CS207285B1 - Method of making the polycrystallic scintillator - Google Patents

Description

The invention relates to a process for the production of a polycrystalline scintillator by means of a programmed heat treatment following a plastic deformation of the single crystal.

Methods for culturing alkali halide scintillation crystals by seed-drawing or crucible methods are known. These methods start from an oriented nucleation and the desired result is a relatively homogeneous single crystal. The cultivation methods are particularly time-consuming and technical in particular for large-size crystals. The largest achievable dimensions of the scintillation crystals thus obtained are limited to the dimensions of the crucibles and furnaces and the material loss during machining to the desired shape is considerable. In addition, monocrystalline large-scale scintillators are very sensitive to mechanical and thermal shocks and easily crack according to areas of perfect cleavage.

However, it is known that polycrystalline material composed of a plurality of randomly crystallographically oriented blocks has satisfactory optical properties, while maintaining perfect optical contact between the individual blocks with respect to the optically isotropic substance.

The alkali halide crystals can be plastically deformed after heating to a temperature near the melting point. After cooling, however, such deformed crystals usually burst due to considerable mechanical stress or eventually disintegrate into individual parts. The luminescence properties of alkali halide scintillation crystals deteriorate sharply after slight plastic deformation.

These drawbacks are overcome by the process for producing the polycrystalline scintillator according to the invention. By starting scintillation material we mean activated alkali halide, which was obtained by melt crystallization in the form of any geometric shape - ingot - with internal monocrystalline or polycrystalline texture. This ingot is plastically deformed. SUMMARY OF THE INVENTION The present invention provides a method for converting a preformed plastically deformed blank by programmatic heat treatment into a polycrystalline aggregate of approximately isometric, angular crystal blocks that are closely related. The plastically deformed blank is annealed at a constant temperature below the melting point and above the forming temperature of the material, recrystallizing and uniformly releasing the mechanical stress in the volume of the plastically deformed blank. The temperature of the annealed blank gradually decreases gradually. Originally homogeneous material is transformed into a cohesive aggregate of randomly oriented, closely related blocks. The size of these blocks and hence the properties of the scintillator can be influenced by the duration

heat treatment.

The polycrystalline aggregate thus formed is optically i.e. equivalent to a single crystal, has comparable scintillation properties, and surpasses the single crystal with shock, vibration and thermal shock resistance.

The process for producing a polycrystalline scintillator according to the invention makes it possible advantageously to produce polycrystalline scintillators in the form of plates, rods and other arbitrary geometric shapes whose dimensions exceed those of the crystals prepared by other known methods. At the same time, in this method of manufacture, the material loss during machining is considerably reduced, since during forming the blank can be manufactured exactly to the desired dimensions with a minimum machining allowance.

The luminescent properties of the polycrystalline aggregate scintillators are equal to or better than the starting scintillation material.

Process for producing a polycrystalline scintillator

Claims (1)

SUBJECT A process for producing a polycrystalline scintillator from activated alkali halides in the form of a melt crystallized and plastically deformed ingot, characterized in that the annealing of the blank at a constant temperature lower than the melting point and higher than the forming temperature of the material has been experimentally tested. A cylindrical single crystal NaJ (T1) grown from a melt in a crucible, whose ratio of the diameter of the cylinder to the height of the cylinder was 1: 1, was used as the starting material. This cylinder was transformed into a circular plate whose diameter to height was in the ratio of 10: 1 under the influence of a generally oriented pressure of the order of 1 to 10 MPa. The single crystal was formed at a temperature of 600 ° C and immediately afterwards the blank was transferred to an annealing furnace where it was kept at a temperature at least 10 ° C lower than the corresponding melting point of NaJ (T1) for 12 hours. After this time, the temperature was lowered at an average rate of 5 ° C / h to ambient temperature. The resulting polycrystalline scintillator was also perfectly cohesive, transparent and consisted of randomly oriented, mostly isometric and angu- lar! broken grains with dimensions of 5 to 10 mm. Its scintillation properties were satisfactory. OF THE INVENTION, followed by a gradual slowing of the temperature to ambient temperature, a cohesive polycrystalline aggregate is formed with randomly oriented angular crystal blocks closely related to each other.