JP2001004753A - Oxide phosphor and radiation detector using it as well as x-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an oxide phosphor whose luminous efficiency with reference to X-rays is increased, whose afterglow is reduced extremely and which can acquire a high-resolution and high-quality tomogram by forming a garnet structure which is constituted of at least elements as Gd, Ce, Al, Ga and O. SOLUTION: In this oxide phosphor which is used for an X-ray CT apparatus or the like, a garnet structure in which Gd at a site (c) is substituted for Ga and/or holes and in which Ga and Al at a site (a) and a site (d) are substituted mutually and substituted for Gd and/or holes is used as a mother crystal, and Ce is used as a luminous component. When the atomic ratio of Ga/(A+Ga+Gd) is at 0.33 to 0.42, a good characteristic is obtained, Ge is set at a molecular ratio of 0.05 to 2.0% with reference to Gd. However, a case in which (Gd+ Ce)/(Al+Ga+Gd+Ce) is at 0.375 is excluded. A heterogeneous amount other than in the garnet structure is less than 20 wt.%, a relative density is 99.0% or higher, a luminous main peak is near 550 nm, and an afterglow attenuation rate after 30 ms of an excitation stop is 10-3 or lower.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide phosphor suitable for use as a radiation detector for detecting X-rays, .gamma.-rays, etc., and more particularly to a radiation detector such as an X-ray CT apparatus or a positron camera, and uses the same. The present invention relates to a radiation detector and an X-ray CT device.

[0002]

2. Description of the Related Art Conventionally, as a radiation detector used in an X-ray CT apparatus, a xenon gas chamber, a combination of bismuth germanate (BGO single crystal) and a photomultiplier, a CsI: Tl single crystal or CdWO are used. Combinations of _four single crystals and photodiodes have been used. In recent years, rare earth phosphors having high conversion efficiency from radiation to light have been developed, and radiation detectors combining such phosphors and photodiodes have been put to practical use.
Rare earth phosphors are those to which an activator, which is a light emitting component based on a rare earth oxide or a rare earth oxysulfide, is added, and is described as a rare earth oxide phosphor in JP-A-3-50991 and the like. A material using yttrium oxide and gadolinium oxide as a base material has been proposed.

[0003]

Generally, characteristics required for a scintillator material used for a radiation detector include high luminous efficiency, short afterglow, and high X-ray stopping power. Some of the above phosphors have high luminous efficiency, but the afterglow time is relatively long. In the application of the X-ray CT apparatus, if the afterglow of the scintillator used in the X-ray detector is large, the obtained information becomes unclear in the time axis direction. In the above conventional materials, afterglow is relatively large as a characteristic of the scintillator material.

An object of the present invention is to provide a phosphor having a high luminous efficiency with respect to X-rays and an extremely low afterglow. In addition, by using this phosphor as a scintillator of a radiation detector having a photodetector, a low afterglow radiation detector having a large light output can be obtained, and this X-ray detector can be applied to an X-ray CT apparatus. Accordingly, the present invention provides a high-resolution and high-quality tomographic image.

[0005]

The phosphor according to the present invention has a garnet structure composed of at least Gd, Ce, Al, Ga and O elements, and has a different phase content other than the garnet structure of 2.
Less than 0 wt%, relative density 99.0% or more, diffuse transmittance 50.0
% Or more of oxides. The oxide has a main peak of the emission spectrum at around 550 nm, the decay rate of afterglow 30 ms after the excitation light is cut off is 10 ^-3 or less, the luminous efficiency is high, and the phosphor with little afterglow is used. The inventors have found that they can be obtained, and have reached the present invention.

FIG. 1 shows the arrangement of metal ions in the garnet structure of the present invention. Garnet-structured metal ions have three types of c, a, and d sites. The size of the site is c>a> d, and from the ion size alone, Ce ³⁺ = 1.07Å and Gd ³⁺ = 0.97
Å, Ga ³⁺ = 0.62Å, Al ³⁺ = 0.51Å, so the c site is C
e ³⁺ and Gd ³⁺ , a site is Ga ³⁺ , and d site is Al ³⁺ , respectively.In this case, the chemical composition is Gd ₃ (Ga ₂ Al ₃ ) O
Represented by ₁₂ . For garnet structures of various compositions consisting of Gd, Ce, Al, Ga, and O elements, crystallographic studies based on the X-ray diffraction method were performed using Rietveld analysis.

As a result, the garnet structure composed of these constituent elements has a stoichiometric composition of (Gd, Ce) ₃ (G
a, Al) ₅ O ₁₂ composition, Gd at c site is replaced by Ga and / or vacancy, Ga and Al at a, d site not only replace each other, but also Gd and / or vacancy It has been clarified that the substitution has been made and the oxygen site is also deficient. Therefore, the actual oxide composed of the elements Gd, Ce, Al, Ga, and O is strictly a stoichiometric composition formula (Gd, Ce) ₃
It cannot be represented by (Ga ₂ Al ₃ ) O ₁₂ .

Based on these results, various compositions of oxide phosphors having a garnet structure composed of elements of Gd, Al, Ga, and O as a mother crystal and Ce as a light emitting component were examined. As a result, Gd / ( When the atomic ratio of (Al + Ga + Gd) was 0.33 or more and 0.42 or less, good X-ray sensitivity and afterglow characteristics were obtained. Ce has an atomic ratio of 0.05% or more and 2.0% or less with respect to Gd, and if it is out of the range of the composition, sufficient luminescence intensity cannot be obtained. However, (Gd +
Except when Ce) / (Al + Ga + Gd + Ce) is 0.375.

The reason why the composition of the present invention is essential is that Gd /
If the atomic ratio of (Al + Ga + Gd) is less than 0.33, the amount of Gd is too small compared to the garnet composition, and a heterogeneous phase such as Ga ₂ O ₃ is generated in the garnet structure matrix, while the amount of Gd is Is 0.42
By weight, of the perovskite crystal structure Gd (Ga, Al) 0 ₃ and G
A d ₄ (Ga, Al) ₂ O ₉ phase forms in the garnet matrix. In any of these cases, even when the relative density is 99.0% or more, the amount of the different phase becomes 2.0 wt% or more and the diffusion transmittance becomes less than 50.0%. The atomic ratio of Ga / Al is 0.20
If it is less than 1, the crystal structure becomes a garnet structure single phase, but sufficient light emission intensity cannot be obtained even with Ce doping, and sufficient light emission intensity is obtained even if the atomic ratio of Ga / Al exceeds 4.0. This is because it cannot be done.

[0010] The phosphor of the present invention is not particularly limited to a crystal form, and may be a single crystal or a polycrystal. However, a polycrystal is desirable in view of easiness of production and less variation in characteristics. . The phosphor material can be obtained from the polycrystal through 1) a synthesis process of a powder used as a raw material for scintillation, and 2) a sintering process using the powder. In order to obtain a desired oxide having a garnet structure single phase, the crystal grain size of the synthetic powder is preferably as small as possible, and desirably 1.0 μm or less.

[0011] Powders can be synthesized by 1) a method mainly based on an ordinary oxide mixing method, 2) a method via a liquid phase such as a coprecipitation method or a sol-gel method, and 3) a further method based on an oxide mixing method. A method of mechanically refining the synthesized powder again,
There is.

In a usual oxide mixing method, it can be produced, for example, as follows. Gd ₂ O ₃ , Ce ₂ O ₃ , Al ₂ O as raw material powder
₃ and Ga ₂ O ₃ are weighed in a predetermined amount, and then wet-mixed with an automatic mortar or the like for about 30 minutes. 1400 ° C
It is baked for several hours in the air at 11700 ° C. to produce a scintillator synthetic powder. If necessary, K ₂ SO as flux
Gd- using potassium compounds such as _{4 and} fluorides such as BaF ₂
The formation of a Ce-Al-Ga-O-based garnet structure can be promoted.

In a process using the coprecipitation method, for example, the following synthesis can be performed. A predetermined amount of gadolinium nitrate, alumina nitrate, gallium nitrate, and cerium nitrate were weighed to form a composite nitrate aqueous solution, and the total amount of metal ion concentration was 15%.
Twice the amount of urea coexists with sulfate ions. This aqueous solution
Heat to 70-100 ° C to hydrolyze urea, Gd-Ce-Al-Ga-O
Precipitate the precursor. Washing of the precipitate is repeated to reduce the concentration of irrelevant anions in the precipitate to less than 1000 ppm, followed by drying at about 120 ° C. and calcination at about 1200 ° C. to obtain a scintillator powder. Drying temperature, moisture evaporates
The temperature may be 90 ° C. or higher, and the calcining temperature may be 900 ° C. or higher at which a garnet structure is generated. Heat treatment at a high temperature at which the generated garnet structure crystal grains grow must be avoided.

The raw materials for the coprecipitation method are not limited to nitrates.
Hydrochlorides, sulfates, oxalates, and the like of various metals can also be used. In some cases, these metal salts may be used in combination of several types. Also, ammonium bicarbonate can be used instead of urea.

Further, as another method, 1) urea and ammonium lactate, 2) ammonium bicarbonate and aqueous ammonia, or 3) ammonium sulfate and aqueous ammonia, 4) hydrogen peroxide as a masking agent The Gd-Ce-Al-Ga-O precursor can also be precipitated by adding water, aqueous ammonia, ammonium sulfate, or the like.

Further, mechanically pulverizing the raw material powder is also a method suitable for obtaining a garnet structure by sintering. That is, similarly to the above-described oxide mixing method, a predetermined amount of the constituent metal component oxide is weighed, and then mixed in an automatic mortar for about 30 minutes. After calcining the mixed powder at a temperature of about 1500 ° C., mechanical pulverization is performed. A pulverization method with higher pulverization energy, such as a ball mill, more preferably a planetary ball mill, is preferred. As a result, the powder particle size is about 0.01 to 0.5 μm
Powder of a certain degree can be easily obtained.

The sintering of the powder synthesized in this manner can be performed by a hot press method, a HIP method, a normal pressure sintering method, or a combination of the normal pressure sintering method and the HIP method. In the hot press method, after the above-mentioned synthetic powder is molded into a molded body at a pressure of about 600 kgf / cm ² and set in a hot press mold, in a vacuum, in the air, or in an atmosphere of oxygen, 1400
Sintering is performed at a sintering temperature of 1 to 700 ° C. for several hours with a pressure of about 300 kgf / cm ² . This makes it possible to easily obtain a phosphor having a relative density of 99.0% or more. On the other hand, in the HIP method,
The synthetic powder is put in a metal capsule made of iron or W, Mo, or the like, sealed in a vacuum, and sintered at a temperature of about 1400 ° C. and a pressure of about 2000 atm.

In the normal pressure sintering method, the synthetic powder is 600 kgf
/ After molding in cm ^2, a pressure, after isostatic pressing at 3000 kgf / cm ^2, the pressure (CIP), from 1,600 to 1700 ° C.
Sintering is performed for several to several tens of hours in the atmosphere before and after or in pure oxygen. If the temperature exceeds 1700 ° C., the sample dissolves. If the temperature is lower than 1600 ° C., the sintered density becomes about 90%, which is not sufficient. Atmosphere in air or pure oxygen is preferred, Ar, in an inert atmosphere or vacuum atmosphere such as N ₂ can not reduce the voids in the sintered body.

In order to obtain a sintered body having a relative density of 99.0% or more by the normal pressure sintering method, 1) a powder to which a sintering aid is added is used as a synthetic powder. Is used. In addition, if a material with a relative density of about 93.0% or more is obtained by normal pressure sintering, it will be closed pores. If necessary, a capsule-free HIP method that does not require metal capsules is added, so that the relative density is 99.0% or more. Can easily be obtained.

According to the present invention, it is possible to obtain an oxide phosphor having a matrix having a garnet structure, a crystal phase other than 2.0% by weight, a relative density of 99.0% or more, and a diffusion transmittance of 50.0% or more. . Since this phosphor has a high luminous output and extremely low afterglow, it is suitable for a radiation detector for detecting X-rays and the like, in particular, a radiation detector such as an X-ray CT apparatus and a positron camera.

The radiation detector of the present invention includes a ceramic scintillator and a photodetector for detecting light emission of the scintillator, and uses the above-described phosphor as the ceramic scintillator. PI as photodetector
Use N-type diode. This photodiode has a high sensitivity, a fast response time, and a wavelength sensitivity in a range from visible light to near-infrared light, so that the matching with the emission wavelength of the phosphor of the present invention is good.

Further, the X-ray CT apparatus of the present invention comprises an X-ray source, an X-ray detector arranged opposite to the X-ray source, and an X-ray source which holds the X-ray source and the X-ray detector. An X-ray CT apparatus comprising: a rotating body driven to rotate around the X-ray detector; and an image reconstruction unit configured to form a tomographic image of the subject based on the intensity of the X-ray detected by the X-ray detector. As the detector, a radiation detector combining the above-described phosphor and photodiode is used. By using this X-ray detector, X-rays can be detected with high detection efficiency, so that the sensitivity can be improved about twice as compared with an X-ray CT apparatus using a conventional scintillator (for example, CdWO ₄ ), and afterglow is reduced. Since the number is extremely small, high-quality and high-resolution images can be obtained.

[0023]

Embodiments of the present invention will be described below.

(Example 1) Gd ₂ O ₃ , Ce ₂
Sample Nos. 1 to 6 were manufactured using O ₃ , Al ₂ O ₃ , and Ga ₂ O ₃ . The powder was weighed in atomic ratio as shown in Table 1 and wet-mixed with an automatic mortar or the like for 30 minutes using ethanol.
To this mixed powder, K ₂ SO ₄ is added as a flux component,
The mixture was filled in an alumina crucible and fired in an atmosphere at 1650 ° C. for 3 hours.

In order to remove the flux component, the powder was washed with pure water six times to obtain a synthetic powder for scintillator. This synthetic powder was molded into a molded body at a pressure of 600 kgf / cm ² , and then set in a hot press die, and at a pressure of 300 kgf / cm ² at 1550 ° C. for 3 hours in a vacuum atmosphere. For hot press sintering. The relative densities were all over 99.9%. These sintered bodies were annealed in air at 1300 ° C. for 3 hours, and then machined to a thickness of 1.8 mm to produce a ceramic scintillator. The amount of the different phase and the diffuse transmittance of this sample were measured using an X-ray diffractometer and a spectrophotometer. In addition, a detector was made by combining these samples with a photodiode, and an X-ray source (120 kV, 150 mA) was used.
The detector was placed at a distance of 0 cm, and the emission intensity and the afterglow were evaluated. The luminescence intensity was a relative value when the value of CdWO ₄ was set to 1, and the afterglow was represented by an attenuation rate 30 ms after the X-ray was cut off. It can be seen that the scintillator having the composition of Example 1 has high emission intensity and has excellent scintillator characteristics with little afterglow.

[Table 1]

Example 2 Sample Nos. 7 to 19 shown in Table 2 were prepared using gadolinium nitrate, aluminum nitrate, gallium nitrate, and cerium nitrate as raw material powders. Each powder was weighed to make a 500 cc aqueous solution of a composite nitrate. Add urea equivalent to 14.5 times the total amount of metal ion concentration, and use concentrated sulfuric acid to allow 1.2 times the amount of sulfate ions to coexist.
Then, the mixture was reacted for 6 hours with stirring and then cooled to room temperature. The Gd-Ce-Al-Ga-O precursor precipitated in this way is subjected to filtration and washing cycles six times, and then 15 g.
Dry at 0 ° C. for 12 hours. The sulfate ion concentration in the obtained precipitate was 900 ppm or less in all samples. This precipitate was calcined in the air at 1200 ° C. for 3 hours to produce a synthetic powder. When observed by SEM, the average primary particle diameter of all samples was around 0.1 μm. 600 kgf / cm ^{2 of} these synthetic powders was
After pressing at a pressure of 3000 kgf / cm ² , isostatic pressing (CIP) was performed, and then normal pressure sintering was performed in pure oxygen at 1650 ° C. for 3 hours. In the same manner as in Example 1, mechanical processing was performed to obtain a scintillator wafer, and then material properties such as the amount of different phases and diffuse transmittance, and characteristics as a scintillator such as light emission intensity and afterglow were evaluated.

[Table 2]

As in the case of Example 1, it can be seen that the material composition of Example 2 exhibits excellent characteristics in both emission intensity and afterglow. Similar results were obtained when a precursor precipitated using ammonium bicarbonate was used instead of urea. In the composite metal solution, 1) urea and ammonium lactate, 2) ammonium bicarbonate and aqueous ammonia,
Alternatively, powders synthesized by adding 3) ammonium sulfate and ammonia water and 4) aqueous ammonia as a masking agent to aqueous ammonia and ammonium sulfate, etc., can obtain submicron-sized powders, and have good scintillator characteristics. Indicated.

(Example 3) Samples Nos. 20 to 31 were manufactured using the same oxide powder of the constituent metal components as in Example 1.
After weighing the powder, it was mixed for about 30 minutes in an automatic mortar. This mixed powder was calcined in air at 1500 ° C. for 3 hours. This calcined powder was dry-pulverized for 30 minutes using an automatic mortar made of alumina to obtain a powder of 0.5 mm or less. This powder was further mechanically pulverized using a centrifugal ball mill. ZrO ₂ balls were used as grinding media. The average particle size of the pulverized powder was almost the same for all samples, and was about 0.08 μm. Since there is concern that ZrO ₂ may be mixed into this powder, a chemical analysis of the No. 22 sample revealed that the Zr content was 0.002 wt%.
Thus, the amount of impurities mixed was a very small value.
Using these synthetic powders, a molded body was produced in the same manner as in Example 2, and a scintillator material was produced by normal-pressure sintering. Table 3 shows the results. All of the materials in the composition range of Example 3 exhibited good scintillator characteristics.

[Table 3]

(Embodiment 4) FIG. 2 shows an example of an X-ray detector using the scintillator of the present invention. The scintillator 11 is adhered to the photodiode 13 and further covered with a shield 12 for preventing the light emitted from the scintillator from escaping to the outside. The shield 12 is made of a material that transmits X-rays and reflects light, such as aluminum. When the scintillator 11 of the present invention absorbs X-rays, the scintillator 11 has a higher luminous output than the conventional scintillator, and has a wavelength of 550 nm which is relatively close to the sensitivity wavelength of the Si photodiode.
Since it has an emission peak in the vicinity, it was photoelectrically converted by the photodiode with high efficiency. Further, the afterglow was extremely small as compared with the conventional scintillator, and exhibited excellent characteristics as an X-ray detector.

(Embodiment 5) FIG. 3 schematically shows an X-ray CT apparatus according to the present invention. This device consists of a gantry section 18 and an image reconstruction section.
The gantry section 18 includes a rotating disk 19 having an opening 20 into which a subject is carried, an X-ray tube 16 mounted on the rotating disk, and an X-ray tube attached to the X-ray tube. A collimator 17 for controlling the radiation direction of the X-ray, an X-ray detector 15 mounted on a rotating disk facing the X-ray tube, and converting the X-ray detected by the X-ray detector 15 into a specific signal. It comprises a detector circuit 21 and a scan control circuit 24 for controlling the rotation of the rotating disk and the width of the X-ray flux. X-rays are emitted from an X-ray tube in a state where the subject lies on a bed set in the opening 20. The X-rays have directivity obtained by a collimator, and X-rays transmitted through the subject are detected by an X-ray detector. By rotating the rotating disk around the subject, X-rays are detected while changing the X-ray irradiation direction, and a tomographic image is created by the image reconstruction unit 22 and displayed on the monitor 23.

[0031]

According to the present invention, it is possible to provide a phosphor having a high luminous efficiency with respect to X-rays and an extremely low afterglow. Further, by using this phosphor as a scintillator of a radiation detector having a photodetector, a low-persistence radiation detector having a large light output can be obtained. By applying this X-ray detector to an X-ray CT apparatus, , High-resolution, high-quality tomographic images can be obtained.

[Brief description of the drawings]

1A is a layout diagram of metal ions having a garnet structure, and FIG. 1B is a layout diagram of oxygen around metal ions having a garnet structure.

FIG. 2 is a schematic diagram of an X-ray detector using the scintillator of the present invention.

FIG. 3 is a schematic diagram of an X-ray CT apparatus according to the present invention.

[Explanation of symbols]

 1 ... c site in garnet structure 2 ... a site in garnet structure 3 ... d site in garnet structure 4 ... oxygen ions 11 ... scintillator 12 ... shielding plate 13 ... photodiode 16 ... X-ray tube 17 ... collimator 18 ... gantry 19 ... rotation Disk 20 ... Opening 21 ... Detector circuit 22 ... Image reconstruction unit 23 ... Monitor 24 ... Scan control circuit

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takaaki Furuhiki 1-1-1 Uchikanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Corporation (72) Inventor Shoki Yamada 1-1-1, Uchikanda, Chiyoda-ku, Tokyo No. 14 F-term in Hitachi Medical Corporation (reference) 2G088 EE02 FF02 GG10 JJ37 4H001 CA08 XA08 XA13 XA31 XA58 XA64

Claims (7)

[Claims] 1. An oxide phosphor comprising at least an element composed of Gd, Ce, Al, Ga, and O, wherein the crystal structure of the oxide is a garnet structure. 2. The oxide phosphor according to claim 1, wherein the amount of the hetero phase other than the garnet structure is less than 2.0 wt% and the relative density is 9%.
An oxide phosphor having a diffuse transmittance of at least 9.0% and a diffuse transmittance of at least 50.0%. 3. The oxide phosphor according to claim 1, wherein the main peak of the emission spectrum exists near 550 nm, and the decay rate of afterglow 30 ms after the excitation light is cut off is 10 ⁻³ or less. An oxide phosphor characterized by the following. 4. The oxide phosphor according to claim 1, wherein an atomic ratio of Gd / (Al + Ga + Gd) is 0.33 or more and 0.42 or less. However, Ce is 0.05% or more and 2.00% or less in atomic ratio with respect to Gd, and (Gd + C
e) Except for the composition in which the atomic ratio of (Al + Ga + Gd + Ce) is 0.375. 5. The oxide phosphor according to claim 1, wherein an atomic ratio of Ga / Al is 0.20 or more and 4.0 or less. 6. A radiation detector comprising a ceramic scintillator and a photodetector for detecting light emission of the scintillator, wherein the oxide phosphor according to claim 1 is used as the ceramic scintillator. A radiation detector, characterized in that: 7. An X-ray source, an X-ray detector placed opposite to the X-ray source, holding the X-ray source and the X-ray source detector, and driven to rotate around a subject. A rotating disk and the X
Image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of the X-rays detected by the X-ray detector.
7. An X-ray CT apparatus, wherein the radiation detector according to claim 6 is used as the X-ray detector in the T apparatus.