JP5100050B2 - Oxide phosphor, radiation detector and X-ray CT apparatus - Google Patents

Description

  The present invention relates to an oxide phosphor used in a radiation detector that detects X-rays, γ-rays, and the like. The present invention also relates to a radiation detector using an oxide phosphor, particularly a radiation detector suitable for an X-ray CT apparatus, and an X-ray CT apparatus using the radiation detector.

In recent years, solid-state detectors combining ceramic scintillators and photodiodes have become mainstream as radiation detectors for X-ray CT apparatuses. In order to obtain a high-quality CT image with little noise in an X-ray CT apparatus, a scintillator with a high light emission output is desired. Further, it is desired that the afterglow of the scintillator is short in order to shorten the scanning time. Specifically, a so-called sub-second scanner is realized in which the imaging time of one slice image is about 0.4 to 0.8 seconds. As a corresponding scintillator, afterglow (attenuation rate%) after 300 ms after X-ray blocking is realized. It is required to be on the order of 4-5 × 10 ⁻³ or less. Further, in order to obtain a stable CT image free of artifacts, an afterglow of about 2 × 10 ⁻³ or less is required.

Conventionally, rare earth oxysulfide phosphors Gd ₂ O ₂ S: Pr, Ce, F and the like have been used as scintillators used in radiation detectors for X-ray CT apparatuses. The present applicant has developed and proposed a garnet-structured oxide phosphor containing Ce as a light-emitting element and containing Gd, Al, Ga, and O (Patent Documents 1 to 3). This oxide phosphor has a high output comparable to Gd ₂ O ₂ S: Pr, Ce, F, and has a small afterglow.
International Publication WO / 1999/033934 JP 2001-4753 A JP 2002-189080 A

The above-described oxide phosphor having a garnet structure has a high light emission output and small afterglow, and thus is an excellent scintillator. However, there is a problem that afterglow varies depending on a sample. For example, depending on the sample, the afterglow after 300 ms after X-ray blocking (attenuation rate%) shows an extremely short afterglow of 1 × 10 ⁻³ or less, but depending on the sample, there were cases where it exceeded 5 × 10 ⁻³ . In other words, conventionally, it has been difficult to ensure that afterglow is in a range that can be applied to a sub-second scanner.

  An object of the present invention is to provide a ceramic scintillator having a high light emission output and stably exhibiting short afterglow. The present invention also provides a radiation detector with high output and excellent time resolution by using such a ceramic scintillator, and also provides an X-ray CT apparatus with high image quality even at high-speed scanning. .

  In order to solve the above-mentioned problems, the present inventors have used an oxide phosphor mainly composed of a base crystal having a garnet structure containing Ce as a light emitting element and containing at least Gd, Al, Ga, and O. As a result of intensive studies on the contained impurity elements, it was found that there is a strong correlation between the Fe content as an impurity component and the afterglow characteristics, and the present invention has been achieved.

  That is, the oxide phosphor of the present invention contains Ce as a light emitting element, contains Gd, Al, Ga, O, and is mainly composed of a host crystal having a garnet structure. It is characterized by being 35 ppm by weight or less.

  The radiation detector of the present invention comprises a phosphor element that emits light by radiation and a photoelectric conversion element that detects light emitted by the phosphor element, and the oxide phosphor of the present invention is used as the phosphor element. And

  Furthermore, the X-ray CT apparatus of the present invention includes a radiation source, a radiation detector disposed opposite to the radiation source, a rotation that holds the radiation source and the radiation detector, and is driven to rotate around the subject. A disk and image reconstruction means for reconstructing a tomographic image of a subject based on the intensity of radiation detected by the radiation detector, wherein the radiation detector of the present invention is used as a radiation detector And

Hereinafter, the oxide phosphor of the present invention will be described in detail.
The oxide phosphor that is the subject of the present invention is a garnet-structured oxide phosphor disclosed in Patent Documents 1 to 3 described above. Specifically, the general formula _{(Gd 1-zx L z Ce} x) 3 Al 5-y Ga y O 12 ( wherein, L is the .z representing a La or Y 0 ≦ z <0.2, x is 0.0005 ≦ x ≦ 0.015, y is a value in the range of 0 <y <5), Ce is a light emitting element, includes Gd, Al, Ga, O, Gd / (Al + Ga + Gd) A phosphor having an atomic ratio of 0.33 or more and 0.42 or less (except for (Gd + Ce) / (Al + Ga + Gd + Ce) = 0.375), Ce as a light emitting element, and Gd, Al, Ga, O A phosphor having an atomic ratio of (Gd + Ce) / (Al + Ga + Gd + Ce) greater than 0.375 and not greater than 0.44, and an atomic ratio of Ce / (Ce + Gd) not less than 0.0005 and not greater than 0.02 Etc. Hereinafter, these phosphors are collectively referred to as garnet phosphors.

  These garnet phosphors are mainly composed of a base crystal having a garnet structure and may contain a phase (second phase) other than the garnet structure. The second phase has, for example, a perovskite structure.

  The Fe component contained in such a garnet phosphor is considered to exist in the crystal as a lattice defect and form a new energy level. It is thought that the energy level due to Fe greatly contributes to the afterglow characteristics of the phosphor and prolongs the afterglow. Therefore, by suppressing the Fe content present in the phosphor, the afterglow of the phosphor can be reduced. This Fe component is introduced into the phosphor due to impurities contained in the raw material powder itself or impurities mixed in during the phosphor manufacturing process. Therefore, it is possible to reduce the amount of Fe impurities contained in the phosphor by reducing the amount of Fe impurities contained in the raw material powder to be used and preventing Fe from being mixed in the phosphor production process.

Next, the manufacturing method of the garnet fluorescent substance of this invention is demonstrated.
The garnet phosphor of the present invention can be manufactured by a general ceramic material manufacturing method comprising (1) mixing and calcining raw material powder and (2) sintering the calcined powder.

As the raw material powder, oxides such as Gd ₂ O ₃ , Al ₂ O ₃ , Ga ₂ O ₃ , and CeO ₂ , hydroxides, oxalates, and the like can be used. Further, a fine powder synthesized by a coprecipitation method, a sol-gel method or the like may be used. The raw material powder is preferably a micron-order fine powder, preferably a submicron powder. The powder purity is 99.99% or more, and the Fe content is 20 wtppm or less, preferably 5 wtppm or less.

  In the manufacturing process, non-ferrous materials such as resin are used for devices such as sieves and medicine spoons, and mixing of Fe is suppressed as much as possible. First, in the calcining step, a predetermined amount of the above-described high-purity raw material powder is weighed and wet-mixed for about 10 hours by ball mill mixing using a resin container such as polyethylene and an alumina ball, for example. After drying this mixed powder, it is put into an alumina crucible and calcined for several hours in an atmosphere containing oxygen at 1400 to 1800 ° C. to obtain a calcined powder. Thus, the calcining step is performed using a ball, a container, and a crucible made of a non-ferrous material.

For the sintering step, a hot pressing method, a hot isostatic pressing (HIP) method, a normal pressure sintering method, a combined method of a normal pressure sintering method and a HIP method, or the like can be employed. In the hot press method, the above-mentioned calcined powder is molded by molding at a pressure of about 300 to 600 kgf / cm ² to form a molded body, then set in a hot press mold, and in an atmosphere in vacuum, air, or oxygen Under sintering at a temperature of 1400-1800 ° C. for several hours at a pressure of about 300-600 kgf / cm ² . Thereby, a sintered body having a relative density of 99% or more can be easily obtained. Also in the sintering step, it is preferable to use a die made of a non-ferrous material or a hot press die. Specifically, nonferrous materials such as beryllium copper alloy, zinc alloy, carbon, TiN coating, and DLC (Diamond Like Carbon) coating can be used as the mold, and nonferrous materials such as carbon can be used as the hot press mold. Further, when a metal material is used, for example, after molding the mold, the surface of the molded body that has been in contact with the mold is scraped so that the Fe component is not mixed.

In the HIP method, a capsule-free method, or a pre-baked powder is placed in a metal capsule or glass capsule such as W or Mo and vacuum-sealed, and a temperature of 1200 to 1800 ° C for several hours, 1000 to 2000 kgf / cm ² Sinter at a moderate pressure. At this time, the incorporation of Fe can be prevented by adopting a capsule-free method or a method using a glass capsule. In the case of a metal capsule, it is desirable to use a metal other than Fe, such as tungsten or molybdenum. Moreover, when using a Fe-type capsule, it is preferable to use what coated the inner surface. Examples of the coating material include zinc, tin, and chromium.

In the normal pressure sintering method, the calcined powder is subjected to cold isostatic pressing (CIP) at a pressure of about 3000 kgf / cm ² and then sintered for several to several tens of hours in an atmosphere containing oxygen at 1400 to 1800 ° C. Do the tie. Also in this case, as in the hot press method, a non-ferrous material is used as the mold or press mold, or the surface of the molded body that has been in contact with the mold after molding is scraped so that Fe is not mixed. .

  The oxide phosphor of the present invention may be annealed for about 2 to 20 hours in an atmosphere containing oxygen at 1000 ° C. to 1600 ° C. after the above-described sintering step. As a result, effects such as improved light emission output, reduced afterglow, and improved transparency can be obtained.

In this way, the Fe content in the phosphor is limited to 35 wtppm or less, preferably 30 wtppm or less by limiting the amount of Fe contained in the raw material powder and suppressing the mixing of Fe in the calcining step and the sintering step. A ceramic scintillator can be obtained.
The garnet phosphor (sintered body) of the present invention thus obtained is a material in which the mother crystal mainly has a garnet structure and the main peak of the emission spectrum exists at about 535 nm, and light emission based on CdWO ₄ Afterglow (% attenuation) after 30 ms after passing through the excitation source (X-ray) is not less than 0.06, and afterglow (% attenuation) after 300 ms is 0.005 or less. Become.

  ADVANTAGE OF THE INVENTION According to this invention, the oxide fluorescent substance by which the mixing amount of Fe was suppressed to very small amount is provided. By using this oxide phosphor as a scintillator, a radiation detector having a high light emission output and a stable short afterglow can be obtained. By using such a radiation detector, a high-quality X-ray CT apparatus can be obtained even at high-speed scanning.

  Hereinafter, embodiments of the radiation detector of the present invention will be described. FIG. 1 shows a cross-sectional view of the radiation detector of the present embodiment. This radiation detector is a single slice type detector in which a plurality of detection elements are arranged in a one-dimensional direction or a multi-slice type detector in which a plurality of detection elements are arranged in a two-dimensional direction. A scintillator element 11 is arranged on the diode array substrate 14 corresponding to each photodiode position. The surface of each scintillator element 11 not facing the photodiode 13 is covered with the light reflecting layer 12. The light reflecting layer 12 is made of a material that transmits X-rays and reflects light, such as titanium oxide or aluminum.

As the scintillator element 11, the above-described ceramic scintillator of the present invention is used. As the photodiode 13, it is desirable to use a PIN photodiode. Since this photodiode has a high sensitivity, a high response speed, and a wavelength sensitivity from the visible light to the near infrared region, matching with the emission wavelength of the scintillator of the present invention is good. For the light reflecting layer 12, it is preferable to use an organic resin mixed with TiO ₂ powder, which has low X-ray absorption and high light reflectance.

  In such a radiation detector, X-rays incident from the direction indicated by the arrow in the figure are transmitted through the light reflecting layer 12 and absorbed by the scintillator element 11. The scintillator element 11 that has absorbed X-rays emits visible light. Visible light emitted from the scintillator element 11 is reflected by the light reflecting layer 12 and converted into an electric signal by the photodiode 13. Here, by using the ceramic scintillator of the present invention for the scintillator element 11, a radiation detector with high output and low afterglow can be obtained.

  FIG. 2 shows an outline of an embodiment of the X-ray CT apparatus of the present invention. The apparatus includes a scan gantry unit 210 and an image reconstruction unit 220. The scan gantry unit 210 is mounted on a rotating disk 211 having an opening 214 into which a subject is carried, and the rotating disk 211. An X-ray tube 212, a collimator 213 attached to the X-ray tube 212 for controlling the radiation direction of the X-ray bundle, an X-ray detector 215 mounted on the rotating disk 211 facing the X-ray tube 212, A detector circuit 216 that converts the X-rays detected by the X-ray detector 215 into a predetermined signal, and a scan control circuit 217 that controls the rotation of the rotating disk 211 and the width of the X-ray bundle are provided.

  The image reconstruction unit 220 includes an input device 221 for inputting a subject's name, examination date and time, examination conditions, etc., and an image computation circuit that performs CT image reconstruction by computing the measurement data S1 sent from the detector circuit 216. 222, an image information addition unit 223 for adding information such as a subject name, examination date and time, and examination conditions input from the input device 221 to the CT image created by the image calculation circuit 222, and image information added And a display circuit 224 that adjusts the display gain of the CT image signal S2 and outputs it to the display monitor 230.

  In this X-ray CT apparatus, X-rays are irradiated from the X-ray tube 212 in a state where the subject is placed on a bed (not shown) installed in the opening 214 of the scan gantry unit 210. The X-rays have directivity by the collimator 213 and are detected by the X-ray detector 215. At this time, by rotating the rotating disk 211 around the subject, the X-ray transmitted through the subject is detected while changing the direction of X-ray irradiation. Based on the measurement data, a tomographic image is created by the image reconstruction unit 220 and displayed on the display monitor 230.

  Here, the X-ray detector 215 has a large number (for example, 960) of detector elements in which scintillators and photodiodes are combined arranged in an arc shape. Many X-ray detectors 215 are configured by arranging a large number of slice detectors in a polygonal shape. The scintillator constituting each detector element is composed of the oxide phosphor having a high light emission output and a short afterglow of the present invention. Therefore, the X-ray CT apparatus of the present invention can provide a high-quality CT tomogram even in high-speed scanning.

  Examples of the present invention will be described below.

<Example 1>
Gd ₂ O ₃ , Al ₂ O ₃ , Ga ₂ O ₃ , and CeO ₂ powders each having a purity of 99.99% and a particle size of about 0.1 to 5 μm were prepared as starting material powders. Gd ₂ O ₃ , Ga ₂ O ₃ , and CeO ₂ powders having Fe contents of 5 wtppm, 4 wtppm, and 5 wtppm, respectively, were used. As the Al ₂ O ₃ powder, five types (3 wtppm, 5 wtppm, 10 wtppm, 20 wtppm, 50 wtppm) having different Fe contents in the range of 3 to 50 wtppm were used. These raw material powders were weighed so as to have a composition of Gd _2.982 Ce _0.018 Al _2.8 Ga _2.2 O ₁₂ . The weighed raw material powder was put into a polyethylene container together with alumina balls and ion-exchanged water, and mixed with a ball mill for about 16 hours. After mixing, the mixture was transferred to an evaporating dish and the mixed powder was dried and sized through a nylon sieve. The sized powder was placed in an alumina crucible and calcined in oxygen at 1500 ° C. for 4 hours. The scintillator calcined powder, after molding at a pressure of 500kgf / cm ^2, 1500 ℃ in vacuum and hot-press sintering at 500 kgf / cm ^2. The sintered body was processed into a 30 × 24 × 2 mm test piece and annealed in oxygen at 1300 ° C. for 4 hours to complete a ceramic scintillator plate.

Luminescence and afterglow when the obtained scintillator plate was irradiated with X-rays having a tube voltage of 120 kV and a tube current of 100 mA were measured with a PIN photodiode. The results are shown in the graph of FIG. Here, the light emission output is expressed as a relative output when the light emission output of the CdWO ₄ scintillator is 1. Afterglow was expressed as the ratio (%) of the output 300 ms after blocking X-rays to the output during X-ray irradiation.

As can be seen from the results shown in FIG. 3, the light emission output hardly changed even when the Fe content in the phosphor was changed, but the afterglow increased with an increase in the Fe content. In particular, when the Fe content in the phosphor exceeds 35 wtppm (more than ₂₀ wtppm in the Al ₂ O ₃ powder), the afterglow exceeds 0.004%. When applied to a radiation detector for X-ray CT, it is a CT image. Image quality will be reduced. On the other hand, when the Fe content was 31 wtppm or less (5 wtppm or less in the Al ₂ O ₃ powder), the afterglow was 0.001%, which was extremely low afterglow as a radiation detector for X-ray CT.

  According to the present invention, it is possible to provide a ceramic scintillator having high light emission output and low afterglow. This ceramic scintillator can be suitably applied to a radiation detector, particularly a radiation detector for an X-ray CT apparatus, and can provide a high-quality image even at high-speed scanning.

Sectional drawing which shows one Embodiment of the radiation detector of this invention. The figure which shows the structure of the X-ray CT apparatus of this invention. The figure which shows the relationship between Fe content of an oxide fluorescent substance, and luminescent property.

Explanation of symbols

11 ... scintillator element, 12 ... reflective layer, 13 ... photodiode, 210 ... scan gantry, 211 ... rotating disk, 212 ... X-ray tube, 214 ... opening 215 ... X-ray detector, 216 ... detector circuit, 217 ... scan control circuit, 220 ... image reconstruction unit, 221 ... input device, 224 ... display circuit, 230: Display monitor.

Claims (6)

The Ce and emitting elements, Gd, comprises Al, Ga, and O, and a predominantly Ri Do from the host crystal of garnet structure, the radiation detector oxide phosphor satisfying any of 3 to no condition 1 below, Fe The content of is 35 wtppm or less with respect to the phosphor weight, and the afterglow at 300 ms after blocking the excitation light (ratio of emission output to emission output upon excitation light irradiation) is 0.005 or less. An oxide phosphor characterized by the following.
Condition 1: Formula _{(Gd 1-zx L z Ce} x) 3 Al 5-y Ga y O 12 ( wherein, L is the .z representing a La or Y 0 ≦ z <0.2, x is 0.0005 ≦ x ≦ 0.015, y is a value in the range of 0 <y <5).
Condition 2: The atomic ratio of Gd / (Al + Ga + Gd) is 0.33 or more and 0.42 or less (except for (Gd + Ce) / (Al + Ga + Gd + Ce) = 0.375).
Condition 3: The atomic ratio of (Gd + Ce) / (Al + Ga + Gd + Ce) is greater than 0.375 and 0.44 or less, and the atomic ratio of Ce / (Ce + Gd) is 0.0005 or more and 0.02 or less.   2. The oxide phosphor according to claim 1, wherein the Fe content contained in each raw material powder for synthesizing the oxide phosphor is 20 ppm by weight or less based on the weight of each raw material powder. An oxide phosphor.   3. The oxide phosphor according to claim 1, wherein the base crystal has a perovskite structure as a second phase.   The oxide phosphor according to any one of claims 1 to 3, wherein afterglow at 300 ms after blocking excitation light (percentage of emission output with respect to emission output upon excitation light irradiation) is 0.004 or less. An oxide phosphor characterized by being.   5. The radiation detector comprising a phosphor element that emits light by radiation and a photoelectric conversion element that detects light emitted by the phosphor element, wherein the phosphor element is oxidized according to claim 1. A radiation detector characterized by being a fluorescent substance.   A radiation source, a radiation detector arranged opposite to the radiation source, a rotating disk that holds the radiation source and the radiation detector and is driven to rotate around the subject, and is detected by the radiation detector An X-ray CT apparatus comprising image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of the emitted radiation, wherein the radiation detector according to claim 5 is used as the radiation detector. X-ray CT system characterized by

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