JP2009227794A - Fluorescent material, its producing method and scintillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a LuAG (lutetium aluminum garnet) sintered compact having sufficient properties as a scintillator to be used in a PET detector. <P>SOLUTION: The raw materials are mixed wetly by using a ball mill using an alumina ball at a mixing step. Ethanol is used as a solvent in this case, for example. The mixing time can be 12-40 hours. When TEOS (tetraethylorthosilicate) is used particularly as a Si raw material, it is easy to mix TEOS uniformly with ethanol since TEOS is liquid at normal temperature. When the Si content is made larger, it can be recognized that the luminuous intensity is decreased and the resolution is also deteriorated gradually (becomes larger). Even when the Si content is <30 ppm, the luminuous intensity is deteriorated. Therefore, there is an optimum range when the Si content is set. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fluorescent material that emits light by absorbing radiation such as γ-rays, a manufacturing method thereof, and a scintillator manufactured using the fluorescent material.

  In recent years, PET (Positron Emission Tomography) has been used as a medical device. PET is a technique for diagnosing a subject by administering a substance that emits positrons to the subject, detecting the positrons emitted from the body, and specifying the distribution. In PET, it is necessary to detect positrons. In this case, γ rays (energy 511 keV) generated when positron annihilation occurs are detected.

  In general, it is difficult to efficiently detect radiation such as X-rays and γ-rays directly with an ordinary photodetector. For this reason, fluorescent materials (scintillators) that absorb these radiations (photons of X-rays and γ-rays) and emit light (photons of ultraviolet rays and visible light) are used. When the scintillator absorbs photons of X-rays or γ-rays, it emits a number of photons of visible light or ultraviolet light corresponding to the energy. These radiations can be detected indirectly by converting the photons of visible light and ultraviolet light into electrical signals, for example, with a photomultiplier tube (photomultiplier). Therefore, in PET, a scintillator that absorbs the above-described energy γ-rays to emit light is required.

As such a scintillator material, for example, BGO (Bi ₄ Ge ₃ O ₁₂ ) is known. However, in order to increase the detection efficiency of this γ-ray, there is a demand for a material that emits more visible light or ultraviolet light from one γ-ray photon, that is, a material having higher luminous efficiency. This light emission occurs every time the photon of one γ ray is absorbed by the scintillator, and the intensity (count number) of the γ ray corresponds to the number of times of this light emission. Here, if the emission time (fluorescence lifetime) due to one absorption of γ-rays is long, it overlaps with the light emission when γ-rays are incident next, making it difficult to count. Therefore, it is also necessary for the fluorescence lifetime to be short. That is, there is a demand for a material having higher luminous efficiency and shorter fluorescence lifetime than BGO.

There is LuAG (Lu ₃ Al ₅ O ₁₂ : lutetium aluminum garnet) as a material satisfying such requirements. LuAG is used by adding cerium (Ce) or praseodymium (Pr) as a light emitting element. LuAG can be preferably used in PET because it has higher luminous efficiency and shorter fluorescence time than BGO. This polycrystal (sintered body) is described in Non-Patent Document 1, and the single crystal is described in Patent Document 1, for example. At this time, since the absorption efficiency of γ rays is low, a thick large scintillator is necessary to obtain sufficient detection efficiency. Further, in order to obtain an image in PET, a large number of detectors arranged in a ring shape are required. Therefore, it is also important to obtain this scintillator at a low price.

  Here, it is described in Non-Patent Document 1 that the LuAG sintered body to which Ce is added can obtain emission intensity about 1.3 times that of the BGO sintered body. On the other hand, Non-Patent Document 2 describes that according to the LuAG single crystal to which Pr is added, emission intensity as high as 3.3 times that of the BGO single crystal can be obtained. Therefore, from the viewpoint of obtaining high emission intensity, LuAG added with Pr is more preferable.

  However, when a single crystal of LuAG doped with Pr is grown, it is difficult to obtain a uniform crystal because Pr segregates. Moreover, the temperature required for this crystal growth is high and requires a long time. Further, since it is necessary to mechanically process the single crystal into a desired shape after that, the manufacturing cost is increased. On the other hand, no segregation of Pr occurs in the sintered body (polycrystal). Moreover, since the process itself of obtaining a sintered body is easier than single crystal growth and can be formed into a desired shape at the time of forming the sintered body, the manufacturing cost of the sintered body is reduced.

  Therefore, the LuAG sintered body to which Pr is added can be a material particularly suitable for PET.

Martin Nikl, Jiri Mares, Natasha Solovieva, Hui-Li Li, Xue-Jian Liu, Li-Ping Huang, Laria Fonana, Mauro Fasoli, Anna Vedda, Ana Vedda, Year) Furukawa Machine Metal Co., Ltd., Materials Research Laboratory, Product Introduction, Internet (Search March 17, 2008, URL: http://www.furukawakk.co.jp/rd/material/rd_product/rd_crystal.html) JP 2006-16251 A

  However, generally, the emission intensity of the polycrystalline material (sintered body) is lower than that of the single crystal material, and it has been difficult to obtain a LuAG sintered body capable of obtaining a high emission intensity. That is, it has been difficult to obtain a LuAG sintered body having sufficient characteristics as a scintillator used for a PET detector.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The gist of the present invention is a method for producing a fluorescent material comprising a LuAG (lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added, the powder being used as a raw material for the LuAG sintered body, The present invention resides in a method for producing a fluorescent material, comprising a mixing step of adding a Si component as much as possible in a silicon (Si) concentration in a range of 30 to 400 ppm.
The gist of the present invention resides in the method for producing a fluorescent material, wherein the Si component is TEOS (tetraethylorthosilicate).
The gist of the present invention is a fluorescent material made of a LuAG (Lu ₃ Al ₅ O ₁₂ : lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added, wherein silicon (Si) has a concentration in the range of 30 to 400 ppm. The fluorescent material is characterized by being added in the above.
The gist of the present invention resides in a fluorescent material manufactured by the above-described method for manufacturing a fluorescent material.
The gist of the present invention is a LuAG (Lu ₃ Al ₅ O ₁₂ : lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added, the Si concentration is 400 ppm or less, and the equivalent circle diameter is 0 in the cross section. The present invention resides in a fluorescent material having an intragranular void density of 500 μm / mm ² or less that is 1 μm or more.
The gist of the present invention resides in a scintillator comprising any one of the fluorescent materials described above.

  Since the present invention is configured as described above, a LuAG sintered body having sufficient characteristics as a scintillator can be easily obtained.

  In order to increase the emission intensity in the scintillator, the luminous efficiency of the fluorescent material constituting the scintillator is high, that is, the number of visible or ultraviolet photons emitted by absorbing one γ-ray photon is large. Required. Further, in the detection of γ-rays, since the absorption of γ-rays in the scintillator material is small, it is necessary to increase the thickness of the scintillator sufficiently to absorb the γ-rays sufficiently. For this reason, the distance that the generated visible light or ultraviolet light passes through the scintillator becomes longer, and the probability that these lights are scattered and absorbed in the scintillator during that time increases. Therefore, it is also required that these light transmittances are high.

The inventor found that Si added as a sintering aid in a LuAG (Lu ₃ Al ₅ O ₁₂ : lutetium aluminum garnet) sintered body to which praseodymium (Pr) was added was effective for both luminous efficiency and light transmittance. By optimizing this concentration, a method for producing a fluorescent material that obtains high emission intensity as a scintillator material was completed.

  That is, the scintillator (fluorescent material) according to the embodiment of the present invention is LuAG to which Pr is added as a light emitting element, and is manufactured as a sintered body. At this time, silicon (Si) is added as a sintering aid. FIG. 1 is a process diagram showing a method of manufacturing a scintillator according to the present invention.

First, the raw materials weighing process, lutetium oxide as the raw materials _(Lu 2 _{O 3)} powder, alumina _(Al 2 _{O 3)} powder, a three praseodymium nitrate _{(Pr (NO 3) 3 ·} 6H 2 O) powder, respectively Weigh as much as possible to a predetermined composition. Here, praseodymium trinitrate powder is a raw material of Pr which becomes a light emitting element. Further, a predetermined amount of TEOS (Si (OC ₂ H ₅ ) ₄ : tetraethylorthosilicate), which is liquid at room temperature, is weighed into these powders as a Si raw material that serves as a sintering aid. By adding this sintering aid, the sintering proceeds properly and a dense LuAG sintered body can be obtained. In addition, as the Si raw material, SiO ₂ powder or the like can also be used.

  In order to obtain a LuAG sintered body having a stoichiometric composition, for example, 1396.17 g of lutetium oxide powder having a purity of 99.999% and an average particle diameter of about 1 μm, a purity of 99.995% and an average particle diameter of 0.1%. 598.03 g of about 4 μm alumina powder is used. For example, in order to set the Pr concentration to 0.9 mol%, 9.185 g of praseodymium trinitrate powder having a purity of 99.9% is used. The amount of TEOS mixed will be described later.

  Next, in the mixing step, the raw materials are wet-mixed using a ball mill using alumina balls. In this case, for example, ethanol is used as the liquid medium, and the mixing time can be 12 to 40 hours. At this time, particularly when TEOS is used as the Si raw material, TEOS is liquid at room temperature. Therefore, if it is mixed with ethanol, it is particularly easy to uniformly mix it. Moreover, since praseodymium trinitrate is soluble in ethanol, it can be uniformly mixed particularly if dissolved in ethanol. Thereafter, ethanol as a medium is removed by drying.

  In this mixing step, in addition to the above components, an organic substance can be added as a binder or a dispersant within a range that does not affect the properties of the obtained phosphor, and granulation can be performed by a spray drying method. In this case, it is preferable to remove the carbon component by a heat treatment at 500 to 800 ° C. after the molding step described below.

Next, in the forming step, the above mixture is formed into a predetermined shape as a scintillator, for example, a rectangular parallelepiped shape. At this time, after first forming a rectangular parallelepiped of a predetermined size by, for example, a pressure of about 0.5 t / cm ² by the uniaxial pressing method, 3 t / cm ² by the CIP (Cold Isostatic Press) method. It is preferable to mold at a moderate pressure because a dense molded body can be obtained. In addition, any method can be used as long as it is a method capable of obtaining a molded body having a predetermined shape, such as cast molding, extrusion molding, or injection molding.

  Next, in the firing step, the formed body is fired. This baking is performed at a temperature of about 1650 to 1800 ° C., for example, in a vacuum. The firing time is 6 to 40 hours. As a result, a PrAG-added LuAG sintered body is obtained. Moreover, this baking can also be performed in an oxygen atmosphere, a hydrogen atmosphere, or the like. Further, a hot press (HP) method or a hot isostatic press (HIP) method may be used.

  The fluorescence characteristics when the LuAG sintered body manufactured by the above manufacturing method is used as a scintillator were measured by an evaluation system having the configuration shown in FIG. Here, the γ-rays to be measured are 662 keV γ-rays emitted from the radiation source 1 (Cs137) which is a radioisotope, and this is incident on the scintillator 2 manufactured by the above-described manufacturing method. As described above, the scintillator 2 emits light (fluorescence) having a wavelength of 310 nm by absorbing the γ rays. The photomultiplier tube (PMT) 3 detected this light, and the pulse waveform output at that time was observed with a digital oscilloscope 4, whereby the time constant (fluorescence lifetime) of the fluorescence in the scintillator 2 was measured. Since this pulse waveform decays exponentially, the fluorescence lifetime is defined as the time until the intensity reaches 1 / e, for example. This light emission occurs every time a photon of γ rays is absorbed in the scintillator 2.

  Further, this output was amplified by the preamplifier 5, shaped by the waveform shaper 6, and the wave height (channel) distribution was measured by the multichannel analyzer 7. An example of the measured wave height distribution (horizontal axis: pulse wave height, vertical axis: count number) is shown in FIG. This wave height (horizontal axis) corresponds to the number of photons with a wavelength of 310 nm generated by one gamma ray, but since the number of photons has a distribution, it has a shape extending from the center pulse wave height, and the center pulse wave height is emitted. Corresponds to strength. However, since a part of γ-ray energy is given to electrons in the sample by the Compton effect, there is a region with a high count number, particularly at a pulse height lower than this.

  The center pulse wave height corresponds to the emission intensity. Also, from this wave height distribution, the resolution of this energy in γ rays was measured as (half width in pulse wave height distribution) / (center pulse wave height).

   In the above measurement, the scintillator 2 had a size of 10 mm × 20 mm × 2 mm, and all surfaces were mirror-polished. In the above measurement, γ rays were incident on a 10 mm × 2 mm surface, and a reflective material was pasted on the surface other than the visible light measurement surface.

FIG. 4 shows the results of measurement of emission intensity and resolution using the system of FIG. 2 when the Si content is changed by changing the TEOS addition amount in the scintillator manufactured by the above manufacturing method. Here, the emission intensity is indicated by channels in the wave height distribution, and the Si addition amount is indicated by weight ppm (wtppm) of Si relative to the unit weight of the sample. For example, 30 wtppm corresponds to 0.003 when the unit weight of the sample is 100. Here, the Si addition amount is the Si amount converted from the TEOS addition amount at the time of mixing using the composition formula. Since the evaporation of TEOS in the mixing and drying process is negligible, the Si content in the sintered body substantially corresponds to this Si addition amount. Further, the carbon component in TEOS is discharged as CO ₂ at the time of sintering or the like.

  From this result, it can be confirmed that when the Si addition amount is large, the light emission intensity decreases and the resolution gradually deteriorates (increases). Also, the emission intensity deteriorates when the Si addition amount is 30 ppm or less. Therefore, there is an optimum range for the Si content. Since the change in the density of the LuAG sintered body due to the Si addition amount is negligible, the amount of γ-ray absorption is considered to be constant within the range shown in FIG. 4 regardless of the Si addition amount.

  When the Si addition amount is large, the emission intensity decreases because a new level is formed by Si added in the LuAG crystal, and the transition through the level resulting from the original emission formed by Pr occurs. This is because it becomes difficult to occur. That is, the luminous efficiency decreases as the Si addition amount increases. Therefore, from this viewpoint, it is preferable that the Si addition amount is small. From the result of FIG. 4, for this purpose, the Si addition amount is preferably 400 ppm or less as a range in which, for example, 70% or more is obtained from the maximum emission intensity. Moreover, 250 ppm or less is more preferable as a range in which 80% or more can be obtained from the maximum emission intensity.

  On the other hand, FIG. 5 shows the results of examining the Si concentration dependence of the light transmittance of 310 nm in this scintillator. Here, the thickness of the scintillator (fluorescent material) was 2 mm. From this result, it can be confirmed that the transmittance is substantially constant when the Si addition amount is 100 ppm or more, but greatly decreases when the Si content is 30 ppm or less.

  Therefore, in this scintillator, when the amount of Si added is small, the transmittance is lowered, so that the emission intensity is considered to be substantially reduced. As a factor that greatly affects the transmittance, there is a void that exists as a light scattering center in the sintered body. There are two types of voids: intragranular voids that exist within a single grain and intergranular voids that exist at grain boundaries.

  The structure of the above LuAG sintered body was observed with an electron microscope, and the average particle diameter and void density (per unit area of the cross section) were examined. FIG. 6 shows a field emission scanning electron microscope (FE-SEM) photograph of this cross section. This photograph was taken with a field emission scanning electron microscope (FE-SEM). In the figure, 8 is a LuAG crystal grain, a portion indicated by 9 is an intragranular void, and a location indicated by 10 is a grain boundary void. FIG. 7 shows the Si addition amount dependence of the density of intragranular voids having an equivalent circle diameter of 0.1 μm or more. FIG. 8 shows the Si addition amount dependency of the density of all voids including intragranular voids and grain boundary voids. Here, the equivalent circle diameter means the diameter of a circle having the same area as the void observed in the cross section. The same applies to the average particle diameter, which is the average value of the diameters of circles having the same area as the observed crystal grains.

  Si, which is a sintering aid, promotes grain growth during sintering, and serves to crush voids when the grain boundary moves with grain growth. The larger the Si addition amount in the range where the Si addition amount is greater than 100 ppm, the larger the average particle size means that sintering proceeds due to the presence of Si, and a dense LuAG sintered body is obtained. Yes. However, as the Si addition amount increases, grain growth proceeds faster than crushing the voids, and voids that could not be crushed are left in the grains to become intragranular voids. The number is increasing. However, as shown in FIG. 8, since the variation rate of the total number of intergranular voids and intragranular voids is small compared to the variation rate of intragranular voids, as shown in FIG. Absent.

  On the other hand, when the Si addition amount is less than 30 ppm, the effect as a sintering aid is insufficient, and the average grain size is slightly increased because the crystal grains grow partly without being densified. is doing. However, in this state, the action of crushing voids due to Si is small, and a dense LuAG sintered body is not formed, so that the number of intragranular voids and total voids are greatly increased. That is, in this case, since the number of voids that become light scattering centers increases, the transmittance decreases as shown in FIG. The decrease in the emission intensity due to the decrease in the transmittance is 40% or more of the maximum emission intensity in the result of FIG. For example, when this fluorescent material is used as a scintillator in PET, a scintillator that is thicker than the sample in this case may be used, and in this case, it is clear that this effect increases as the thickness increases. In order to suppress a substantial decrease in light emission intensity due to the decrease in the transmittance, it is preferable from this result that the Si addition amount is in a range of 30 ppm or more.

  As mentioned above, in this LuAG sintered compact, it is preferable that Si addition amount exists in the range of 30-400 ppm. A LuAG sintered body in which the amount of Si added is within this range has sufficient characteristics as a scintillator.

  Moreover, from the result of FIG. 4, the dependence of the resolution on the Si addition amount is small, and a good value of about 10% is obtained in the above range. Furthermore, the fluorescence lifetime was measured as described above, and a good value of about 23 ns was obtained within the above range.

  In the above example, the concentration of Pr, which is a light-emitting element, is 0.9 mol%. However, the effect of Pr concentration on the sinterability in the LuAG sintered body is small, so the same applies to other Pr concentrations. is there.

Incidentally, Si addition amount, can be adjusted by adjusting the TEOS or SiO ₂ powder added amount, in the case that contains the Si component to other raw material powder, the total amount of the Si component after firing of the It is preferable to adjust as much as possible.

Further, the intragranular void density corresponding to the lower limit of the Si addition amount is about 500 / mm ² from the result of FIG. That is, in the LuAG sintered body in which the Si addition amount is 400 ppm or less, a preferable intragranular void density is 500 pieces / mm ² or less.

  In the above manufacturing method, as shown in FIG. 1, the emission intensity can be increased only by adjusting the addition of the Si component as a sintering aid in the ordinary sintered body manufacturing method. In addition, a desired shape can be easily obtained in the molding process. Therefore, this phosphor and scintillator can be obtained at low cost.

It is process drawing which shows the manufacturing method of the fluorescent substance used as embodiment of this invention. It is a figure which shows the structure of the evaluation system of fluorescent substance. It is an example of the measured pulse wave height distribution. It is a figure which shows the light emission intensity (wave height channel) in LuAG sintered compact, and the Si addition amount dependence of the resolution. It is a figure which shows the Si addition amount dependence of the transmittance | permeability of the light of wavelength 310nm in a LuAG sintered compact. It is an example of the field emission type | mold scanning electron micrograph of the cross section of a LuAG sintered compact. It is a figure which shows the Si addition amount dependence of the average particle diameter and the intragranular void density in a LuAG sintered compact. It is a figure which shows the Si addition amount dependence of the average particle diameter and total void density in a LuAG sintered compact.

Explanation of symbols

1 radiation source 2 scintillator 3 photomultiplier tube (PMT)
4 Digital Oscilloscope 5 Preamplifier 6 Waveform Shaper 7 Multichannel Analyzer 8 LuAG Grain 9 Intragranular Void 10 Grain Boundary Void

Claims (6)

A method for producing a fluorescent material of a LuAG (lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added,
A method for producing a fluorescent material comprising a mixing step of adding a Si component to a powder as a raw material of the LuAG sintered body so that a silicon (Si) concentration in the sintered body is in a range of 30 to 400 ppm. .   The method for producing a fluorescent material according to claim 1, wherein the Si component is TEOS (tetraethylorthosilicate). A fluorescent material comprising a LuAG (Lu ₃ Al ₅ O ₁₂ : lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added,
A fluorescent material, wherein silicon (Si) is added at a concentration in the range of 30 to 400 ppm.   A fluorescent material manufactured by the method for manufacturing a fluorescent material according to claim 1. A LuAG (lutetium aluminum garnet) sintered body to which praseodymium (Pr) is added,
Si concentration is 400 ppm or less,
A fluorescent material characterized in that, in a cross section, an intra-particle void density having an equivalent circle diameter of 0.1 μm or more is 500 pieces / mm ² or less. A scintillator comprising the fluorescent material according to any one of claims 3 to 5.