US2210683A - Photoelectric tube - Google Patents

Description

. Aug. 6, 1940. e. MAURER PHOTOELECTRIC TUBE Filed April 21, 1938 /w r mww M0 M07 u w M 0 d 5 m @mm 5 6 57 6 7/ WITNESSES: fi QZ 2Z4 ATTORN Y Patented Aug. 6, 1940 UNITED STATES PHOTOELECTRIC TUBE Georg Maurer, Stuttgart-Botnang, Germany, as-

signor to Siemens & Halskc, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application April 21,

1938, Serial No. 203,339

In Germany May 19, 1937 2 Claims.

The invention relates to photo-tubes and especially the photo-sensitive material in such tubes.

An object of the invention is to obtain a more sensitive photo-tube than those now in use.

Other objects and advantages will be apparent from the following description. Compounds of alkali metals or of alkaline earth metals, such as, for instance, caesium oxide, are often used in photo-electric tubes for the purpose of supportingthe photo-electric material, for instance, caesium. Now it has been found that tubes having an exceptionally high sensitivity will be obtained if metal salts, especially alkaline earth metal salts of acids formed by elements belonging to Groups V and VI of the periodic table-and especially of acids formed by the metals chromium, tungsten, mo-

lybdenum, or uranium, are used for the purpose ofsupporting the photo-electric material, which can consist of caesium, rubidium, sodium, potassium, barium, and so on. Itis advantageous to deposit said supporting material, or mixture of materials, from the vapor phase by sublimation onto a layer of conductive material, such as, for instance, silver, and to produce in this manner on the conductive base layer a thin layer of the supporting material. It is equally well possible to use a base layer of copper, nickel, chromium, tungsten, platinum, or other metals instead of the base layer of silver just referred to. It is advisable to mix with the material constituting the supporting layer for the photo-electric material a heavy metal, such as, for instance, a silver, copper, nickel, tungsten, or chromium, in a finely distributed form. This can be done by condensing vapors of these metals on the base layer simultaneously with the vapors of the alkali metal or alkaline earth metal compounds.

It is not necessary that in all cases these heavy metals be present throughout the supporting layer; in many cases it will be already sufiicient if they are only present at the surface of this layer. It is further possible, of course, tohave two heavy metals, for instance, silver and copper, or silver and tungsten, or more than two of these metals, mixed with the substance constituting the supporting layer.

As'stated before, the supporting layer for the photo-electric material is produced most adequately by evaporating the substance in question. The most advantageous thickness of the supporting layer for the photo-electric material has different values, depending on the nature of the substance of which it is made; it depends further on the nature of the process used in depositing on the supporting layer the photoelectric material by sublimation, and on the nature and the quantity of the heavy metal (for instance, silver) "to be embedded in the supporting layer. The thickness obtained can be estimated by observing the 'temper-colors of the material. A suitable method of producing the supporting layer consists in fastening a quantity of the material for this layer, for instance, barium sulphate or barium phosphate, in the form of pearl-shaped, granules, to a wire that can be heated. In those cases where the supporting layer must contain a plurality of alkaline earth metal compounds, these compounds can either be used in this way in the form of a mixture, or each compound can be evaporated separately; in-the latter case it is possible, of course, to evaporate the compounds in question simultaneously. The process of separate evaporation has the advantage that the velocity of evaporationof each compound, which depends on the temperature of evaporation, can be chosen to suit prevailing conditions.

One or more heavy metals can be embedded in a finely distributed state in the supporting layer for the photo-electric material by depositing also these metals fromthe Vapor phase by sublimation. If it is desired to deposit by sublimation a plurality of heavy metals, the procedures of depositing a plurality of alkaline earth metal compounds from the vapor phase described in the foregoing can be followed also in depositing these heavy metals. The heavy metals can be deposited by sublimation simultaneously with the alkaline earth metal compounds, such as, for instance, barium dichromate, which must constitute the supporting layer for the photo-electric material. If, however, it is desired to deposit heavy metals, such as silver, chromium, copper, or tungsten, only at the surface of the supporting layer, the desired result will be obtained by depositing these metals by sublimation only towards the end of the process of depositing the supporting layer proper, or after the supporting layer proper has been deposited. In the latter case, the body in question can be subjected to a suitable thermal treatment to make that part of the deposited. heavy metal penetrate into the supporting layer by diffusion.

If the base layer for the supporting layer proper is going to be made of the same metal that will be embedded in a finely distributed form in the supporting layer, the process of producing these layers can be carried out as follows. In the first place the metal in question, for instance, silver, is evaporated, and then, after this metal has formed by sublimation a layer of sufficient thickness, the alkaline earth metal compound, for instance, barium sulphate, is also caused to evaporate. At the same time the velocity of evaporation of the material that is forming the base layer (for instance, silver) can be reduced somewhat.

After the supporting layer for thephoto-electric material has been produced by either one of the methods described in the foregoing, it is finally coated in a well-known manner with the photo-electric material, for instance, caesium or rubidium, by bringing these metals in the form of vapor into the tube. This process can be started already before the formation of the supporting layer has been completed. It is advisable to raise the temperature of the tube, for instance, up'to before thephoto-electric material is brought in the form of vapor into the cell, becausethe production of a non-uniform deposit of the photo-electric substance and of changes in the characteristics of the cell when the latter is operating will be avoided in this case. Since the photo-electric effect is a maximum at a quite definite value of the thickness of the photo-electric layer, it is advisable'to see-to it that the most favorable layer-thickness is actually obtained. This most favorable value can be determined by observing the photo-electric emission in the course of the process of manufacturing the tube.

A relatively large number of chemical compounds of the alkali metals can be used for producing the supporting layer; they must satisfy the requirement of being readily evap-orable. Although it is especially advantageous to work with compounds that will not decompose in the course of the distillation process, it is equally well possible to use compounds that decompose when they are evaporated, provided that the product formed by decomposition can be used in building up the supporting layer. For instance, nitrates of the alkali metals can be precipitated from the vapor phase obtained by evaporating the nitrates of these metals. Oxides of the alkali metals can be obtained by evaporating the carbonates. It has been stated already that oxides of alkali metals, such as, for instance, the oxides of caesium: CszO, CszOz, CszOs, C5204, are suitable compounds to be used for the supporting layer. The compounds of alkali metals with sulphur can be .used for the same purpose. For instance, caesium disulphide, caesium trisulpbide, caesium tetrasulphide, and caesium pentasulphide are suitable substances. Other substances that can be used for the purpose just outlined are the halogenides of alkali metals, such as, for instance, caesium chloride, caesium bromide, and similar compounds, and further polyhaloids such as CsClBm. Other suitable substances are the nitrogen-compounds of the alkali metals, especially those that will not decompose in the course of the process of distillation.

Many variations may be made in the specific embodiments above described Without departing from the spirit of the invention.

I claim as my invention:

1. A photo-electric tube having photo-electric material supported by a mixture of a heavy metal in a finely distributed form and a substance from the group of acidsformed by elements belonging to Groups V and VI of the periodic table.

2. A photo-electric tube having a photo-electrio material supported by a metallic backing having a coating consisting of an alkaline earth metal salt of an acid formed by one of the elements belonging to Groups V and VI of the periodic table, at least the surface of said coating containing an admixture of a finely divided heavy metal, and a layer of a light sensitive metal superposed on said surface. I GEORG MAURER.

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