JP4906322B2 - Oxide phosphor, radiation detector and X-ray CT apparatus - Google Patents

Description

  The present invention relates to a radiation detector that detects X-rays, γ-rays, and the like, and more particularly to a radiation detector suitable for an imaging apparatus such as an X-ray CT apparatus. The present invention also relates to an X-ray CT apparatus using the radiation detector.

In recent years, solid-state detectors used in imaging devices such as X-ray CT apparatuses are mainly a combination of a ceramic scintillator and a photodiode, and a detector element array substrate in which a plurality of scintillator elements and photodiodes are arranged. A structure in which a plurality of the tubes are arranged on an arc centered on the X-ray tube focal point is often used. The characteristics required of the detector of the imaging device include high output, short afterglow, and the like. A scintillator material capable of obtaining excellent characteristics has been developed. For example, the present applicant has proposed an oxide phosphor made of a base crystal having a garnet structure containing Ce as a light emitting element and containing Gd, Al, Ga, and O (Patent Document 1, Patent Document 2, etc.). With this oxide phosphor, a light emission output 1.5 times or more that of a conventional single crystal scintillator such as CdWO ₄ can be obtained.

  On the other hand, the processing accuracy of the scintillator and the assembly accuracy of the detector element array have a great influence on the detector characteristics. The detector element array can be assembled by attaching a scintillator plate to the photodiode array substrate, then grooving the scintillator plate in correspondence with the photodiode array, and separating the metal separator plate or resinous channel into the groove. A method of inserting a material, or a method of bonding a scintillator material and a channel separation material alternately to form a scintillator array and adhering it to a photodiode array substrate is used. In any of the assembling methods, high-precision processing for the scintillator material is required. For example, in the groove processing method, when the groove is bent and processed, the channel shape deviates from an ideal geometric position. In such a case, the geometric positional relationship between the detector element array and the scattered radiation elimination collimator is shifted, which causes artifacts. Also in the system in which the scintillator array is bonded to the photodiode array substrate, each scintillator material is required to have high processing accuracy.

  In general, various sintering aids are used in the manufacturing process of the ceramic material. The purpose of using the sintering aid is various, such as suppressing crystal grain growth during sintering, inhibiting pore formation, growing crystal grains, and improving the optical characteristics of the material to be sintered. For example, Patent Document 3 describes that optical properties are improved by using an alkali metal oxide as a firing aid for YAG ceramics, and Patent Document 4 discloses a firing process for lowering the firing temperature of YAG ceramics. It has been proposed to use aluminum nitride as a binder.

The processability and properties of the oxide phosphor may also be improved by the sintering aid, but as described above, the function of the sintering aid is various, and the function depends on the material to be sintered. Since the function can also change, it is difficult to search for a sintering aid suitable for each purpose. Furthermore, the sintering aid must be one that does not interfere with the properties of the intended ceramic material.
JP 2001-4753 A JP 2002-189080 A Japanese Patent Laid-Open No. 5-286761 JP 2005-154155 A

  An object of the present invention is to provide an oxide phosphor that has an excellent function as a scintillator, has good processability, and is easy to process precisely. Another object of the present invention is to provide an inexpensive radiation detector provided with the oxide phosphor and processed with high accuracy. Another object of the present invention is to provide an imaging apparatus such as a high-performance and inexpensive X-ray CT apparatus that includes the radiation detector.

  The inventors of the present invention have made studies on the additive added in the process of synthesizing an oxide phosphor composed of a base crystal having a garnet structure containing Ce as a light emitting element and containing Gd, Al, Ga, and O. As a result, when sintering this phosphor powder, by adding a Na compound as a sintering aid and sintering, the crystal grain size can be increased, and the characteristics of the phosphor itself, particularly the residual The present inventors have found that light can be improved and have arrived at the present invention.

  That is, the oxide phosphor of the present invention is 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, and contains Na. It is characterized by.

  In the oxide phosphor of the present invention, the content of Na is preferably 500 wtppm or less with respect to the weight of the oxide phosphor. In the oxide phosphor of the present invention, the base crystal may have a perovskite structure as the second phase.

Further, the method for producing the oxide phosphor of the present invention is a method for producing an oxide phosphor mainly composed of a garnet-structured base crystal containing Ce as a light-emitting element and containing at least Gd, Al, Ga, and O. The method includes a step of mixing and calcining the raw material powder and a step of sintering the calcined powder, and adding a Na compound to the raw material powder or the calcined powder.
The Na compound is, for example, one or more compounds selected from the group consisting of Na ₂ B ₄ O ₇ , NaCl, NaBr, and Na ₂ CO ₃ .

  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. Features.

  An X-ray CT apparatus according to the present invention includes a radiation source, a radiation detector disposed opposite to the radiation source, a rotation circle that holds the radiation source and the radiation detector, and is driven to rotate around a subject. A plate and image reconstruction means for reconstructing a tomographic image of the subject based on the intensity of radiation detected by the radiation detector, and using the radiation detector of the present invention as a radiation detector. Features.

  The imaging apparatus of the present invention creates an image of the subject based on a radiation source, a radiation detector disposed opposite to the radiation source with the subject interposed therebetween, and the intensity of the radiation detected by the radiation detector. And a radiation detector according to the present invention as a radiation detector.

  The oxide phosphor of the present invention is a scintillator material for a radiation detector, and when the oxide phosphor is described as a scintillator material for a radiation detector in this specification, it is also called a ceramic scintillator.

  According to the present invention, it is possible to provide a ceramic scintillator having good processability and easy precision processing. In addition, by using the ceramic scintillator, a radiation detector processed with high accuracy can be provided with high processing yield, low cost, and high productivity. In addition, it is possible to provide a medical imaging apparatus such as a high-performance and inexpensive X-ray CT apparatus that includes the radiation detector.

Embodiments of the present invention will be described below.
First, the oxide phosphor of the present invention will be described. An oxide phosphor that is an object of the present invention is 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. WO99 / according No. 33934, the general formula _{(Gd 1-zx L z Ce} x) 3 Al 5-y Ga y O 12 ( wherein, L is La or Y, 0 ≦ z <0.2, x is 0.0005 ≦ x ≦ 0.02, y is a value in the range of 0 <y <5), described in JP-A-2001-4753, having a garnet crystal structure, Gd / (Al + Ga + A phosphor mainly composed of phosphors other than (Gd + Ce) / (Al + Ga + Gd + Ce) = 0.375 with an atomic ratio of (Gd) of 0.33 or more and 0.42 or less, in JP-A-2002-189080 And a phosphor having a host crystal having a perovskite structure as a second phase and a garnet structure including Ce as a light-emitting element, at least Gd, Al, Ga, and O as a first phase.

Na is added to the phosphor as a Na compound when the phosphor is manufactured. The Na compound may be added as a flux to the raw material powder, or may be added as a sintering aid. Examples of the Na compound include Na ₂ B ₄ O ₇ , NaCl, NaBr, Na ₂ CO ₃ , (Na ₃ PO ₄ , Na ₂ SO ₄ ), etc., but particularly Na ₂ B ₄ O ₇ , NaCl, NaBr, Na ₂ CO ₃ is preferred. By adding Na, the crystal grain size can be increased and the afterglow can be reduced without lowering the light emission output of the phosphor.

  The function of the Na compound is considered as follows. Na compound acts as a sintering aid to promote crystal grain growth, and remains mainly at the crystal grain boundary and also enters the crystal. In the phosphor of the present invention in which grains are grown by Na compound, lattice defects at the grain boundaries are reduced, and afterglow caused by this can be reduced. Moreover, it is surmised that the non-radiative energy loss process has been achieved by the trapped state of electrons, which has been the cause of afterglow, due to Na entering the crystal. Conversely, afterglow may increase due to the addition of an auxiliary agent. This is presumably because the additive enters the crystal and newly generates an electron trap level that causes afterglow. That is, it is considered that the Na compound is a sintering aid and also works as an additive for reducing afterglow.

The content of Na in the phosphor is 40 wtppm or more and 50 wtppm or less . Since the content of impurity elements inevitably contained in the phosphor is several wtppm, if it is too small, the effect of addition is unclear. On the other hand, the more the content of Na, afterglow tends but to reduce the emission intensity decreases, exceeds 500wtppm afterglow almost not decrease the emission intensity you decrease further.

  The oxide phosphor of the present invention is produced by the same production method as that of the oxide phosphor containing at least Gd, Al, Ga, and O using Ce as a light emitting element except that a Na compound is added. be able to. For example, as with a general ceramic material, a manufacturing method including (1) mixing raw material powder and calcining step and (2) sintering step of calcining powder can be employed. An example of a manufacturing method for obtaining the ceramic scintillator of the present invention will be described below.

First, a predetermined amount of raw powders such as high-purity Gd ₂ O ₃ , Al ₂ O ₃ , Ga ₂ O ₃ , and CeO ₂ are weighed and then wet-mixed for about 10 hours, for example, by ball mill mixing. 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. The raw material powder is preferably a micron-order fine powder, preferably a submicron powder. In addition to the oxide, hydroxide, oxalate, etc., the raw material powder may be a fine powder synthesized by a coprecipitation method or a sol-gel method. The powder purity is desirably 99.99% or more. This is because if the powder purity is low, the light emission characteristics such as light emission intensity and afterglow deteriorate.

  A Na compound weighed in a predetermined amount is added to the scintillator calcined powder thus produced, and is wet-mixed for about 10 hours by, for example, a ball mill and then dried. The additive mixed powder thus produced is sintered. The sintering method is not particularly limited, and sintering can be performed by a hot pressing method, a hot isostatic pressing (HIP) method, a normal pressure sintering method, a combination method of a normal pressure sintering method and a HIP method, or the like. Is possible.

In the hot press method, the above-mentioned additive mixed powder is molded by molding at a pressure of about 300 to 600 kgf / cm ² to form a molded body, and then set in a hot press mold, and in vacuum, air, or oxygen Sintering is performed at a temperature of 1400-1800 ° C. for several hours under 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.

In the HIP method, an additive mixed powder is placed in a metal capsule such as Fe, W, or Mo, vacuum sealed, and a temperature of 1200 to 1800 ° C. for several hours, 1000 to 2000
sintered at kgf / cm ² about pressure.
In normal pressure sintering, the additive powder mixture is subjected to cold isostatic pressing (CIP) at a pressure of about 3000 kgf / cm ² and then sintered in an atmosphere containing oxygen at 1400 to 1800 ° C. for several to several tens of hours. Do the tie.

  The ceramic scintillator thus obtained contains Na and has a crystal grain size as large as 1 to 500 μm. The large growth of the crystal grain size improves the workability and facilitates precision processing. Further, the afterglow is smaller than that of a conventional ceramic scintillator that does not use a Na compound, and the deterioration of the emission intensity is small.

  This manufacturing method does not limit the manufacturing method for obtaining the ceramic scintillator of the present invention, and the ceramic scintillator of the present invention can be manufactured by other manufacturing methods. For example, the ceramic scintillator of the present invention can also be produced by mixing and mixing Na compounds when calcining the raw material powder, calcining, and sintering this, and the same effect can be obtained. The ceramic scintillator of the present invention can also be obtained by omitting the calcining step and directly sintering the mixture of the raw material powder and the Na compound.

The oxide phosphor of the present invention has a light emission wavelength in the visible to near-infrared region, and a light output of 1.5 times or more that of CdWO ₄ can be obtained. Further, the afterglow is extremely short, and the afterglow becomes less than 1 × 10 ⁻³ % in about 300 ms after the X-ray is turned off. Therefore, it is extremely useful as a scintillator for a radiation detector. Further, the oxide phosphor of the present invention has a larger particle size than that containing no Na. Specifically, it is 10 times or more (20 μm or more) of the oxide phosphor having the same composition and not containing Na. Therefore, it is excellent in workability, and high-precision machining can be performed efficiently.

  Next, an embodiment of the radiation detector of the present invention will be described. FIG. 2 shows an embodiment of the radiation detector of the present invention. FIG. 2 shows a cross-sectional view of the radiation detector of the present invention. On the photodiode array substrate 24 having a plurality of photodiodes 23, scintillator elements 21 are arranged corresponding to the respective photodiode positions. ing. The surface of each scintillator element that does not face the photodiode is covered with the light reflecting layer 22.

As the scintillator element 21, the above-described ceramic scintillator of the present invention is used. As the photodiode 23, it is desirable to use a PIN photodiode. Since the PIN photodiode has high sensitivity, high response speed, and wavelength sensitivity in the visible to near infrared region, it matches well with the emission wavelength of the scintillator of the present invention. The light reflecting layer 22 is made of a material that absorbs less X-rays and has a high light reflectance. For example, an organic resin mixed with TiO ₂ powder is used.

  X-rays incident from the direction indicated by the arrow in the figure are transmitted through the light reflecting layer 22 and absorbed by the scintillator element 21. The scintillator element 21 that has absorbed X-rays emits visible light. Visible light emitted from the scintillator element 21 is reflected by the light reflecting layer 22 and converted into an electric signal by the photodiode 23.

  Although the manufacturing method of the radiation detector of this invention is not specifically limited, For example, scintillator material (bulk) is processed and the scintillator board of predetermined thickness is created. After this scintillator plate is bonded to the photodiode array substrate 24 on which a plurality of photodiodes 23 are formed, the scintillator plate is grooved corresponding to the photodiode array. A metal separator plate or a resinous channel separator is inserted into the created groove as necessary. Finally, the entire grooved scintillator 21 is covered with the material constituting the light reflecting layer 22 to obtain a radiation detector. Alternatively, a scintillator plate having a thickness corresponding to the size of the photodiode 23 and a channel separating material are alternately bonded, and then cut into a predetermined thickness to produce a scintillator array. The scintillator array is adhered to a photodiode array substrate 24 on which the photodiodes 23 are formed, and the entire scintillator array is covered with a material for the light reflecting layer 22 to obtain a radiation detector.

  In the manufacturing process of such a radiation detector, by using the ceramic scintillator of the present invention having excellent processability as a scintillator, a scintillator element with high processing accuracy can be easily produced. Therefore, the radiation detector assembled with high accuracy can be easily and inexpensively manufactured.

  Next, an embodiment of the imaging apparatus of the present invention will be described. FIG. 3 is a diagram showing an outline of an X-ray CT apparatus as an embodiment of the imaging apparatus of the present invention.

  This apparatus includes a scan gantry unit 310 and an image reconstruction unit 320. The scan gantry unit 310 is mounted on a rotating disk 311 having an opening 314 into which a subject is carried, and the rotating disk 311. An X-ray tube 312, a collimator 313 which is attached to the X-ray tube 312 and controls the radiation direction of the X-ray bundle, an X-ray detector 315 mounted on the rotating disk 311 facing the X-ray tube 312, A detector circuit 316 that converts the X-rays detected by the X-ray detector 315 into a predetermined signal, and a scan control circuit 317 that controls the rotation of the rotating disk 311 and the width of the X-ray bundle are provided.

  The image reconstruction unit 320 includes an input device 321 for inputting a subject's name, examination date and time, examination conditions, and the like, and an image computation circuit that performs CT image reconstruction by computing the measurement data S1 sent from the detector circuit 316 322, an image information adding unit 323 for adding information such as a subject name, examination date and time, and examination conditions input from the input device 321 to the CT image created by the image arithmetic circuit 322, and image information added And a display circuit 324 that adjusts the display gain of the CT image signal S2 and outputs it to the display monitor 330.

  In this X-ray CT apparatus, X-rays are irradiated from an X-ray tube 312 in a state where a subject is placed on a bed (not shown) installed in an opening 314 of a scan gantry unit 310. The X-ray is obtained by the collimator 313 and is detected by the X-ray detector 315. At this time, the direction of X-ray irradiation is changed by rotating the rotating disk 311 around the subject. The X-ray transmitted through the subject is detected. A tomographic image created by the image reconstruction unit 320 based on the measurement data is displayed on the display monitor 330.

  Here, the X-ray detector 315 includes a large number (eg, 960) of detector elements in which a scattered radiation removing collimator, a scintillator, and a photodiode are combined, arranged in an arc shape, and the radiation detector of the present invention is used. Yes. For this reason, the X-ray CT apparatus of the present invention has a radiation detector assembled with high precision, and can provide a high-quality CT tomogram.

Examples of the present invention will be described below.
[Example 1]
As the starting material powder, Gd ₂ O ₃ , Al ₂ O ₃ , Ga ₂ O ₃ , CeO ₂ powder having a purity of 99.99% and a particle size of about 0.1 to 5 μm was used, and Gd _2.982 Ce _0.018 Al _2.8 Ga _2.2 O ₁₂ Each was weighed to have a composition. This 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. To this scintillator calcined powder, 0.0005 to 0.01 mol of Na ₂ B ₄ O ₇ was added to 1 mol of the scintillator powder, and wet-mixed with a ball mill for about 16 hours and dried. The additive powder mixture, 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 mm × 24 mm × 2 mm test piece and annealed in oxygen at 1300 ° C. for 4 hours to complete a ceramic scintillator plate.

The obtained scintillator plate was measured for the residual amount of Na, the crystal grain size, and the light emission characteristics. The results are shown in Table 1. The residual amount of Na was measured by ICP emission analysis. The crystal grain size was measured by observing the surface of the test piece with a scanning electron microscope. The emission intensity was measured by irradiating X-rays with a tube voltage of 120 kV and a tube current of 100 mA. In Table 1, the emission intensity is expressed as a ratio to the emission intensity of the CdWO ₄ scintillator. The afterglow was expressed as the ratio of the output at 300 ms after X-ray irradiation to the output during irradiation.
Furthermore, in order to evaluate workability, each test piece was lapped with a load of 100 g, and the rate of decrease in the weight of the test piece was measured. The results are shown in Table 1. The higher the processing speed, the better the workability of the material. FIG. 1 shows changes in crystal grain size and processing speed when the amount of Na ₂ B ₄ O ₇ added is changed.

From Table 1 and FIG. 1, it can be seen that as the amount of Na ₂ B ₄ O ₇ added increases, the crystal grain size grows and the workability also increases accordingly. Moreover, the afterglow is reduced as the amount of the auxiliary agent added is increased. However, as the amount of auxiliary added increases, the emission intensity also decreases slightly. Compared with the sample to which the auxiliary of Sample No. 1 is not added, up to the added amount of 0.002 mol, the decrease in the emission intensity is within about 10%, which is a range where the effect of the present invention can be sufficiently obtained. At this time, the residual amount of Na is 500 wtppm. When Na ₂ B ₄ O ₇ is further added, the emission intensity further decreases and the afterglow hardly decreases. Therefore, it can be said that the Na content in the scintillator of the present invention is preferably 500 wtppm or less.

[Example 2]
A scintillator calcined powder was prepared in the same manner as in Example 1, and different types of Na compounds (NaCl, NaBr, Na ₂ CO ₃ , Na ₃ PO ₄ .12H ₂ O, Na ₂₎ were used in this scintillator calcined powder. A scintillator plate was prepared in the same manner as in Example 1 except that SO ₄ ) was added. The amount of Na compound added was such that Na ion was 0.001 mol per 1 mol of scintillator powder. The obtained scintillator plate was measured for the residual amount of Na, the crystal grain size, the light emission characteristics, and the processing speed in the same manner as in Example 1. The results are shown in Table 2. Table 2 also shows the results of sample numbers 1 and 2 of Example 1 for reference.

[Comparative Example 1]
A scintillator calcined powder was produced in the same manner as in Example 1, and the Li compound shown in Table 2 was added to the scintillator calcined powder, and a scintillator sample was produced in the same steps as in Example 1. The amount of Li compound added was such that the amount of Li ions was 0.001 mol per mol of scintillator powder. For the obtained scintillator sample, the residual amount of Li, the crystal grain size, the light emission characteristics, and the processing speed were measured in the same manner as in Example 1. The results are also shown in Table 2.

As can be seen from the results in Table 2, when NaCl, NaBr, Na ₂ CO ₃ is used as the Na compound (sample numbers 6 to 8), as in sample number 2, crystal grain growth, workability improvement, and remaining There is an effect of reducing light, and the decrease in emission intensity is less than 10%. On the other hand, when Na ₃ PO ₄ .12H ₂ O and Na ₂ SO ₄ are used (sample numbers 9 and 10), the processability is improved, but compared to sample number 1 where no Na compound was added. The afterglow has increased or the emission intensity has decreased. Further, when Li compound was used (Sample Nos. 11 and 12), the afterglow was larger than that of Sample No. 1, and in the case of LiCl, the particle size was not increased and the workability was not improved.

Diagram showing the relationship between Na content, crystal grain size and processing speed The figure which shows the cross section of the radiation detector of this invention The figure which shows the structure of the X-ray CT apparatus of this invention

Explanation of symbols

21 scintillator element, 22 reflective layer, 23 photodiode, 24 photodiode array substrate, 310 scan gantry, 311 rotating disk, 312 X-ray tube, 313 collimator, 314 aperture, 315 X-ray detector, 316 detector circuit , 317 scan control circuit, 320 image reconstruction unit, 321 input device, 322 image calculation circuit, 323 image information addition unit, 324 display circuit, 330 display monitor

Claims (4)

It consists of a base crystal of garnet structure, which contains Ce as a light emitting element and contains at least Gd, Al, Ga, O , and is selected from the group consisting of Na ₂ B ₄ O ₇ , NaCl, NaBr and Na ₂ CO ₃ An oxide phosphor for a radiation detector produced by adding a seed or two or more kinds of Na compounds ,
An oxide phosphor containing Na in a range of 40 wtppm to 50 wtppm . A method for producing 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, mixing raw material powders, and calcining; 1 or 2 or more types of Na compounds selected from the group consisting of Na ₂ B ₄ O ₇ , NaCl, NaBr and Na ₂ CO ₃ . And an oxide phosphor for a radiation detector containing Na in an amount of 40 wtppm to 50 wtppm .   A 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 oxide phosphor according to claim 1 is used as the phosphor element. Characteristic radiation detector.   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 3 is used as the radiation detector. X-ray CT apparatus characterized by this.

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