RU2164562C1 - Method of producing lead-tungstate scintillation single crystal - Google Patents

Abstract

FIELD: methods of crystal production, particularly, methods of production of lead-tungstate single crystals; applicable in manufacture of scintillation members used in detectors of high-energy ionizing radiations operating under conditions of high dose loads in registration circuits requiring high time resolution. SUBSTANCE: PWO single crystal is produced by Czochralski method on seed oriented at an angle up to 10 deg to crystallographic axis "a" under conditions of suppression of oxidation of crystal-forming ions and stoichiometrization of melt and alloying with admixtures of lanthanum, yttrium and niobium in amount of 0.001-0.1 wt.%, and admixture of molybdenum in the form of PbMoO₄ in amount to obtain molybdenum content in finished crystal of 0.01-0.1 wt.%. Scintillation single crystal of lead-tungstate produced of proposed method possesses high service properties such as increased light output of scintillations and luminescence spectrum displaced to green region relative to material described in prototype that extends the field of its application. Scintillation single crystal may be used for registration and spectrometry of particles and quanta in devices of physics of high, medium and low energies (above 100 MeV), particularly, in detectors of electromagnetic calorimeters of colliders, positron emission tomography using time-of-flight method of registration. EFFECT: higher efficiency. 1 tbl, 1 ex

Description

 The invention relates to methods for producing crystals, and in particular to a method for producing single crystals of lead tungstate, and can be used in the manufacture of scintillation elements used for registration and spectrometry of particles and quanta in high, medium and low energy physics devices (more than 100 MeV), in particular in detectors of electromagnetic calorimeters of accelerators in oncoming beams, in systems for detecting explosives at airports, in positron emission tomography using the time-of-flight method registration.

Single crystals of various types are obtained by the method of drawing from the melt according to the Czochralski method, for example, bismuth orthogermanate Bi ₄ Ge ₃ O ₁₂ (BGO) [1], sodium double tungstate - bismuth NaBi (WO ₄ ) ₂ [2], lead tungstate PbWO ₄ (PWO) [3, 4] and others used as scintillation elements in ionizing radiation detectors.

 Lead tungstate scintillation single crystal (PWO) has significant advantages over other known scintillation single crystals [5, 6] used in experimental high-energy physics, which allows its successful use in experiments in elementary particle physics conducted on modern accelerators at high dose loads in registration paths providing high time resolution (about 130 ps).

A known method of producing a single crystal of lead tungstate PWO according to the Czochralski method by pulling from a melt a seed, oriented at an angle from zero to ten degrees to the crystallographic axis “a”, from platinum or iridium crucibles, in which lead oxide Pb ₃ O is included in the initial charge ₄ , (minium) in a molar ratio with tungsten oxide WO ₃ from 0.317 to 0.351 and impurities of metal oxides: lanthanum La, yttrium Y and niobium Nb in an amount such that in the finished crystal the total content of these metals is from 0.001 to 0.1 wt.%, process single crystal growing is performed in a gaseous medium consisting of inert gases or nitrogen with an oxygen content in the range of 10 ^-3 to 1 vol.% at a melt temperature of the mixture from 1110 to 1150 ^o C at a speed of pulling the seed from the melt from 1.0 to 12.0 mm / h with the speed of rotation of the seed from 5 to 50 min ^-1 and with the cooling rate of the single crystal after the end of the growing process no more than 150 ^o C / h, [4], adopted as a prototype.

 As a result, single crystals with a diameter of up to 50 mm and a length of up to 270 mm are obtained.

The PWO single crystal obtained in this way has a high density (8.28 g / cm ³ ), a large effective atomic number (Z = 72), a small radiation length (X ₀ = 0.9 cm), high radiation resistance to ionizing radiation, and a high rate of scintillation emission (from units up to tens of ns), which allows its use in scintillation detectors for detecting high-energy particles.

 However, the scintillation single crystal obtained in this way has significant drawbacks: low scintillation light output, and the maximum of the luminescence spectrum is in the region of 420 nm, which makes it difficult to use this scintillation single crystal together with semiconductor photodetectors.

 Such a crystal with a length of 230 mm at room temperature is characterized by a light output of 10 to 14 photoelectrons / MeV and is of little use for detecting gamma rays with energies less than 1 GeV when used in conjunction with semiconductor photodetectors such as avalanche photodiodes (APDs) with a small gain (50 - 150). The position of the maximum of the luminescence spectrum in the region of 420 nm does not correspond to the position of the maximum of the spectrum of the conversion efficiency of APD 450 - 600 nm [7].

 The technical problem that this invention solves is to create a method for producing scintillation lead tungstate single crystal (PWO) having high consumer properties by changing the growing technology by changing the chemical composition of the initial charge.

 The proposed method for producing a scintillation single crystal of lead tungstate provides a significant increase in the scintillation light yield, as well as a shift in the luminescence maximum to the green region of the spectrum (up to 520 nm).

To achieve the indicated technical results, the proposed method for producing lead tungstate single crystal (PWO) is used according to the Czochralski method by drawing from a melt onto a seed oriented at an angle from zero to ten degrees to the crystallographic axis “a”, from platinum or iridium crucibles, in which to the initial charge includes oxide-lead oxide Pb ₃ O ₄ (minium) in a molar ratio with tungsten oxide WO _3, from 0.317 to 0.351 and an impurity metal oxides: lanthanum La, yttrium Y, and niobium Nb in an amount to total in the finished crystal said metals content was from 0.001 to 0.1 wt. %, the process of growing a single crystal is carried out in a gas medium consisting of inert gases or nitrogen with an oxygen content in the range from 10 ^-3 to 1 vol.% at a melt temperature of the mixture from 1110 to 1150 ^o C at a speed of pulling the seed from the melt from 1.0 to 12.0 mm / h with a speed of rotation of the seed from 5 to 50 min ^-1 and with a cooling rate of a single crystal after the end of the growing process is not more than 150 ^o C / h, characterized in that an impurity of molybdenum in the form of PbMoO ₄ is introduced into the initial charge in an amount that the finished crystal the molybdenum content was from 0.01 to 0.1 weight%.

 As a result, single crystals with a diameter of up to 50 mm and a length of up to 270 mm are obtained.

The use of molybdenum as a dopant is due to the following. It is known [3] that scintillating tungstates belong to the class of so-called self-activated scintillators with strongly quenched luminescence. Their radioluminescence is due to radiative transitions of anionic regular WO ₄ ² -oxy complexes. An admixture of molybdenum creates additional luminescent centers of MoO ₄ ^2– in PbWO ₂ crystals. MoO ₄ ^2– impurity centers proper, at a Mo concentration of 0.01–0.1 wt% in PbWO ₄ crystals, exhibit rapidly decaying green luminescence with radiative times from 10 to 300 ns under intracenter photoexcitation. The MoO ₄ ^2– complex has an energy structure similar to the tungsten oxycomplex WO ₄ ^2– ; however, it has a lower energy of the radiative state, which determines the green luminescence of this center. At low concentrations of molybdenum up to 0.1 weight. %, this center makes an additional contribution to the total luminescence. At higher concentrations, the MoO ₄ ^2– complex acts as a quencher of luminescence of tungsten oxy complexes. Thus, in the indicated range of activator concentrations, scintillation luminescence is a superposition of luminescence of regular and molybdenum groups, which leads to an increase in light output and a shift in the maximum of total luminescence to 520 nm.

The obtained single crystals are controlled by the content of the molybdenum impurity by the magnitude of the light output of scintillations, the time of emission of the pulse, and radiation resistance. In this case, the atomic absorption analysis method, the standard method for measuring the light yield from the peak of total absorption of γ rays of a ⁶⁰ Co source, the one-electron method when excited by α particles from a Pu ²³⁸ source, and measuring the optical transmittance of samples before and after γ irradiation with a fixed dose of source Co ⁶⁰ .

To measure scintillation characteristics, crystals are made of crystals with a length of 230 mm and a cross-sectional area of 24x24 mm ² , the planes of which are polished according to the class R _z 0.025. Additionally, material samples are taken to control the content of alloying metals. On the manufactured elements, the radiation resistance of the material is also controlled by changing the optical transmission spectra. The spectra are measured on a spectrophotometer at the wavelength of the maximum scintillation emission band of 440 nm before and after irradiation of the element from a source of γ-rays with an energy of 1.22 MeV (Co ⁶⁰ ) with an absorbed dose of 100 krad.

Example 1. A mixture of tungsten oxide WO ₃ and meerkat Pb ₃ O ₄ of the OCP grade is made in a stoichiometric molar ratio of 0.33 and impurities of the oxides of metals La, Y and Nb, and platinum or iridium crucible is fused. Additives PbMoO ₄ in various amounts are introduced into a part of the main charge mixtures. Single crystals are obtained from mixtures with a molybdenum impurity content of 0.01 to 0.1 wt.% By the Czochralski method (see table). When the content of the dopant is more than 0.1 weight. %, the luminescence of regular groups is quenched by an admixture of molybdenum, and the intrinsic luminescence intensity of molybdenum groups significantly decreases due to the intercenter interaction, which leads to a total decrease in the light output to 5 photoelectrons / MeV. When the concentration of the molybdenum impurity is less than 0.01 wt.%, Molybdenum groups do not make a significant contribution to the light output.

It follows from the table that, when molybdenum impurities are introduced into the indicated amount into PWO single crystals grown from a mixture of tungsten oxide WO ₃ and meerkat Pb ₃ O ₄ grade OCP in a stoichiometric molar ratio of 0.33, the resulting scintillation material has a high light output. The maximum of the luminescence spectrum is in the range of 440 - 520 nm, depending on the concentration of molybdenum.

 The technical result of this invention is the creation of a method for producing a scintillation single crystal of lead tungstate (PWO) with high consumer properties: increased light output of scintillations and shifted to the green region by the position of the maximum of the luminescence spectrum 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 devices of high, medium and low energy physics (more than 100 MeV), in particular in detectors of electromagnetic calorimeters of accelerators in oncoming beams, in detection systems for explosives in airports, in positron emission tomography using time-of-flight registration methods. In this case, the increased light output and the luminescence spectrum shifted toward the green region make it possible to use this scintillation single crystal together with semiconductor photodiodes, in particular, avalanche photodiodes as photodetectors.

Literature
1. A method of producing crystals of bismuth orthogermanate. Copyright certificate SU 1745779 Al.

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

 3. P. Lecoq, I. Dafinei, E. Auffay et al., NIM A365 (1995) 291-298.

 4. A method for producing a scintillation single crystal of lead tungstate. Patent of the Russian Federation. RU 2132417 C1.

 5. V.G. Baryshevsky, M.V. Korzhik, V.I. Moroz et al. Single crystals of tungsten compounds as promising materials for the total absorption detectors of the e.m. calorimeters. Nucl. Instr. and Meth. Vol. A322 (1992) 231.

 6. R. Novotny, W. Doring, K. Mengel, V. Metag, Response function of PbW04 - Detectors to Electrons and Photons between 50 and 855 MeV Energy. In Proc. SCINT'97 September 22-25, 1997, Shanghai, China. P.21.

 7. CMS. The Electromagnetic Calorimeter Technical Design Report. / CERN / LHCC 97-33. CMS TDR4. - December 15, 1997.

Claims (1)

A method of producing a scintillation single crystal of lead tungstate (PWO) according to the Czochralski method by drawing from a melt onto a seed, oriented at an angle from 0 to 10 ^o to the crystallographic axis "a", from platinum or iridium crucibles, in which lead oxide Pb is included in the initial charge ₃ O ₄ (red oxide) in a molar ratio with tungsten oxide WO ₃ from 0.317 to 0.351 and impurities of metal oxides: lanthanum La, yttrium Y and niobium Nb in an amount so that in the finished crystal the total content of the mentioned metals is from 0.001 to 0.1 weight .%, the process of growing Bani single crystal is produced in a gas atmosphere consisting of inert gases or nitrogen with an oxygen content ranging from 10 ^-3 to 1 vol.% at a melt temperature of the batch from 1110 to 1150 ^o C at a speed of pulling the seed from the melt between 1.0 and 12, 0 mm / h with a speed of rotation of the seed from 5 to 50 min ^-1 and with a cooling rate of a single crystal after the end of the growing process is not more than 150 ^o C / h, characterized in that an impurity of molybdenum in the form of PbMoO _{4 is} introduced in the quantity so that in the finished crystal, the molybdenum content was from 0.01 to 0.1 wt.% .

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