RU2233916C1 - Method of production of scintillation monocrystalline lutecium-yttric aluminate - Google Patents

Abstract

FIELD: production of chips.

SUBSTANCE: the invention presents a method of production of crystals, in particular it is dealt with production of monocrystals of lutecium - yttric aluminate and may be used at production of the scintillation elements used in detectors of ionizing radiations in medical troubleshooting instrumentation. The method of production of a monocrystal of lutecium - yttric aluminate, activated with admixture of Се by Chokhralsky method, provides for growing up a monocrystal by its drawing from the charge melt containing Al ₂ O ₃ aluminum oxide as inoculum from iridic crucibles at the temperature of the melt from 1900 up to 2000°С, and the rate of the inoculum drawing out of the melt of 0.1 up to 8.0 mm / h, the rate of its rotation - from 2.5 up to 30 minutes^-1 and the rates of the monocrystal cooling after the end of the growing process - from 50 up to 500° С/h. At that as the inoculum they use a monocrystal of aluminate of yttrium or a monocrystal of aluminate of yttrium with the content of admixture of the rare-earth ions of no more than 1 mass percent. The initial charge is added with oxide of lutecium Lu₂O₃ and oxide of yttrium Y₂O₃ in a molar ratio from 1 up to 20 and with the molar ratio of their total content with the above mentioned aluminum oxide Al₂O₃ from 0.95 up to 1.05 of a stoichiometry and admixture of ceric oxide CeO₂, in the amount providing in an off-the shelf crystal the share of Се from 0.01 up to 0.9 mass percents. The growing is conducted in a gaseous medium composed of the noble gases with a share of oxygen - in the limits from 10^-6 up to 1 volumetric percent. The offered method provides substantial improvement of the optical properties of the crystals and an increase of their light output.

EFFECT: method provides substantial improvement of the optical properties of the crystals and an increase of their light output.

5 tbl, 3 ex

Description

The invention relates to methods for producing crystals, namely to a method for producing single crystals of lutetium-yttrium aluminate, and can be used in the manufacture of scintillation elements used in ionizing radiation detectors in medical diagnostic equipment [1, 2].

Single crystals of various types are obtained by the method of drawing from the melt according to the Czochralski method, for example, yttrium aluminate YАlO ₃ : Ce (IAP) [3], bismuth orthogermanate Bi ₄ Ge ₃ O ₁₂ (BGO) [4], double sodium tungstate - bismuth NaBi (WO ₄ ) ₂ [5], lead tungstate PbWO ₄ (PWO) [6], lutetium aluminate LuАlO ₃ : Се [7] and others used as scintillation elements in ionizing radiation detectors.

There is a method of producing a single crystal of yttrium aluminate, activated by an admixture of cerium, according to the Czochralski method by drawing from a melt from iridium crucibles [8, 9]. The yttrium aluminate single crystal obtained in this way has a light output of 9000 to 14800 ph / MeV [7, 10]. However, the relatively low density of 5.55 g / cm ³ does not provide sufficient detection efficiency of γ-quanta with an energy of more than 500 keV, which leads to a deterioration in the spatial resolution of the tomographic detector.

The scintillation single crystal of lutetium aluminate has significant advantages over the widely used single crystals of yttrium aluminate used in ionizing radiation detectors [3]. In particular, having comparable characteristics of the light output, a single crystal of lutetium aluminate has a higher density of 8.34 g / cm ³ and an effective charge, and therefore provides better detection efficiency of γ-quanta with an energy of more than 500 keV, which allows to improve sensitivity and spatial resolution due to the possibility of reducing the size of the detector elements of diagnostic equipment based on scintillation detectors.

A similar set of essential features is the method for producing a single crystal of lutetium aluminate according to the Czochralski method by drawing from a melt from iridium crucibles onto a rotating iridium wire, in which Al ₂ O ₃ and lutetium oxide Lu ₂ O ₃ , as well as an admixture of cerium oxide are included in the initial charge CeO ₂ in an amount so that the cerium content in the melt is not more than 5%, and the process of growing a single crystal is carried out at a charge melt temperature exceeding the melting temperature of lutetium aluminate by ve magnitude from 5% to 7% (from 2060 to 2100 ° C) [7].

A single crystal of lutetium aluminate obtained in this way has a density of 8.34 g / cm ³ and a light output of 9000 to 14200 ph / MeV, which allows its use in scintillation detectors for positron emission tomography (PET) [7].

However, when a scintillation single crystal is drawn onto an iridium wire, spontaneous crystallization takes place, which leads to a significant decrease in the reproducibility of the optical properties of crystals and, as a result, to a decrease in the efficiency in using crystalline material for the production of detector elements. Pulling a single crystal of lutetium aluminate onto an oriented seed from the same material is impossible due to the peculiarities of the phase state diagram in the Al ₂ O ₃ -Lu ₂ O _{3 system} .

The technical problem that this invention solves is to create a method for producing a scintillation single crystal of lutetium-yttrium aluminate (Lu-Y) AlO ₃ , which has higher consumer properties compared to analogs, by changing the growing technology by changing the chemical composition of the initial charge and physical chemical growing conditions.

The proposed method for producing a scintillation single crystal of lutetium-yttrium aluminate provides a significant improvement in the optical properties of crystals and an increase in their light output in comparison with analogs, as well as an improvement in reproducibility of properties from crystal to crystal.

To achieve these technical results, the proposed method is used to obtain a single crystal of lutetium-yttrium aluminate activated by an admixture of cerium according to the Czochralski method by pulling from a melt from iridium crucibles to a seed, in which Al ₂ O ₃ aluminum oxide enters the initial charge, and the process of growing a single crystal is carried out at a temperature the melt of the charge from 1900 to 2000 ° C at a speed of pulling the seed from the melt from 0.1 to 8.0 mm / h with a speed of rotation of the seed from 2.5 to 30 min ^-1 and with a cooling rate of monocri steel after the end of the growing process from 50 to 500 ° C / h, using seed from a single crystal of yttrium aluminate or a single crystal of yttrium aluminate with an impurity content of rare earth ions of not more than 1 weight percent, lutetium oxide Lu ₂ O ₃ and oxide are introduced into the initial charge yttrium Y ₂ O ₃ in a molar ratio of from 1 to 20 and in the molar ratio of their total content with the aforementioned aluminum oxide Al ₃ O ₃ from 0.95 to 1.05 from stoichiometry and impurities of cerium oxide CeO ₂ in an amount such that in the finished crystal cerium content ranged from 0.01 to 0.9 ve ovyh percent, cultivation is carried out in a gas atmosphere consisting of inert gas with an oxygen content ranging from 10 ^-6 to 1 volume percent.

As a result, single crystals of lutetium-yttrium aluminate are obtained with a diameter of up to 40 mm and a length of up to 200 mm. The density of single crystals obtained in this way is from 6.5 to 8.2 g / cm ³ , and the light output is 9000-15000 ph / MeV.

, то кристаллы лютециевого и иттриевого алюминатов имеют различные параметры решетки и затравливание кристалла лютециевого алюмината на затравку из иттриевого алюмината не происходит. Добавление в шихту оксида иттрия в указанном выше соотношении позволяет увеличить параметры решетки кристалла и делает возможным тем самым затравливание кристалла иттрий-лютециевого алюмината на затравку из иттриевого алюмината.The use of oxides of lutetium Lu ₂ O ₃ and yttrium Y ₂ O ₃ in a ratio of 1 to 20 and in the molar ratio of their total content with aluminum oxide Al ₂ O ₃ from 0.95 to 1.05 from stoichiometry is due to the following. It is known that in order to preserve the structural perfection of the crystal, the crystal lattice parameters of the seed material and the grown material must match or be close. In addition, the crystallization temperature of the grown material should not be higher than the melting temperature of the seed material. Since the ionic radius of the Lu ³⁺ ion is 0.97 A °, and Y ³⁺ is 1.02

, then the crystals of lutetium and yttrium aluminates have different lattice parameters and seeding of the crystal of lutetium aluminate to seed from yttrium aluminate does not occur. The addition of yttrium oxide in the mixture in the above ratio allows increasing the lattice parameters of the crystal and thereby makes it possible to seed yttrium-lutetium aluminate crystal with yttrium aluminate seed.

It is also known that the melting point of crystalline compounds based on rare-earth metal oxides decreases when they are activated by impurities that isomorphically replace metal ions and have an entry coefficient less than 1. The coefficient of yttrium in lutetium aluminate is less than 1, since the ionic radius of yttrium is greater than that of lutetium in one and the same oxygen coordination, therefore, the addition of yttrium oxide to the charge in the indicated molar ratio allows the crystallization temperature of lutetium-yttrium aluminate to be lower e than the melting temperature of the pure or lightly doped yttrium aluminate and enables seeding crystal lutetium yttrium aluminate on a seed from yttrium aluminate.

The use of an admixture of cerium oxide in an amount so that the cerium content in the finished crystal is from 0.01 to 0.9 weight percent is due to the fact that the greatest scintillation efficiency in aluminate crystals is achieved when the content of the finished crystal Ce from 0.01 to 0.9 weight percent [11, 12].

The use of a gas medium depleted in oxygen during the growth is due to the need to localize cerium ions in the trivalent state in the crystals grown. It is known that cerium ions are localized in oxygen compounds in the two-, three-, and tetravalent states, depending on the growing conditions [3]. Ce ⁴⁺ ions are not scintillating centers and their presence in the crystal decreases the scintillation efficiency in aluminate crystals. The use of a gas medium depleted in oxygen during the growth prevents the appearance of Ce ⁴⁺ ions in the crystal, which makes it possible to obtain the highest scintillation efficiency of crystals.

The obtained single crystals are controlled by the content of cerium impurity, by the magnitude of the light output of scintillations. In this case, the atomic absorption analysis method and the standard method for measuring the light yield from the peak of the total absorption of γ-rays of a ⁶⁰ Co source are used respectively.

To measure the scintillation characteristics, crystals are made of crystals with elements 10 mm long and a cross-sectional area of 10 mm ² , the planes of which are polished according to the class R _z 0.025. Additionally, material samples are taken to control the cerium content.

Example 1. An initial batch is prepared containing alumina Al ₂ O ₃ , into which various mixtures of lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ of the OCP grade are introduced in a molar ratio of from 0.5 to 25 and in a molar ratio of their total content with the mentioned alumina Al ₂ O ₃ from 0.5 to 1.5 from stoichiometry and iridium crucible is deposited. From these mixtures, the Czochralski method produces single crystals of lutetium-yttrium aluminate by drawing a single crystal onto a wire and using seeds from a single crystal of yttrium aluminate YAlO ₃ and a single crystal of yttrium aluminate with a content of rare-earth ions from 0.5 to 2 weight percent (Table 1-3) . As can be seen from Table 1, with a molar ratio of lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ from 1 to 20 and a molar ratio of their total content with alumina Al ₂ O ₃ from 0.95 to 1.05 from stoichiometry crystals have the best combination of parameters, including high density and light output, while achieving the highest yield of single crystals. Tables 2 and 3 show that the use of a seed from a single crystal of yttrium aluminate YАlO ₃ or a single crystal of yttrium aluminate with an impurity content of rare-earth ions of not more than 1 weight percent leads to an improvement in optical and scintillation properties and an improvement in the reproducibility of parameters from crystal to crystal compared to single crystals obtained by drawing on a wire (Table 3). It is also seen from Tables 1 and 2 that when the ratio of lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ exceeds 20, the yield of suitable crystals sharply decreases due to changes in the structural lattice.

Example 2. An initial batch is prepared containing alumina Al ₂ O ₃ , into which various mixtures of lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ of the OCP grade are introduced in a molar ratio of 1 to 20 and in a molar ratio of their total content with the mentioned aluminum oxide Al ₂ About ₃ from 0.95 to 1.05 from stoichiometry and carry out the deposition of the iridium crucible. Single crystals of lutetium-yttrium aluminate are grown from these mixtures by seed using the Czochralski method, using various compositions of the gaseous medium during cultivation (Table 4). As can be seen from Table 4, single crystals grown in the atmosphere with an oxygen content of more than 1 volume percent have a low light output (less than 7000 photons / MeV). Single crystals grown in the atmosphere from inert gases with oxygen contents ranging from 10 ^-6 to 1 volume percent have a high light output (more than 9000 photons / MeV).

Example 3. An initial batch is prepared containing alumina Al ₂ O ₃ , into which various mixtures of lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ of the OCP grade are introduced in a molar ratio of 1 to 20 and in a molar ratio of their total content with said alumina Al ₂ O ₃ from 0.95 to 1.05 from stoichiometry and iridium crucible is deposited. Various amounts of cerium oxide are added to the starting mixture. Single mixtures of lutetium-yttrium aluminate are obtained from mixtures by the Czochralski method for inert gas seeding with inert gases with oxygen contents ranging from 10 ^-6 to 1 volume percent (Table 5). As can be seen from Table 5, single crystals grown from a mixture with an amount of cerium impurity in an amount so that the cerium content in the finished crystal is from 0.01 to 0.9 weight percent has a light output of 9000 to 15000 photons / MeV. With a dopant content of 1 or more weight percent - less than 7000 photons / MeV.

The technical result of this invention is to provide a method for producing a scintillation single crystal of lutetium-yttrium aluminate having high consumer properties: increased light output and increased reproducibility of optical and scintillation properties from crystal to crystal, relative to the material described in the prototype, which extends the range of its application. This scintillation single crystal can be used for registration and spectrometry of particles and quanta in medical diagnostic devices, in particular in positron emission tomography detectors. In this case, an increased light output will provide an increase in the sensitivity of the recording system, and an increased reproducibility of optical properties will provide greater efficiency in the use of crystalline material for the production of detector elements.

Literature

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2. Korzhik M.V., Lobko A.S., Misevich O.V., Fedorov A.A. Matrix detector for positional emission tomography. Fundamental and applied physical research. 1986-2001: Collection of works. Ed. prof. V.G.Baryshevsky. - Мn .: BSU, 2001, pp. 424-453.

3. M.V. Korzhik, S. A. Smirnova. YAP scintillators: Ce and their application. Proceedings of VNIISIMS. The synthesis of minerals and methods for their research, Alexandrov, Russia, 1997, p.206-225.

4. A method of obtaining crystals of bismuth orthogermanate. A.S. SU 1745779 A1.

5. Scintillation material. Patent of the Russian Federation. RU 2059026 C1.

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7. Method of growing lutetium aluminum perovskite crystals and apparatus including lutetium aluminum perovskite crystal scintillators. US Patent 5961714.

8. A. Annenkov, MV Korzhik, A. Tkachev, P. Lecoq, E. Auffiay, Production of REAlO ₃ : Ce scintillators by Czochralski method, Scint2001 Conference Records, 16-21, September, 2001 in Chamonix, France, p. 26.

9. VGBaryshevsky, BIMinkov, P. Dorenbos, CWEvan Eijk, MV Korzhik et al., Spectroscopy and scintillation properties of cerium doped YAlO ₃ single crystals. J.Phys. Condens. Matter, 5 (1993) 788.

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Claims (1)

A method for producing a single crystal of lutetium-yttrium aluminate activated by an impurity of cerium Ce according to the Czochralski method, which consists in growing a single crystal by pulling a mixture containing aluminum oxide Al ₂ O ₃ from a melt for seeding from iridium crucibles at a melt temperature from 1900 to 2000 ° C, speed pulling the seed from the melt from 0.1 to 8.0 mm / h, its rotation speed from 2.5 to 30 min ^-1 and the cooling rate of the single crystal after the end of the growing process from 50 to 500 ° C / h, using seed from single crystal aluminate um of yttrium aluminate single crystal with a rare earth ion impurity content of not more than 1 wt.%, lutetium oxide Lu ₂ O ₃ and yttrium oxide Y ₂ O ₃ are introduced into the initial charge in a molar ratio of from 1 to 20 and in the molar ratio of their total content with the aforementioned alumina Al ₂ O ₃ from 0.95 to 1.05 from stoichiometry and impurities of cerium oxide CeO ₂ in an amount providing Ce in the finished crystal from 0.01 to 0.9 wt.%, and the growth is carried out in a gaseous medium, consisting of inert gases with an oxygen content ranging from 10 ^-6 to 1 vol.%.