Method of producing a crystal for a scintillation crystal detector and a crystal for a scintillation crystal detector

Abstract

The invention relates to a method of producing a crystal from a material with the general composition of Ce.sub.xGd.sub.yY.sub.1?x?yAlO.sub.3 known to the professional public for scintillation crystal detectors, which has not yet been industrially produced by the Czochralski method. The invented method makes it possible to produce crystals with a diameter larger than units of mm. In particular, the invention adds to the initial Czochralski method the steps of annealing the input raw materials as well as the controlled flow of a reducing hydrogen-argon atmosphere through a crystal growth furnace.

Claims

1. A method of producing a crystal for a scintillation crystal detector consisting in producing a crystal with the general composition of Ce.sub.xGd.sub.yY.sub.1?x?yAlO.sub.3 by the Czochralski method by pulling from a molybdenum or tungsten crucible under a reducing atmosphere of a crystal growing furnace, where x is from the range of 0.005 to 0.015 and y is from the range of 0.4 to 0.6, and within the framework of which a) the input raw materials are prepared, b) the input raw materials are placed in the crucible, c) the content of the crucible is melted under the reducing atmosphere of the crystal growing furnace under the action of heat and a crystal is produced by pulling, characterized in that as part of process step a), the input raw materials are annealed in the presence of fluoride ions, and, during process step c), the reducing atmosphere of the crystal growing furnace consists of a gaseous mixture of argon and hydrogen, while the reducing atmosphere is allowed to flow through the crystal growth furnace, and at the same time the flow rate of the reducing atmosphere ranges from 1.67?10.sup.?7 m.sup.3/s to 1.39?10.sup.?5 m.sup.3/s.

2. The method according to claim 1, characterized in that argon makes up 5-95% of the volume of the reducing atmosphere and hydrogen makes up 95-5% of the volume of the reducing atmosphere, while the composition of the reducing atmosphere remains the same throughout the crystal production.

3. The method according to claim 1, characterized in that the input raw materials are Gd.sub.2O.sub.3, Y.sub.2O.sub.3, Al.sub.2O.sub.3 and CeO.sub.2.

4. The method according to claim 1, characterized in that process step c) is followed by process step d), in which the produced crystal or semi-finished products prepared from the crystal are annealed in a circulating reducing atmosphere consisting of hydrogen with 0-99% by volume of at least one complementary gas from the group of argon, helium, neon, krypton, xenon.

5. The method according to claim 4, characterized in that as part of process step d) the reducing atmosphere has a temperature in the range from 1000? C. to 1500? C., while the annealing time ranges from 50 hours to 100 hours.

6. The method according to claim 1, characterized in that NH.sub.4F is used as part of process step a).

7. The method according to claim 6, characterized in that the concentration of NH.sub.4F in step a) is 0.1 to 1% by weight in proportion to the input raw materials.

8. The method according to claim 1, characterized in that the ratio of gadolinium to yttrium is set in the range of 0.4<y<0.6 as part of process step b).

9. A crystal with the general composition of Ce.sub.xGd.sub.yY.sub.1?x?yAlO.sub.3 for a scintillation crystal detector, where x is in the range from 0.005 to 0.015 and y is in the range from 0.4 to 0.6, produced by the method according to claim 1, characterized in that its diameter ranges from 30 to 60 mm.

Description

EXPLANATION OF DRAWINGS

[0020] The present invention will be explained in detail by means of the following figures where:

[0021] FIG. 1 presents a graph comparing the radioluminescence spectra of the crystal produced in accordance with the invention (Example 1) and the reference material,

[0022] FIG. 2 presents a graph describing the scintillation decay of the crystal produced in accordance with the invention (Example 1),

[0023] FIG. 3 presents a graph comparing the energy spectra of the crystal produced in accordance with the invention (Example 1) and the reference materials.

EXAMPLE OF THE INVENTION EMBODIMENTS

[0024] It shall be understood that the specific cases of the invention embodiments described and depicted below are provided for illustration only and do not limit the invention to the examples provided here. Those skilled in the art will find or, based on routine experiment, will be able to provide a greater or lesser number of equivalents to the specific embodiments of the invention which are described here.

[0025] To produce a crystal with the general composition of Ce.sub.xGd.sub.yY.sub.1?x?yAlO.sub.3, a crystal growth furnace was modified to perform the Czochralski method, which is commonly used, for example, to grow single crystals of garnet or perovskite structure. A circuit for the circulation of the reducing atmosphere was connected to the crystal growth furnace, including cylinders containing hydrogen and argon and a mixing valve for precise adjustment of the ratio of the two gases. The circulation circuit included an adjustable valve and a flow meter for setting the flow rate of the gaseous mixture of the reducing atmosphere through the crystal growth furnace. Crystals were successfully produced at proportions of hydrogen within 5 to 95% of the total volume of the reducing atmosphere and argon within 5 to 95% of the total volume of the reducing atmosphere. Once the reducing atmosphere with the gas ratio in the crystal growth furnace was prepared, the argon-to-hydrogen ratio was no longer varied.

[0026] As for the parameters of circulation of the reducing atmosphere through the crystal growth furnace, the flow rate of the reducing atmosphere was chosen in the range from 1.67?10.sup.?7 m.sup.3/s to 1.39?10.sup.?5 m.sup.3/s. Higher circulation rates disrupted the thermodynamic stability of the pulled crystal, while lower circulation rates resulted in a higher occurrence of undesired inclusions in the produced crystals.

[0027] For the industrial production of crystals, the input raw materials Gd.sub.2O.sub.3, Y.sub.2O.sub.3, Al.sub.2O.sub.3 and CeO.sub.2 have been prepared, but it is not impossible that the expert could suggest alternative input raw materials, or other suitable input raw materials could be found in the future. Crystals for which the input raw materials met the condition of maintaining the gadolinium-to-yttrium ratio in the range of 0.4<y<0.6 showed better results in the first reference scintillation tests than crystals produced from input raw materials with different gadolinium-to-yttrium ratio.

[0028] The input raw materials were annealed in the presence of fluoride ions, while practical experiments showed that NH.sub.4F was the most suitable material for the formation of fluoride ions, which substantially affected the initial reactivity of the input raw materials. The range of concentration of NH.sub.4F material was determined by experiment from 0.1 to 1% by weight in proportion to the input raw materials.

[0029] During the production of crystals and semi-finished products prepared from the crystals, it was necessary to heat the crystals and semi-finished products prepared from the crystals in a circulating reducing atmosphere, which was composed of hydrogen and of 0 to 99% by volume of at least one complementary gas from the group of argon, helium, neon, krypton, xenon. The annealing reducing atmosphere prevented the formation of structural changes in the structure of the produced crystals, as the structure of freshly produced crystal is sensitive to external factors, both in terms of chemical substances and sudden temperature changes. Hydrogen is used as a reducing gas, while the presence of a complementary gas, which is at least one non-reactive noble gas, serves to prevent undesired chemical reactions from occurring.

[0030] It turned out that even after the crystal has been pulled, there are processes taking place in the crystal structure that need sufficient temperature and enough time to complete them successfully. Based on the experiments, annealing intervals in the temperature range from 1000? ? C. to 1500? C. and 50 hours to 100 hours time duration were statistically determined.

[0031] Industrially produced crystals with the general composition of Ce.sub.xGd.sub.yY.sub.1?x?yAlO.sub.3 were pulled in the form of single crystals, the diameter of which was from 30 mm to 60 mm.

Example 1

[0032] Ce.sub.0.009Gd.sub.0.5Y.sub.0.491AlO.sub.3 crystal is grown by the Czochralski method at a flow rate of 2?10.sup.?7 m.sup.3/s in a protective atmosphere with a composition of 40% argon+60% hydrogen. The growth takes place in a 0.4 dm.sup.3 molybdenum crucible in a furnace with resistance heating formed by tungsten loops. Raw material, prepared by isostatic pressing and annealing a mixture of oxides Y.sub.2O.sub.3, Al.sub.2O.sub.3, Gd.sub.2O.sub.3 and CeO.sub.2 at 1500? C. with the addition of 0.5% by weight NH.sub.4F in the above stoichiometric ratio is weighed into the crucible in an amount of 5800 g. After melting, the raw material is homogenized by natural convection of the melt for 12 hours. After the melt is homogenized, a sample is taken out for stoichiometry analysis of the melt and the crystal growth starts at an oriented YAP <010> crystal seed rotating at 2 rpm. The velocity of pulling the crystal is 1.5 mm/hour. Crystal growth is controlled automatically by monitoring and evaluating weight increase over time.

[0033] After reaching the required length of the crystal, the crystallization process is completed by detouching the crystal from the melt. The crystal is then annealed in several steps.

[0034] In the temperature range of 1900? C.-1500? ? C. at a rate of 2.7?10.sup.?2 s/K, in the range of 1500? C.-1000? C. at a rate of 0.08 s/K, and in the range of 1000? ? C. to room temperature at a rate of 2.7?10.sup.?2 s/K. The temperature is monitored using a two-beam pyrometer.

[0035] The result is a clear single crystal with a weight of 950 g and a diameter of 32 mm.

[0036] A double-sided ?10?1 mm polished plate was prepared from the initial part of the crystal, on which the radioluminescence spectrum and scintillation decay were measured, from the latter the 1/e decay time was calculated. The energy (pulse-height) spectrum was measured as well from which the light yield and energy resolution at 662 keV were calculated. The graphs (FIGS. 1 to 3) and the table below present a comparison of the parameters of the crystal grown using the invention and the reference scintillators BGO and YAP:Ce with the same dimensions.

TABLE-US-00001 EnRes Spectrum - 1/e decay Light yield (%) maximum Sample time (ns) (photons/MeV) @662 keV (nm) GYAP:Ce 63.4 23990 4.1 356 14210/1A + YAP:Ce 35.1 19330 4.0 360 reference BGO reference 308 7380 8.0 480

Example 2

[0037] Ce.sub.0.009Gd.sub.0.55Y.sub.0.441AlO.sub.3 crystal is grown by the Czochralski method at a flow rate of 1?10.sup.?6 m.sup.3/s in a protective atmosphere with a composition of 25% argon+75% hydrogen. The growth takes place in a 3 dm.sup.3 molybdenum crucible in a furnace with resistance heating formed by tungsten loops. Raw material, prepared by isostatic pressing and annealing a mixture of oxides Y.sub.2O.sub.3, Al.sub.2O.sub.3, Gd.sub.2O.sub.3 and CeO2 at 1500? ? C. with the addition of 0.8% by weight NH.sub.4F in the above stoichiometric ratio is weighed into the crucible in an amount of 12400 g. After melting, the raw material is homogenized by natural convection of the melt for 12 hours. After the melt is homogenized, a sample is taken out for stoichiometry analysis of the melt and the crystal growth starts at an oriented YAP <010> crystal seed rotating at 2 rpm. The velocity of pulling the crystal is 1 mm/hour. Crystal growth is controlled automatically by monitoring and evaluating weight increase over time.

[0038] After reaching the required length of the crystal, the crystallization process is completed by detouching the crystal from the melt. The crystal is then annealed in several steps.

[0039] In the temperature range of 1900? C.-1500? ? C. at a rate of 2.0?10.sup.?2 s/K, in the range of 1500? C.-1000 C at a rate of 0.1 s/K, and in the range of 1000? ? C. to room temperature at a rate of 2.0?10.sup.?2 s/K. The temperature is monitored using a two-beam pyrometer.

[0040] The result is a clear single crystal with a weight of 4750 g and a diameter of 60 mm.

[0041] Subsequently, a ? 1?1 cylinder with polished faces was made and similar measurements as in Example 1 were made with the results presented in the table below.

TABLE-US-00002 EnRes Spectrum - 1/e decay Light yield (%) maximum Sample time (ns) (photons/MeV) @662 keV (nm) GYAP:Ce 67.5 17,640 7.1 358 14210/1A +

INDUSTRIAL APPLICABILITY

[0042] A method of producing a crystal for a scintillation crystal detector and a crystal produced by the invented method will find their application in penetrating ionizing radiation detectors, in particular in research and industry.