JPH11315278A - Scintillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a scintilator to greatly improve the light emission strength, to improve the relative light emission strength without increasing afterglow, and well enable the scintilator to deal with a high resolution and high-speed scanning, by adding Tb<3+> to impart a specific formula and a specific relation. SOLUTION: This scintillator is represented by the formula: (Gd1-x-y-z Prx Tby Cez )2 O2 S, with 0.0003<=X<=0.004, 0.00002<=y<=0.004, and 0<z<=0.0004. An aq. Ce(NO3 )3 .6H2 O soln. was added to a given amt. of Gd2 O3 , Pr6 O11 and Tb4 O7 , mixed together and then the mixture was dried. Na2 CO3 , Li2 B4 O7 , K3 PO4 .3H2 O, NaBF4 and S were added to the mixture, dry-mixed powder was heat-treated at 1,300 deg.C for 8 hr. After cooling, it was washed with pure water, 4N hydrochloric acid, and warm water successively to give a scintillator powder of (Gd0.9985 Pr0.001 Tb0.0005 Ce0.00001 )2 O2 S with an average particle size of 40 μm. Then the powder was hot isostatic pressing(HIP) sintered at 1,300 deg.C and under 1,000 atm for 3 hr. The sintered material thus obtd., after being machined into a wafer shape, heat-treated in an Ar gas comprising a trace of oxygen to give a ceramic scintillator.

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 scintillator having a large emission intensity and a good afterglow characteristic which is optimal for a CT apparatus.

[0002]

2. Description of the Related Art As one of X-ray diagnostic apparatuses, there is a computed tomography apparatus (hereinafter referred to as a CT apparatus). This CT apparatus comprises an X-ray tube having a fan-shaped X-ray tube for generating fan-beam X-rays and an X-ray detector provided with a large number of X-ray detection elements arranged at the center of the tomographic plane of the subject. X-ray absorption data is detected and collected by irradiating a fan beam X-ray from an X-ray tube toward the target and moving the angle, for example, by one degree with respect to a tomographic plane each time irradiation is performed. The X-ray absorptance at each position on the tomographic plane is calculated by analyzing the
An image corresponding to the absorption rate is created.

Conventionally, a xenon gas detector has been used in this CT apparatus. This xenon gas detector encloses xenon gas in a gas chamber and irradiates it with x-rays while applying a voltage between a large number of arranged electrodes. Can be taken out, thereby forming an image. But,
In this xenon gas detector, a thick window is required to enclose high-pressure xenon gas in the gas chamber, and therefore, there is a problem in that the use efficiency of X-rays is low and the sensitivity is low. Further, in order to obtain a high-resolution CT apparatus, it is necessary to reduce the thickness of the electrode plate as much as possible. If the electrode plate is made thinner as described above, 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 crystals and G
A detector that combines a ceramic scintillator sintered with d ₂ O ₂ S: Pr phosphor powder and a silicon photodiode has been developed and put into practical use. 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. However, recently, there is a trend that a CT apparatus is required to further improve the resolution and reduce the exposure dose to the human body. In order to improve the resolution, it is necessary to reduce the size of the detection element.However, when the detection element is reduced in size, the amount of X-ray incident on one element decreases, and the output of a practical scintillator decreases. It has been pointed out that a high resolution cannot be obtained. In addition, to reduce the human exposure dose, it is necessary to shorten the scanning time,
Since one irradiation time is short, the output of a practically used scintillator is reduced, and there is a problem that a sufficient resolution cannot be obtained. The present invention has been made in view of the conventional problems described above, and has high resolution,
An object of the present invention is to provide a scintillator for an X-ray CT apparatus compatible with high-speed scanning.

[0005]

In order to solve the problems of the prior art, the present inventor has been studying various composition systems and trying to develop new materials for many years. Registered patent number 12
No. 84949 discloses a technique for reducing the afterglow by adding Ce to the Gd ₂ O ₂ S: Pr composition and improving the emission intensity by further adding a halogen element. However, in the course of examining the effect of adding the rare earth element, it was found that the rare earth element causes energy resonance with the luminescent element Pr ^3+, and the luminescence intensity is improved. In particular, it was found that Tb ³⁺ had the largest sensitizing effect, and the present invention was conceived.

FIG. 1 shows the relative emission intensity characteristics for Tb ³⁺ . Tb ³⁺ addition is the general formula (Gd _0.999-y Pr _0.001 Tb _y Ce _0.00001 )
_As shown in ₂ O ₂ S, the effect of the emission intensity is recognized when y is 0.00001, and the output monotonously increases with an increase in the amount of Tb ³⁺ added. When the added amount of Tb ³⁺ was increased to around 0.0005, the improvement effect was large, and it was confirmed that the emission intensity was improved by about 15% as compared with the case where Tb ³⁺ was not added. This is in Gd ₂ O ₂ S
It is considered that the fluorescence spectrum of Tb ³⁺ and the excitation spectrum of Pr ³⁺ have an overlapping portion, which causes energy resonance. On the other hand, Dy ³⁺ , Nd ³⁺ , Sm ³⁺ , Ho ³⁺ , Er ³⁺ and Tm ³⁺
Similarly, when y = 0.00005, the emission intensity was observed to increase by about 8% at the maximum, but the rate of increase of the emission intensity was smaller than when Tb ³⁺ was added. From the above examination results, Gd ₂ O ₂ S: P
It was experimentally confirmed that adding a predetermined amount of Tb ³⁺ to the r composition was effective.

However, like an X-ray CT apparatus, X
In a scintillator used for measuring a change in the intensity of a line, it is also important that the afterglow (a phenomenon of luminescence that continues even after X-ray irradiation is stopped) is small, like the luminescence intensity. FIG. 2 shows Tb ³⁺
Shows the relationship between the amount of addition and the afterglow. The figure shows the measurement results at 3 ms and 30 ms after stopping the X-ray irradiation. As can be seen from the figure, the afterglow characteristics 30 ms after the stop of X-ray irradiation have no effect on the Tb ³⁺ addition amount, but the afterglow characteristics after 3 ms monotonically increase with the increase in Tb ³⁺ addition amount. To increase. And the value of y is
When the afterglow exceeds 0.004 and becomes 0.25% or more, the image resolution of the CT apparatus starts to decrease. Therefore, the limit value of y is considered to be 0.004. Further, when the value of y is less than 0.00002, the improvement in the emission intensity associated with Tb ³⁺ is small. As a result, the optimum range of the Tb ³⁺ added in the general formula _{(Gd 1-xyz Pr x Tb} y Ce z) 2 O 2 S,
It was found that the range of 0.00002 ≦ y ≦ 0.004 was more preferable, and the range of 0.00005 ≦ y ≦ 0.002 was more preferable.

On the other hand, FIG. 3 shows the relationship between the amount of addition of the light emitting element Pr and the light emission intensity. An upwardly convex characteristic curve having a maximum value near x = 0.001. When x exceeds 0.004 or is less than 0.0003, the emission intensity becomes less than 80% of the emission intensity around x = 0.001. Formula The results _{(Gd 1-xyz Pr x Tb} y
It was found that the range of x shown in Ce _z ) ₂ O ₂ S was 0.0003 ≦ x ≦ 0.004, and more preferably 0.0005 ≦ x ≦ 0.002. FIG. 4 shows the relationship between the amount of added Ce, which is an afterglow reducing element, and the emission intensity and afterglow. It can be seen that Ce is very effective in reducing the afterglow, but at the same time, Ce works in the direction of lowering the emission intensity. When z exceeds 0.00004, the emission intensity is lower than 80% as compared with the case where Ce is not added. Therefore, the general formula (Gd _1-xyz Pr _x Tby _y Ce
_{In z} ) ₂ O ₂ S, it was found that the value of z was in the range of 0 <z ≦ 0.00004, and more preferably in the range of 0.000005 ≦ z ≦ 0.00002.

[0009]

(Embodiment 1) An embodiment of a scintillator according to the present invention will be described. Gd ₂ O ₃ 361.96 g, Pr ₆ O ₁₁ 0.1.
34 g and 0.187 g of Tb ₄ O ₇ were weighed. Next, 1.3016 g of Ce (NO ₃ ) ₃ .6H ₂ O was dissolved in 500 cc of pure water, 4 ml of the solution was added to the raw material with a pipette, and the mixture was wet-mixed and dried. And to this raw material, 95.72 g of Na ₂ CO ₃ and 10.10 of Li ₂ B ₄ O _{7
g, K 3 PO 4 · 3H} 2 O to 32.33G, the NaBF ₄ to 3.29g and S 105.49G
Add and dry mix. Next, this raw material mixed powder was put into an alumina crucible, and after covering with alumina, the mixture was heated at 1300 ° C. for 8 hours.
Fired. After cooling, the crucible and the fired product were left in pure water for 1 hour to loosen the raw materials. This raw material is thoroughly washed with pure water,
Next, using a stirrer, washing was performed for 2 hours with 4N hydrochloric acid and for 1 hour with warm water at 90 ° C. Thus, (Gd _0.9985 P
r _0.001 Tb _0.0005 Ce _0.00001 ) ₂ O ₂ S scintillator powder was obtained. 0.1% by weight of Li ₂ GeF ₆ was added to this powder as a sintering aid, and the powder was filled in a mild steel capsule and then vacuum sealed. Then, under the conditions of 1300 ° C, 1000atm and 3h, hot isostatic pressing (HI
P) Sintered. After the obtained sintered body was machined into a wafer shape of 30 × 26 × t1.25 mm, it was subjected to 1100 in Ar gas containing a trace amount of oxygen.
Heat treatment was performed at 30 ° C. for 30 minutes to obtain a ceramic scintillator. X-ray with a tube voltage of 120 kV and a tube current of 5 mA
Table 1 shows the measurement results of the emission intensity when irradiating (W target) and the afterglow 30 ms after stopping the X-ray excitation. In Examples 2 to 5, raw material powders and sintered bodies were produced in the same procedure as in Example 1. However, raw material powders were prepared by changing the addition amounts of Gd ₂ O ₃ , Pr ₆ O ₁₁ , and Tb ₄ O ₇ . Table 1 shows the composition of the obtained scintillator and the measurement results of the luminescence intensity and the afterglow.

(Comparative Example 1) 361.42 g of Gd ₂ O ₃ and 1.02 g of Pr ₆ O ₁₁
g was weighed. Then, the pure water 500 cc Ce a _{(NO 3) 3 · 6H 2} O 1.30
After dissolving 16 g, 6 ml of the solution was added to the raw material with a pipette, then wet-mixed and dried. And, to this raw material, Na ₂ C
The O ₃ 95.72g, 10.10g a Li ₂ B ₄ O _7, the _{K 3 PO 4 · 3H 2 O} 32.33g,
3.29 g of NaBF ₄ and 105.49 g of S were added and dry-mixed. Next, this raw material mixed powder was placed in an alumina crucible, covered with alumina, and fired at 1300 ° C. for 8 hours. After cooling, the crucible and the fired product were left in pure water for 1 hour to loosen the raw materials. This raw material was thoroughly washed with pure water, and then washed with 4N hydrochloric acid for 2 hours and warm water at 90 ° C. for 1 hour using a stirrer. Thus, a (Gd _0.997 Pr _0.003 Ce _0.000015 ) ₂ O ₂ S scintillator powder having an average particle size of 40 μm was obtained. Li ₂ G is added to this powder as a sintering aid.
The eF ₆ was added 0.1 wt%, after filling the soft steel capsule, vacuum sealing. Then, hot isostatic pressing (HIP) sintering was performed at 1300 ° C., 1000 atm, and 3 hours. 30 × 26 × t
After machining to a 1.25 mm wafer shape, Ar containing a small amount of oxygen
Heat treatment was performed at 1100 ° C for 30 minutes in a gas to obtain a ceramic scintillator. The composition of the obtained scintillator and the measurement results of the emission intensity and afterglow are shown together with other comparative examples in Table 1.

[0011]

[Table 1]

It is very important for a scintillator used in an apparatus that detects a change in the intensity of radiation at a high speed, such as an X-ray CT apparatus, that the emission intensity with respect to the radiation be large and the afterglow be small. However, as shown in FIG. 5, in the existing scintillators, those having a large luminous intensity tend to have a large afterglow, and those having a small luminescence have a tendency to have a low sensitivity, and thus have not sufficiently satisfied the requirements for characteristics. on the other hand,
The composition system disclosed in Registered Patent No. 1284949 has a relatively small afterglow and a relatively large emission intensity,
It has been put to practical use for high-resolution X-ray CT systems. However, for high-speed scanning to further increase the resolution and reduce the exposure dose to the human body, it has been required to improve the emission intensity.
In addition, Gd ₂ O ₂ S: Tb
And Gd ₂ O ₂ S: Eu, but the former has a large decay time constant of emission (time until emission intensity becomes 1 / e after stopping X-ray excitation),
The latter has a large afterglow and cannot be used for an X-ray CT apparatus. According to the present invention, Gd ₂ O2S: Pr composition system based on conducted detailed studies, the general formula _{(Gd 1-xyz Pr x Tb} y Ce z) 2 O 2 S,
0.0003 ≦ x ≦ 0.004, 0.00002 ≦ y ≦ 0.004, and 0 <z ≦ 0.0
By including the range of 0004, the afterglow characteristics can be further reduced, and the emission intensity can be improved by about 20%. This makes it possible to improve the resolution of the CT apparatus and realize high-speed scanning.

[0013]

As described above, it is apparent from the detailed description of the present invention that the addition of Tb ³⁺ to the conventional scintillator significantly improves the emission intensity and increases the afterglow. It is possible to provide a scintillator that improves the relative luminous intensity without the need.

[Brief description of the drawings]

FIG. 1 shows emission intensity characteristics with respect to addition of Tb according to the present invention.

FIG. 2 shows the afterglow characteristics with respect to Tb addition according to the present invention.

FIG. 3 shows emission intensity characteristics of Pr which is a light emitting element according to the present invention.

FIG. 4 shows emission intensity and afterglow characteristics with respect to Ce addition according to the present invention.

FIG. 5 shows emission intensity and afterglow characteristics of various scintillators according to the present invention.

Claims (4)

[Claims] 1. A scintillator characterized by being represented by a general formula (Gd _{1 -XYZ} Pr _X Tb _Y Ce _Z ) ₂ O ₂ S. However, x, y and z are respectively 0.0003 ≦ x ≦ 0.004, 0.00002 ≦ y ≦ 0.00
4, 0 <z ≦ 0.00004. 2. The method according to claim 1, wherein x in the general formula is 0.0
A scintillator characterized by the range of 005 ≦ x ≦ 0.002. 3. The method according to claim 1, wherein
A scintillator, wherein y in the general formula is in the range of 0.00005 ≦ y ≦ 0.002. 4. The scintillator according to claim 1, wherein z in the general formula is in the range of 0.000005 ≦ z ≦ 0.00002.