Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Nonresinous material only
Patent
1995-11-22
1997-12-23
Czaja, Donald E.
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Nonresinous material only
264 3123, 264 122, 264 21, 264310, 264332, 117 81, 117 83, B29D 1100, G02B 100
Patent
active
057004048
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Large scintillator plates composed of sodium iodide doped with thallium are required in nuclear medicine for the Anger camera for detecting gamma rays. These detectors are used to observe the distribution in the human body of radioactive labelled or radioactive marker substances which have previously been introduced into the body. The change or the accumulation of the radioactive marker substances may provide information about metabolic processes and about, in particular, active local disease foci, for example about cancerous ulcers. In applications of the Anger camera to date, rectangular or circular plates with dimensions of up to 40 cm by 60 cm or up to 60 cm diameter, with a thickness of 10 to 12 mm, are required as scintillator plates.
The disadvantage of detectors of these dimensions is that it is always possible to observe only a particular section of the patient's body in one investigation. However, for certain medical investigations it is an advantage to be able to image the distribution of radioactive-labelled substances or drugs in the patient's body by a whole-body film. It would thus be possible to investigate functions which embrace the entire body, for example arterial, nervous and lymphatic systems, without the necessity for repeated introduction of the substances labelled with short-lived radionuclides, which result in the patient being heavily exposed to radiation. Another problem is that the imaging is greatly affected by reflections at the edge of the scintillator plate. In this respect a very large scintillator plate with which the reflecting edge is as far as possible from the region of the plate on which gamma rays impinge is more favourable.
German Patent DE 40 02 320 C2 proposes the production of salt bodies which are suitable as luminescent material from doped alkali metal halides by allowing a doped melt of the salt to solidify. To do this, the melt of the salt is, immediately after the addition of the volatile thallium halide which is added as doping substance, rapidly cooled to the extent that at least the surface of the melt is solidified. The salt body obtained after complete cooling can be detached from the melting crucible or the casting mould only by destruction thereof because alkali metal halides on solidification adhere extremely well to many materials. However, since the inert materials suitable for containing an alkali metal halide melt, such as, for example, quartz glass, graphite or noble metals, are relatively costly materials, loss of the casting mould after production of the salt bodies makes the process considerably more costly. In addition, reworking of all the surfaces of the resulting salt body is necessary.
For conventional scintillator plates, initially clear, bubble-free blocks of NaI:Tl are produced. Single crystals are obtained by the Bridgeman, Stockbarger or Czochralski process, whereas polycrystalline blocks can be produced by extrusion. Because all these blocks can be produced with only limited dimensions, to produce large-area plates they must be thermoformed to the required diameter by compression at temperatures above 300.degree. C. in an additional step. This represents an industrially elaborate process and is, moreover, unsuitable for producing scintillator plates with dimensions of 60 cm by 200 cm, which would be required for whole-body investigations.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process for the production of a polycrystalline salt body which can be carried out more simply and at lower cost than known processes and nevertheless leads to optically clear crystal bodies without opacities, cracks and the like.
This object is achieved according to the invention by a process having the steps of producing a salt melt by melting a starting material with a composition intended for the salt body in a melting crucible which is inert to the salt melt; introducing the salt melt into a casting mould having a base kept at a temperature which is 1.degree. C. to 50.degr
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patent: 4382838 (1983-05-01), Authier
patent: 5178719 (1993-01-01), Pandelisev
Colaianni Michael
Czaja Donald E.
Siemens Aktiengesellschaft
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