US2147669A - Secondary electron emitting electrode - Google Patents

Description

i n 0 M @5 4 m 7 EMA MMTM m0 (Ittorneg E. R. PIORE Feb. 21, 1939.

SECONDARY ELECTRON EMITTING ELECTRODE Filed Jan. so, 1957 Patented Feb. 21, 1939 2,147,6tii

SECONDARY ELECTRON EMITTING ELECTRODE Emanuel R. Piore, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1937, Serial No. 123,203

8 Claims.

My invention relates to electric discharge devices, particularly to secondary-electron emitting electrodes for use therein, and has for its principal object the provision of improvements in electrodes of the general type disclosed in a copcnding application Serial No. 77,092 to Vladimir K. 'Zworykin and Louis Malter, filed April 30, 1986, and assigned to Radio Corporation of America.

Prior to the Zworykin-Malter discovery of the so-called anomalous emission efiect, described in the above-identified application, caesiated silver was generally considered to be the best surface material for secondary-emissive electrodes. Electrodes constructed in accordance with the prior art, of silver, oxidized and then sensitized by the formation thereon of a layer of caesium oxide, appear to be capable of emitting no more than substantially nine or ten secondary electrons to each impinging primary electron. As a consequence, wherever copious secondary emission is desired, the prior art dictates the use of a number of secondary emitting electrodes in cascade. Such an arrangement is quite well exemplified by French Patent No. 582,428, which discloses an electron multiplier comprising three secondary electron emitting electrodes.

By providing an electrode capable of emitting many hundredand even several thousand secondary electrons to each impinging primary electron, Zworykin and Malter achieve an output with a single multiplying electrode equivalent to that heretofore achieved only in tubes employing many electron multiplying stages.

How ZWorykin and Malter obtain an emissive ratio of, say 3000 to 1, whereas prior experimenters could obtain a ratio of no more than substantially 10 to 1, may be summarized as follows:

(1) They provide an electrode which is constituted of a layer of an oxide of an alkali metal, a conductive base, and an interposed insulating layer of substantially microscopic thinness.

(2) They provide means, such, for example, as an electron beam, which, according-to one hypothesis, polarizes the layer of alkali-metaloxide electrically positive with respect to the conductive base, the interposed insulating material permitting this. As long as this polarization. effect remains on the electrode (it persists after. the beam is removed), electrons, according to this hypothesis, are drawn from the base through the interposed layer of insulating material to appeal-on and'adjacent the outer alkali-metal-oxide layer from whence they may be drawn to a more positively charged remote anode or collector electrode.

The present invention is predicated upon my discovery that an electrode treated with certain materials other than those described in the Zworykin and Malter disclosure may exhibit the above-described. anomalous emission efiect.

Specifically, I have discovered that an electrode comprising a conductive base covered with an insulating layer of an alkaline earth metal derivative and a superimposed layer of an alkali metal, such, for example, as rubidium, caesium, or the oxides thereof, is capable of emitting a copious flow of secondary electrons.

The intermediate, insulating layer may be constituted of a borate of any one, or mixture of two or more, alkaline earth metals and, as above indicated, the outer surface layer, which receives the positive charge, may be constituted of caesium or its oxide. The base upon which the insulating layer is imposed may be formed of almost any conducting material, such, for example, as nickel, aluminum, silver, or alloys thereof.

In the drawing:

Figure l is a diagrammatic view of an electron multiplier system wherein an electrode constructed according to my invention may advantageously be utilized, and

Figure 2 is a diagrammatic, enlarged, sectional View of a multiplying electrode constructed in accordance with the principle of my invention.

Referring now to Fig. 1 of the drawing, an electron multiplier in which an electrode formed according to my invention can be utilized may comprise a Y-shape evacuated container 1 within which, adjacent to the extreme end of one of the arms, is mounted a photosensitive cathode 3 and within the end of the other arm of which is mounted an output electrode 5. The cathode material may be silver, having a surface layer comprising caesium oxide. The photosensitive cathode which is the source of primary electrons 'may be replaced by any other source of electrons such as a thermionic cathode or another secondary emitting electrode.

An electrode 1 capable of emitting secondary electrons is disposed within the stem of the container in such position that it is accessible to electrons emitted from the photosensitive cathode 3 and is also visible from the output electrode 5. For purposes of convenience the electrode l mounted in the stem of the container will be referred to as a multiplying electrode. A variab-leor a constant light source may be s0. disposed with respect to the container that light therefrom falls upon the photosensitive cathode. In the drawing, such light source is exemplified by a light 8 connected in circuit with a battery 9 and a variable rheostat I l, and a lens Under the influence of light from the source, electrons leave the surface of the photosensitive cathode in random directions. Since it is desirable to focus all such electrons upon the multiplying electrode an electromagnetic coil I may be disposed around one arm of the container between the photosensitive cathode and the multiplying electrode. A similar coil l'l may be disposed around the other arm of the container for the purpose of focusing secondary electrons upon the output electrode.

The several focusing coils may be provided with uni-directional potential from a battery l9 or the like. In the drawing these coils are exemplified as being connected in parallel to the battery, a potential divider 2| and a plurality of contact devices 23 and 25 being utilized for the purpose of individually controlling the magnitude of the several focusing fleld currents. It is my understanding that the polarity of the coils is immaterial. Alternatively, electrostatic focusing of the electrons may be resorted to, or a combination of electrostatic and magnetic focusing.

In the operation of an electron multiplier of the type under discussion the output electrode 5 may be connected to any suitable utilization circuit, such as a relay 27. When utilizing a multiplier of the type shown, the photosensitive cathode may be connected to the negative terminal of a potential divider 29 that is connected across a source of uni-directional potential 3|, the output electrode 5 connected to the positive terminal of the potential divider and the multiplying electrode 1 connected to an intermediate point thereon. The relative potentials shown in the drawing are to be construed solely as illustrative.

In accordance with my invention, referring to Fig. 2, the multiplying electrode 1 may have a base portion 33 made of a sheet of nickel, aluminum, silver, copper or any other metal which is readily degassed, or it may be formed of glass or the like plated with metal to give it a conductive surface. This surface 33 carries an insulating layer or film 35 constituted essentially of a borate of an alkaline-earth-metal, such, for example, as barium borate, strontium borate, calcium borate, or mixtures of the same. Superimposed on the borate film is a very thin layer 31 of an emissive material selected from the group comprising the alkali-metals and the oxides thereof. Each layer is of a thickness preferably no greater than .001 of an inch.

The alkaline-earth-metal-borate layer 35 may be applied to the conductive surface 33 prior to being mounted within the tube I. This may be accomplished by mounting the surface 33 in an evacuated vessel containing a filament of spiral or cylindrical contour supporting, as between the turns, a small quantity of fused borate and then evaporating the borate as by heating the filament. One advantage of coating the metal in a separate tube is that several electrodes, or a large surface which may be later cut up to electrode size, may be treated at one time. If it is desired to coat the surface 33 with the electrodes all mounted in place in the device of Fig. 1 the tube may be provided with a neck portion (not shown), near the electrode 1, in which the auxiliary heater supporting the borate'is mounted.

The neck will ordinarily be sealed off after the borate coating is applied.

In manufacturing a device of the type shown, after the borate treated electrode 1 and the other electrodes are mounted in place, the tube is highly exhausted and. a slight amount of alkali-metal, such as sodium, potassium, rubidium or caesium is distilled in to it. Caesium is preferred. sufficient alkali metal is used to form a thin coating upon the borate layer on electrode 1 and to sensitize the cathode 3. The caesium coating should preferably be no thicker than substantially .001" and may be comprised of no more than a molecularly thin coating. The coating on electrode 1, in this case, is probalby discontinuous and it may be that it consists of spaced apart molecules of the metal, insulated by the borate from each other and from the underlying conductive base 33.

It may also be that the caesium interpenetrates the borate layer to some extent. The terms "layer, film, surface coating and the like, here used, are not to be construed as necessarily implying physical continuity and homogeneity, but are to be given as broad an interpretation as is consistent with my disclosure.

The tube, after the introduction of the alkali metal, is preferably baked at 200 C. for approximately ten minutes and then permitted to cool to room temperature. Subsequent to cooling, pure oxygen is admitted into the tube to react with the caesium and is permitted to remain therein for a short time. The tube is then re-evacuated to a pressure sufficiently low to prevent ionization during use. Presumably, the alkali metal which deposits on the alkaline-earth-metal-borate layer 35 is oxidized and the ability of the electrode 1 to emit true secondary electrons is thereby enhanced. It is to be borne in mind, however, that the baking of the caesium and borated covered metal surface 33--35--31 even without the introduction of the oxygen gives rise to a surface which is vastly more efficient than surfaces known to the prior art, although it may not be quite as eificient as the surface consisting of an oxidized alkali metal on an alkaline-earth-metal-borate coating.

My invention is not to be limited by the theory of operation described herein, or in the Zworykin and Malter application, as such theory is given merely for purposes of aiding in the explanation of the inventive concept. Neither is the invention to be limited to any specific mode of use or type of discharge tube in which the described electrodes may be employed.

What is claimed is:

1.. Method of preparing an electrode having a metal surface, which comprises depositing a thin layer .of a borate of an alkaline-earth metal upon said surface and subsequently depositing a thin layer of an electron-emissive material upon said alkaline-earth-metal borate layer.

2. Method of preparing an electrode having a metal surface, which comprises depositing a thin layer of a borate of an alkaline-earth metal upon said surface, subsequently depositing a thin layer of an alkali metal upon said alkaline-earthmetal-borate layer and thereafter oxidizing the said alkali metal layer.

3. An electron emissive electrode comprising a conductive base, an electron-emissive surface layer and an intermediate layer constituted essentially of a borate of an alkaline-earth metal.

4. The invention as set forth in claim 3 wherein said intermediate layer is constituted essentially of barium borate.

5. The invention as set forth in claim 3 wherein said intermediate layer is constituted essentially of strontium borate.

6. The invention as set forth in claim 3 wherein said intermediate layer is constituted essentially of calcium borate.

'7. An electron-emissive electrode comprising a conductive base, an electron-emissive surface layer formed of a material selected from the group comprising the alkali-metals, and an intermediate layer constituted essentially of a borate of an alkaline-earth metal.

8. An electron-emissive electrode comprising a conductive base, an electron-emissive surface layer constituted of an oxide of an alkali metal, and an intermediate layer constituted essentially of a borate of an alkaline-earth metal.

' EMANUEL. R. PIORE.

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