JP5218891B2 - Method for producing fluorescent material - Google Patents

Description

  The present invention relates to a method for producing a fluorescent material for a scintillator that absorbs radiation such as X-rays and emits light.

  One of X-ray diagnostic apparatuses is X-ray CT (Computed Tomography). This CT is composed of an X-ray tube that radiates a fan-shaped fan beam X-ray and an X-ray detector provided with a large number of X-ray detection elements. This apparatus is an apparatus that irradiates a human body with X-rays, detects transmitted X-rays by light emitted from a scintillator as a detector, and performs image processing on the output of the photomultiplier tube.

  Conventionally, a xenon (Xe) gas detector has been used as the X-ray detector. This xenon gas detector encloses a xenon gas in a gas chamber, applies a voltage between a large number of arranged electrodes, and irradiates X-rays. The X-rays ionize the xenon gas and a current signal corresponding to the intensity of the X-rays. Can be retrieved, thereby constructing an image. However, this xenon gas detector requires a thick window to enclose high-pressure xenon gas in the gas chamber, so that there is a problem that the utilization efficiency of X-rays is poor 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 as such, there is a problem that the electrode plate vibrates due to external vibration and noise is generated. .

On the other hand, as the scintillator, CdWO ₄ single crystal, Gd ₂ O ₂ S: Pr, Ce, F, (Y, Gd) ₂ O ₃ : Eu, Pr, Gd ₃ Ga ₅ O ₁₂ : Cr, Ce, etc. The body is used. Such phosphors are required to have high uniformity of materials, small variation in X-ray characteristics, small radiation degradation, little change in characteristics with respect to environmental changes, There is a demand for chemical stability without deliquescence.

  In such an X-ray detector, the higher the intensity (light emission intensity) of light emitted by the scintillator in accordance with X-ray absorption, the higher the sensitivity. In order to increase the emission intensity, it is necessary to sufficiently absorb X-rays. In addition, if this absorption is small, the X-ray dose that passes through the scintillator increases, which becomes a noise source of the silicon photodiode, which causes a decrease in sensitivity. In order to reduce the X-ray dose transmitted through the scintillator, it is necessary to increase the thickness of the scintillator. However, if this is done, the detection element cannot be reduced in size and the cost increases. Therefore, in order to sufficiently absorb X-rays with a thin fluorescent material, it is necessary that the X-ray absorption coefficient is large. Further, if the transmittance of this light in the fluorescent material is low, the amount of generated light that does not reach the photodiode increases, so that the emission intensity substantially decreases. Therefore, in order to increase the emission intensity, the fluorescent material used as the scintillator material is required to have (a) a large X-ray absorption coefficient and (b) a high transmittance of emitted light.

  Further, X-ray CT is required to improve the resolution, that is, 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 integration time in one detection element is shortened, and the total amount of X-rays absorbed during the integration time is reduced, so that particularly high light emission efficiency (high light emission intensity) is required. Furthermore, in order to increase the time resolution of the detection element, it is necessary to instantaneously reduce the light emission (afterglow) after the X-ray irradiation is stopped. For this purpose, it is necessary that the decay time constant of light emission and the afterglow intensity are small. Here, the decay time constant of light emission is the time until X-ray irradiation is stopped and the light emission intensity becomes 1 / e of the light emission intensity during X-ray irradiation, and the afterglow intensity is the X-ray irradiation. The ratio of the emission intensity after stopping for a certain period of time to the emission intensity during X-ray irradiation is expressed. If the attenuation is completely exponential, the afterglow intensity will inevitably be low if the attenuation time constant is small, but actually the decay of the afterglow is not exponential. Therefore, in order to obtain a high-performance X-ray CT apparatus with reduced afterglow, it is necessary to use a fluorescent material having both a small decay time constant and afterglow intensity. Table 1 shows the emission intensity, decay time constant and afterglow intensity after 30 ms in various conventionally used fluorescent materials.

  As a process of the polycrystalline scintillator, each raw material is weighed and then put into a container filled with media such as ceramic balls. Then, after rotating and mixing for a predetermined time and drying the slurry, the dried powder is pressed into an arbitrary shape and sintered. A scintillator element is formed by machining the sintered body (for example, Patent Documents 1, 2, and 3).

JP 2002-189080 A JP 2001-4753 A JP 2008-24739 A

  In high-performance X-ray CT, the scanning time is being further shortened in order to reduce the influence of body movement in order to obtain a clear image and to minimize the exposure dose to the human body. In order to realize these two points, it is necessary to increase the light emission efficiency as much as possible during the short integration time (the light emission efficiency is large) and to improve the time resolution accordingly. To increase the time resolution, X-ray irradiation is stopped. Subsequent light emission is required to be small (afterglow is small).

  At present, the afterglow after a relatively long time of 30 to 300 ms after stopping X-ray irradiation (long afterglow) and the afterglow after a short time of 1 to 10 ms after stopping X-ray irradiation (short afterglow) 2) are parameters for evaluation of afterglow, but the demand for short afterglow has become stricter as the scanning time is shortened. Patent Documents 1, 2, and 3 do not suggest a method for further reducing afterglow. Therefore, an object of the present invention is to provide a method for manufacturing a fluorescent material for obtaining a scintillator with reduced afterglow.

In order to solve the above-mentioned problems, the present inventor has found that trace element components resulting from an increase in afterglow can be removed by heat-treating Al ₂ O ₃ used as a raw material in a vacuum, and the present invention has been completed.

Gist of the present invention, the Ce and emitting elements, Gd, Ga, Al, a method for producing polycrystalline fluorescent material for a scintillator of garnet structure containing O, the Al Motogenryo in a vacuum, 1300 ° C. or higher A method for producing a polycrystalline fluorescent material, characterized in that a powder obtained by ball milling alumina powder heat-treated at 1600 ° C. or less with an alumina ball is used . Furthermore, the alumina balls used for ball milling may be heat treated in advance in a vacuum.

The inventors have found that Fe ions in GGAG: Ce greatly contribute to afterglow. The fluorescence spectrum and 3 ms afterglow profile at each Fe addition amount are shown in FIGS. The synthesized GGAG: Ce always contains Fe, and if the Fe content exceeds 0.05 to 1 massppm, the 3 ms afterglow exceeds 800 ppm, which is higher than the allowable level, so the upper limit of the Fe content is 1 massppm. (Preferably the upper limit is 1.0 mass ppm). However, the raw material used for the synthesis of the fluorescent material already contains several mass ppm to several tens mass ppm of Fe. Therefore, the fluorescent material synthesized using these raw materials contains Fe of several mass ppm to several tens of mass ppm. Therefore, high-purity Gd ₂ O ₃ , Al ₂ O ₃ , and Ga ₂ O ₃ are required to eliminate the Fe element in the raw material as much as possible. Among them, the Fe content contained in Al ₂ O ₃ is relatively large, and it has an influence even after material synthesis.

  On the other hand, when the phosphor is synthesized, the raw material powder is put into a container filled with media and mixed for a predetermined time. In addition, when the particle size of a certain raw material powder is relatively large compared to the particle size of other powders, the same method as mixing or a method of contacting the target powders without using a medium such as a jet mill, Only the powder can be pulverized in advance to adjust the particle size.

  When mixing and pulverizing using a ceramic ball as a medium, the ball surface may be worn due to collision between the ball and hard particles, or collision between balls, and the wear powder may be mixed into the slurry. The alumina balls used contain about 16 mass ppm of Fe element, and the Fe contamination rate increases as the ball wear rate increases.

  A scintillator with little afterglow can be provided by the method for producing a fluorescent material of the present invention.

The manufacturing method of one embodiment of the present invention includes S1, S2, S3, S4, S5, S6, S7 and S8 among the following steps, and is performed in the order of the arrows shown in FIG.
(S1): The raw materials (Gd ₂ O ₃ , Ga ₂ O ₃ , Lu ₂ O ₃ , Sc ₂ O ₃ , Ce (NO ₃ ) · 9H ₂ O) are weighed.
(S2): Al ₂ O ₃ used as a raw material is heat-treated in a vacuum.
(S3): Al ₂ O ₃ heat-treated in S2 is wet-ground with alumina balls.
(S4): Al ₂ O ₃ pulverized in S3 is put into the raw material weighed in S1, and wet mixed with alumina balls.
(S5): The slurry after mixing is dried and granulated.
(S6): The granulated powder is uniaxially pressed and CIP molded.
(S7): The molded body is sintered in oxygen.
(S8): The sintered body is machined to form a scintillator element.
(S21): The alumina balls used in S3 or S4 are heat-treated in a vacuum.

  The manufacturing method of other embodiment of this invention is equipped with S1, S2, S21, S3, S4, S5, S6, S7, and S8 among the said processes, and is performed in the order shown in FIG. In the step S3, the alumina ball of S21 is used. Furthermore, the alumina ball of S21 can be used in the step of S4.

Examples will be described below. However, the present invention is not necessarily limited to these examples.
Example 1
The Al ₂ O ₃ raw material having an Fe content of 2.1 ppm or less and a specific surface area of 4.0 m ² / g was heat-treated at 1300, 1400, 1500, and 1600 ° C. for 3 hours in vacuum (−10 Pa). The 1300 ° C. treated powder was 20 h, the 1400 ° C. treated powder was 30 h, the 1500 ° C. treated powder was 40 h, and ball milling was performed with an alumina ball having a diameter of 5 mm. Using the resulting _Al 2 _{O 3} pulverized powder, 117.63G the _Gd 2 _{O 3,} 4.45g of _{Lu 2 O 3, Ce (NO} 3) · 9H 2 O to 0.777G, the _Sc 2 _{O 3} 0 925 g, Al ₂ O ₃ 32.35 g, and Ga ₂ O ₃ 43.87 g. These raw materials were put into a container filled with 5 mm diameter alumina balls, mixed for 12 hours and dried. After uniaxial pressure molding by adding 1 wt% of pure water at 500 kg / cm ² in the dry powder was subjected to cold isostatic pressing at 3000kg / cm ² (CIP). The molded body was sintered in O ₂ at 1675 ° C. for 12 hours, and the obtained sintered body was subjected to hot isostatic pressing (HIP) in Ar at 1500 ° C. and 1000 atm (1.01 × 10 ⁵ Pa) for 3 hours. It was. The HIP sintered body was machined to □ 10 mm (10 mm × 10 mm) and thickness t2 mm, mirror-polished, and heat-treated at 1500 ° C. for 2 h in O ₂ to prepare a polycrystalline scintillator.

The obtained scintillator was evaluated using the radiation detectors of FIGS. The radiation detector is composed of 24 sliced scintillators 2 arranged at a pitch of 1.2 mm, and light reflection obtained by applying a mixture of TiO ₂ and epoxy resin to the top and side surfaces of the arranged scintillators 2 and curing them. Corresponding to the arrangement of the film 3 and the scintillator 2, the size is 1 mm × 30 mm, the pitch is 1.2 mm, and the light-receiving part is positioned so that the scintillator 2 and the light-receiving surface exactly coincide with each other. A 24-channel silicon photodiode 5 fixed with resin and a wiring substrate 4 to which the 24-channel silicon photodiode 5 is electrically connected are configured. According to such a radiation detector, the scintillator 2 is excited and emits light by X-ray irradiation from the X-ray source 1, and the light is detected by the photodiode 5, whereby the characteristics of the scintillator can be confirmed.

FIG. 4 shows the Fe content and specific surface area of the alumina powder under each heat treatment condition. The unit of Fe content is mass ppm (parts per million by mass), and is omitted from the figure and labeled as ppm.
The composition analysis of the main component was performed by ICP-AES (High Frequency Inductively Coupled Plasma Emission Spectroscopy, OPTIMA-3300XL manufactured by PerkinElmer), and Fe was analyzed by GDMS (Glow Discharge Mass Spectrometry, VG Elemental: VG9000). ).
The emission intensity by X-ray irradiation and the afterglow after 3 ms were measured. In this measurement, the radiation detector shown in Example 1 was manufactured, a tungsten target X-ray tube was used as the X-ray source, and the scintillator of the radiation detector was irradiated with X-rays under the conditions of a tube voltage of 120 kV and a tube current of 5 mA. And evaluated. The 3 ms afterglow intensity is obtained by dividing the emission intensity when 3 ms has elapsed from the stop of X-ray irradiation by the relative emission intensity with Gd ₂ O ₂ S: Pr. The unit is ppm (parts per million). The relative light emission intensity is the light emission intensity when the light emission intensity of the GOS: Pr scintillator is 100%.

  FIG. 5 shows the pulverization time, the Fe content after pulverization, the specific surface area, and the afterglow intensity of the pulverized powder obtained by ball milling the alumina powder with alumina balls under each heat treatment condition. It can be seen that the Fe content in the scintillator is reduced to 1 ppm or less by performing the vacuum heat treatment. Moreover, since the afterglow intensity | strength of the sample using the alumina which does not require the grinding | pulverization before vacuum heat processing is 521 ppm, it turns out that the afterglow is falling by heat processing.

(Example 2)
Further, an alumina ball having a Fe content of 16 ppm or less and a diameter of 5 mm was heat-treated at 1500 ° C. for 3 hours in a vacuum (−10 Pa). Using the heat-treated balls, the Al ₂ O ₃ powder heat treated at 1400 ° C. of Example 1 was pulverized for 30 hours at a rotation speed of 100 rpm using a pot of 90 mm in diameter and 1 liter (1 liter). Further, Al ₂ O ₃ powder whose heat treatment temperature was changed to 1500 ° C. was pulverized for 40 hours at a rotation speed of 100 rpm using the same pot. Using each pulverized powder, a polycrystalline scintillator was produced in the same manner as in Example 1.

  FIG. 6 shows the pulverization time, crushed Fe content, specific surface area, and afterglow intensity of the pulverized powder ball milled with alumina balls vacuum-heated at 1500 ° C. under each heat treatment condition of the alumina powder. It can be seen that the afterglow is reduced when the alumina balls are heat-treated in addition to the heat treatment of the alumina powder.

FIG. 8 is a diagram showing afterglow profiles at a wavelength of 800 nm of fluorescent materials having different Fe contents. A sample having an Fe content of 1.0 mass ppm or less was prepared by the method of the present invention. FIG. 8 shows the results of measuring afterglow profiles at a wavelength of 800 nm for samples with an Fe content of 1.3 massppm and samples with 0.7 massppm. It can be seen that the light-emitting component of Fe almost disappears in 20 ms, but has a very large effect on the afterglow after 3 ms. In addition, the 1.0 massppm sample has improved afterglow emission intensity by about half compared to the Femass content 1.3 massppm sample. Furthermore, compared with a sample with an Fe content of 0.85 massppm, the sample with 0.05 massppm to 0.7 massppm has a smaller change in emission intensity between the emission intensity after 1 ms and the emission intensity after 5 ms. When the rate of change in emission intensity is small, the influence of afterglow on the signal is also reduced, and the scanning time of the radiation detector can be further shortened, and the CT image resolution can be further improved. If the Fe content is in the range of 0.05 massppm to 0.4 (more specifically 0.35) massppm, the afterglow intensity can be sufficiently suppressed, which is more preferable.
Here, the rate of change in emission intensity = (emission intensity after 1 ms−emission intensity after 5 ms) / emission intensity after 1 ms × 100 (%). Specifically, the following major differences occur.
1.0 massppm: emission intensity change rate = 44.4%
0.7 mass ppm: rate of change in emission intensity = 4.6%
0.05 massppm: rate of change in emission intensity = 7.0%

It is a flowchart figure which shows the example of the manufacturing method of this invention. It is a figure which shows an example of the structure of the radiation detector concerning this invention. It is sectional drawing of an AA cross section in FIG. It is the table | surface which showed the Fe content rate (massppm) and specific surface area (m < ² > / g) of the alumina powder heat-processed in the vacuum in this invention. 3 is a table showing pulverization time, crushed Fe content, specific surface area and afterglow intensity of pulverized powder obtained by ball milling of heat-treated alumina powder with alumina balls in the present invention. 3 is a table showing the pulverization time, the Fe content after pulverization, the specific surface area, and the afterglow intensity of the pulverized powder obtained by heat-treating alumina balls used for ball mill pulverization in vacuum and ball milling with the balls in the present invention. It is a graph which shows the emission spectrum in Fe addition amount. It is a graph which shows the afterglow profile in Fe addition amount.

Explanation of symbols

1 X-ray source 2 Scintillator 3 Light reflecting film 4 Wiring substrate 5 Silicon photodiode

Claims (2)

A method for producing a polycrystalline fluorescent material for a scintillator having a garnet structure containing Ce as a light-emitting element and containing Gd, Ga, Al, and O, and heat-treated at 1300 ° C. to 1600 ° C. in vacuum as an Al raw material A method for producing a polycrystalline fluorescent material, characterized by using powder obtained by ball milling alumina powder with alumina balls . A method for producing a polycrystalline fluorescent material, wherein the alumina balls used for ball milling according to claim 1 are heat-treated in advance in a vacuum.

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