JP2011202118A - Single crystal scintillator material and manufacturing method therefor, radiation detector and pet device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single crystal scintillator material which has a larger amount of emissions than before, and outstanding fluorescence decay characteristics, and to provide its manufacturing method, radiation detector, and PET device.SOLUTION: The manufacturing method for single crystal scintillator material includes steps of mixing Ce and Lu compounds into a solvent containing Pb, at least one species selected from Li, Na, K, Rb, and Cs, W and/or Mo, and B and oxygen, and heating it to a temperature of ≥800°C and ≤1,350°C to melt the compounds, and carrying out the deposition growth of single crystal expressed by the composition formula (CeLu)BOand satisfying the composition ratio x of Ce 0.0001≤x≤0.05 by cooling the molten compound.

Description

  The present invention relates to a single crystal scintillator material for a positron emission nuclide tomography apparatus and a method for producing the same.

  In recent years, diagnosis by a positron emission tomography (hereinafter referred to as “PET”) has been widely performed in the medical field, and an excellent scintillator for realizing a PET apparatus with higher performance. The search for materials is ongoing.

  PET scintillator materials need to detect γ-rays, and so far BGO (bismuth germanium oxide), LSO (lutetium silicon oxide), GSO (gadolinium silicon oxide), LYSO (lutetium yttrium silicon oxide), etc. Single crystal scintillator materials have been applied to PET. The characteristics of the scintillator material are evaluated by the amount of luminescence (fluorescence output), the fluorescence decay time, the energy resolution, and the like, but the above-described single crystal material is excellent in the characteristics necessary for application to PET. As methods for producing these single crystals, melt growth methods such as the Czochralski method and the Bridgman method are widely used commercially.

  In order to popularize PET, it is necessary to improve diagnostic throughput. To that end, it is essential to develop a single crystal scintillator material that has a larger light emission amount and a shorter fluorescence decay time than conventional scintillator materials.

  Patent Document 1 describes an example of GSO activated with Ce (cerium). On the other hand, Patent Document 2 and Patent Document 3 disclose cerium-activated lutetium borate-based materials. Cerium-activated lutetium borate is expected to be an excellent scintillator material because it has a large light emission amount and a short fluorescence decay time. Patent Document 3 also proposes application of cerium-activated lutetium borate materials to PET, but the cerium-activated lutetium borate materials disclosed in these documents are only powders. As described above, the methods described in Patent Document 2 and Patent Document 3 could not form a single crystal of cerium-activated lutetium borate having a size that can be used for PET.

  Lutetium borate has a phase transition point (about 1350 ° C.) with a large volume change in a temperature region lower than the melting point (1650 ° C.). For this reason, when a conventional single crystal growth method that requires heating or melting the starting material to a high temperature is used, volume expansion occurs when passing through the phase transition point during cooling of the melt. There was a problem that the crystals collapsed. Patent Document 4 discloses a method of manufacturing a single crystal material for scintillator by adding any element of Sc, Ga, and In to a lutetium borate-based material to suppress a phase transition of the crystal material. ing. However, the lutetium borate single crystal formed by the method described in Patent Document 4 suffers from characteristic deterioration such as a decrease in density due to an additive element and a decrease in light emission amount.

  In order to solve these problems, the applicant of PCT / JP2008 / 1717 (filed on July 1, 2008) created a cerium-activated lutetium borate prepared by a flux method using a lead borate-based solvent. A single crystal is disclosed.

Japanese Patent Laid-Open No. 2003-300795 JP 2005-298678 A JP 2006-52372 A JP 2007-224214 A

  According to the method disclosed in PCT / JP2008 / 1717, a cerium-activated lutetium borate single crystal material having a calcite-type crystal structure can be obtained by a simple method without adding an element such as Sc. it can. However, according to this method, there is a possibility that an extremely small amount of a solvent component is mixed in the produced cerium-activated lutetium borate single crystal and the original characteristics cannot be exhibited. For this reason, the inventors have studied to reduce the amount of solvent components mixed in the cerium-activated lutetium borate single crystal when producing a cerium-activated lutetium borate single crystal with a lead borate-based solvent. It was.

  Accordingly, an object of the present invention is to provide a single crystal scintillator material having a larger light emission amount and superior fluorescence decay characteristics, a method for producing the same, a radiation detector, and a PET apparatus.

The method for producing a single crystal scintillator material according to the present invention comprises a solvent containing Pb, at least one selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, and B and oxygen. A step of mixing the Ce compound and the Lu compound with the solvent, heating the compound to a temperature of 800 ° C. to 1350 ° C. and melting the compound, and cooling the molten compound, is represented by _{(Ce x Lu 1-x)} BO 3, and a step of precipitating growing a single crystal having a composition ratio x of Ce satisfies 0.0001 ≦ x ≦ 0.05.

In a preferred embodiment, in the step of preparing the solvent and the step of melting the compound, the compound forming the solvent, the Ce compound, and the Lu compound are mixed and heated to a temperature of 800 ° C. or higher and 1350 ° C. or lower. It is a process.
In the step of preparing the solvent, the boric acid-based material, the lead-based material, and the non-lead-based material are mixed to constitute the solvent. However, all the materials for the solvent may be mixed at the same time, or a part thereof The material may be added later.
In a preferred embodiment, the Ce composition ratio x satisfies 0.001 ≦ x ≦ 0.03.
In a preferred embodiment, the step of depositing and growing the single crystal is performed by a TSSG method.
In a preferred embodiment, in the step of precipitating and growing the single crystal, the molten compound is cooled to a temperature of 750 ° C. or higher and lower than 1350 ° C. at a temperature lowering rate of 0.001 ° C./hour or more and 5 ° C./hour or less.
In a preferred embodiment, the precipitation growing step is performed over 80 hours or more.

The single crystal scintillator material of the present invention has a single crystal part represented by a composition formula (Ce _x Lu _1-x ) BO ₃ and a composition ratio x of Ce satisfying 0.0001 ≦ x ≦ 0.05, The content of Pb in the single crystal part is 100 ppm or less by mass ratio. That is, in the unit mass of 100% of the single crystal part, the Pb content is suppressed to 100 ppm or less by mass ratio.

In a preferred embodiment, the single crystal portion has a calcite type crystal structure.
In a preferred embodiment, the transmittance at a wavelength of 280 nm of the single crystal portion mirror-finished to a thickness of 0.5 mm is 20% or more.

  The radiation detector of the present invention includes a single crystal scintillator material according to the present invention and a detector that detects light emission from the single crystal scintillator material.

  The PET apparatus of the present invention is a PET apparatus that includes a plurality of radiation detectors arranged in a ring shape and detects γ rays from a subject, each of the plurality of radiation detectors according to the present invention. It is a radiation detector.

  According to the present invention, it is possible to obtain a colorless and transparent single crystal scintillator material having no emission of impurities that are colored and having a larger light emission amount than conventional ones.

It is a graph which shows the transmittance | permeability of the single crystal by this invention, and a comparative example. It is a figure which shows the crystal growth apparatus used by this invention. It is a graph which shows the heat pattern of the crystal growth in Example 1 of this invention. It is a photograph which shows the crystal body produced in Example 1 of this invention. ¹ shows a γ-ray excited emission spectrum in which the crystal of Example was emitted by γ-ray excitation from a ¹³⁷ Cs ray source. It is a figure which shows the crystal growth apparatus used by the comparative example. It is a graph which shows the heat pattern of the crystal growth in the comparative example 1 of this invention. It is a graph which shows the heat pattern of the crystal growth in Example 2 of this invention. It is a graph which shows the lead borate solvent ratio dependence of the X-ray excitation light emission amount excited by the CuK (alpha) ray in Examples 2-4 of this invention. It is a perspective view which shows the structural example of a scintillator array. It is sectional drawing which shows the structural example of the radiation detector of this invention. It is sectional drawing which shows an example of PET apparatus of this invention.

The method for producing a single crystal scintillator material according to the present invention comprises a solvent containing Pb, at least one selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, and B and oxygen. A step of mixing the Ce compound and the Lu compound with the solvent, heating the compound to a temperature of 800 ° C. to 1350 ° C. and melting the compound, and cooling the molten compound, is represented by _{(Ce x Lu 1-x)} BO 3, and a step of precipitating growing a single crystal having a composition ratio x of Ce satisfies 0.0001 ≦ x ≦ 0.05. More specifically, it is desirable to cool the molten compound and precipitate and grow the single crystal at a temperature lower than the phase transition point from the high-temperature vaterite phase to the calcite phase of cerium-activated lutetium borate.

The solvent used in the production method of the present invention contains Pb and contains at least one alkali metal selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, and B and oxygen. And the amount of Pb contained in the solvent is 35 mol% or less in terms of PbO.
When Pb is contained in the solvent, a single crystal with few defects is likely to be formed. Therefore, Pb is preferably contained.

  Further, when about 0.1 mass% is mixed in the single crystal material to be produced, the single crystal material is colored. As a result, the original characteristics may not be exhibited. On the other hand, if the amount of Pb in the solvent exceeds 35 mol% in terms of PbO, the amount of emitted light decreases with time when the generated single crystal material is irradiated with radiation such as X-rays. A single crystal material produced from a lead borate-based solvent containing 70 mol% of Pb in terms of PbO, when irradiated with X-rays from a CuKα radiation source, resulted in a 10% decrease in light emission after 1 minute. It has been. Therefore, the Pb content in the solvent needs to be 50% by weight or less in order not to cause coloring or performance degradation. Since the solvent containing Pb tends to have a high viscosity, it is preferable to contain an alkali metal in order to adjust the temperature dependence of the viscosity and solubility of the solvent. Further, in order to lower the melting point of the solvent, it is desirable to contain two or more alkali metals, and Li and Na can be suitably used. It is known that the above-mentioned alkali metals and their compounds are very similar in nature, and even if K, Rb, or Cs is used, the same effect as that obtained when Li or Na is used can be expected. .

Furthermore, the solvent used in the production method of the present invention contains W and / or Mo as an element that forms a compound having a low melting point with an alkali metal. From the viewpoint of bringing the density of the solvent close to the density of the single crystal material to be produced, it is desirable to form a low melting point compound between an element having a larger atomic weight and an alkali metal. For this reason, W is more preferably adopted than Mo. However, Mo is also a heavy element of the same genus as W, and its properties are similar to W. Therefore, even if Mo is used together with W or instead of W, the same effect can be expected.
B contained in the solvent is an essential element as a component of the produced single crystal. This B forms an alkali metal and a boric acid compound, forms lead and lead borate, and becomes a component of the solvent.
A raw material that becomes a single crystal component, such as a Ce compound and a Lu compound, is melted in the solvent, and then slowly cooled to precipitate a single crystal.

According to the production method of the present invention, a colorless and transparent single crystal part represented by a composition formula (Ce _x Lu _1-x ) BO ₃ and having a Ce composition ratio x satisfying 0.0001 ≦ x ≦ 0.05 is obtained. A single crystal scintillator material can be obtained. In the present invention, the typical shape of the single crystal portion after precipitation and crystal growth is generally a flat plate shape, but it does not exclude processing into other shapes.

Typically, the entire single crystal scintillator material of the present invention is composed of the above “single crystal part”, but a part other than the “single crystal part”, for example, a part that is polycrystallized is a single crystal. It may be included in the scintillator material, or a protective film or the like may be attached to the single crystal scintillator material.
The rare earth-activated lutetium borate has a scintillation characteristic of absorbing ultraviolet radiation or visible light by absorbing radiation such as X-rays. In particular, lutetium borate activated with Ce can function as a very excellent scintillator material in that it emits a large amount of light and has a short fluorescence decay time.

  The composition ratio x in the above composition formula indicates the ratio that Ce is substituted with Lu sites. When the composition ratio x is less than 0.0001, a sufficient amount of light emission cannot be obtained because Ce as a light emitting element is small. On the other hand, when the composition ratio x exceeds 0.05, the transmittance decreases, and the light emission amount also decreases.

  In the single crystal scintillator material according to a preferred embodiment of the present invention, Ce is almost uniformly doped throughout the single crystal, and the composition range is satisfied in all regions in the single crystal. Thereby, the whole single crystal can be made into the desired Ce substitution amount, and excellent fluorescence attenuation characteristics can be exhibited as the whole single crystal.

  The lutetium borate single crystal has a calcite structure at a temperature lower than the phase transition point existing near 1350 ° C., and a vaterite structure at a temperature higher than the phase transition point. As will be described later, in the present invention, since a single crystal is precipitated and grown by dissolving and cooling a lutetium borate-based material in a solvent at a temperature of 1350 ° C. or less, a large volume change caused by a phase transition in the cooling process. Does not occur. As a result, a calcite-type lutetium borate single crystal can be grown greatly.

  The single crystal scintillator material of the present invention thus produced exhibits a high transmittance for visible light. For example, in a single crystal having a thickness of 2 mm or less, it is considered that the transmittance at the peak of the emission wavelength reaches 50% or more. Although the peak value of the emission wavelength depends on the composition of the single crystal, the peak of the emission wavelength of the cerium-activated lutetium borate single crystal in the composition range is in the range of 350 nm to 450 nm.

  Further, the single crystal scintillator material of the present invention has a feature that the transmittance is sufficiently high in a short wavelength region of 260 nm to 300 nm. FIG. 1 shows the transmittance (thick line) of a single crystal scintillator material (Example 1) according to the present invention and the transmission of a single crystal scintillator material (Comparative Example 1) produced by the method disclosed in PCT / JP2008 / 1717. It is a graph which shows a rate.

As can be seen from FIG. 1, the transmittance of the single crystal scintillator material of the present invention takes a minimum value near the wavelength of 340 nm, but realizes a relatively high value in the wavelength region other than the wavelength near 340 nm. In the single crystal scintillator material of the present invention, the transmittance in a short wavelength region of 260 nm to 300 nm is equal to or higher than the transmittance of a wavelength of 340 nm.
On the other hand, the transmittance of the single crystal scintillator material manufactured by the method disclosed in PCT / JP2008 / 1717 decreases monotonously as the wavelength decreases in a short wavelength region of 350 nm or less. Such a decrease in transmittance in the comparative example is attributed to the relatively large amount of Pb contained in the single crystal scintillator material.

As described above, the transmittance of the single crystal scintillator material is expressed by the composition formula (Ce _x Lu _1-x ) BO ₃ and the Ce composition ratio x satisfies 0.0001 ≦ x ≦ 0.05. Even if it is a case, it may change a lot depending on the content of Pb.

  According to the present invention, the lead content can be reduced to 100 ppm or less by mass ratio. Therefore, a transmittance of 20% or more (preferably 30%) at a wavelength of 280 nm, which is a substantially median value of a wavelength region of 260 nm to 300 nm. Can be realized.

  In this specification, the intensity ratio of outgoing light (light emitted from the sample) when the intensity of light incident on the sample is 100 is defined as “transmittance”. Specifically, the measurement is performed as follows.

  As a measurement sample, a single crystal scintillator material was cut out parallel to the (001) plane, and then the surface was flattened by mirror polishing, and the surface roughness was adjusted to 0.005 μm or less and a thickness of 0.5 mm. Use things. In general, the magnitude of transmittance is affected not only by absorption inside the sample, but also by reflection at the surface of the sample. However, by adjusting the surface roughness of the sample to the above level, the influence of reflection in each sample can be regarded as the same level. Therefore, it is possible to compare the absorption characteristics of the samples with sufficiently high accuracy by the transmittance measured for the sample adjusted as described above. An ultraviolet-visible spectrophotometer is used for the transmittance measurement.

Next, the method for producing the single crystal scintillator material of the present invention will be described in more detail.
As described above, the lutetium borate single crystal takes a vaterite structure at a temperature higher than the phase transition point existing near 1350 ° C., and takes a calcite structure at a temperature lower than the phase transition point. In the conventional single crystal manufacturing method, it is necessary to melt the raw material at a high temperature, and thus it is inevitable that the temperature of the crystal precipitated by cooling the melt passes through this phase transition point in the cooling process. . Therefore, there is a problem that the volume of the precipitated crystal is greatly changed by the phase transition and the crystal collapses, and it is extremely difficult to produce a single crystal of lutetium borate having a phase transition point lower than the melting point. It was.

  The above-described flux method using lead borate as a solvent, disclosed in PCT / JP2008 / 1717, solves this problem and can easily produce a single crystal of cerium-activated lutetium borate, which has been difficult in the past. It was a very good method. However, the single crystal of cerium activated lutetium borate prepared by this method was slightly colored yellow. According to the analysis by the inventors, there is a possibility that a very small amount of lead, which is a component of the solvent, is mixed in the single crystal, and it is estimated that the single crystal is colored. In addition, it is considered that the mixing of lead has an influence on the light emission amount of the single crystal to be generated.

  Therefore, as a result of further studies on the solvent used in the flux method, the present inventors have determined that Pb, an alkali metal such as Li, and W and / or Mo as a metal that forms a low melting point compound with this alkali metal. A cerium-activated lutetium borate system is prepared by dissolving a lutetium borate-based material in a solvent at a temperature lower than the phase transition point after using a solvent containing B, B and oxygen as a main solvent. It has been found that a single crystal of the material can be deposited. According to the present invention, unlike the single crystal scintillator material disclosed in PCT / JP2008 / 1717, which is produced by the flux method using lead borate as a solvent, the lead content is 100 ppm or less by mass ratio, A colorless and transparent single crystal scintillator material can be obtained.

Hereinafter, the manufacturing method of the single-crystal scintillator material of this invention is demonstrated concretely.
[Starting materials (lutetium borate materials and solvents)]
As a starting material, a Pb compound and an alkali metal compound selected from Li, Na, K, Rb, and Cs, a W and / or Mo compound, a boron compound, a Ce compound, and a Lu compound are mixed at a required ratio. Use what you did.

As the Pb compound, lead monoxide, lead trioxide, lead fluoride and the like can be used. Among them, lead monoxide is preferable in terms of ease of handling. As the alkali metal compound, carbonates, bicarbonates, hydroxides and oxides such as Li ₂ CO ₃ , NaHCO ₃ , KOH and Cs ₂ O can be used, but carbonates are preferable because of easy handling. . Alkali halides such as NaCl, KBr, LiF, and CsI can also be used as the alkali metal compound. Alkali halides may be used alone or in combination with an alkali carbonate or the like. In order to lower the melting point of the compound serving as the solvent, it is desirable to include two or more kinds of alkali metals.

As a compound of W and / or Mo, WO ₃ or MoO ₃ can be used. As the boron compound, B ₂ O ₃ , H ₃ BO _3, or the like can be used.
Examples of the Ce compound include CeO ₂ , Ce (OH) ₃ , and Ce ₂ O _3. Among them, CeO ₂ and Ce ₂ O ₃ are preferable in that high-purity mass-produced products are distributed and easily available. preferable. As the Lu compound, Lu ₂ O ₃ is preferably used.

  These starting materials are adjusted in the following proportions. First, boron and W and / or Mo are blended in a molar ratio of 10:90 to 80:20 to obtain a solvent. From the viewpoint of precipitating larger crystals, it is preferable that the compounding ratio of boron and W and / or Mo is 30:70 to 60:40. It is preferable to mix | blend the alkali metal so that it may become 0.5 mol-2 mol with respect to the total 1 mol of W and / or Mo and boron. Further, a lead borate composition in which Pb and boron are blended so that the molar ratio is 1: 0.2 to 1: 1.6 is added at a ratio of 10 to 50% by weight with respect to the solvent.

The solvent may be referred to as at least one selected from alkaline earth metal compounds (hereinafter referred to as “BaCO ₃ etc.”) such as BaCO ₃ , SrCO ₃ , CaCO ₃ for the purpose of adjusting the melting point and viscosity. ) May be included. When BaCO ₃ or the like is contained in the solvent, it is preferable to blend such that BaCO ₃ or the like is 0.1 mol or less with respect to 1 mol of the alkali metal.

In this solvent, each compound is mixed in such a ratio that Lu is 0.002 to 0.3 mol and Ce is 0.0001 to 0.5 mol with respect to 1 mol of the total amount of Pb and alkali metal. More preferably, each compound is added to the solvent in such a ratio that 0.02 to 0.3 mol of Lu and 0.0001 to 0.5 mol of Ce to 1 mol of Lu with respect to 1 mol of the total amount of Pb and alkali metal. Mix. When these mixtures are heated, the compound for the solvent is melted, and the Lu compound and the Ce compound are dissolved in the melted solvent. As starting material, may be blended so that the above ratio with a previously separately regulated lead _{borate, Na 2 WO 4, Li 2} B 2 O 4 or the like.

[Temperature control for crystal growth]
(1) Temperature rising / temperature holding The starting material is heated to a temperature of 800 ° C. or more and 1350 ° C. or less, which is a phase transition point of lutetium borate, at a heating rate of 50 ° C./hour to 500 ° C./hour, The temperature is kept as it is for 1 to 12 hours to melt the whole. The melting point of lutetium borate is 1650 ° C, but it dissolves in the solvent melted at a temperature lower than the above range, so that a single crystal having a calcite structure is precipitated without passing through the phase transition point of the crystal in the cooling step. Can be made.

In the temperature raising / temperature holding step, the temperature may be once raised to a temperature higher than the holding temperature and then held in the above temperature range. Further, the temperature may be raised in several stages from a relatively high temperature increase rate to a relatively low temperature increase rate.
If the holding / cooling (slow cooling) for the purpose of crystal growth is 1350 ° C. (phase transition temperature) or lower, the temperature may be raised to 1350 ° C. or higher and then held at a temperature lower than 1350 ° C.

  When the Ce compound and the Lu compound were mixed with the solvent and heated to a temperature above 1350 ° C. and below the boiling point of lutetium borate, the compound became a liquid phase when passing through a temperature of 500 ° C. to 1350 ° C. All melts in solvent. For example, even if the temperature is raised to 1400 ° C., the melting process itself is performed during the temperature increase.

(2) Cooling Subsequently, the molten solution is cooled from the holding temperature (800 ° C. or higher to 1350 ° C. or lower) to a temperature lower than the holding temperature of 750 ° C. or higher and lower than 1350 ° C. (Referred to as a cold temperature region), preferably from 0.001 ° C./hour to 5 ° C./hour, and more preferably from 0.003 ° C./hour to 2 ° C./hour. By slowly cooling the initial cooling stage at such a low rate, the precipitated crystals can be grown greatly. The temperature may be maintained for 30 minutes or longer within a temperature range of 800 ° C. or higher and lower than 1350 ° C. in order to grow crystals greatly. In addition, from the viewpoint of growing crystals larger, it is desirable that the first slow cooling temperature region is gradually cooled in several stages from a relatively low cooling rate to a relatively high cooling rate.

  After the slow cooling in the first slow cooling temperature region, further until the temperature of the molten solution reaches a temperature of 500 ° C. or higher and 800 ° C. or lower (the temperature range is hereinafter referred to as a second slow cooling temperature region). It may be gradually cooled at a temperature decreasing rate of 01 ° C / hour or more and 30 ° C / hour or less, preferably 0.1 ° C / hour or more and 20 ° C / hour or less. After completion of the slow cooling (after completion of slow cooling in the first slow cooling temperature region or after completion of slow cooling in the first slow cooling temperature region + second slow cooling temperature region) Cooling may be performed at a relatively high temperature decrease rate of time to 300 ° C./hour.

  Such temperature control is not limited to controlling the entire melt and melt at the same temperature, and it is only necessary to control at least the portion of the entire melt that precipitates crystals. For the purpose of controlling the size of the crystal to grow, the temperature of the molten solution may be controlled at partially different temperatures. For example, a large crystal can be formed by controlling the portion where the crystal is grown within the above temperature range and controlling the other portion where the crystal is not desired to be precipitated at a higher temperature. Also in a method using a seed material or the like such as a TSSG method to be described later, the temperature of only the seed material or the like may be controlled instead of the entire melted melt in the crucible.

  Since the solid crystal is attached to the single crystal in the crucible or the single crystal taken out from the crucible, the cerium-activated boric acid is removed from the solvent by immersing the single crystal in hydrochloric acid, acetic acid, nitric acid or an aqueous solution thereof. The lutetium single crystal can be separated and taken out. Before the separation, the molten solvent may be poured out by reheating to a temperature of 500 ° C. or higher and 700 ° C. or lower. In the course of cooling to precipitate and grow a single crystal, the temperature of the mixture is kept at a temperature of 500 ° C. or higher and 700 ° C. or lower (for example, 550 ° C.) for several hours (for example, 5 hours), and then taken out and melted. You may start flowing out.

[Crystal growth method]
Specific examples of the crystal growth method include a flux method (slow cooling method, temperature difference method), a Bridgman method, and a TSSG (Top Seed Solution Growth) method. According to the TSSG method, a large crystal can be grown, and the grown crystal and the solution can be easily separated. Hereinafter, a specific example of crystal growth by the TSSG method will be described with reference to FIG.

  FIG. 2 shows a crystal growth apparatus using the TSSG method. The apparatus of FIG. 2 has an electric furnace 3 whose temperature can be controlled by a heater 2, and a platinum crucible 1 in which a raw material solution 7 is placed on a crucible base 4 ′ in the electric furnace 3 is installed. In the apparatus having such a configuration, the adjusted raw material is put into the crucible 1 and the heater 2 is heated to melt the raw material. The seed material 6 attached to the tip of the pulling shaft 5 is brought into contact with the raw material solution 7, and the crystal is grown while being held or pulled as it is. As the seed material 6, it is common and desirable to use the same type of crystal as the crystal to be grown. However, a different kind of crystal that is difficult to dissolve in the raw material solution 7, platinum, or the like is often used as the seed material 6. .

  In order to grow a large crystal, it is necessary to enlarge the crucible. On the other hand, when a large crucible is used, an unintended temperature distribution or concentration distribution may easily occur in the solution. Under such an environment, it is difficult to reproducibly grow high quality crystals. In order to solve such problems and always maintain a solution suitable for growth, it is preferable to perform crystal growth while stirring the solution.

The cerium-activated lutetium borate single crystal obtained by the above production method is a colorless and transparent hexagonal plate-like single crystal having a transmittance of 40% or more at an emission wavelength peak, and no contamination of coloring impurities is observed. It has a structure. The light emission by X-ray excitation has a high emission amount of about 350% or more with respect to BGO having a peak wavelength of 365 nm to 410 nm and the same volume. In addition, the fluorescence decay time due to the γ-ray excited emission from the ¹³⁷ Cs radiation source is 25 nsec or less, which is very fast with respect to 300 nsec of BGO.

Another method for producing a single crystal scintillator material according to the present invention contains Pb, at least one selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, and B and oxygen. Preparing a solvent to be used;
A Ce compound and a Lu compound were mixed with the solvent, and a single crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ and satisfying 0.0001 ≦ x ≦ 0.05 was obtained from the high-temperature vaterite phase. Heating the compound at a temperature that does not cause a phase transition with a significant volume change to the phase during the cooling process; and
By cooling the molten compound, a single crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ and having a Ce composition ratio x satisfying 0.0001 ≦ x ≦ 0.05 is precipitated and grown. And a process.

  As described above, the phase transition point is 1350 ° C. However, when the pressure applied from the atmosphere to the molten compound and solvent is changed from normal pressure, the phase transition temperature shifts from 1350 ° C. Even when an electric field is applied to the molten compound and solvent, the phase transition temperature shifts from 1350 ° C. In these cases, the temperature for precipitation growth is set to a temperature lower than the shifted phase transition point.

Example 1
In this example, a crystal was grown by the TSSG method using the apparatus shown in FIG. A platinum crucible having a diameter of 75 mm and a depth of 75 mm was prepared, Na ₂ CO ₃ : 174.21 g, Li ₂ CO ₃ : 122.44 g, WO ₃ : 508.09 g, PbO: 193.86 g, BaCO ₃ : 1.94 g. , B ₂ O ₃ : 107.57 g, Lu ₂ O ₃ : 40.53 g, CeO ₂ : 0.106 g were weighed. Then, it mixed with the mortar and filled in the said crucible. The temperature of the crucible 1 was controlled by a heater 2 disposed around the crucible 1, and crystals were grown with the heat pattern shown in FIG. The vertical axis in FIG. 3 is temperature, and the horizontal axis is time. The description is omitted during the holding process or during the cooling process.

  In this example, as can be seen from FIG. 3, the temperature was raised to 1150 ° C. at 150 ° C./Hr. After maintaining at 1150 ° C. for 24 hours, the temperature was lowered to 1021 ° C. at 25 ° C./Hr. After holding for 2 hours, at the same timing as the crucible, the seed material 6 (5 mm wide, 1 mm thick lutetium borate crystal attached to the tip of the pulling shaft 5 rotated at 30 rpm so that the rotation direction is opposite to that of the crucible ) Was lowered from the top of the crucible and brought into contact with the solution surface. In this state, the temperature is further maintained for 2 hours, and then the temperature is lowered to 0.05 ° C./1000° C., then the pulling shaft 5 is raised to separate the platinum plate from the solution surface, and then the temperature is continuously lowered at 200 ° C./Hr. It was. The target cerium-activated lutetium borate single crystal grew on the tip of the seed material. After cooling, the grown crystal was washed with water and acetic acid to remove the attached solvent, and the crystal was taken out.

  FIG. 4 is a photograph showing the obtained crystal. In FIG. 4, the crystal was photographed obliquely from above with a grid for indicating the size of the crystal. The grid interval of the graph paper (the size of the squares) is 1 mm. Crystals grew on the seed material supported at the tip of the pulling shaft. The lower right part of the crystal is transparent, and part of the grid can be seen through.

As a result of measuring the obtained crystal with an X-ray diffractometer, it was confirmed that the crystal had a LuBO ₃ calcite structure. Other phases such as the vaterite structure were not included. Moreover, when the Ce density | concentration with respect to the whole rare earth of the obtained crystal | crystallization was measured with the electron beam microanalyzer (EPMA), it confirmed that it was 0.1 at% or more. That is, the composition ratio x of Ce in the composition formula (Ce _x Lu _1-x ) BO ₃ was 0.001 or more, and the relationship of 0.0001 ≦ x ≦ 0.05 was satisfied. Further, when the content of Pb was measured by ICP emission spectroscopic analysis, it was 100 ppm or less by mass ratio.

FIG. 5 shows a case in which only a transparent portion is cut out from the obtained crystal, a BaSO ₄ reflective material is formed on a sample processed into a 2 mm square × 0.7 mm thick rectangle, and excited by γ-ray excitation from a ¹³⁷ Cs radiation source. It is a graph which shows the gamma ray excitation luminescence spectrum made to light-emit. This graph shows the wave height distribution obtained by detecting the light emission output of the crystal with a photomultiplier tube (PMT) and measuring with a multichannel analyzer. Channel on the horizontal axis represents the intensity of light emission. The larger the number, the stronger the light emission. The number of counts (Count) corresponding to each channel is shown on the vertical axis. As can be seen from FIG. 5, the emission peak was confirmed by γ-ray excitation, indicating that it can be suitably used as a scintillator material for PET.

(Comparative Example 1)
In this comparative example, a crystal was grown by the TSSG method using the apparatus shown in FIG. A platinum crucible 1 having a diameter of 50 mm and a depth of 47 mm was prepared, and PbO: 130 g, B ₂ O ₃ : 18 g, Lu ₂ O ₃ : 10 g, BaCO ₃ : 1.0 g, and CeO ₂ : 0.1 g were weighed. . Then, it mixed with the mortar and filled in the said crucible. The temperature of the crucible 1 was controlled by a heater 2 disposed around the crucible 1, and crystals were grown with the heat pattern shown in FIG. In FIG. 7, the vertical axis represents temperature, and the horizontal axis represents time. The description is omitted during the cooling process.

  In this comparative example, as can be seen from FIG. 7, the temperature was first raised to 500 ° C. at 50 ° C./Hr, then raised to 850 ° C. at 87.5 ° C./Hr, and then 1150 at 150 ° C./Hr. The temperature was raised to ° C. After holding at 1150 ° C. for 8 hours, the seed material 6 (platinum plate having a width of 5 mm and a thickness of 0.5 mm) attached to the tip of the pulling shaft 5 rotated at 30 rpm is lowered from the upper part of the crucible and comes into contact with the solution surface I let you. After maintaining for 6 hours in that state, the temperature was lowered to 940 ° C. at 1 ° C./Hr, and then the pulling shaft 5 was raised to separate the platinum plate from the solution surface, and then the temperature was continuously lowered at 200 ° C./Hr.

A crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ grew on the tip of the platinum plate that had been in contact with the solution. After cooling, the platinum plate and the grown crystal were washed with hydrochloric acid to remove the attached solvent, and the crystal was taken out.

As a result of measuring the obtained crystal with an X-ray diffractometer, it was confirmed that the crystal had a LuBO ₃ calcite structure. Moreover, when the Ce concentration with respect to the whole rare earth of the obtained crystal | crystallization was measured with the electron beam microanalyzer (EPMA), it confirmed that it was 0.05 at% or more in all the area | regions. That is, the composition ratio x of Ce in the composition formula (Ce _x Lu _1-x ) BO ₃ was 0.0005, which satisfied the relationship of 0.0001 ≦ x ≦ 0.05. Moreover, when the content of Pb was measured by ICP emission spectroscopic analysis, it was 400 ppm by mass ratio.

  Furthermore, when the crystal of the comparative example was caused to emit light by X-ray excitation from a CuKα radiation source, the peak wavelength of light emission was 365 nm. FIG. 1 shows the result of measuring the transmittance by the same method as in Example 1. The transmittance at a wavelength of 280 nm was 0%, and it was confirmed that the transmittance decreased on the short wavelength side. Even when the conditions were changed, the transmittance was a low value of less than 5%.

In the examples described below, light emission by X-ray excitation from a CuKα radiation source using crystals grown by a flux method (slow cooling method) in which a lead borate solvent A and a non-lead solvent B are mixed at a predetermined ratio. The amount dependency of the lead borate solvent A ratio on the amount was investigated.
Here, the lead borate solvent A is composed of 70.8 mol% Pb in terms of PbO, 28.4 mol% boron in terms of B2O3, and 0.8 mol% Ba in terms of BaO. Lead-free solvent B, and 25.0 mol% of Na in terms of Na ₂ O, and 25.0 mol% of Li in Li ₂ O in terms, and 33.3 mol% of W in terms _of WO _3, B 2 _{O 3} in terms of 16.7 mol% boron.

(Example 2)
In this example, 10% by weight of lead borate solvent A and 90% by weight of non-lead solvent B are mixed. A platinum crucible having a diameter of 44 mm and a depth of 47 mm was prepared. Na ₂ CO ₃ : 17.82 g, Li ₂ CO ₃ : 12.42 g, WO ₃ : 51.96 g, PbO: 8.81 g, B ₂ O ₃ : 9 0.53 g, Lu ₂ O ₃ : 3.62 g, CeO ₂ : 0.009 g were weighed. Then, it mixed with the mortar and filled in the said crucible. The temperature of this crucible was controlled by a heater arranged around the crucible, and crystals were grown in a heat pattern shown in FIG. The vertical axis in FIG. 8 is temperature, and the horizontal axis is time. The description is omitted during the cooling process. In this example, as can be seen from FIG. 8, the temperature is first raised to 400 ° C. at 100 ° C./Hr, then raised to 600 ° C. at 50 ° C./Hr, and further up to 700 ° C. at 25 ° C./Hr. The temperature was raised to 1300 ° C. at 300 ° C./Hr. Thereafter, the temperature was not maintained at 1300 ° C., and the temperature was decreased to 900 ° C. at 2 ° C./Hr, and then the temperature was continuously decreased at 43.8 ° C./Hr.

From the mixture melt-dissolved at a temperature around 1300 ° C., a crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ grew by cooling.
After cooling, the solidified material in the crucible was washed with water and an aqueous acetic acid solution to remove the solvent component, and the crystal body remaining in the crucible was taken out. When the same measurement as in Example 1 was performed, the crystal had a calcite structure. Other phases such as the vaterite structure were not included. The composition ratio x of Ce in the composition formula (Ce _x Lu _1-x ) BO ₃ of this crystal was 0.001, and the content of Pb was 100 ppm or less by mass ratio.

(Example 3)
In this example, 20% by weight of lead borate solvent A and 80% by weight of non-lead solvent B are mixed. A platinum crucible having a diameter of 44 mm and a depth of 47 mm was prepared. Na ₂ CO ₃ : 15.84 g, Li ₂ CO ₃ : 11.04 g, WO ₃ : 46.19 g, PbO: 17.62 g, B ₂ O ₃ : 9 0.78 g, Lu ₂ O ₃ : 3.69 g, CeO ₂ : 0.010 g were weighed. Then, it mixed with the mortar and filled in the said crucible. The temperature of this crucible was controlled by a heater arranged around the crucible, and crystals were grown in a heat pattern shown in FIG. The vertical axis in FIG. 8 is temperature, and the horizontal axis is time. The description is omitted during the cooling process. In this example, as can be seen from FIG. 8, the temperature is first raised to 400 ° C. at 100 ° C./Hr, then raised to 600 ° C. at 50 ° C./Hr, and further up to 700 ° C. at 25 ° C./Hr. The temperature was raised to 1300 ° C. at 300 ° C./Hr. Thereafter, the temperature was not maintained at 1300 ° C., and the temperature was decreased to 900 ° C. at 2 ° C./Hr, and then the temperature was continuously decreased at 43.8 ° C./Hr.

From the mixture melt-dissolved at a temperature around 1300 ° C., a crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ grew by cooling.
After cooling, the solidified material in the crucible was washed with water and an aqueous acetic acid solution to remove the solvent component, and the crystal body remaining in the crucible was taken out. When the same measurement as in Example 1 was performed, the crystal had a calcite structure. Other phases such as the vaterite structure were not included. The composition ratio x of Ce in the composition formula (Ce _x Lu _1-x ) BO ₃ of this crystal was 0.001, and the content of Pb was 100 ppm or less by mass ratio.

Example 4
In this reference example, 50% by weight of lead borate solvent A and 50% by weight of non-lead solvent B are mixed. A platinum crucible having a diameter of 44 mm and a depth of 47 mm was prepared. Na ₂ CO ₃ : 9.90 g, Li ₂ CO ₃ : 6.90 g, WO ₃ : 28.87 g, PbO: 44.06 g, B ₂ O ₃ : 10 0.52 g, Lu ₂ O ₃ : 3.90 g, CeO ₂ : 0.010 g were weighed. Then, it mixed with the mortar and filled in the said crucible. The temperature of this crucible was controlled by a heater arranged around the crucible, and crystals were grown in a heat pattern shown in FIG. The vertical axis in FIG. 8 is temperature, and the horizontal axis is time. The description is omitted during the cooling process. In this example, as can be seen from FIG. 8, the temperature is first raised to 400 ° C. at 100 ° C./Hr, then raised to 600 ° C. at 50 ° C./Hr, and further up to 700 ° C. at 25 ° C./Hr. The temperature was raised to 1300 ° C. at 300 ° C./Hr. Thereafter, the temperature was not maintained at 1300 ° C., and the temperature was decreased to 900 ° C. at 2 ° C./Hr, and then the temperature was continuously decreased at 43.8 ° C./Hr.

From the mixture melt-dissolved at a temperature around 1300 ° C., a crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ grew by cooling.
After cooling, the solidified material in the crucible was washed with water and an aqueous acetic acid solution to remove the solvent component, and the crystal body remaining in the crucible was taken out. When the same measurement as in Example 1 was performed, the crystal had a calcite structure. Other phases such as the vaterite structure were not included. The composition ratio x of Ce in the composition formula (Ce _x Lu _1-x ) BO ₃ of this crystal was 0.001, and the content of Pb was 100 ppm or less by mass ratio.

FIG. 9 is a graph showing the dependence of the amount of luminescence by the X-ray excitation from the CuKα radiation source on the lead borate solvent A ratio in the crystals grown in Example 2, Example 3 and Example 4.
As can be seen from FIG. 9, compared to Example 4 in which the lead borate solvent A ratio is 50% by weight, Example 3 in which the lead borate solvent A ratio is 20% by weight has a higher light emission amount. Show. Moreover, compared with Example 3 in which the lead borate solvent A ratio is 20% by weight, Example 2 in which the lead borate solvent A ratio is 10% by weight shows a higher light emission amount.
From this fact, the smaller the ratio of the lead borate solvent A, that is, the greater the ratio of the non-lead solvent B, the less the solvent component that reduces the light emission amount is mixed into the grown crystal, and the higher the light emission amount is likely to be obtained. It is shown that.

FIG. 10 is a perspective view showing a configuration example of a scintillator array according to the present invention. In the configuration example shown in this figure, a plurality of bar-shaped scintillator crystals 11 are arranged in a grid (a grid pattern) via a reflector. Each scintillator crystal 11 is a single crystal scintillator material of the present invention.
A space (gap) between adjacent scintillator crystals 11 is filled with a reflective material 12, and the reflective material 12 is also disposed on the outer periphery of the scintillator array. The reflector 12 is made of a material that transmits γ rays but has a high reflectance at the wavelength of light emitted from the scintillator crystal 11.

  In this scintillator array, five of the outer peripheral surfaces are covered with the reflecting material 12, but only one surface is not covered with the reflecting material 12, but is exposed. More specifically, one end surface of the scintillator crystal 11 is not covered with the reflector 12, and light can be emitted to the outside from this end surface.

  FIG. 11 is a cross-sectional view showing a configuration example of a radiation detector according to the present invention. This radiation detector includes a single crystal scintillator material according to the present invention and a known detector that detects light emission from the single crystal scintillator material. The illustrated radiation detector includes an array of scintillator crystals 11 shown in FIG. 10 and a photomultiplier tube 13. Specifically, the scintillator array and the photomultiplier tube 13 are joined via the optical grease 14 so that the end face of the scintillator crystal 11 and the light receiving side surface of the photomultiplier tube 13 are optically coupled. ing. The scintillator crystal 11 is covered with the reflective material 12 on the side on which the γ rays 15 are incident.

FIG. 12 is a diagram showing a configuration example of a PET apparatus according to the present invention. A plurality of radiation detectors are arranged so as to form a ring. Each radiation detector has the configuration shown in FIG.
On the inner periphery of the ring, scintillator crystals 11 coated with a reflecting material are arranged. Photomultiplier tubes 13 are arranged on the outer periphery of the ring. A subject 16 stands by near the center of the ring. The subject 16 is administered a drug labeled with a radioisotope that emits positrons.
A pair of γ rays 15 are generated by positron annihilation at the affected part of the subject 16 and emitted in two directions. The γ-ray 15 is converted into light by the scintillator crystal 11. The light is amplified by the photomultiplier tube 13 and detected by an electric signal output from the photomultiplier tube 13.

  The cerium-activated lutetium borate single crystal scintillator material obtained by the production method of the present invention has a larger amount of light emission than conventional single crystal materials for scintillators and has excellent scintillator characteristics. Can be used.

1 crucible 2 heater 3 electric furnace 4 crucible base 4 'crucible base 5 pulling shaft 6 seed material 7 raw material solution 11 scintillator crystal 12 reflecting material 13 photomultiplier tube (PMT)
14 Optical grease 15 γ-ray 16 Subject

Claims (11)

Preparing a solvent containing Pb, at least one selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, B and oxygen;
Mixing a Ce compound and a Lu compound with the solvent and heating the compound to a temperature of 800 ° C. or higher and 1350 ° C. or lower to melt the compound;
By cooling the molten compound, a single crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ and having a Ce composition ratio x satisfying 0.0001 ≦ x ≦ 0.05 is precipitated and grown. Process,
A method for producing a single crystal scintillator material comprising:   The step of preparing the solvent and the step of melting the compound are steps of mixing the compound forming the solvent, the Ce compound, and the Lu compound, and heating to a temperature of 800 ° C. or higher and 1350 ° C. or lower. Item 2. A method for producing a single crystal scintillator material according to Item 1. Preparing a solvent containing Pb, at least one selected from the group consisting of Li, Na, K, Rb, and Cs, W and / or Mo, B and oxygen;
Mixing a Ce compound and a Lu compound with the solvent and heating to melt the compound;
By cooling the molten compound, a single crystal represented by the composition formula (Ce _x Lu _1-x ) BO ₃ and having a Ce composition ratio x satisfying 0.0001 ≦ x ≦ 0.05 is precipitated and grown. Process,
Including
The method for producing a single crystal scintillator material, wherein the single crystal precipitation growth is performed at a temperature lower than a temperature of a phase transition from a high-temperature vaterite phase to a calcite phase.   The method for producing a single crystal scintillator material according to claim 1, wherein the step of precipitating and growing the single crystal is performed by a TSSG method.   2. The method according to claim 1, wherein in the step of precipitating and growing the single crystal, the molten compound is cooled to a temperature of 750 ° C. or higher and lower than 1350 ° C. at a temperature decreasing rate of 0.001 ° C./hour or more and 5 ° C./hour or less. Manufacturing method of single crystal scintillator material.   The method for producing a single crystal scintillator material according to claim 5, wherein the precipitation growing step is performed over 80 hours or more. It has a single crystal part represented by a composition formula (Ce _x Lu _1-x ) BO ₃ and a composition ratio x of Ce satisfies 0.0001 ≦ x ≦ 0.05, and the content of Pb in the single crystal part Is a single crystal scintillator material having a mass ratio of 100 ppm or less.   The single crystal scintillator material according to claim 7, wherein the single crystal portion has a calcite type crystal structure.   The single crystal scintillator material according to claim 7, wherein a transmittance at a wavelength of 280 nm of the single crystal portion mirror-finished to a thickness of 0.5 mm is 20% or more. A single crystal scintillator material according to any of claims 7 to 9,
A radiation detector comprising: a detector for detecting light emission from the single crystal scintillator material. A PET device comprising a plurality of radiation detectors arranged in a ring shape and detecting γ rays from a subject,
Each of the plurality of radiation detectors is a PET apparatus, which is the radiation detector according to claim 10.