JP6776671B2 - Fluorescent materials, ceramic scintillators and radiation detectors, and methods for manufacturing fluorescent materials - Google Patents

Description

The present disclosure relates to a fluorescent material of a garnet-type oxide having a composition containing Gd, Al and Ga, a ceramic scintillator and a radiation detector, and a method for producing the fluorescent material.

The radiation image system irradiates a subject with radiation such as α-rays, β-rays, γ-rays, and X-rays, and images the radiation transmitted through the subject. Radiation imaging systems are used in various application fields such as medical fields such as computed tomography, industrial fields such as non-destructive inspection, security fields such as baggage inspection, and academic fields such as high energy physics.

Currently, radiation imaging systems that are mainly used commercially use radiation detectors that convert the intensity of radiation into electrical signals. The radiation detector includes a scintillator for converting the intensity of radiation into light and a photodetector such as a CCD for converting light into an electric signal. For example, Patent Document 1 discloses a scintillator for an X-ray detector using a fluorescent material having a garnet structure containing Ce as a light emitting element and containing at least Gd, Al, Ga and O, and Y. When radiation enters the scintillator, it is converted to light, and the converted light is converted into an electrical signal by a photodetector. As a result, an image having a gradation corresponding to the signal strength is generated.

In various fields, there is a demand for a radiographic image system capable of acquiring images with higher definition and lower radiation dose. As such a radiation image system, a photon counting type radiation detector that detects radiation by using a photon counter capable of measuring the number of photons has been studied. For example, Patent Document 2 discloses an X-ray CT apparatus using a CdTe scintillator capable of directly detecting the intensity of radiation as an electric signal. By detecting the intensity of radiation as the number of photons by photon counting, it is expected that an image having a large S / N ratio and high resolution can be obtained. Moreover, since the intensity of the radiation used for the measurement can be reduced, the measurement can be performed with a low radiation amount. Furthermore, since it is possible to decompose the energy of radiation and measure the intensity, it is possible to obtain an image system that decomposes and draws tissues such as distinguishing between bone and blood in a living body.

Japanese Patent No. 5729352 Japanese Unexamined Patent Publication No. 2012-34901

The X-ray CT apparatus disclosed in Patent Document 2 uses a CdTe scintillator as a radiation detector. CdTe scintillators are relatively expensive and toxic. Moreover, since it is not easy to obtain a large-sized CdTe single crystal, it is difficult to enlarge the radiation detection surface.

On the other hand, in the case of the indirect conversion method as in Patent Document 1, a scintillator having a high emission intensity during irradiation and a small afterglow, which is a characteristic several ms after irradiation, is used. However, as will be described in detail below, in conventional fluorescent materials for radiation detectors, the decay time constant of fluorescence, which is an order of magnitude smaller than the afterglow, that is, a characteristic of several hundred μs after irradiation, is Not considered. Therefore, it is difficult to use it as a scintillator for measuring the number of photons. Therefore, considering manufacturing cost and commercial use, it is desirable to develop an indirect photon counting type radiation detector that detects radiation by combining a polycrystalline ceramic scintillator and a photodetector instead of a single crystal scintillator. Is done.

In view of such problems, the present disclosure provides fluorescent materials, ceramic scintillators and radiation detectors that can be used for photon counting type radiation detectors, and methods for producing fluorescent materials.

Fluorescent materials of the present disclosure consists of a composition represented by _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12, wherein L is Y, and It is at least one of Lu, and the a, b, α, β, u, and v satisfy the following range.
0 ≤ a ≤ 0.1,
0 ≦ b ≦ 0.1,
0 ≤ α ≤ 0.9,
0.0167 <β,
0 ≦ u ≦ 0.686,
0 ≦ v ≦ 0.024

The β may satisfy the following range.
0.033 ≤ β ≤ 0.1

The L may be Y, and the α may satisfy the following range.
0.7 ≤ α ≤ 0.9

The ceramic scintillator of the present disclosure contains the fluorescent material according to any one of the above, and has a relative density of 99% or more.

The radiation detector of the present disclosure includes the ceramic scintillator and a photoelectric conversion element that converts light into an electric signal, a current value, or a voltage value.

Method of manufacturing a phosphor material of the present disclosure is represented by _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12, a, b, α, A step of preparing a raw material containing Gd, Lu, and at least one of L, Ce, Al, Ga, and Sc at a ratio in which β, u, and v satisfy the following ranges. ,
0 ≤ a ≤ 0.1,
0 ≦ b ≦ 0.1,
0 ≤ α ≤ 0.9,
0.0167 <β,
0 ≦ u ≦ 0.686,
0 ≦ v ≦ 0.024

A step of mixing and crushing the raw materials to obtain a raw material powder, a step of molding the raw material powder to obtain a molded product, and a step of sintering the molded product at a temperature of 1650 ° C. to 1700 ° C. in an oxygen atmosphere. Including.

According to the present disclosure, a fluorescent material having a small fluorescence attenuation time constant and usable for a photon counting type radiation detector, and a method for producing the fluorescent material can be obtained. Further, a ceramic scintillator and a radiation detector containing a fluorescent material having such characteristics and capable of increasing the area can be obtained.

_{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, when L is Lu, Ce It is a figure which shows the relationship between the quantity β and the decay time constant τ of fluorescence. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, when L is Y, Ce It is a figure which shows the relationship between the quantity β and the decay time constant τ of fluorescence. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, Y amount alpha and decay time constant of the fluorescence It is a figure which shows the relationship with τ. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, the decay time constant of the Ga amount u and fluorescence It is a figure which shows the relationship with τ. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, the decay time constant of the Sc amount v and fluorescence It is a figure which shows the relationship with τ. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, when L is Lu, Ce It is a figure which shows the relationship between the quantity β and the relative emission intensity. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by a composition formula, when L is Y, Ce It is a figure which shows the relationship between the quantity β and the relative emission intensity. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by the composition formula, the Y amount alpha and relative emission intensity It is a figure which shows the relationship. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by the composition formula, the Ga amount u and the relative luminous intensity It is a figure which shows the relationship. _{(Gd 1-α-β L} α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O 12 A fluorescent material represented by the composition formula, the Sc amount v and the relative luminous intensity It is a figure which shows the relationship. It is a figure which shows the example of the emission spectrum of the fluorescent material of an Example.

The inventor of the present application examined the physical properties required for a fluorescent material and a ceramic scintillator that can be used for a photon counting type radiation detector, and examined in detail the composition of the fluorescent material that can realize the physical properties.

A conventional intensity-integrated radiation detector irradiates an object with radiation at a predetermined time interval on the order of microseconds to milliseconds (this time interval is called one frame), and the radiation transmitted through the object is incident. The fluorescence emitted by the scintillator is detected by a photodetector and converted into an electric signal. The radiation irradiates the object for a period shorter than the predetermined time interval described above.

The fluorescent material of the scintillator emits strong fluorescence when irradiated with radiation, but the fluorescence does not become zero immediately after the irradiation is stopped, and weak fluorescence continues. This is called afterglow. Afterglow example, using an emission intensity I ₀ in the radiation, and the emission intensity I _r after a predetermined time has elapsed from the radiation irradiation stop is defined by I _r / I ₀ (ppm). The fixed time is selected on the order of milliseconds (ms) in consideration of the signal processing speed from the photodetector and the exposure of the object.

In order to reduce the influence of noise, etc., the radiation imaging system subtracts the electric signal obtained from the radiation detector when the radiation irradiation is stopped from the electric signal obtained from the radiation detector when the radiation is irradiated in each frame. Treat the signal as the intensity of the detected radiation. Therefore, when the afterglow is large, the detected radiation intensity becomes relatively small. For this reason, it is preferable that the afterglow of the fluorescent material is small in the conventional intensity-integrated radiation detector.

On the other hand, photon counting type radiation detectors include photomultiplier tubes and multi-pixel photon counters (silicon photomultipliers) that can measure the number of light emitted (photons) in a very short time of fluorescent materials. The number of light emission is read out as a pulse signal. Since the photon counting type radiation detector reads out by setting a certain threshold value for the emission intensity, the emission intensity of the fluorescent material does not have to be as high as the fluorescent material required for the above-mentioned conventional intensity integration type radiation detector. .. Further, even if the afterglow is generated, if the intensity of the afterglow is equal to or less than the above-mentioned threshold value, the afterglow is not detected, so that the presence or absence of the afterglow does not significantly affect the detection of the light emission.

On the other hand, in a radiation image system equipped with a photon counting type radiation detector, the number of emissions is proportional to the intensity of radiation. Therefore, if the fluorescence decay time is long, the emissions overlap and the accurate number of emissions cannot be detected. .. For this reason, scintillators and fluorescent materials used in photon counting type radiation detectors have a tendency to attenuate in a very short time, specifically, the emission intensity is about 1 / e in a time on the order of about nanoseconds. A characteristic that attenuates to is required. As described above, the fluorescent material preferably used for the photon counting type radiation detector is required to have completely different characteristics from the fluorescent material for the conventional radiation detector.

A positron emission tomography system has been put into practical use as an image system using a radiation detector similar to a photon counting type. In a positron tomographic imaging system, it is necessary to detect γ-rays with high temporal resolution and positional resolution. Positron tomographic imaging systems include single crystals such as Gd ₂ SiO ₅ : Ce (GSO), Bi ₄ Ge ₃ O ₁₂ (BGO), Li ₂ SiO ₅ : Ce (LSO), Lu _2-x YSiO ₅ (LYSO). A scintillator composed of oxides is used. These single crystal oxides can achieve a fluorescence decay time constant of about submicroseconds, but a photon counting type radiation detector is required to have a smaller decay time constant. Moreover, since it is a single crystal oxide, it may be difficult to manufacture a large scintillator. Therefore, it is difficult to use a conventional scintillator in an image system using a photon counting type radiation detector, which has a large radiation detection surface and considers cost and commercial use.

In view of these problems, the inventor of the present application has conceived a novel fluorescent material, a ceramic scintillator and a radiation detector, and a method for manufacturing the fluorescent material. Hereinafter, the fluorescent material, the ceramic scintillator and the radiation detector of the present disclosure, and the method for producing the fluorescent material will be described in detail.

(Composition and physical properties of fluorescent material)
Fluorescent materials of the present disclosure, the general formula (hereinafter, referred to as the general formula _{(1)) :( Gd 1-} α-β L α Ce β) 3 + a (Al 1-uv Ga u Sc v) 5-b O _It consists of the composition shown in ₁₂ . L is at least one of Y and Lu, and a, b, α, β, u, and v each satisfy the following ranges.
0 ≤ a ≤ 0.1
0 ≦ b ≦ 0.1
0 ≤ α ≤ 0.9
0.0167 <β
0 ≦ u ≦ 0.686
0 ≦ v ≦ 0.024

In the above general formula (1), the composition ratio of oxygen is defined as 12. This is because the fluorescent material of the present disclosure has a garnet-type crystal structure, and the composition ratio is determined based on oxygen. However, the fluorescent material of the present disclosure does not have to have a complete garnet structure. Depending on the values of a and b, the fluorescent material may also have a garnet structure that is oxygen deficient or excess oxygen. That is, when the composition formula is determined assuming that the composition ratio of oxygen is 12, regardless of whether oxygen is deficient or excess oxygen, a, b, α, β, u, and v in the composition formula are in their respective ranges. It suffices if it satisfies.

A fluorescent material composed of a garnet-type oxide is a metal oxide that is stable to radiation. The emission of the fluorescent material is generated by, for example, the combination of electrons and holes generated by X-ray excitation at the luminescent ions. In a garnet-type oxide containing Gd, Al and Ga, Ce functions as a luminescent ion. One of the features of the fluorescent material of the present disclosure is that it contains more Ce than the fluorescent material used in the conventional intensity-integrated radiation detector. As a result, the generated hole-electron pair binds at Ce in a shorter time and emits light. Therefore, the decay time constant of fluorescence becomes small.

The fluorescent material of the present disclosure comprises the elements contained in the above general formula (1), and does not contain any other elements except unavoidable impurities. This is because the decay time constant changes depending on the element including the element, so that the decay time constant may become large when an element other than the elements of the present disclosure is included.

In the present disclosure, the fluorescence attenuation time constant τ is the ratio of the emission intensity of fluorescence after the stop of X-ray irradiation to the emission intensity of fluorescence during X-ray irradiation, where the irradiation stop time of X-rays, which is the excitation light, is set to zero. It is defined by the time when it becomes 1 / e (= 0.3679). More specifically, for example, when a pulse X-ray tube is used to generate X-rays at a tube voltage of 30 kV and the fluorescent material is irradiated with X-rays, the emission intensity becomes 1 / e after the X-ray irradiation is stopped. The time until decay to is defined as the time constant τ. Since the unit of the decay time constant is time, it is not the intensity ratio (ppm) like the afterglow, but nanoseconds (ns).

Β indicating the amount of Ce in the general formula (1) satisfies 0.00167 <β. When β satisfies this condition, the decay time constant τ becomes small enough to be used in a photon counting type radiation detector. As described above, the decay time constant changes depending on the elements contained in the material, but by satisfying the condition β of the general formula (1), the fluorescence in the currently generally available fluorescent materials for scintillators It is about 1/10 of the decay time constant τ. When β is 0.00167 or less, it is not sufficiently small. On the other hand, the larger β is, the smaller the decay time constant of fluorescence is. Therefore, there is no upper limit to β from the viewpoint that the decay time constant is small. However, as β increases and the content of Ce increases, grain growth tends to occur in the fluorescent material and heterogeneity tends to occur, so that it becomes difficult to obtain a dense sintered body. Further, as the content of Ce increases, the distance between Ce atoms becomes smaller, and the emission intensity due to energy migration (so-called concentration quenching) decreases. Therefore, β preferably satisfies β ≦ 0.1.

Α indicating the amount of the L element (Y or Lu) is 0 ≦ α ≦ 0.9. Preferably, α satisfies 0.7 ≦ α ≦ 0.8. Α, which indicates the amount of the L element, also correlates with the fluorescence decay time constant τ, and within the above range, when the value of α is large, the fluorescence decay time constant τ becomes small. Further, as the value of α increases, the emission intensity decreases.

The decay time constant of a scintillator used in PET or the like as a photon counting type radiation detector is about 40 ns. In the present disclosure, the L element is Lu and 0.033 ≦ β ≦ 0.1, or the L element is Y and 0.69 ≦ α and 0.16 ≦ β ≦ 0.1. When any of the conditions is satisfied, the decay time constant τ becomes 40 ns or less, which is preferable. Further, when the L element is Y and any of 0.066 ≦ β ≦ 0.1 and 0.58 ≦ u ≦ 0.69 is satisfied, the decay time constant τ becomes 20 ns or less, which is more preferable.

U indicating the amount of Ga is 0 ≦ u ≦ 0.686. U, which indicates the amount of Ga, also correlates with the fluorescence decay time constant τ, and within the above range, when the value of u is large, the fluorescence decay time constant τ becomes small. Further, as the value of u increases, the emission intensity decreases.

The v indicating the Sc amount is 0 ≦ v ≦ 0.024. The amount of Sc does not significantly affect the fluorescence attenuation time constant τ and the fluorescence emission intensity in the range of 0 ≦ v ≦ 0.024, and affects the crystal grain size of the sintered body (ceramic). At v = 0, abnormal grain growth occurs, and the density of the fluorescent material tends to decrease. Therefore, when v = 0, it is preferable to improve the uniformity of the composition and perform precise temperature control during sintering. On the other hand, when the value of v exceeds 0.024, the emission intensity decreases.

The ranges of a and b are both 0 ≦ a ≦ 0.1 and 0 ≦ b ≦ 0.1. It is preferable that a and b have the same value, but they may have different values due to the solid solution of impurity elements such as Si and Fe contained in the raw material and the weighing error. When a ≠ b, oxygen defects are likely to occur in the crystal, and the emission intensity may decrease.

When a is a negative number less than 0, ion vacancies are generated at the site occupied by rare earth elements (Gd _1-α-β L _α Ce _β ) in the garnet-type crystal structure, and the afterglow increases. In addition, the emission intensity is extremely reduced. Therefore, a is set to 0 or more. In industrial mass production, it is preferable to set 0 <a, 0 <b, more preferably 0.0001 ≦ a, 0.0001 ≦ b in consideration of the variation in composition. However, when a and b exceed 0.1, a perovskite-type crystal structure phase (GdAlO ₃ ) having a different phase is likely to be generated. Since this different phase has a different refractive index from the phase of the garnet-type crystal structure of the fluorescent material of the present disclosure, light scattering occurs and the emission intensity is lowered.

In addition to having a small decay time constant, in order to satisfy particularly high emission intensity and low afterglow characteristics at the same time, a should be in the range of 0 <a≤0.07 and b should be in the range of 0 <b≤0.07. It is more preferable, and it is particularly preferable that the range is within the range of 0.0001 ≦ a ≦ 0.05 and 0.0001 ≦ b ≦ 0.05.

The density of the fluorescent material of the present disclosure is preferably 99% or more in the evaluation of the relative density described below. The calculation method of the relative density is as follows. First, in the general formula (1), when a = 0, b = 0, α = 0, β = 0, u = 2/5, v = 0 (composition formula: Gd ₃ Al ₃ Ga ₂ O ₁₂ ). The lattice constant is quoted from the data of ICDD (International Center for Diffraction Data), and the volume is calculated based on the data. Next, the formula amount is calculated as mass from the composition formula of the sample for which the relative density is to be calculated. Then, the density obtained from the volume and the formula quantity is used as the theoretical density. Next, the measured density of the fluorescent material is measured and divided by the theoretical density to calculate the relative density. When the relative density is small, sufficient X-ray absorption is not performed, so 99% or more is preferable. Further, in order to replace the Gd element with the L element based on the value of α, based on the lattice constants of a = 0, b = 0, α = 0, β = 0, u = 2/5, v = 0. It is calculated, and in some cases, the relative density may exceed 100%. However, if it exceeds a large amount, it is highly possible that the crystal structure has changed. From the examples of the present disclosure, it is confirmed by XRD (X-Ray Diffraction) that the garnet structure is provided if the relative density is 102.5% or less.

The fluorescent material of the present disclosure does not contain sulfur. Therefore, unlike Gd ₂ O ₂ S-based fluorescent materials, by not using sulfide as a raw material, a high-density sintered body can be obtained, which increases the light transmittance and realizes high emission intensity. obtain.

The fluorescent material of the present disclosure has a peak in the emission spectrum in the range of about 530 nm or more and about 575 nm or less. The peak wavelength can be adjusted, for example, by changing u, which indicates the amount of Ga. The peak wavelength in the emission spectrum of the fluorescent material of the present disclosure is longer than the peak wavelength of the single crystal oxides such as GSO, BGO, LSO, and LYSO described above, and is shifted to the long wavelength side (red side). Therefore, in particular, a photon counting type radiation detector can be realized by combining with a photodetector having high sensitivity to infrared and near infrared light such as a Si avalanche photodiode.

(Manufacturing method of fluorescent material)
An example of the method for producing a fluorescent material according to the present disclosure will be described. First, a raw material containing Gd, at least one of Lu and Y, Ce, Al, Ga, and Sc at a ratio represented by the general formula (1) is prepared. Specifically, raw materials such as oxides or carbonates of elements of gadolinium, yttrium and / or lutetium, cerium, aluminum, gallium and scandium are prepared, and these elements have a composition represented by the general formula (1). Weigh the raw materials so that they have a ratio. Next, if necessary, a solvent is added to the weighed raw material, and the mixture is mixed and pulverized with a ball mill or the like. Raw material powder is obtained by drying the mixture. A molded product is obtained by granulating the raw material powder with an appropriate sieve and press molding. The molded product is then sintered by holding it in an oxygen atmosphere at a temperature of 1650 ° C to 1700 ° C for 0.5 to 12 hours. As a result, a polycrystalline fluorescent material can be obtained. In the method for producing a fluorescent material according to the present disclosure, the element ratio of the obtained fluorescent material does not deviate significantly from the ratio of the elements of the raw material used. Therefore, the composition ratio of the fluorescent material obtained by obtaining the ratio of Gd, at least one of Lu and Y, Ce, Al, and Ga and Sc in the raw material can be used.

In addition to the above production method, the fluorescent material of the present disclosure can also be produced by an inorganic salt method or the like.

Since the fluorescent material of the present disclosure can be produced by the same process as general ceramics, it is a low-cost and highly productive fluorescent material, and a large-area ceramic scintillator can be relatively easily produced. It is possible to make it. Therefore, by using the ceramic scintillator of the present disclosure, unlike a single crystal scintillator, by arranging a large amount of fluorescent materials or processing a large-area ceramic scintillator, a radiation detector having a large radiation detection surface can be easily obtained. Obtainable.

(Embodiment using fluorescent material)
[Ceramic scintillator]
The obtained polycrystalline fluorescent material is cut as a plate having an appropriate thickness with an inner slicer, for example, and held in an oxygen atmosphere at a temperature of, for example, 1250 ° C to 1350 ° C for 0.5 to 12 hours. Apply heat treatment. Then, the surface is optically polished to obtain a ceramic scintillator. When the fluorescent material obtained by sintering has a desired shape, it is possible to obtain a ceramic scintillator by performing the above-mentioned heat treatment and optical polishing.

[Radiation detector]
A radiation detector can be constructed by combining the ceramic scintillator of the present disclosure with a photomultiplier tube capable of measuring light with high sensitivity and a photodetector such as a multi-pixel photon counter (silicon photomultiplier). it can. For example, the photodetector includes a photomultiplier tube having a light receiving surface, a photodetector such as a multi-pixel photon counter, and a ceramic scintillator arranged on the light receiving surface of the photodetector. As the ceramic scintillator, the ceramic scintillator of the present disclosure described above can be used. The photodetector is preferably highly sensitive to light and can measure light emission in a very short time. More preferably, it has detection sensitivity in the wavelength range of about 530 nm or more and about 600 nm or less. As described above, the fluorescence attenuation constant in the fluorescent material constituting the scintillator is small. Therefore, it is possible to detect radiation in combination with a photodetector with high time resolution, and it is possible to suitably combine it with a photomultiplier tube, a multi-pixel photon counter, etc. that can measure the number of photons. Is.

(Example)
The results of preparing fluorescent materials having various compositions and examining their characteristics will be described.

[Sample preparation condition 1]
Using Lu as the L element, samples 1 to 4 having different Ce amounts were prepared as shown in Table 1, and the relationship between the Ce amount and the fluorescence attenuation time constant τ and the emission intensity was investigated. 200 g of raw materials (lutetium oxide, gadolinium oxide, cerium oxide, aluminum oxide, gallium oxide and scandium oxide) are weighed in a resin pot having a capacity of 1 liter so as to have the composition shown in Table 1, and a high-purity alumina ball having a diameter of 5 mm is used. 1250 g and 200 ml of ethanol were put together, mixed and pulverized for 40 hours using a ball mill, and then dried. After granulating this raw material powder with a sieve having a mesh size of 150 μm, uniaxial press molding is performed at a pressure of 500 kg / cm ² , and cold hydrostatic pressing is further performed at a pressure of 3000 kg / cm ² , with respect to the theoretical density. A molded product having a relative density of 56% was obtained. This molded product was placed in an alumina pan, covered with a lid, and sintered in a 100 vol% oxygen atmosphere at a temperature of 1650 to 1700 ° C. for 12 hours to obtain a sintered body. The obtained sintered body is machined into a plate having a width of 20 mm, a length of 30 mm, and a thickness of 1.75 mm using an inner slicer, and then heat-treated at a temperature of 1300 ° C. for 2 hours in a 100 vol% oxygen atmosphere. Was done. After the heat treatment, the surface was optically polished to obtain a ceramic scintillator made of a polycrystalline fluorescent material.

[Sample preparation condition 2]
Samples 5 to 9 in which Y was used as the L element and the amount of Ce was different as shown in Table 2 were prepared. Samples 5 to 9 were prepared in the same manner as in Sample Preparation Condition 1 except that yttrium oxide was used as a raw material.

[Sample preparation condition 3]
Samples 11 to 15 in which Y was used as the L element and the amount of Y was different as shown in Table 3 were prepared. Samples 11 to 15 were prepared in the same manner as in Sample Preparation Condition 1 except that yttrium oxide was used as a raw material.

[Sample preparation condition 4]
Samples 16 to 21 in which Y was used as the L element and the amount of Ga was different as shown in Table 4 were prepared. Samples 16 to 21 were prepared in the same manner as in Sample Preparation Condition 1 except that yttrium oxide was used as a raw material.

[Sample preparation condition 5]
Samples 22 to 25 in which Y was used as the L element and the amount of Sc was different as shown in Table 5 were prepared. Samples 22 to 25 were prepared in the same manner as in Sample Preparation Condition 1 except that yttrium oxide was used as a raw material.

[Measurement of characteristics]
The fluorescence decay time constant τ of the prepared samples 1-9 and 11-25 was determined. Using a pulsed X-ray tube, generate X-rays at a tube voltage of 30 kV, irradiate samples 1 to 9, 11 to 25 with X-rays, and after stopping X-ray irradiation, until the emission intensity attenuates to 1 / e. Was calculated as the decay time constant τ. For the measurement of fluorescence, Quantaurus-τ (Quantaurus-Tau) of a fluorescence lifetime measuring instrument manufactured by Hamamatsu Photonics was used.

The emission intensity was measured using a silicon photodiode (S2281 manufactured by Hamamatsu Photonics). Gd ₂ O ₂ S: Arranged by converting to a relative value (%) when the emission intensity of Pr was 100%. Table 6 shows the measurement results of the attenuation time constant τ and the relative emission intensity.

Further, the relationship between the Ce amount β and the attenuation time constant τ when L is Lu and Y is shown in FIGS. 1 and 2, respectively. The relationship between the Y quantity α and the decay time constant τ is shown in FIG. The relationship between the Ga amount u and the decay time constant τ is shown in FIG. The relationship between the Sc amount v and the decay time constant τ is shown in FIG.

Further, the relationship between the Ce amount β and the relative emission intensity when L is Lu and Y is shown in FIGS. 6 and 7, respectively. The relationship between the amount of Y α and the relative emission intensity is shown in FIG. The relationship between the amount of Ga u and the relative emission intensity is shown in FIG. The relationship between the Sc amount v and the relative emission intensity is shown in FIG.

The measured densities of the obtained samples 1 to 9 and 11 to 25 were determined by an in-liquid weighing method using water based on Archimedes' principle. The relative density was determined by dividing the measured density by the theoretical density. It was confirmed that all the samples had a relative density of 99% or more.

The spectra (wavelength distribution of intensity) of samples 16 and 21 were measured. The results are shown in FIG. A small spectroscope (USB2000 manufactured by Ocean Optics) was used for the measurement.

[Discussion]
As can be seen from FIG. 1, when the L element is Lu, the decay time constant τ of fluorescence becomes smaller than 40 ns when the Ce content β becomes larger than 0.0167. Further, as can be seen from FIG. 2, when the L element is Y, if the Ce content β is larger than 0.167, the fluorescence attenuation time constant τ becomes smaller than 40 ns. Further, regardless of whether the L element is Lu or Y, if the Ce content β is 0.033 or more, the decay time constant τ becomes smaller than 40 ns. It can be seen that this value is about the same as or higher than that of single crystal LSO and LYSO, and that β is 0.033 or higher, so that a ceramic scintillator and a radiation detector with high time resolution can be realized.

As shown in FIG. 3, the attenuation time constant τ decreases as the Y content α increases. If 0.7 ≤ α, it can be seen that the decay time constant τ is smaller than 40 ns. Similarly, as shown in FIG. 4, as the Ga content u increases, the decay time constant τ decreases. On the other hand, as shown in FIG. 5, the Sc content v does not significantly affect the decay time constant τ.

As shown in FIGS. 6 and 7, the emission intensity decreases as the Ce content β increases. Regardless of whether the L element is Lu or Y, if the Ce content β is larger than 0.167, the relative emission intensity becomes 70% or less. As shown in FIGS. 8, 9 and 10, the Gd content α, the Ga content u and the Sc content v have a small effect on the relative emission intensity.

FIG. 11 shows (Gd _0.169 Y _0.797 Ce _0.0332 ) _3.01 (Al _0.401 Ga _0.587 Sc _0.012 ) _4.99 O ₁₂ (Sample 20) and (Gd _0.169 Y _0.797 Ce _0.0332 ) _3.01 (Al _0.988 Sc _0.012 ) _4.99 O ₁₂ (Sample 16). ) Is shown. It has peaks at 530 nm and 575 nm, respectively. As a result of measuring the spectrum other than the sample shown in FIG. 11, it was found that the spectrum had a peak in the range of about 530 nm or more and about 575 nm or less by having the composition represented by the general formula (1).

From the above results, according to the fluorescent material of the present disclosure, it is possible to obtain a fluorescent material having the composition of the general formula (1), which has a small attenuation time constant τ and can be used for a photon counting type radiation detector. It could be confirmed.

The fluorescent materials, ceramic scintillators and radiation detectors of the present disclosure, and methods for producing fluorescent materials are suitably used for fluorescent materials, ceramic scintillators and radiation detectors for various purposes, for example, a photon counting type for a radiographic imaging system. Suitable for radiation detectors.

Claims (4)

_{(Gd 1-α-β Y} α Ce β) 3 + a (Al 1-uv Ga u Sc v) consists composition represented by _5-b O _12,
A fluorescent material in which the a, b, α, β, u and v satisfy the following ranges.
0 ≤ a ≤ 0.1,
0 ≦ b ≦ 0.1,
0.69 ≤ α ≤ 0.9,
0.066 ≤ β ≤ 0.1 ,
0 ≦ u ≦ 0.686,
0 ≦ v ≦ 0.024 A ceramic scintillator containing the fluorescent material according to claim 1 and having a relative density of 99% or more. The ceramic scintillator according to claim 2 and
A radiation detector with a photoelectric conversion element that converts light into either an electrical signal, a current value or a voltage value. _{(Gd 1-α-β Y} α Ce β) 3 + a (Al 1-uv Ga u Sc v) is represented by _{5-b O 12, a,} b, α, β, u, v are satisfy the following ranges A step of preparing a raw material containing Gd, Y , Ce, Al, Ga, and Sc, respectively, at the ratio of
0 ≤ a ≤ 0.1,
0 ≦ b ≦ 0.1,
0.69 ≤ α ≤ 0.9,
0.066 ≤ β ≤ 0.1 ,
0 ≦ u ≦ 0.686,
0 ≦ v ≦ 0.024
A step of mixing and pulverizing the raw materials to obtain a raw material powder, and
The process of molding the raw material powder to obtain a molded product,
A step of sintering the molded product at a temperature of 1650 ° C. to 1700 ° C. in an oxygen atmosphere.
A method for producing a fluorescent material including.

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