US2257827A - Electron discharge tube - Google Patents

Description

Oct. 7, 1941 G. J. WEISSENBERG 2,257,827

ELECTRON DI SGHARGE TUBE Filed Aug. 23, 1959' ZLUMINOUS LAYER BASE METAL FIG. I TRANSLUCENT SEPARATING LAYER Fig.3

INVENTOR. qusiav Joseph 'u'llssenb'erg BY %u ATTORNEY.

Patented Oct. 7, 941

ELECTRON DISCHARGE TUBE Gustav Joseph Weissenberg, Berlin-Mariendorf, Germany, asslgnor to Radio Patents Corporation Application August 23, 1939, Serial No. 291,501

17 Claims.

This invention relates to electron emitting bodies and methods of manufacturing the same, more particularly to electron emitters of the type requiring an outside agency for liberating electrons from the surface of the bodies such as light rays as in the case of photoelectric cells or a primary electron beam as in the case of electron multipliers.

As is well known, the property to emit electrons of a photoelectric layer or of a secondary emitting cathode, when excited by light rays of extremely short wave lengthsuch as X-rays or by high velocity primary electrons, respectively, is exceedingly small or to all practical purposes nil. In order to utilize short wave length radiation and high velocity electrons to liberate electrons from the surface of an electronically active body, it has already been proposed to convert the incident light or electron beam into visible light suited for exciting a photoelectric layer. Thus, it

Germany October 21, 1938 has been suggested to provide a luminescent material on either the inner or outer surface of or within the glass wall of a photoelectric cell suited to convert light of short wave length into light coming within the range of the visible spectrum. The converted light in turn serves to excite the photoelectric substance. These prior attempts have various disadvantages due mainly to the fact that the luminous material is too far removed from the photoelectrically active layer. For this reason it has not been possible, without the interposition of an optical lens system, to direct the luminous image formed by the light of short wave length or the high speed electrons upon the photoelectric layer.

Accordingly, it is an object of the present invention to provide an electron emitter and a method of manufacturing such emitters by which the above drawbacks and defects are substantially overcome. With this object in view the invention contemplates the construction of cathodes comprising a base metalwhich may be in the form of a thin plate and preferably consists of a heavy metal or an alloy of heavy metals, such as tungsten (W) tantalum (Ta), molybdenum (Mo), gold (Au) platinum (Pt) and the like. To this base there is applied a thin layer of a luminescent substance. The latter is in turn coated with a thin metallic layer producing negligible light absorption, which metallic layer is covered with the photoelectrically active material. The incident light rays of extremely low wave length such as X-rays or a high velocity electron beam impinged upon such a structure will pass through the photoelectric layer and the thin metal layer without appreciable absorption and strike the layer of luminescent material thereby exciting the latter to luminesce within the visible spectrum. The light thus produced will in turn penetrate through the thin metal layer and affect the photoelectric layer, thus causing the latter to liberate electrons. Due to the fact that the distance between the source or origin of the visible light and the photoelectric layer is extremely small, practically a fraction of a thousandth of a millimeter, the image produced upon the luminous layer will be faithfully depicted upon the photoelectrically active layer. The same considerations apply analogously to secondary emitting cathodes excited by a high velocity electron beam.

The manufacture of cathodes of the aforedescribed type may be carried out in accordance with any one of the known processes in the art. Thus, a layer of luminescent material may be applied to the carrier metal in finely divided condition and coated with thin metallic carrier and separating layer such as by an electrolytic process, by cathode disintegration or sputtering or in any other suitable manner. The thus prepared cathode is then mounted in a utilization vessel or bulb and coated with a photoelectrically active layer in accordance with one of the known processes.

According to a specially advantageous process proposed by the invention, the cathode is completed in a vessel separate from the utilization vessel in which case it is recommendable to apply a further covering layer overlying the photoelectric layer to prevent attack by air and to enable diffusion of or formation of mixed crystals with the photoelectric layer by moderate. heating. The thus prepared cathode is then removed from the manufacturing vessel, wrought and shaped in a desired manner for mounting in a photoelectric tube or the like. After the latter has been exhausted, the cathode is heated by means of a high frequency induction coil or in any other manner to activate the same by diifusion or crystal formation in the manner mentioned above. It has been found advisable in order to prevent a direct. contact of the photoelectric layer with the luminous layer after the heating process, to

'1 sectional views, respectively, of an arrangemen Figures 3, 4 and 5 show a crosssection ofa. cathode produced according to'a'modifled-process in accordance with the invention.

Figures 6 and 7 are longitudinal and cross for manufacturing cathodes according to the in.- vention.

Referring more particularly to F1 is shown at I a metal basa qr carrier which may be a sheet or plate of tungsten or the like. The latter is coated with a layer 2 of luminescent material deposited thereon by any suitable method;

e 1, there obtained cathodes may! then be cut or otherwise formed into a desired shape for mounting in an electronic device. Such cathodes are shown schematically in Figures 3 and 4.

According to this modification the luminescent layer 2 is applied to the carrier metal I within or outside the manufacturing vessel. It is recommendable when using zinc sulphate which may be easily evaporated to apply, during the evaporation under a considerably lower partial pressure the activating metal for the zinc sulphate such as silver. As carrier metal for the photoelectric layer there is then applied by evaporation or by 1 means of a cathode sputtering process a separating and carrier layer I0 of such a metal which when moderately heated will act with the photoelectric layer to form an alloy, chemical compound or a mixed crystalline structure, or which The luminescent material is selected depending upon the purpose and use to which the cathode is to be put. In this connection both. the period of after-luminescence and the spectralfrange of the luminous light rays produced are to be conpermits diffusion of the photoelectric layer. The layer l0 should have such a thickness as to prevent an appreciable absorption of the light passing through it. When using caesium or rubidium as photoelectric material silver has been found j to behighly suitable as a separating and carrier sidered. For a cathode intendedforv use with X-rays it has been found that potassium,tung ..v

state produces favorable results which has both a favorable spectral range (blue) and an extremely small period of after-luminescence. It the after luminescence is of no importance zinc sulphate activated with silver may be usedfior various other known luminescent materials. The luminescent layer 2 is covered with a thin. metal layer 3 serving as a carrier for the photoelectric layer 4 and causing only slight ornegligible absorption of light passed through it. .As an example, a layer of silver having a thickness of only a few atoms may be deposited upon the luminous layer, thus on the one hand insuring a minimum of light absorption and at the same time providing a suitable base for the photoelectric layer which may consist of caesium, rubidium or any other suitable material. The light beam of extremely short wave length indicated by the arrows at passes through the layers 3 and 4 practically without absorption in such a manner that it will strike the layer 2 thereby causing the latter to luminesce. The luminous rays produced by the layer 2 in turn pass through the layer 3 as indicated by the arrows I), thus causing electrons to be liberated by the layer 4 in proportion to their intensity. In place of a light beam of extremely short wave length, a high velocity electron beam may be caused to impinge upon the luminous layer 2 to produce a secondary electron beam by way of conversion into visible light for use in electron multipliers or in other devices making use of secondary electron emission.

In the manufacture of cathodes of the above type as shown in Figure 1, the luminescent material 2 is applied to the carrier metal I and subsequently coated with the layer 3 by an electrolytic process, cathode sputtering, evaporation or in any other suitable manner. The thus prepared cathode is then mounted in the tube of the photocell, as shown in Figure 2, whereupon the tube is evacuated and the photoelectric, material deposited upon the layer 3 and the tube sealed oil in the usual manner.

According to a modified process as proposed by the invention, the cathodes are produced in large sheets in a vessel separate from the utilization vessel or tube of the electron device. The t us using an additional glow discharge. so

material.

Finally, the photoelectric layer 3 is applied such as by evaporation under a relatively low partial pressure of an activating gas such as O: or by Under given circumstances, the partial pressure of the metal may be reduced by a determined amount and the layer 3 evaporated immediately after sufllcient metal for the layer III has been deposited. In this manner an intimate mixture of the substances later to react with each other (metal and photoelectric substance) is obtained. Finally. a thin layer ll of the same metal as the layer III is deposited upon the photoelectric layer serving to protect the latter against attacks by air or other objectionable agencies. The thus produced cathode sheet is then removed from the vacuum vessel and shaped such as by cutting, drawing, pressing, punching for mounting as a cathode in an electronic tube. In order to prevent the uppermost layer I I from oxidizing by prolonged storage, the same may be coated with a suitable lacquer. This coating may be applied within the manufacturing vessel. The thus obtained cathode is then mounted within the bulb of the electronic device and after evacuation of the latter subjected to a heat treatment such as by the aid of a high frequency induction coil. As a result of the heat treatment, a homogeneous layer I2 is formed out of the layers Ill, 3 and II as shown in Figure 4 whereby the photoelectric material is brought to the surface of the cathode.

Since in general luminescent substances are bad conductors for electric currents it is recommendable to coat the cathodes before their mounting in the utilization vessel at the sides and on the upper and lower edge portions with a layer of highly conducting material preferably by means of an electrolytic process. This conducting layer is shown at I3 in Figure 5. After the heat treatment in the vacuum tube, the layer I3 wil be in intimate contact with the layer I2. t

It may be advisable to prevent a diffusion 0 crystal formation between the layers 3 and III by employing for this layer a metal capable of preventing such diffusion or crystal formation. In Figure 5 this layer is designated by the numeral III which will remain after completion of the heat treatment.

Referring to Figures 6 and 7 there is shown an arrangement especially suited for carrying out the process oi. the invention. Item l4 represents a glass vessel the end ll of which may be connected to a high vacuum pump. A small hollow drum I8 is rotatably mounted within the vessel N by means of a suitable support having bearings l1 and I8 and the drum rotated by the aid of a magnet I! carried on its axis and driven by a cooperating magnet 20 arranged outside the vessel and arranged to form a closed magnetic circuit with the inner magnet IS. /The magnet 20 is mounted upon a shaft rotated by a suitable prime mover through a pulley or the like 2| resulting in a rotation of the magnet l9 and in turn the drum l6 within the vessel l4. The latter is provided with four depending extensions as shown in Figure '7 receiving the materials to be evaporated into the drum l6. Thus extension 22 may contain the photoelectric material which may be heated inductively by means of a winding indicated in the drawing. Extension 23 may contain the luminescent material, while extensions 24 and contain wires consisting ot -the-metal such as silver to be deposited by passing a current through the wires to cause cathode disintegration or sputtering in a known manner.

The operation of the arrangement according to Figures 6 and 7 is as follows. A carrier metal which may be in the form of a tungsten sheet is mounted upon the drum l6 which latter contains a suitable amount such as a few cubic centimeters of liquid air. The vessel I4 is then connected to the pump and evacuated. After a suflicient degree of vacuum has been reached, the drum is rotated rapidly, at about 3000 revolutions per minute and the luminescent material, such as zinc sulphate or the like, in the extension 23, heated by passing current through its heating coil. The luminous material is thus evaporated and condensed immediately upon the drum by reason of the fact that the latter has a substantially lower temperature than the other parts in the vessel. Due to the rapidrotation of the drum the luminous material is condensed and deposited evenly upon the surface of the metal sheet carried by the drum I 6. After suflicient luminescent material has been deposited upon the metal sheet, the heater winding at 23 is disconnected and the silver deposit started by heating the filaments in 23 and 24. After the silver layer has assumed a sufiicient thickness, the

heating of the wires in 24 and 25 is interrupted whereupon the evaporation of the photoelectric material in 22 is started. After the photoelectric layer has assumed its required thickness, the, heating in 22 is interrupted and a final metallic covering layer applied by heating the wires in either 24 or 25 or both. After this process has been completed finally the drum is stopped, the vacuum released and the cathode removed from the drum. The thus prepared cathode may be stored for an indefinite period.

It may be cut or otherwise worked into a de- I metal may be given a sufficiently large crosssection. Thus, it may be formed with grooves or ribs increasing its rigidity and also improving its heat conduction. In place of sheets, plates or the like, the carrier body may be in the form 01' a wire or strip or of any other desired construction.

The novel electron emitting cathodes de-.

scribed/by the invention have various uses such as for determining the intensity of X-rays in electron microscopes for converting light of extremely short wave length into an electron beam, as secondary emitting cathodes in electron amplifiers and for many other applications.

It will be evident from the above that the invention is not limited to the specific details and steps described and disclosed herein for illustration but that the novel principle and the underlying inventive thought are susceptible of numerous modifications and variations coming within the broader scope of the invention as defined by the appended claims. Thus applicants process of producing cathode sheets on a large scale outside the utilization vessel has general application other than limited to the fabrication of cathodes of the specific type described herein. For instance, ordinary cathodes may be prepared in the form of large sheets by directly depositing upon a metal sheet a layer of light sensitive material in accordance with any one of the known processes, coating this layer with a protective metal covering as described by the invention and storing or directly utilizing the thus obtained cathode sheets after removal from the manufacturing vessel for constructing cathode electrodes. The latter, after being mounted in the tube or other vessel of the photoelectric device are then activated by heating in the manner described hereinabove.

The specification and drawing are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A cathode structure comprising a conducting base plate, a layer of luminescent material applied to said base plate, and a translucent layer capable of emitting photo-electrons when struck by light applied to said luminescent layer.

2. A cathode structure comprising a conducting base plate, a layer of luminescent material applied to said base plate, a translucent separating layer applied to said luminescent layer, and a translucent layer capable of emitting photo-electrons applied to said separating layer.

3. A cathode structure comprising a conducting base plate, a layer of luminescent material applied to said base plate, a translucent metallic separating layer applied to said luminescent layer. and a translucent layer capable of emitting photo-electrons when struck by light applied to said separating layer.

4. A cathode structure comprising a conducting base plate of heavy metal, a layer of luminescent material applied to said base plate, a translucent separating layer of silver applied to said luminescent layer, and a translucent layer capable of emitting photo-electrons applied to said separating layer.

5. A cathode structure comprising a conducting base plate, a layer of potassium tungstate applied to said base plate, a translucent separating layer applied to said tungstate layer, and a translucent layer capable of emitting photo-electrons applied to said separating layer.

6. A cathode structure comprising a conducting base plate, a layer of zinc sulphate applied to said base plate, a translucent separating layer applied to said sulphate layer, and a translucent I layer capable of emitting photo-electrons applied to said separating layer.

7. A cathode structure comprising a conducting base plate, a layer of luminescent material applied to said base plate, a translucent silver layer applied to said luminescent layer, and a translucent layer capable of emitting photo-electrons applied to said silver layer.

8. A cathode structure comprising a base plate of tungsten, a layer of luminescent material applied to said base plate, a translucent separating layer applied to said luminescent layer, and a translucent layer capable of emitting photo-electrons applied to said separating layer.

9. A cathode structure comprising a conducting base plate, a layer of luminescent material applied to said base plate, a translucent metallic separating layer of molecular thickness applied to said luminescent layer, and a translucent layer capable of emitting photo-electrons applied to said metallic layer.

The method of manufacturing photo-electric tubes comprising the steps of applying a layer of light sensitive material to a comparatively large, sheet-like base conductor, producing a protective covering layer upon said light sensitive layer, subdividing and shaping the coated conductor into a plurality of electrode elements, mounting the thus produced electrode elements in the tubes together with other cooperating elements, exhausting the tubes, and subjecting the electrode elements to final heat treatment to allow the light sensitive material to diffuse to the surface of said covering layer.

11. The method of manufacturing photoelectric tubes comprising the steps of applying a layer of light sensitive material to a comparatively large, sheet-like base conductor, producing a thin metallic protective covering layer upon said light sensitive layer, subdividing and shaping the coated conductor into a plurality of electrode elements, mounting the thus produced electrode elements in the tubes together with cooperating elements, exhausing the tubes, and subjecting the electrode elements to final heat treatment to allow the light sensitive material to diffuse to the surface of said covering layer.

12. The method of manufacturing photoelectric tubes comprising the steps of applying a layer of light sensitive material to a comparatively large, sheet-like base conductor, producing a thin silver layer upon said light sensitive layer, subdividing and shaping the coated conductor into a plurality of electrode elements, mounting the thus produced electrode elements in the tubes together with other cooperating elements, exhausting the tubes and subjecting the electrode elements to final heat treatment to allow the light sensitive material to diffuse to the surface of said silver layer.

13. The method of manufacturing electronically active electrodes adapted to be excited to electron emission byan outside agency comprising applying a layerof luminescent material to a base conductor, producing a translucent metallic layer upon said luminescent layer, covering said metallic layer with light sensitive material, further producing a protective layer upon said photoelectric layer, mounting the thus obtained electrode structure in a utilization vessel, exhausting said vessel, and subjecting the electrode to final heat treatment.

14. The method. of manufacturing lectronically active electrodes adapted to be excited to electron emission by an outside agency comprising applying a layer of luminescent material to a base conductor, producing a translucent metallic layer upon said luminescent layer, covering said metallic layer with light sensitive material, further producing a metallic layer upon said photoelectric layer, mounting the thus obtained electrode structure in\a utilization vessel, exhausting said vessel, and subjecting the electrode to final heat treatment.

15. The method of manufacturing electron tubes comprising the steps of applying a layer of electronically active material to a comparatively large sheet-like base conductor, applying a protective covering layer to said first layer, subdividing and shaping the coated conductor into a plurality of cathode elements, mounting said cathode elements in the tubes together with other cooperating elements, evacuating the tubes and subjecting the cathode elements to final heat treatment to allow the electronically active material to diffuse to the surface of said covering ayer.

16. In the art of manufacturing cathodes, the steps of electronically activating the surface of a cathode in a vacuum, applying a protective metallic coating to the activated surface of said cathode, removing the cathode from said vacuum and mounting it with cooperating electrodes in an envelope, exhausting said envelope and subjecting the cathode to final heat treatment to allow the electronically active material to diffuse to the surface of said coating.

GUSTAV JOSEPH WEISSENBERG.

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