Electronic high vacuum tube and method of providing a coating therefor

Abstract

An electronic high vacuum tube has a high vacuum container containing two conducting parts of different electrical potentials which produce between these parts high electrical field strength. The invention is particularly characterized in that at least the part which lies upon the high potential has upon its outer surface an electrically insulating layer which binds alkali metals. This coating is produced by immersing the part to be coated into a bath consisting of a solution of partially condensed polyimides followed by special treatment.

Description

This invention relates to an electronic high vacuum tube, the high vacuum case of which contains two conducting parts of different electrical potentials, which produce between them a high electrical field strength. The invention also relates to a method of making a coating upon one of the parts.

Tubes of this type are, for example, electronically optical image amplifiers wherein electrodes subjected to high voltage produce an image of released electrons upon a sheet-like photocathode. However, they can be also used in other high vacuum tubes to eliminate undesired effects, since no secondary electrons are released.

The quality of electronic image amplifiers, particularly X-ray image amplifiers is determined primarily by the amplifying factor and the properties with which the images are transmitted. A measure for these properties is the visually measured analysis and the modulation transmission function which provides contrast transmission depending upon the size of details. Since electronic optical image transforming tubes and image amplifiers carry voltages of 15 to 35 kv for electronic optical magnification, disturbing field emissions can take place. In order to avoid them as much as possible, the surfaces of the voltage carrying parts, such as grids, actuating electrodes etc., are made very flat. For example, the surfaces are smoothed to such an extent that all unevenesses amounting to more than 1 .mu. m disappear. Despite this smoothing electronic emissions or spontaneous flashovers take place. The undesired effects consist primarily in that the flashovers or other discharges are followed by appearance of light which acts upon the emission layer of the photocathode and releases electrons. These electrons produce intensive light appearances upon the viewing screen and disturb considerably the image reproducing properties of the tube.

An object of the present invention is to eliminate to the greatest extent the release of electrons by electrodes which are subject to voltage and by current carrying parts.

Other objects will become apparent in the course of the following specification.

In the accomplishment of the objectives of the present invention it was found desirable to provide the outer surface of at least that part which is subject to the higher potential with an electrically insulating layer which binds the alkali metals.

A simple reason for the detrimental effects can lie in the manufacture of the photocathode. In the known process antimony and caesium, or several alkali metals are steamed on in the tube upon a carrier. Alkali metals particularly caesium are unavoidably spread over surfaces of other parts of the tube. On surfaces covered with alkali metals, the work function of electrons is diminished to the work function of electrons on surfaces of alkali metals. The coating of the present invention acts against the diminution.

In an electronic image amplifier the greatest voltage difference is located as a rule between the third electrode and the following anode. According to an embodiment of the present invention the layer of an organic substance applied upon this electrode prevents discharges in that the outer surface is smoothed, thereby diminishing in the known manner the outflow of electrons. On the other hand the alkali metals used in the manufacture of photocathodes are bound by the organic substances, thereby preventing the outgoing of the electrons at the outer surface.

As useful vacuum fixed substances were found to be organic substances, such as thin layers of polyimide. They consist of condensation products of organic acids, for example, pyromelilotic acid or maleic acid with amines, for example, 1, 4 -diaminobinzol. These layers bind alkalimetal gases and consequently those of caesium, which appear during the making of the photocathode by a mechanism wherein side bindings are acceptable or in the form of fixed solutions. In addition to the diminution of surface rawness, high fluorated hydrocarbons, such as, for example, polytetrafluorethylene, polysulfon, particularly polyarylsulfon, and also silicon resin, also cause the binding of alkali metals. This has the property of binding alkali metal gases, for example, by a substitution, whereby the freed hydrogen is removed during the evacuation of the tube.

The invention will appear more clearly from the following detailed description when taken in conjunction with the accompanying drawing the sole FIGURE of which shows by way of example only, a section through an X-ray image amplifier constructed in accordance with the present invention .

The drawing shows an amplifier having electrodes 1, 2 and 3 located between the cathode 4 and the anode 5 behind the inlet window 6 of a high vacuum tube 7 consisting of glass.

The photocathode consists of a carrier 8, a luminous layer 9 and the actual photocathode layer 10. On the other side of the electrode structure the anode 5 is closed by the luminous layer 11 located in front of the end window 12 of the tube. The cathode is connected by a line 13 to the ground potential of current supplying means (not shown). The connection 14 of the electrode 1 is subjected to 80 v., the connection 15 of the electrode 2 to a potential of 300 v., the connection 16 of the electrode 3 to a potential of 3.5 kv., and the connection 17 of the anode to a potential of 25 kv. The coating of the present invention is applied to the electrode 3 and is indicated as 18. It consists of polyimide and is applied by a process described hereinbelow.

If an X-ray quantum passes in the known manner through the ray inlet window and the carrier 10 and strikes the luminous layer 9, it produces there a luminous action which releases electrons in the photocathode layer 10. These electrons are actuated by the potentials located at the electrodes 1, 2 and 3 and the anode 5 and are directed to the luminous layer 11. They receive the greatest acceleration in the range of electrode 3 and can produce there at the outer surfaces electrons which do not come from the photocathode and which cause appearances releasing additional electrons at the photocathode. These would produce on the screen 11 representations which are not released by the X-rays. This is prevented by the coating 18 which covers the outer surface of the electrode 3. The coating has a thickness of 5 .mu. m and it prevents the release of an uncontrollable amount of electrons.

The method of making the polyimide layer is as follows:

The inner ring is inserted with its bent edge extending downwardly into a bath containing a solution of partly condensed polyimide dissolved in a diluter, as, for example, dimetylformamide. It was found advantageous to use twice as much of the diluter as of the partial condensate, since then the solution at room temperature has useable own viscosity. The amount of liquid in the bath can be used to determine the duration of the application of coating upon the electrode 3. After immersion the ring is removed from the bath, is mounted upon a holding device to remove the excessive drops and is dried while being rotated by an infrared radiator. After the resin during drying has reached a viscosity wherein the layer does not run any more, the lacquer film is heated in a warming cabinet in air for 1 hour at 100.degree.c and thereupon for 2 hours at 300.degree.c.

A study of experiments as the result of which the present invention was produced, showed that the thickness of layers should be in general between 0.1.mu. and 950 .mu.. In case of thinner layers there is the danger that holes or the like could be still present in the layer. Thicker layers could have a negative effect upon the photocathode. Specifically layers out of materials with a strong binding effect upon alkalimetals can bind a great deal of metal in case of a large amount of material due to the final steam pressure of the alkalimetal and thus cause a reduction of alkalimetal in the photocathode. A particularly suitable range for conditions in image transformers lies between 0.5 .mu. and 10 .mu. , particularly at 5 .mu. , since on the one hand the creation of interfering caesium layers is effectively avoided and on the other hand no disturbance of the photocathode takes place as yet. The application of layers of polyimide and silicon resin can take place in known manner by brushing, immersing, spraying, splashing etc. In case of electrodes used in image amplifiers the immersing process is advantageous since a well reproduceable layer thickness is produced with small technical exertion. This is particularly the case when excessive amount of coating is cast off from the parts being coated by rotation. After the application the resins can be condensed and then degassed by heating in air and in high vacuum. Polytetrafluorethylene is advantageously applied by sintering whirling resin powder or by spraying a suspension followed by sintering. Silicon resin is particularly well deposited by electrophosetic deposit upon the electrons.

Claims

What is claimed is:

1. An electronic high vacuum tube, comprising a high vacuum container, two conducting members located within said container, means connected with said members and providing them with different electrical potentials which produce high electrical field strength between said members, and an electrically insulating coating carried by at least the member having a higher potential, said coating binding alkali metals and consisting of a compound selected from the group consisting of polytetrafluorethylene, polysulfon, polyarylsulfon and silicon resin.

2. A tube in accordance with claim 1, wherein said coating has a thickness ranging between 0.5 .mu. to 10 .mu..

3. A tube in accordance with claim 1, wherein said coating has a thickness amounting to 5 .mu..