RU2555901C1 - Method of obtaining scintillation monocrystals based on lanthanum bromide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be applied in the production of sensitive elements of a gamma- and roentgen radiation detector. Scintillation monocrystals La_(1-m-n)Hf_nCe_mBr_(3+n), where m is the molar part of La substitution with cerium (0.0005≤m≤0.3), n is the molar part of La substitution with hafnium (0≤n≤0.015), are obtained from a mixture of metal bromides. The charge is loaded in a quartz ampoule with inoculum, the ampoule is vacuumised, soldered, placed in a growth installation, heated to the charge melting, exposed until an equilibrium condition is set in the liquor, a monocrystal is grown by the creation of a gradient temperature section in the ampoule and cooled, with the application of a multi-zone growth installation with the electrodynamic movement of a temperature gradient in the longitudinal-axial direction. To melt the charge the temperature of the installation heater in the zone of the inoculum t₁ is selected from the interval of 685°C<t₁<720°C, the temperature of the following heater t₂ - from the interval of 770°C≤t₂≤790°C. After the charge melting the ampoule is exposed for not less than 10 hours, the monocrystal growing is realised by the movement of the temperature gradient along the longitudinal axis of the installation at a rate of 0.3 mm/h≤v_tg≤0.5 mm/h. Boundary values of temperatures of so-called cold t_cz and hot t_hz zones of the gradient section are selected from the intervals of 720°C<t_cz≤740°C and 790°C≤t_hz≤820°C, with the ampoule cooling being realised at a rate of not higher than 15°C/h.

EFFECT: accuracy of supporting temperature fields, stability of their movement at all stages of the crystal growth, strict control of temperature and time parameters of the growth process, obtaining monocrystals with specified optical characteristics and sizes with high output.

2 cl, 2 tbl, 9 ex

Description

The invention relates to a technology for producing scintillation single crystals based on lanthanum bromide and can be used mainly in the manufacture of sensitive elements of gamma radiation, x-ray radiation, cosmic rays, low-energy electromagnetic waves, etc.

A known method for producing scintillation single crystals of the general formula M ₍₁₎ Ce _x Br3 (US Patent No. 7067816 B2. IPC G01K 11/85 (2006.01). Scintillation crystals, methods for their manufacture and their use. Published on June 27, 2006), where M is an element selected from the group La, Gd, Y, or a mixture thereof, x is the mole fraction of the substitution of M with cerium. According to this invention, a scintillation single crystal is obtained from a mixture of powders MBr ₃ and CeBr ₃ by Bridgman crystallization in a vacuum sealed quartz ampoule. The specific parameters at which the method for producing said crystals is carried out are not given in the description of the invention.

A known method of producing scintillation single crystals of the general formula Ln _(1-y) CeyX ₃ : M (US Patent No. 7692153 B2. IPC G01T 1/202 (2006.01). Scintillation crystal and radiation detector. Published on 04/06/2010), where Ln is one or more elements selected from the group of rare-earth elements, X is one or more elements selected from halogens, M is a dopant to the material of the crystal matrix selected from the group of elements Li, Na, K Rb, Cs, Al, Zn, Ga, Be, Mg, Ca, Sr, Ba, Sc, Ge, Ti, V, Cu, Nb, Cr, Mn, Fe, Co, Ni, Mo, Ru, Rh, Pb, Ag, Cd, In, Sn, Sb, Ta, W, Re, Os, Ir, Pt, Au, Hg, TiBi, where y is a number satisfying the conditions 0,0001≤u≤1 Twa. The scintillation single crystal based on lanthanum bromide according to this invention is obtained mainly by the Bridgman method. The growth furnace, which is used to implement the method, is configured to vertically move the crucible in it. The furnace contains a crucible in which the source material (charge) is placed, a heater for forming a temperature gradient decreasing in the downward direction along the vertical axis, a rod for moving the crucible in the vertical direction, an insulating element surrounding the structural elements listed above. The initial charge, which is a mixture of commercially available compounds LaBr ₃ and CeBr ₃ and a dopant, is heated to melt, and then cooled to crystallize. As a dopant, a substance containing elements such as Na, Fe, Cr or Ni, preferably NaBr, FeBr ₂ , CrBr ₂ and NiBr _{2, is used} . For example, a quartz glass ampoule is used in a crucible, the initial charge is loaded, the ampoule is sealed at a pressure of ≤1 Pa, the ampoule is installed in a growth plant, heated to a temperature of ~ 800 ° C and kept at this temperature until the charge is melted. After melting the charge, the ampoule is gradually lowered down along the longitudinal axis of the growth furnace, while slowly lowering the melt temperature below the melting point with a temperature gradient of ~ 3 ° C / cm ÷ 10 ° C / cm. The lowering speed of the ampoule is preferably not more than 3 mm / h, most preferably not more than 0.5 mm / h to prevent cracking of the crystal. As a result, crystallization from the melt occurs in a section with a directed temperature gradient (hereinafter, referred to as a "gradient section"). After the lowering of the ampoule is completed, the heater is turned off, the ampoule is kept at the lower point reached after lowering, cooling to room temperature, the result is a single crystal. The more precisely the temperature and the speed of the ampoule are controlled, the more the resulting crystal corresponds to the required parameters.

The closest technical solution is a method for producing scintillation single crystals, preferably based on lanthanum bromide of the general formula La _(1-mn) Hf _n Ce _m Br _{(3 + n)} (RF Patent No. 2426694. IPC С01F 17/00 (2006.01). Inorganic scintillation material, crystalline scintillator and radiation detector. Published on 08/20/2011), where m is the mole fraction of La substitution with cerium, n is the mole fraction of La substitution with hafnium. The material of the present invention is obtained in the form of a single crystal by growing by known methods, in particular by the Bridgman method. To grow a single crystal by the Bridgman method in evacuated quartz glass ampoules (quartz ampoules), the starting materials (purity 99.999) are used. The loading of the quartz ampoule with growth material, which is a mixture of the starting metal bromides in the required ratio, is carried out in a dry box. The ampoule with growth material is evacuated and placed in a growth zone installation, which is a two-zone growth furnace, structurally containing hot and cold zones (melting zone and crystallization zone). The speed of lowering the ampoule from the hot zone to the cold is ~ 2 mm / h. The result is a transparent single crystal based on lanthanum bromide.

In the analogs described above, directional crystallization is carried out by the Bridgman method in growth plants or the so-called growth furnaces. The growth furnace has at least two characteristic zones: a heating zone, where the initial billet (charge) is melted, and a cooling zone, where the crystal is “pulled”. Moreover, in the crystal-melt system, the gradient temperature field necessary for directed crystallization is created. When the crucible is slowly lowered (hereafter, the crucible can be understood as a quartz ampule) from the hot zone of the growth furnace to the colder, a seed crystal forms on its sharp bottom, which can grow into a large single crystal during further lowering of the crucible. The common disadvantages of the described known methods for producing single crystals based on lanthanum bromide include the following. In a dual-zone growth furnace, it is difficult to control the temperature with the necessary accuracy, which can lead to either melting of the seed or insufficient melt cooling to start crystallization. The main difficulty in growing single crystals according to the Bridgman method is the need to create a small temperature gradient along the crucible, in which the smoothest layer-by-layer growth of the single crystal occurs. In this case, insufficient subcooling of the charge melt is possible to start crystallization. At the same time, if significant subcooling is created in the melt, and the temperature gradient is relatively small, then the entire mixture may be subcooled below the melting temperature even before the appearance of the first crystallite. The nucleation of a crystal under such conditions leads to a very rapid growth of crystals in the melt and the inevitable formation of small crystals of unsatisfactory quality. In addition, the use of the mechanical method of moving the ampoule during high-temperature crystallization leads to the difficulty of positioning the ampoule due to the opacity of the walls of the growth furnace, and also does not allow observing the position of the crystallization front and, accordingly, influencing it.

The authors were faced with the task of eliminating these shortcomings and developing a method for producing scintillation single crystals based on lanthanum bromide, easily scalable in the future to industrial technology and making it possible to obtain single crystals with desired characteristics and required optical properties. An integral part of the task was to determine the operating parameters of the method for producing high-quality single crystals suitable for use in scintillation radiation detectors.

, гдеTo solve this problem, a method for producing scintillation single crystals based on lanthanum bromide having the general formula

where

m is the molar fraction of the replacement of lanthanum with cerium, a number from the interval

0.0005≤m≤0.3, preferably from the interval 0.03≤m≤0.05,

n is the molar fraction of the substitution of lanthanum with hafnium, a number from the interval

0≤n≤0.015, preferably 0≤n≤0.01,

according to which single crystals of the general formula (1) are obtained from a charge, which is a mixture of metal bromides, the charge is loaded into a crucible, preferably a quartz ampoule in which a seed crystal is placed, the ampoule with a charge is vacuumized, sealed, installed in a growth unit, heated until the charge is melted , kept in a growth plant until the equilibrium state is established in the melt, a single crystal is grown by creating a gradient temperature section in the ampoule, after which the ampoule is cooled. The proposed method is characterized in that a multi-zone growth installation with electrodynamic movement of the temperature gradient in the longitudinal-axial direction is used as a growth installation, to melt the charge, the temperature of the installation heater in the seed crystal placement zone (t ₁ ) is selected from the interval 685 ° C≤t ₁ ≤720 ° C, and the temperature of the next heater of the growth unit (t ₂ ) is selected from the interval 770 ° C ≤t ₂ ≤790 ° C, after melting the charge, the ampoule is kept for at least 10 hours in isothermal words, the single crystal is grown by moving the temperature gradient along the longitudinal axis of the multiband growth setup, the boundary values of the temperature t _xz and gradient portion is selected from the ranges 720 ° S≤t _xz ≤740 ° C and 790 ° C≤t _rs ≤820 ° C, where t _xs is the temperature of the so-called cold zone of the growth plant, and t _gz is the temperature of the so-called hot zone of the growth plant, while the temperature gradient velocity v _{tg is} selected from the interval 0.3 mm / h ≤v _tg ≤0.5 mm / h, and the ampoule is cooled by heat ode a rate not exceeding 15 ° C / h.

The boundary values of the temperatures t _xs and t _{gs of the} gradient portion are preferably selected such that the temperature gradient is 10 ° C / cm ÷ 15 ° C / cm.

The quality of a single crystal is determined by the absence of defects, stresses, cracks in it, the size of the single crystal, as well as its scintillation characteristics, such as energy resolution and light output.

In the proposed method, these problems are solved by choosing a growth unit and correctly determining the operating parameters of the process, while achieving the following technical result. The use of a multi-zone growth installation with electrodynamic movement of the temperature gradient in the longitudinal-axial direction allows us to ensure the accuracy of maintaining temperature fields, the stability of their movement at all stages of crystal growth, and strict control of the temperature and time parameters of the growth process. In addition, the implementation of the method in such an installation eliminates the need for complex and expensive precision devices for the mechanical movement of the ampoule. Correct determination of the operating parameters of the proposed method made it possible to obtain, with a high yield, single crystals based on lanthanum bromide with predetermined optical characteristics and required sizes.

The temperature profile of a multi-zone growth installation (hereinafter referred to as the "multi-zone growth furnace") with electrodynamic movement of the temperature gradient is formed by several independent heating elements (hereinafter referred to as k), which allows not only to create a temperature profile of complex shape, but also to move it along the longitudinal axis of the furnace by independent, in particular software control of heaters. The concept of a zone in a growth plant is rather arbitrary, since there is a single space in the furnace in which k heaters create the temperature, forming k corresponding conventional heating zones. For convenience of presentation, hereinafter sequentially located heaters and their corresponding conditional temperature zones will be denoted by the same alphanumeric numbers, for example, the heater of the first zone and the seed placement zone - k ₁ heater of the next zone and the corresponding zone - k ₂ , etc. Since the zones are not isolated from each other, their boundaries are blurred, as a result of which, when creating a gradient section, the thermal effect of each of the temperature sections on the adjacent zones inevitably occurs.

The method is as follows. Below is a brief description of the main stages of obtaining single crystals based on lanthanum bromide in accordance with the claimed method.

The melting of the charge. The charge is melted inside the ampoule due to the slow heating of the multi-zone growth furnace to a given temperature. Since in the inventive method, the growth of a single crystal occurs on an oriented seed crystal (seed), which is placed in the first zone of the furnace, it was necessary to experimentally find such a temperature of this zone at which there is no melting of the seed, and the charge in contact with the seed is in molten state at a certain temperature (t ₂ ) of the neighboring zone k ₂ . Here, the temperature of the first zone is understood to mean the temperature t _{1 of a} particular heater (k ₁ ) of a multi-zone growth furnace at the location of the seed. During the entire process of growing a single crystal, the temperature t _{1 is} kept constant. It was experimentally established that the operating value of t ₁ should be selected from the interval 685 ° C <t ₁ <720 ° C. It was found that at t ₁ <685 ° C the charge near the seed does not completely melt, resulting in spontaneous crystallization, which leads to the formation of small crystals of poor quality. At a temperature t ₁ > 720 ° C, the seed melts, which also leads to spontaneous crystallization. For the same reason, the temperature of the next heating element k ₂ should be slightly lower than the melting temperature of the mixture. The temperature value t _{2 of the} heater k _{2 is} selected from the interval 770 ° C ≤t ₂ ≤790 ° C. Moreover, the charge in the zone k ₂ is in the molten state due to convection, since there is heat transfer from the subsequent temperature zone k ₃ - the melt overheating zone, which corresponds to the heater k ₃ , to the zone of the heater k ₂ ; the seed does not melt. The temperature values of subsequent heating elements (k ₃ , k ₄ , etc.) and the corresponding zones of the growth furnace correspond to t _gz .

Thermal stabilization - holding the ampoule in the furnace after the charge is melted for a certain period of time at a constant temperature to establish the equilibrium state of the melt inside the ampoule. It was experimentally established that the exposure time of the ampoule at the temperature reached at the stage of melting the charge should be at least 10 hours, followed by the stage of growing a single crystal.

Single crystal cultivation. The single crystal growth is ensured by shifting the temperature gradient along the longitudinal axis of the furnace upward at a certain speed. To obtain scintillation single crystals based on lanthanum bromide with given sizes and the required optical characteristics, it was necessary to create a minimum gradient section with a certain temperature change along the longitudinal axis of the furnace. A gradient plot is created as follows. The heater k _{2 is} gradually cooled to the temperature of the cold zone (t _xs ), while the gradual formation of the cold zone occurs, and the heater k _{3 is} cooled to the temperature t ₂ . As a result, the gradient section moves up along the axis of the ampoule, and a crystallization zone is created between the cold and hot zones due to a change in temperature from the temperature of the cold zone t _xz to the temperature of the hot zone t _gz with a smooth temperature gradient. It was experimentally determined that when growing single crystals based on lanthanum bromide, the gradient section should preferably be characterized by a temperature change of ~ 70 ° C in the section of the furnace ~ 5 cm long along its longitudinal axis. It was experimentally established that t _xs should be selected from the interval 720 ° C≤t _xz ≤740 ° C, t _gz - from the interval 790 ° C≤t≤820 ° C, and the speed of the temperature gradient (v _tg ) - from the interval 0, 3 mm / h≤v _tg ≤0.5 mm / h. Thus, at the beginning of the growth process, the gradient temperature section starts from the first zone of the growth furnace and gradually moves upward along the longitudinal axis of the ampoule. As a result of the movement of the gradient, as the single crystal grows, the cold zone increases in length, and the hot zone (melt zone), respectively, decreases in length. After completion of the growth process, only the cold zone remains in which the obtained single crystal is located. The described process of growing a single crystal based on lanthanum bromide according to the claimed method is illustrated by the example below in table 1.

The table shows the temperature values that form the gradient region on which crystallization occurs.

Gradual cooling of the ampoule. Reaching room temperature with a certain cooling rate. This is important because rapid cooling leads to cracks in the grown single crystal due to the anisotropy of the linear coefficient of thermal expansion and to stresses in the single crystal, which can lead to cracking during subsequent crystal processing operations (cutting, grinding, polishing). Too slow cooling leads to an unjustified increase in the duration of the process of growing a single crystal, as a consequence of this - a decrease in the number of single crystals obtained in one installation for the same time and, accordingly, the cost of the final product. To eliminate the possibility of the formation of the above defects and to reduce the time required to complete this step, it was necessary to determine the optimal cooling rate of the ampoule. It was experimentally established that the cooling rate should not exceed 15 ° C / h.

The inventive method is illustrated by the following examples, but is not limited to. The execution of the method described in the examples for the production of single crystals of the general formula (1) includes the following general part for all the described examples.

The quartz ampoule is washed with hydrofluoric acid (HF) and rinsed with distilled water. The ampoule is placed in an oven for ~ 10 hours at a temperature of 250 ° C, and then transferred to a dry box, where a seed crystal is placed in the nozzle of the ampoule and a charge is loaded, which is a sample of a mixture of the starting metal bromides (purity 99.99%, production company Sigma Aldrich) in the ratio of metal bromides in the mixture calculated in accordance with the single crystal formula.

The ampoule with the loaded charge is evacuated to a pressure of ~ 1 Pa, hermetically sealed and installed in the lodgement of a multi-zone growth unit with electrodynamic movement of the temperature gradient. For the implementation of the claimed method can be used in particular a multi-zone installation MellenEDG11 Sunfire, which contains 32 inch zones. When growing single crystals by the present method, as a rule, 7-8 zones were involved. The use of such or a similar installation provides a smooth transition from one stage of the method to another due to the integrated control unit for the process of growing a single crystal. The preferred heating rate of thermocouples is ~ 120 ÷ 135 ° C / h, in which case the exit time to the mode is about six hours. After a series of experiments to optimize the speed and temperature conditions of the process, it was possible to achieve stable seeding of the crystal. Subsequently, by selecting the speed and temperature conditions, the problem was solved of obtaining homogeneous single crystals based on lanthanum bromide with desired optical properties.

Below are the results of some experiments to determine the operating parameters for producing single crystals using La _0.95 Ce _0.05 Br ₃ and La _0.949 Ce _0.05 Hf _0.001 Br _{3.001 as examples} (the experimental results are similar for both single crystals, therefore the examples are given without separation according to the formulas of single crystals )

Example 1

Key process parameters:

- temperature of the first zone - 725 ° C

- temperature of the cold zone - 740 ° С

- hot zone temperature - 810 ° C

- temperature gradient shift rate - 0.5 mm / h

- heating rate - 120 ° C / h

The result of the experiment: the seed melted, the single crystal did not grow.

Example 2

Key process parameters:

- temperature of the first zone - 721 ° C

- cold zone temperature - 730 ° C

- hot zone temperature - 800 ° C

- temperature gradient shift rate - 0.4 mm / h

- heating rate - 120 ° C / h

The result of the experiment: the seed melted, the single crystal did not grow.

Example 3

Key process parameters:

- temperature of the first zone - 690 ° C

- temperature of the second zone - 770 ° C

- cold zone temperature - 718 ° C

- hot zone temperature - 790 ° C

- rate of temperature gradient displacement - 0.45 mm / h

- cooling rate - 20 ° C / h

- heating rate - 120 ° C / h

The result of the experiment: seeding, partial formation of small crystals.

Example 4

Key process parameters:

- temperature of the first zone - 690 ° C

- temperature of the second zone - 780 ° C

- cold zone temperature - 725 ° C

- hot zone temperature - 805 ° C

- rate of temperature gradient displacement - 0.45 mm / h

- cooling rate - 17 ° C / h

- heating rate - 120 ° C / h

The result of the experiment: successful seeding, a single crystal grew, the crystal cracked upon cooling.

Example 5

Key process parameters:

- temperature of the first zone - 680 ° C

- temperature of the second zone - 770 ° C

- cold zone temperature - 730 ° C

- hot zone temperature - 800 ° C

- rate of temperature gradient displacement - 0.45 mm / h

- heating rate - 120 ° C / h

- cooling rate - 15 ° C / h

The result of the experiment: the mixture near the seed did not melt completely, and small crystals were obtained.

Example 6

Key process parameters:

- temperature of the first zone - 690 ° C

- temperature of the second zone - 790 ° C

- cold zone temperature - 740 ° C

- hot zone temperature - 800 ° C

- temperature gradient shift rate - 0.55 mm / h

- heating rate - 120 ° C / h

- cooling rate - 15 ° C / h

The result of the experiment: successful seeding, a single crystal grew, having dark inclusions and cracks.

Example 7

Key process parameters:

- temperature of the first zone - 685 ° C

- temperature of the second zone - 780 ° C

- cold zone temperature - 740 ° C

- hot zone temperature - 825 ° C

- temperature gradient shift rate - 0.4 mm / h

- heating rate - 120 ° C / h

- cooling rate - 15 ° C / h

- single crystal growing time - 17 900 min

The result of the experiment: successful seeding, a single crystal grew, the number of color centers decreased significantly.

Example 8

Key process parameters:

- temperature of the first zone - 700 ° C

- temperature of the second zone - 785 ° C

- cold zone temperature - 730 ° C

- hot zone temperature - 810 ° C

- rate of temperature gradient displacement - 0.46 mm / h

- heating rate - 135 ° C / h

- cooling rate - 15 ° C / h

- single crystal growing time ~ 18 300 min

The result of the experiment: successful seeding, a single crystal has grown, there are practically no centers of color.

Example 9

Key process parameters:

- temperature of the first zone - 690 ° C

- temperature of the second zone - 770 ° C

- cold zone temperature - 740 ° C

- hot zone temperature - 810 ° C

- rate of temperature gradient displacement - 0.46 mm / h

- cooling rate - 15 ° C / h

- heating rate - 135 ° C / h

- single crystal growing time ~ 18 500 min

The result of the experiment: successful seeding, a single crystal has grown, there are no color centers, no cracks.

The experimentally established operating parameters given in the claims and, in particular, in table 1, were used to obtain single crystals of La _0.949 Ce _0.05 Hf _0.003 Br _3.003 , La _0.948 Ce _0.05 Hf _0.002 Br _3.002 , La _0.96 Ce _0.04 Br ₃ , La _0.949 Ce _0.05 Hf _0.001 Br _3.001 , La _0.97 Ce _0.03 Br ₃ . As a result, scintillation single crystals based on lanthanum bromide with the characteristics shown below in table 2 are obtained.

The advantages of the proposed method for producing single crystals include the possibility of continuous monitoring of the process at all its stages, which reduces the time of growing single crystals and the number of unsuccessful growth experiments and, consequently, reduces the number of defective crystals. The use of experimentally found operating parameters of the process allowed us to develop a computer program for growing single crystals, with the help of which the control computer of a multi-zone growth unit automatically controls the process of producing single crystals at all its stages. In addition, the control computer can control the simultaneous growth of single crystals in two or more plants.

Claims (2)

1. The method of obtaining scintillation single crystals based on lanthanum bromide, having the General formula

where
m is the molar fraction of the replacement of lanthanum with cerium, a number from the interval
0.0005≤m≤0.3, preferably from the interval 0.03≤m≤0.05,
n is the molar fraction of the substitution of lanthanum with hafnium, a number from the interval
0≤n≤0.015, preferably 0≤n≤0.01,
according to which single crystals of the general formula (1) are obtained from a charge, which is a mixture of metal bromides, the charge is loaded into a crucible, preferably a quartz ampoule in which a seed crystal is placed, the ampoule with a charge is vacuumized, sealed, installed in a growth unit, heated until the charge is melted , kept in a growth plant until the equilibrium state is established in the melt, a single crystal is grown by creating a gradient temperature section in the ampoule, after which the ampoule is cooled, characterized in that as a growth installation, a multi-zone growth installation with electrodynamic movement of the temperature gradient in the longitudinal-axial direction is used, to melt the charge, the temperature of the installation heater in the seed crystal placement zone (t ₁ ) is selected from the interval 685 ° C≤t ₁ ≤720 ° C, and next growth heater temperature setting (t ₂₎ is selected from the range of 770 ° C≤t ₂ ≤790 ° C, after the melting of the charge vial incubated for at least 10 hours under isothermal conditions, a single crystal is grown by eremescheniya temperature gradient along the longitudinal axis of the multiband growth setup, the boundary temperature values t _xs and t _rs gradient portion is selected from the ranges 720 ° C≤t _xs ≤740 ° C and 790 ° C≤t _rs ≤820 ° C, wherein t _xs is the temperature of the so-called cold zone of the growth plant, at _rz is the temperature of the so-called hot zone of the growth plant, while the temperature gradient velocity v _{tg is} selected from the interval 0.3 mm / h ≤v _tg ≤0.5 mm / h, and cooling ampoules are carried out by heat removal at a rate of not more than 15 ° C / h. 2. A method according to claim 1, characterized in that the boundary values of the temperature t and t _{rs xz} gradient portion is preferably selected so that the temperature gradient is 10 ° C / cm to 15 ° C / cm.