cleaning. The acid cleaning is preferably performed with dilute nitric acid (or dilute hydrochloric acid). After cleaning with dilute nitric acid, cleaning is performed with pure water. By repeating this process several times, the remaining reaction accelerator can be removed. This process is also performed until the pH of the pure water after cleaning reaches 6 or more. For example, BaF ２ Since 100% of the garnet compound powder is used, the remaining elements are Ba and F. At this time, the garnet compound powder is added to the cleaning container at a ratio of 5 to 30 vol % of the volume, and then diluted nitric acid or pure water is added to clean it. After stirring for a certain period of time, the diluted nitric acid or pure water is discarded. This process is repeated several times.

[0044] F is an element that easily reacts with water, so it is relatively easy to remove. Furthermore, by performing the cleaning step ST4 five or more times, the amount of F can be reduced to zero (below the detection limit). On the other hand, the less residual Ba, the better the ceramic characteristics. However, since Ba is less reactive with water than F and is an element that is easily incorporated into Al, a considerable number of cleanings are required to reduce the amount to zero (below the detection limit). On the other hand, if the residual Ba is less than 10 ppm by mass, the Ba that was once incorporated into Al will be removed, resulting in small pores and defects, which will cause a decrease in transparency. Therefore, it is preferable to perform the cleaning step ST6 five or more times and up to ten times.

[0045] Here, since it is desirable that the residual Ba [mass ppm] after the aforementioned cleaning step ST4 is small, in the aforementioned baking step ST3, Ba (Al, Ga) ７．５～１０ O ｘ By the firing step ST3, the Ba compound has a composition of BaF ２ , Ba(Al,Ga) ０．２～２．５ O ｘ , and Ba(Al,Ga) ７．５～１０ O ｘ The garnet compound powder after washing is dried to produce a scintillator material (phosphor powder).

[0046] In the fifth step, the molding step ST5, the product obtained in the cleaning step ST4 is molded. In the molding step ST5, a molded body may be produced using a mold having the shape of the desired molded body, 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), or may include a degreasing step of degreasing the molded body. The degreasing step is preferably carried out in nitrogen gas at a temperature of 400 to 800°C.

[0047] In the sixth step, the sintering step ST6, the product obtained in the molding step ST5 is sintered to produce a ceramic scintillator as a sintered body. The sintered body becomes a polycrystalline body. In other words, the solid scintillator is a sintered body of a polycrystalline body. Sintering methods include hot pressing, HIP (hot isostatic pressing), vacuum sintering, SPS (spark plasma sintering), and sintering by 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 more. The sintering atmosphere may be an inert atmosphere such as Ar, or a vacuum (10 －２ It is preferable that the viscosity is 100 Pa or less.

[0048] In the mixing step ST2, BaF ２ Contains only AlF ３ The one that does not contain Ba ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ Ba(Al,Ga) in which y / x is 2.5 or more ０．２－０．６ O ｘ and Ba(Al,Ga) １．５－２．５ O ｘ It is known that garnet compounds are likely to contain Ba. The composition of the Ba compound can be determined from the results of mapping each element in the polycrystalline body by energy dispersive X-ray diffraction (EDX) of the observed image.

[0049] In the sintering step ST6, it is preferable to perform the treatment by the HIP method at 1550°C or higher for 3 hours under a pressure of 50 MPa. In this case, the composition (Lu, Pr) of the normal part is ３ (Al, Ga) ５ O １２ On the other hand, Ba(Al,Ga) ０．２－０．６ O ｘ The relative luminescence of the ceramic scintillator can be increased to about 100% by including Ba(Al,Ga) in the garnet compound. Even when the ceramic scintillator is processed by the HIP method at 1500°C or less for 3 hours under a pressure of 50 MPa, the Ba compound Ba(Al,Ga) １．５－２．５ O ｘThe relative luminescence intensity of the ceramic scintillator can be increased to about 70% by including the compound in the garnet compound.

[0050] Here, it is desirable that the residual Ba (ppm by mass) after the sintering step ST6 be small. By performing steps ST1 to ST6, the Ba ratio (ppm by mass) can be reduced to about 1 / 100 of the amount at the time of mixing the raw materials.

[0051] The fluorescent material is composed of the elements contained in the above composition and contains no other elements except for unavoidable impurities. Because the decay time constant of luminescence varies depending on the elements contained, containing a large amount of impurities can increase the decay time constant. Note that the material may contain impurities in amounts of 100 ppm or more as long as it satisfies the required luminescence intensity and / or the required decay time constant of luminescence. Here, when Si functions as a sintering aid, it remains as an impurity in the grain boundaries of the finished ceramic scintillator. Therefore, if Si remains in the grain boundaries, it can be determined that it has been added as a sintering aid, and if it is present in the base material, it can be determined that it has been added as a raw material. In the ceramic scintillator according to the embodiment, except when present at tens of ppm as an impurity, Sn, Si, Ge, or a compound containing at least one of them is not present in the grain boundaries, and the base material is a garnet compound containing Si, Ge, or Sn.

[0052] (Conditions 1 to 3) The relationship between the proportion of residual Ba [mass ppm] in the composition of the garnet compound, the relative light emission amount relative to the relative decay time constant (relative light emission amount / relative decay time constant), and the relative radiation degradation maintenance rate [%] will be described with reference to FIGS. 4 to 8. Here, the relative light emission amount refers to the proportion [%] of the light emission amount when the light emission amount in Example 1 is set as the reference (100). The relative decay time constant refers to the proportion of the decay time constant when the decay time constant in Example 1 is set as the reference (100). Furthermore, the radiation degradation maintenance rate refers to the ratio (light emission amount after exposure to 1 kGy of radiation) / (light emission amount before exposure). The relative radiation degradation maintenance rate refers to the proportion of the radiation degradation maintenance rate when the radiation degradation maintenance rate in Example 1 is set as the reference (100).

[0053] 4 and 5 will be explained. 4 and 5 show Examples 1 to 25 in which the elements R and M in the composition satisfy the above conditions 1 and 2, and Examples 26 to 29 in which the elements R and M satisfy the above conditions 1 to 3. For example, Example 1 shows a garnet compound containing Lu and Pr as the element R in a mass ratio of 0.998:0.002, and Al and Ga as the element M in a mass ratio of 0.9:0.1. Examples 1 to 29 satisfy condition 1, that is, contain 250 mass ppm or less of Ba, and ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ The ratio y / x of y to x is 2.5 or less. Generally, there is a trade-off between the luminescence decay time constant and the luminescence output. Therefore, whether the characteristics are superior or not is considered based on the relative luminescence output relative to the relative decay time constant (relative luminescence output / relative decay time constant). In Examples 1 to 29, the relative luminescence output / relative decay time constant is 0.90 or more. In this case, the decay time constant is approximately 17 [nsec] or less, and the luminescence output is 10,000 [ph / MeV] or more, which is within the preferable range of characteristics. Comparing Examples 26 to 29, when element M contains a tetravalent element (Examples 27 to 29), the relative luminescence output / relative decay time constant is equal to or greater than the case when element M does not contain a tetravalent element (Example 26). In other words, it can be said that it is preferable for element M to contain a tetravalent element.

[0054] In the condition 3, the element M in the composition may contain one or more of tetravalent elements Si, Ge, and Sn. ３＋ and Pr ４＋ does not emit light but absorbs visible light. ４＋ The presence of Pr is a factor that reduces the light emission amount and decay time constant characteristics of the scintillator. ３＋ Since it replaces Pr ３＋ However, Pr ４＋ However, when a divalent element is added, Pr tends to become tetravalent in order to maintain the charge balance, which further reduces the light emission amount and decay time constant characteristics of the scintillator. On the other hand, when a tetravalent element is added, Pr tends to become trivalent in order to maintain the charge balance, which further reduces the light emission amount and decay time constant characteristics of the scintillator. ４＋By suppressing the residue of Pr ３＋ However, the element M in the composition is not limited to a tetravalent element.

[0055] Furthermore, as mentioned above, in Examples 1 to 29, the decay time constant was approximately 17 nsec or less and the light emission amount was 10,000 ph / MeV or more, which is within the preferred range of characteristics, and in this case, the relative radiation degradation maintenance rate was 0.90 or more.

[0056] 6 and 7 will be explained. Fig. 6 and Fig. 7 show Comparative Examples 1 to 33 which do not satisfy the above-mentioned condition 1. That is, Fig. 6 and Fig. 7 show the results of Comparative Examples 1 to 33 in which the residual Ba exceeds 250 mass ppm or the Ba content is ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ The comparative examples 1 to 33 show comparative examples 1 to 33 in which the ratio y / x of y to x exceeds 2.5. The types and mass proportions of the elements R and M are different in the comparative examples 1 to 33. In the cases of the comparative examples 1 to 33, the relative light emission amount / relative decay time constant is less than 0.9, making it difficult for the characteristics to fall within the preferred range.

[0057] 8 shows a table of the relationship between Examples 1 to 12 and Comparative Examples 1 to 18. As shown in the table, when the element R is Lu 0.998 and Pr 0.002, the residual Ba is classified into four groups: 100 ppm by mass or less, more than 100 ppm by mass and 250 ppm by mass or less, more than 250 ppm by mass and 1000 ppm by mass or less, and more than 1000 ppm by mass. ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ The y / x ratio in the above formula is classified into four categories: 1.5 or less, more than 1.5 and 2.5 or less, more than 2.5 and 10 or less, and more than 10. Some of the combinations of the four categories of residual Ba and the four categories of y / x are associated with examples or comparative examples.

[0058] Residual Ba is 250 mass ppm or less, and Ba ｘ (Al, Ga) ｙ O ｘ＋１．５ｙIn this example, if y / x is 2.5 or less, the relative light emission amount / relative decay time constant and the relative radiation degradation maintenance rate will each be 0.90 or more, regardless of the ratio of Al to Ga in element M. As shown in Figures 4 and 5, even if the type of element R is changed from Examples 1 to 12 (Examples 13 to 25) or the type of element M is changed from Examples 1 to 12 (Examples 26 to 29), as long as the residual Ba is 250 ppm by mass or less and y / x is 2.5 or less, the relative light emission amount / relative decay time constant and the relative radiation degradation maintenance rate will each be 0.90 or more.

[0059] The ceramic scintillator made of the Pr-containing garnet compound described above can provide a fluorescent material with a short decay time constant of luminescence. Furthermore, by applying this fluorescent material as a ceramic scintillator in a medical X-ray detector, it is possible to meet the required reactivity.

[0060] According to at least one of the embodiments described above, it is possible to provide a ceramic scintillator that stabilizes the garnet phase without degrading performance, a photon-counting X-ray detector including the same, and a method for manufacturing the ceramic scintillator.

[0061] The ceramic scintillator 11 is not limited to being applied to a photon-counting X-ray detector equipped with a silicon photomultiplier in an X-ray CT device. For example, the ceramic scintillator 11 may be applied to an X-ray detector equipped with a photodiode in an X-ray CT device. The ceramic scintillator 11 may also be applied to a flat panel detector (FPD) equipped with a complementary metal oxide semiconductor (CMOS) in an X-ray diagnostic device. Furthermore, the ceramic scintillator 11 may be applied to a photon-counting detector equipped with a silicon photomultiplier in a PET (Positron Emission Tomography) device. The ceramic scintillator 11 may also be applied to imaging for industrial purposes such as baggage inspection and non-destructive testing.

[0062] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

Claims

1. A ceramic scintillator made of a garnet compound containing Pr, containing 250 mass ppm or less of Ba, wherein the composition formula of the Ba-containing Ba compound is Ba ｘ (Al, Ga) ｙ O ｘ＋１．５ｙ wherein the ratio of y to x, y / x, is 2.5 or less.

2. The composition formula of the garnet compound is R ３ M ５ O １２ 2. The ceramic scintillator according to claim 1, wherein the garnet compound has a composition represented by the formula: wherein an element R in the composition of the garnet compound includes Pr and at least one of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and an element M in the composition includes Al and Ga.

3. The composition formula of the garnet compound is R ３ M ５ O １２ 2. The ceramic scintillator according to claim 1 , wherein the garnet compound has a composition represented by the formula: wherein an element R in the composition of the garnet compound includes Pr; and an element M in the composition includes Al, Ga, and at least one of Sc, Si, Ge, and Sn.

4. A photon-counting X-ray detector comprising the ceramic scintillator according to any one of claims 1 to 3 and a photoelectric conversion element.

5. A radiation imaging device comprising the photon-counting X-ray detector according to claim 4.

6. A method for manufacturing the ceramic scintillator according to any one of claims 1 to 3, wherein the garnet compound has a composition formula R ３ M ５ O １２ A mixture of oxide powders of elements R and M in the composition of BaF with a maximum particle size of 30 μm. ２ a firing step of firing the mixture obtained in the mixing step at a temperature of 1300°C or higher, filling the alumina container with the resulting product, and firing the resulting product in a nitrogen-hydrogen mixed atmosphere at a temperature of 1200°C or higher; a cleaning step of cleaning the product obtained in the firing step; a molding step of molding the product obtained in the cleaning step; and a sintering step of sintering the product obtained in the molding step to produce the ceramic scintillator.

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