DE2134762A1 - PHOTOCATHOD - Google Patents

Description

Siemens Aktiengesellschaft Erlangen, July 6, 1971

Henkestrasse 127

VPA 71/5092 Kn / Kli

«4.

Photocathode

The invention relates to photocathodes which comprise a carrier layer, a luminous layer and a photocathode layer, wherein the carrier layer has an intermediate layer on which the luminous layer is applied. Such photocathodes are known to be used, for example, for the visible reproduction of X-ray images Used in vacuum image intensifiers or to build up a charge image in television pick-up tubes.

Known photocathodes consist of an aluminum plate which a layer of phosphor is vapor-deposited. A photocathode layer, e.g. Contains alkali metals, applied. The problem here is that, on the one hand, the carrier layer reflects light and causes blurred images through scattering in the luminous layer. On the other hand, the luminescent layer does not adhere well the document. For this reason, documents that have been etched etc. have already been used. But this has the disadvantage that the base and the applied phosphor, in particular

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when using aluminum sheet as a base, there are large differences in the expansion coefficient. When using CsI: Na as the phosphor and aluminum as the carrier, there is, for example, a difference in expansion of 1: 2. As a rule, this leads to cracks and cracks during manufacture, which result in a disruption of the image. In addition, more or less well-insulated islands are created in the photocathode, which disturb the image due to electron depletion. The alkali metals usually used in photocathodes can penetrate through the cracks etQ. * _{R to the aluminum carrier.} Because of the reaction that occurs, this then leads, at least in part, to destruction of the photocathode, which results in further strong disturbances of the image.

According to the invention is a well-adhering, even with prolonged use crack-free luminous layer can be obtained on a carrier with a coefficient of expansion that differs from the luminous layer, in that the intermediate layer consists of a substance that is between the carrier and the luminous layer allowed displacements in that it is elastically or plastically deformable. The advantageous one The effect of this structure is explained in a simple manner by the fact that the intermediate layer has the expansion properties of the carrier and the luminescent layer compensates for by their deformability * because then the two parts against each other at least can be shifted against each other to a certain extent.

When using a carrier of aluminum sheet is formed, an intermediate layer of sufficient adapt, for example of polyimide or silicone resin, a well-retaining layer of PoIyimid for example, is obtained by applying to the roughened concave surface of the Al-cap teilkondensiertes polyimide in the form of a Gewichtsverhältalis>: 1 to 1: 2 lacquer dissolved in dimethylformamide is applied . It becomes partially condensed

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Resin used, which is commercially available. By rotating the concave side of the dome around the vertical, which goes through its lowest point, the liquid layer is evenly distributed. The speed of rotation should be with a screen diameter of 190 to 200 mm and a curvature depth of about 20 mm at the beginning of the spinning process are at approx. 1500 revolutions and are gradually reduced. This ensures that the coverage runs smoothly with the lacquer towards the edge of the dome without becoming streaky.

The average coverage with synthetic resin when using aluminum as a carrier and a vapor-deposited luminous layer should be 0.5 to 5 mg / cm, whereby the most favorable range is 1.0 to 1.5 because of the smoothing that can be achieved on sufficiently rough surfaces with suitable plastics mg / cm. It is advantageous to thicken the paint by blowing it with hot air while it is being spun off. This ensures in a simple manner that the lacquer layer remains evenly distributed on the dome surface even after the spin drive has been switched off and does not flow together in the lowest point because the viscosity increases considerably as a result of evaporation of the solvent and partial condensation of the resin. After the polyimide lacquer has become a uniform, dust-dry coating, it is heated to 110 to 130 for about an hour so that solvent residues can escape. In a ⁰ to 300 G draw forming temperature, the polyimide resin is finally condensed in the course of about two hours, that is, cured. The times indicated for the hardening and the driving out of the solvent residues are naturally changeable within wide limits, in particular they can be extended without the result being adversely affected.

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In. With comparable technology, instead of the polyimide ^{resin x} , a silicone resin such as methylphenylsiloxane in a suitable solvent - for example toluene or perchlorethylene - can be spun on using the method described above. The solution should contain from $ 5 to $ 10 resin in order to have adequate viscosity. To harden this resin, the dried layer is ^{heated to 300 °} C. for five to six hours.

On the synthetic resin layer is carried out in a known manner. Vapor deposition in a high vacuum apparatus, the luminous layer, for example a layer of cesium iodide activated with sodium (OsJtNa), evaporated in the required layer thickness. Purely for use with X-rays, which are used in medical diagnostics, it should be from 40 μm to 150 μm thick .., - , '■

Further details and advantages of the invention are provided below based on the figures in connection with a Illustrated embodiments of the invention illustrated vacuum image intensifier.

In Fig. 1 is a schematic cross section through a X-ray image intensifier shown and

in Fig. 2 is a partial cross section through a .. the carrier that the intermediate layer .and the Luminous layer carries.

1 with the glass vacuum bulb is referred to, the photocathode 2, the electrodes 3 to 5 of the electron-optical in a known manner Image arrangement as well as the viewing screen contains. The photocathode 2 constructed according to the invention consists from a curved support, the spherical cap 7 made of aluminum sheet

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0.5 mm thick, with a radius of curvature of 150 mm has a diameter of 200 mm. On its concave side, the dome 7 carries the intermediate layer 8 on which the Luminous layer 9 lies, which is covered by the photocathode layer 10.

The intermediate layer 8 consists of an elastic, .bzw. plastic deformable polyimide or silicone resin and is on the surface 7 'of the carrier 7 roughened by etching with hydrochloric acid one of the above methods applied at a thickness of 1.5 mg / cm. Subsequently, for the production of the light

-5 layer 9, the phosphor CsI: Na at less than 10 torr and a speed of 10 / rpm. In conclusion the photocathode layer 10 made of cesium and antimony (CsSb) is then vapor-deposited in a manner known per se and formed.

When encountering x-ray or similar penetrating As is known, rays which pass through the spherical caps become a light image with a corresponding intensity distribution obtained in the luminescent layer 9. This then releases electrons in the layer 10 in a corresponding distribution. Under The electron image is displayed on the screen 6 under the effect of the voltages applied to the electrodes 3 to 5. there is known because of the reduction in size and acceleration a bright image of the rays hitting the combination.

When the screen 6 is designed as a target of a television scanning tube, the advantages of the invention can also be used, for example, for the Television technology can be made usable. .

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Claims (6)

- 6 claims Photocathode, which consists of a combination of layers, which comprises a carrier layer, a luminous layer and a photocathode layer, the carrier layer being an intermediate layer has, on which the luminous layer is applied, which in turn carries the photocathode layer, ■ characterized in that the intermediate layer (8) consists of an elastically or plastically deformable material. 2. photocathode according to claim 1, characterized in that that the intermediate layer (8) consists of a synthetic resin such as polyimide or silicone. 3. photocathode according to claim 2, characterized in that when using a carrier (7) made of aluminum and one vapor-deposited luminous layer (9) made of cesium iodide / sodium die Intermediate layer (8) has a coverage of 0.5 to 5, in particular 1.0 to 1.5 mg / cm on v / e. 4. Method of making an intermediate layer for the photocathode according to claim 1, characterized in that when the concave surface is occupied by a curved. (7) this is set in rotation and the material from which the intermediate layer (8) is to be made in solution in the upwards turned concave recess is poured and that then during des.Abschlenderns the paint is blown with hot air. 5. A process for the preparation of the intermediate layer of a photocathode according to claim 1 and 2, characterized in that with use of polyimide by application of a partially condensed polyimides in solution, and dried 60 min at 110 to 130 ⁰ C. and then the temperature for ca. is increased to 30O ⁰ C for two hours. 209884/0544 - 7 - 6. A method for producing the intermediate layer of a photocathode according to claim 1 and 2, characterized in that when using a silicone such as methylphenylsiloxane which was applied dissolved in a liquid such as toluene or Perchiοräthy1en, is cured ^{at 300 0 C after drying.} 209884/0544 Lee rs e i t e