JP2001181043A - Transparent polycrystalline garnet scintillator, powder for scintillator and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide ceramic scintillator having a high emission intensity, a low decay time constant and low afterglow at a relatively low cost in order to improve the resolution of an X ray computed tomography(CT). SOLUTION: This scintillator is characterized in that the scintillator is represented by the general formula Gd3-x-yCexYyAl5O12 (wherein (x) and (y) are each within the range of 0.001<=x<=0.05 and 0.7<=y<=2) and is produced by adding BaF2 as a flux in an amount of 0.05-1.5 mol based on 1 mol of garnet thereto, mixing both, then calcining the resultant mixture at 1,400-1,600 deg.C, producing a garnet scintillator powder, further adding 10-1,000 ppm of SiO2 as a sintering aid during sintering, press forming the obtained mixture and then sintering the press formed compact at a temperature within the range of 1,600-1,800 deg.C in a vacuum or a hydrogen, a nitrogen or an argon atmosphere or carrying out the hot isostatic press sintering at 1,400 to 1,600 deg.C temperature in an argon atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scintillator used for a radiation detector for detecting X-rays, and more particularly to a low-cost scintillator which does not contain expensive Ga applied to a radiation detector used for X-ray CT. The present invention relates to a high-performance scintillator, a scintillator powder, and a method for producing the same.

[0002]

2. Description of the Related Art As one of X-ray diagnostic apparatuses, there is a computed tomography apparatus (hereinafter referred to as CT). This CT is composed of an X-ray tube that irradiates a fan-shaped fan beam X-ray and an X-ray detector with a number of X-ray detection elements arranged opposite to the center of the tomographic plane of the subject. X-ray absorption data is collected by irradiating a fan beam X-ray from the X-ray tube and changing the angle, for example, by 1 degree with respect to the tomographic plane each time irradiation is performed. Is analyzed by a computer to calculate the X-ray absorptance at each position of the tomographic plane, and constructs an image corresponding to the absorptivity.

Conventionally, a xenon gas detector has been used for this CT. This xenon gas detector encloses xenon gas in a gas chamber, applies voltage between a large number of arranged electrodes, and irradiates x-rays. When x-rays ionize xenon gas, a current signal corresponding to the intensity of x-rays Can be taken out, thereby forming an image. However, this xenon gas detector requires a thick window to enclose high-pressure xenon gas in the gas chamber, and thus has a problem that the use efficiency of X-rays is low and the sensitivity is low. In addition, in order to obtain a high-resolution CT, it is necessary to reduce the thickness of the electrode plate as much as possible. If the electrode plate is thinned in such a manner, there is a problem that the electrode plate vibrates due to external vibration and noise is generated. .

[0004]

On the other hand, CdWO ₄ single crystal and (Y, Gd) ₂ O ₃ : Eu, Pr and Gd ₂ O
_{2 S:} Pr, Ce, a detector that combines ceramic scintillator and a silicon photodiode F composition has been developed and put to practical use. In addition, as a description method of the notation of the fluorescent substance, in the composition formula, the base material was described on the left side and the luminescent ion was described on the right side with ":". In a detector using these materials, it is easy to reduce the size of the detection element and increase the number of channels, so that it is possible to obtain an image with higher resolution than a xenon gas detector. The characteristics required of the X-ray CT scintillator include (1) a high emission intensity with respect to X-rays. (2) The decay time constant of light emission and the afterglow are small. (Here, the decay time constant of light emission is a time until X-ray irradiation is stopped and the light emission intensity becomes 1 / e of the light emission intensity during X-ray irradiation. (The ratio indicates the ratio of the emission intensity after a certain period of time to the emission intensity during X-ray irradiation.) (3) The uniformity of the material is high and the variation in X-ray characteristics is small. (4) Radiation degradation is small. (5) The light emission characteristics should not change much with changes in environment such as temperature. (6) Good workability and little processing deterioration. (7) No chemical absorption and no deliquescent. Is mentioned.

In recent years, there has been a demand for further improvement in the resolution of CT, and it is necessary to reduce the size of the detection element and to shorten the scanning time in order to reduce the influence of body movement. In this case, the X-ray dose incident on one element is reduced.
The CdWO ₄ single crystal scintillator has a low light emission intensity, and there remains a problem in meeting this demand. In addition, there is a problem that the workability is poor due to the cleavage property, and that it contains a highly toxic ion called Cd. On the other hand, (Y, Gd)
_The 2O ₃ : Eu, Pr ceramic scintillator has a large decay time constant, and there is still a problem in reducing the scanning time. The Gd ₂ O ₂ S: Pr, Ce, F ceramic scintillator has a problem that the manufacturing process is complicated and the cost is high because it is not an oxide. X-ray CT has entered the diffusion period, and cost reduction is important. Existing scintillators do not sufficiently satisfy these requirements. The present invention has been made in view of the above conventional problems, and has an object to provide an oxide ceramic scintillator having a large emission intensity, a small attenuation time constant and afterglow, and a relatively low cost. I do.

[0006]

Means for Solving the Problems In order to achieve the above object, the present inventors have studied various oxide composition systems. As a result, a (Gd, Y) ₃ Al ₅ O ₁₂ : Ce garnet composition having a high luminous intensity was found. C
In an e-containing Gd oxide scintillator, Al is usually converted to G
a, and sufficient addition intensity was not obtained unless complex addition was performed, but a transparent polycrystalline garnet scintillator with sufficient emission intensity was obtained by setting the predetermined amount of Gd to Y without using expensive Ga. Was done. The crystal structure of this composition system is cubic, the optical anisotropy is small, and a sintered body with high translucency can be produced in principle. Therefore, a ceramic scintillator having high light emission intensity is realized by using a sintered body having high transmittance and low light scattering even if the powder characteristics are not as high as that of Gd ₂ O ₂ S: Pr, Ce, F which is hexagonal. it can. Also, Ce as a light emitting element is 5d → 4f of Ce ^3+.
Light is emitted by the transition, and the decay time constant is several tens to several hundred ns and 1 μm.
Very small than s.

Gd₂O₃, Al₂O₃, Y₂O₃and
After mixing, for example, cerium oxalate as a cerium salt, the lid
Calcined at 1500 ℃ for 2 hours in an attached aluminum crucible
Powder scinti with different composition ratio of Gd and Y
Light intensity (Gd₂O ₂S: Pr, Ce, F thin
FIG. 1 shows the ratio of the luminous intensity of the chiller powder. G
d_2.995-yCe_0.005Y_yAl₅O₁₂composition
, The value of y is 0 and 0.7 to 2 and the emission intensity is relative
Showed a large value. However, Gd not including Y₃Al
₅O₁₂The powder is a metastable phase and can reach temperatures above 1400 ° C
GdAlO when heated₃And Al₂O₃Decompose into
(“Carnet-type Gd prepared by amorphous citric acid complex method_ThreeAl_FiveO₁₂Powdered
Adjustment ”, Journal of the Ceramic Society of Japan 10
0 [11] 1381-1383 (1992)). Sintering with increased density
Requires sintering temperature higher than 1400 ℃
The composition powder with the value y of 0 is practically usable as a sintered body.
I can't. FIG. 2 shows the luminescence intensity of Ce as a luminescence element.
4 shows the composition dependence of the scintillator powder. Depends on Ce concentration
Is not very large, but Gd_2-xCe_xYAl₅
O₁₂In the composition, x is suitably 0.001 to 0.05.

Further, a flux for synthesizing a powder of this composition was examined, and a Ba compound, particularly B
It has been found that aF ₂ is effective in increasing the compositional homogeneity, reducing crystal defects in the material, and improving the emission intensity. FIG _{. 99} Ce _0.01 YAl ₅
The luminescence intensity of the powder scintillator in which the addition rate of BaF ₂ added during the powder synthesis of O ₁₂ was changed is shown. Gd _1.99
To Ce _0.01 YAl ₅ O ₁₂ composition 1 mole, BaF
₂ was added in an amount of 0.05 to 1.5 mol, the emission intensity increased,
When moles were added, the emission intensity was at a maximum. FIG. 4 shows the calcination temperature dependency of the light emission intensity. From this figure, it is understood that the calcination temperature is preferably set to 1400 to 1600 ° C. The average particle size of the powder scintillator thus obtained was 10
The powder may be directly sintered, but it is preferable that the powder is formed into a powder having an average particle size of 2 μm or less by a method such as ball milling using alumina ball.

Before molding and sintering, 50
It is preferably added to the silicon-based compounds, including SiO ₂ of ～1000Ppm. A sintering aid such as ethyl silicate may be used. SiO ₂ has the function of suppressing the growth of crystal grains during sintering and the generation of intragranular pores. Si
If O ₂ is less than 50 ppm, there is almost no effect on reduction of intragranular porosity, and if it is added in excess of 1000 ppm, the emission intensity will be significantly reduced. The ground powder can be molded to a density of 60 to 70% of the theoretical density by performing a uniaxial press or a uniaxial press and a cold isostatic press. This molded body is placed in an alumina mortar with a lid, and is placed in a vacuum or in an atmosphere of hydrogen, nitrogen or argon in the range of 1600-18.
Sintering at 00 ° C. increases the density to 95-98% of the theoretical density. Sintering in air or oxygen is
The valence of Ce in the light emitting component is changed from trivalent to tetravalent, and the luminous intensity is lowered, which is not preferable. Regarding the sintering temperature, if the temperature is less than 1600 ° C., the sintered body density does not improve, and open pores remain, and if the temperature exceeds 1800 ° C., the volatilization amount of various components is large, and the composition cannot be controlled. Absent.

The sintered body is further subjected to hot isostatic pressing (HIP) at 500 to 2,000 atm in argon gas at a temperature of 1400 to 1600 ° C. to obtain an optical transparency of 99.5% or more of the theoretical density. A sintered body can be obtained.

[0011]

(Embodiment 1) Next, an embodiment of the scintillator of the present invention will be described. 90.17 g of Gd ₂ O ₃ and Ce ₂ (C ₂ O
_{4) 3} · 9H ₂ O and 0.883 g, 28.23 g of Y ₂ O _3, the Al ₂ O ₃ 63.73g and
21.92 g (0.5 mol / mol) of BaF ₂ was weighed. Next, these raw materials were wet-mixed and then dried. Next, this raw material mixed powder was placed in a B5 size alumina crucible, covered with alumina, and calcined at 1500 ° C. for 2 hours. After cooling, the raw materials were loosened and washed with 4N hydrochloric acid for 2 hours using a stirrer. 100 ppm of SiO ₂ dispersed in ethanol was added to the scintillator powder washed and dried with pure water. Then, alumina balls having a diameter of 5 mm were put into a polyethylene pot, and ball milling was performed for 24 hours. Thus, a scintillator pulverized powder having a particle size of 1 to 2 μm was obtained. To this pulverized powder, add 5 wt% of pure water,
Uniaxial press molding at a pressure of 500 kg / cm ²
perform cold isostatic pressed at ton / cm ^2, the theoretical density to 64%
Was obtained. This compact was placed in a boron nitride mortar, covered, and subjected to primary sintering at 1750 ° C. for 3 hours in a vacuum to obtain a sintered body having a theoretical density of 98.5%. In addition, when using an alumina mortar when sintering in a vacuum, the amount of oxygen dissociated from alumina is large and Ce causes a valence change, so boron nitride was used as the material of the mortar.
(When sintering in a hydrogen, nitrogen or argon atmosphere, use an alumina mortar.) Further, in order to obtain a uniform sintered body having a high density, the heating rate at 1350 ° C. or higher was 50 ° C./h. The sintered body was subjected to hot isostatic press sintering at 1550 ° C. for 3 hours at 1000 atm. The resulting sintered body had the same density as the theoretical density. This sintered body is 3mm thick
After machining to the wafer shape, the characteristics were evaluated. Diffuse transmittance of 550nm using spectrophotometer, tube voltage 120kV, tube current
Emission intensity (CdWO ₄ ) when irradiated with 5 mA X-ray (W target)
Table 2 shows the ratio of the emission intensity of the scintillator to the emission intensity), the afterglow measurement results after 30 ms from the stop of the X-ray excitation, and the measurement results of the emission decay time constant when irradiated with 60 KeV γ-ray emitted from ²⁴¹ Am. . For comparison, Table 1 shows other polycrystalline scintillators as CdWO ₄ , (Y, Gd) ₂ O ₃ : Eu, P
The main emission wavelength, relative emission intensity, decay time constant, afterglow, and cost of r, Gd ₂ O ₂ S: Pr, Ce, F are also shown. The relative issuing strength and cost are comparisons based on a CdWO ₄ sintered body.

[0012]

【table 1】

(Examples 2 to 8 and Comparative Examples 1 to 4) Raw material powders and sintered bodies were produced in the same procedure as in Example 1. However, Gd
Raw material powders having different compositions were prepared by changing the addition amounts of ₂ O ₃ and Y ₂ O ₃ , the addition ratio of BaF _{2 and} the addition ratio of SiO ₂ . Further, a sintered body sample as a polycrystalline scintillator was prepared by changing the sintering temperature and atmosphere. Diffuse transmittance of the obtained polycrystalline scintillator,
Table 2 shows the measurement results of the relative luminescence intensity, decay time constant, and afterglow.

[0014]

[Table 2]

Example 9 90.17 g of Gd ₂ O ₃ , Ce ₂ (C ₂ O ₄ ) ₃
・ 0.883 g of 9H ₂ O, 28.23 g of Y ₂ O ₃ , 63.73 g of Al ₂ O ₃ and BaF
₂ was weighed 21.92 g (0.5 mol / mol). Next, these raw materials were wet-mixed and then dried. Next, this raw material mixed powder is
It was placed in a 5-size alumina crucible, covered with alumina, and calcined at 1500 ° C. for 2 hours. After cooling, the raw materials were loosened and washed with 4N hydrochloric acid for 2 hours using a stirrer. 100 ppm of SiO ₂ dispersed in ethanol was added to the scintillator powder washed and dried with pure water. Then, alumina balls having a diameter of 5 mm were put into a polyethylene pot, and ball milling was performed for 24 hours. Thus, a powder for a transparent polycrystalline garnet scintillator having a particle size of 1 to 2 μm was obtained. Gd ₂ O ₂ S:
FIG. 5 shows the relative luminous intensity with the Pr, Ce, and F powders. For comparison, the characteristics of a transparent polycrystalline garnet scintillator powder produced in the same manner except that other fluxes were used are also described. Those using a fluorinated compound had high emission intensity, and those using BaF ₂ were particularly excellent.

[0016]

The scintillator according to the present invention has a Gd ₂ O ₂ S: P
r, Ce, F Ceramics scintillator can be manufactured at 1/3 the cost, but the characteristics are the same, and the manufacturing cost is the same as CdWO ₄ or (Y, Gd) ₂ O ₃ : Eu, but the relative emission intensity And the decay time constant is very small. As described above, as is clear from the description of the present invention, a powder and a scintillator for a transparent polycrystalline garnet scintillator having excellent cost performance and high performance can be provided with respect to the scintillator according to the related art.

[Brief description of the drawings]

FIG. 1 shows the relative emission intensity characteristics of (Gd, Y) ₃ Al ₅ O ₁₂ : Ce scintillator powder with respect to Gd ₂ O ₂ S: Pr, Ce, F scintillator powder in which the composition ratio of Gd and Y is changed.

FIG. 2 Gd ₂ YAl ₅ O in which the composition ratio of Ce as a light emitting element is changed
₁₂ shows the relative luminescence intensity characteristics of ₁₂ : Ce scintillator powder to Gd ₂ O ₂ S: Pr, Ce, F scintillator powder.

FIG. 3 shows the relationship between the amount of BaF ₂ added as a flux during the synthesis of Gd ₂ YAl ₅ O ₁₂ : Ce scintillator powder and the emission intensity.

FIG. 4 shows the relationship between the calcination temperature and the emission intensity during the synthesis of Gd ₂ YAl ₅ O ₁₂ : Ce scintillator powder.

FIG. 5 shows the relationship between the flux used at the time of synthesizing the Gd ₂ YAl ₅ O ₁₂ : Ce scintillator powder and the emission intensity.

Claims (4)

[Claims] 1. A compound of the general formula Gd _3-xy Ce _x Y _y Al ₅
A transparent polycrystalline garnet scintillator characterized by being represented by O ₁₂ (however, 0.001 ≦ x ≦ 0.05 and 0.7 ≦ y ≦ 2). 2. Gd _3-xy Ce _x Y _y Al ₅ O ₁₂
In order to obtain a garnet composition, an appropriate amount of gadolinium oxide, aluminum oxide, yttrium oxide and cerium salt, and 0.05 to 1.5 mol of a fluorinated compound with respect to 1 mol of garnet as a flux, and after mixing,
A method for producing a powder for a transparent polycrystalline garnet scintillator, comprising calcining at 0 to 1600 ° C. 3. Gd _3-xy Ce _x Y _y Al ₅ O ₁₂
A sintering aid of a silicon compound is added to the powder as a sintering aid.
A transparent polycrystalline garnet which is sintered in a vacuum or in a hydrogen, nitrogen or argon atmosphere in a temperature range of 1600 to 1800 ° C. to obtain a sintered body. Manufacturing method of scintillator. 4. The sintered body is further subjected to an argon atmosphere.
The method for producing a transparent polycrystalline garnet scintillator according to claim 3, wherein hot isostatic press sintering is performed at a temperature of 00 to 1600C.