RU1789578C - Method of growing single crystals of bismuth germanate - Google Patents

Abstract

Usage: obtaining crystals that are used in high-energy physics. SUMMARY OF THE INVENTION: A single crystal is drawn from a melt onto a seed. The growth of the upper conical part is carried out within a solid angle of 130-160 degrees. A crystal is obtained with a diameter of 55 mm and a length of 200 mm without cracks, transparent, without distortion of a cylindrical shape. 4 ill., 1 tab. ate with

Description

The invention relates to a technology for producing bismuth germanate single crystals with the structure of eulitin B 4 without Och 2 (BGO) and can be used in the industrial production of crystals, which are increasingly used in high energy physics with rapidly growing demand in the world market. The invention can also be used for growing other crystals by the Czochralski method.

Single crystals of bismuth germanate are grown by the Czochralski method (melt drawing), which includes the following operations. After the melt is obtained in the crucible of the crystallization unit, etching is performed by immersing the rotating seed in the melt. Then the formation of the upper cone,

The spinning seed is turned upward while the melt temperature is lowered (by means of a process control system). Then they grow with a constant diameter, pulling the crystal and changing the temperature in accordance with the properties of specific crystals and the design of the crystallization unit (mainly with the help of process control systems). After obtaining crystals of a given length, the crystal is separated from the melt, and finally, the crystal is gradually cooled according to a given ACCS program.

One of the problems in obtaining high-quality large-sized D ≈ 50 mm, LK 150 mm bismuth germanate single crystals for high energy physics is the distortion of their shape (geometry) due to the manifestation of the effect of the formation of the surface of the crystal nonequivalent

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growth forms that change their morphological expression as the crystal is drawn under varying thermal conditions. Moreover, the distortion of the cylindrical shape of the crystals often takes the form of a screw (see Fig. 1). As a result of this, the coefficient of utilization of the material of the crystal Q is significantly reduced (defined) as the ratio of the mass (or volume) of the scintillator to the mass of the crystal. This is especially evident in the manufacture of bulk scintillators of large diameter and length (close to the diameter and length of the crystal) (see Fig. 2). This affects the technical and economic indicators of the production of scintillators. Methods for eliminating such a phenomenon have not yet been developed, but growing an oversized crystal to compensate for the distorted shape in the manufacture of bulk scintillators impairs their quality.

An analysis of the growing experience and the results of specially set up experiments showed that in order to eliminate such manifestations of facet growth in BGO crystals, it is necessary to create thermal conditions that exclude the predominant growth of atomically smooth F-faces, but provide, along with these faces, the surface of the crystals with other forms of growth (for faces of the S- and K-classification of faces, see 1JJ. In this case, the cross-sectional shape of the crystal will approximately correspond to the isotherms of the thermal field at the crystallization front. Moreover, taking into account the inheritance In order to obtain growth of crystal faces, gray etching, it is necessary to create such conditions at the stage of etching and growing of the upper cone. In known growing methods, these aspects were not given due attention. Basically, the technology of etching and growing is aimed at obtaining dislocation-free and block-free crystals. a method of growing crystals by the Czochralski method, in which, after seeding, the diameter of the growing crystal is narrowed to 1-2 mm and then expanded stepwise 2. This reduces the likelihood of dislocations, but the use of this method for growing scintillating crystals considerably decreases the utilization ratio of the crystal material, since the extended top cone is not suitable for the manufacture of large diameter scintillators of large diameter.

Other methods are known for growing the Czochralski method of a series of crystals, including bismuth germanate 3, but

in them, an increase in the utilization rate of the material of the crystal is solved by improving its quality (preventing cracking). However application of these

The methods do not exclude in many cases distortions in the shape of the crystal (screw, etc.) and, therefore, a decrease in the utilization of the material of the crystal for this reason. In the descriptions of these analogues

The parameters for the expansion of the upper cone were not specified, but usually the solid angle in them is 60-110 °.

Closest to the proposed one is a method of growing crystals by the Czochralski method, in which, in order to improve the quality of the crystal, after the seeding, the upper cone is uniformly grown (exit to diameter) with a solid angle of the conical part not exceeding 12 ° 4. In some cases x this angle may be increased to 30 °.

This method of growing single crystals includes the following operations:

5 Etching Upper cone expansion Growth with a constant diameter

0 Lower cone formation

Separation of the crystal from the melt Crystal cooling

5 In this method, to eliminate the dislocation structure, the expansion of the upper cone is carried out within a rather acute solid angle of "15-30 °." However, the application of this method to

0 growing single crystals of bismuth germanate does not exclude distortion of the crystal shape, an extended upper cone is not used to produce bulk scintillators. In addition, with such a growth in the volume of the cylindrical part of the crystal of bismuth germanate in the region adjacent to the upper cone, a so-called inverse cone is formed - a cone-shaped accumulation of defects of the type

0 inclusions (for example, with a crystal diameter of DK 60 m, the length of this zone Нк is Ј -30 mm). All these factors significantly (by 15-30%) reduce the utilization rate of the crystal material.

5 An object of the invention is to increase yield by improving the quality of the cylindrical portion of a single crystal. This goal is achieved by the fact that in the method of growing single crystals of bismuth germanate by the Czochralski method,

The crystal rotates and extends from the melt

including spinning and spinning seed etching, expansion of the upper cone, growth with a constant diameter, separation of the crystal from the melt and its subsequent cooling, according to the invention, expansion of the upper cone is carried out within a solid angle of 130-160 °.

The proposed method is based on the following physical phenomena. The solid angle of expansion of the upper cone corresponds to a certain rate of decrease in the temperature of the melt during the expansion and, therefore, to certain thermal conditions at the crystallization front. The specific nature of these conditions determines the growth pattern of the F, S, and K faces. Since, as a preliminary analysis showed, in order to achieve this goal, it is necessary to exclude the predominant growth of F-faces, but to ensure, along with these faces, the formation of the crystal surface by other forms of growth (S- and K-faces). As experiments have shown, in the case of the growth of the upper cone with a solid angle between 130 - 160 °, the shape of the crystals of bismuth germanate is close to a circular cylinder with traces of exits of F faces, but without curvature and screws (see Fig. 3). In this case, The defect area of the cone does not exceed 10 mm.

It has been experimentally established that for bismuth germanate crystals with a diameter of DK 55 - 65 mm, such a range of the solid angle is ensured by a decrease (during growth) of the melt temperature by 50 - 70 ° C for 3 - 7 hours. (The growth process is controlled using process control systems a set of the appropriate program with the given parameters of the crystal (drawing and rotation speeds, crystal diameter, exit time to the diameter, length of the cylindrical part of the crystal, etc.) For crystals of a different diameter (approx. 55 mm and DK 65 mm), the temperature This may differ from the above, however, as growing experience shows, maintaining the solid growth angle within the above limits also provides a stable crystal shape (close to a circular cylinder) without significant distortions and distortions with a minimum length of the defect zone of the inverse cone.

The values of the solid angle of more than 160 ° are difficult to implement in practice, since the relatively high cooling rate of the melt (more than 15 - 20 ° C / h) is necessary for this, leading to an unstable swarm

(stiffness of the cone) and the inability to maintain controlled growth neither automatically, nor even more so in manual mode.

5With a solid angle of 130 ° and. less distorted are the crystal shapes (screw, distortion, etc.).

Thus, as a result of the expansion of the upper cone with a solid angle of 10 within the range of 130-160 degrees, it is possible to avoid distortion of the shape (curvature of the generatrix) of the growing bismuth germanate crystal, to reduce the length of the defect zone at the upper cone, and thereby increase the efficiency of using the material the manufacture of volumetric scintillator of large diameter and length from it. In this case, in comparison with the prototype and analogues, an increase in the 20 coefficient of utilization of the material of the crystal will be 10-30% (see Figs. 2 and 4).

The claimed method includes the following operations.

25 Seeding1 Growth of the upper j cone j Crystal Growth with constant rotation and diameter

30 Separation of the crystal from the melt Crystal cooling

stretched out

The crystal rotates. The claimed method has common operations with the prototype and analogues, however, the parameters (modes) of the implementation of the expansion of the upper cone are different from the known ones and give a significant positive effect. The difference between these modes is significant, since only the development of the growth of the upper cone with such parameters allows one to achieve the indicated positive effect.

Example. Charge overlay crucible

45 bismuth germanate is placed in the crystallization unit of the Crystal-ZM installation, bismuth germanate is melted in a platinum crucible with a diameter of 100 and a height of 120 mm in an oxidizing atmosphere, a single-crystal seed crystal with a diameter of 15 mm and a diameter of 15 mm rotating at a speed of 40 rpm is lowered into the melt and extruded with speed of 1.5 mm / h. They grow the upper cone with the help of an automatic process control system, setting the diameter output

55 55 mm for 15 hours (corresponding to the solid angle of the upper cone 140 °), then continue to grow the crystal on a constant diameter of 55tO, 5 mm to a length of 200 m. Then, the extrusion speed is increased to 2000 mm / h (and thereby tear

it from the melt) and raise to a height of 50 mm. Immediately after detaching the crystal (controlled by USUT devices), the high-frequency power supplied to the inductor is reduced (according to the specified ACS program). After the crystal has cooled, it is removed from the crystallization unit and its quality is evaluated. A bismuth germanate crystal with a diameter of 55 mm and a length of 200 mm without cracks, transparent, with an upper cone height of 10 mm (and a solid angle of 140 °), with a defect zone near the upper cone with a length of Hk no more than 10 mm, without distortion the shape of the side surface with traces of the exit of F-faces - it is completely suitable (taking into account the machining allowance) for the manufacture of a volume scintillator in the form of a hexagonal prism in a U section with a diameter of a circumscribed circle of DC 50 mm and a height of 180 mm for a high-energy photon spectrometer and . The utilization of the material of the crystal is Q 1 - QI -. .

The test results are shown in the table.

Comparative tests of the proposed method were carried out. A total of 25 experiments were carried out according to the given regimes (or close to them) for bismuth germanate single crystals of various diameters and lengths (mainly DK 50–70 mm, to 100–200 mm). More than 50 experiments were conducted on the prototype and analogues.

All crystals obtained in accordance with the claims and the description of the embodiment, had no cracks and a defective lower cone, colorless,

transparent, without distortion of the cylindrical shape of the side surface of the crystal with traces of the exit of F-faces of type (112), with a height of the upper cone of 10 -15 mm and a defective zone of the inverse cone of no more than 10-15 mm - were suitable for the manufacture of volumetric long-length scintillators . The utilization of the material of the crystal was Q 1 - SI - Q2 60% (SI 8 - 10% is the upper cone and the defective area adjacent to it; Q2 30-32% are the processing allowances - the difference in cylinder volume and hexagonal cross-section prisms, cutting, grinding, polishing, etc. If the shape of the crystal is distorted, this fraction increases to Q2 40-42%).

Thus, tests have shown that the claimed method can significantly increase the utilization of the material of the crystal. The specified method can be used when growing other, similar in properties to BGO, single crystals.

Claims (1)

SUMMARY OF THE INVENTION A method of growing bismuth germanate single crystals, comprising etching by a rotating seed, expanding the upper conical part of a single crystal with a given solid angle and drawing a cylindrical part, separating the crystal from the melt and cooling it, characterized in that, in order to increase the yield suitable by improving the quality of the cylindrical part of the single crystal, the growth of the upper conical part is carried out within a solid angle of 130-160 degrees. Table continuation FIG FIG. 2