US2244720A - Photocathode - Google Patents

Description

June 10,1941. E. A. MAssA Erin.

P'HOTOCATHODE Filed March 28, 1940 '11! I'IIIIIIIIIIIIII" lwi 15 [[1 15b C(tforneg Patented 1m 10, ion

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W.Pibo,.8warthmcrc,Pa-, mofmaccrporationcfneiawarc ma num alllsllcntolladio s amumaaimJe-umrmu locum. (Cl. 250-105) This invention relates to improvements in electrondischargedevicesandhasspecialreference to the provision of improvements in photoelectric cathodes for use therein.

It is an object of the invention to provide a photocathode which shall exhibit maximum sensitivity in the blue and violet regions (e. g., of the order of 4000:5000 Angstrom units) of the vinble spectrum.

Another and related object of the present invention is to provide a photocathode which shall have a high response to the blue, a contrastingly low response to the red (e. g., of the order of 6500 I Angstrom units), and substantially zero response to the infra-red (e. g., above 7200 Angstrom units) portions of the spectrum, whereby the electron-emissive ratio of the said photocathode for these diflerent colors follows more closely the color response of the human eye than do cathodes which have been sensitized in. accordance with the prior art.

Another object of the invention is to provide a transparent photocathode which shall possess the foregoing color response characteristic and one which at the same time shall exhibit a high conductivity for extremely thin transparent films.

Still another object of the invention is to provide a simple, inexpensive and trouble-free method of manufacturing photosensitive cathodes having theabove-mentioned characteristics, and one which in its practice results in the production of the minimum number of nonusuable elements.

The foregoing objects are simply achieved in accordance with the invention by applying to a suitable base a film or layer of an alloy constituted of silver and antimony, and subsequently photosensitizing this mm by a layer or deposit of caesium or other alkali Contrary to the procedure followed in the manufacture of present-day caesiated photocathodes. it is unnecessary to oxidize either the underlying layer or the surface layer in order to achieve a copius flow of toelcctrons. 8cm details of construction, together with other objects and advantages. will be appa and the invention itself will be best understood by reference to the following specification and to the sccompany s d win w r n:

Figure 1 is a sectional view of one type of electron image tube wherein a photoelectric cathode constructed according to the invention may advantaseoualy be utilized, and

Figure 2 is a view taken on the line 2-2 of Fig. 1, and

' George A. Morton. Sufllce it here to say that. an

Pigureflsachartcomprisingaseriesofcomparative curves indicative of the color-response characteristic of various types of escalated cathodes. g

ReferringtoFig. 1,ane1ectronimagetubein which an electrode formed according to the invention can be utilized may comprise a highly evacuated cylindrical envelope T having a transparent window W at one end and a semitransparent fluorescent screen 8 at the other end.

The inner surface of this glass or Pyrex window W may comprise a supporting surface for the transparent photoelectric cathode hereinafter described. Alternatively, and as shown in U. S. Patent No. 2,189,322 to Leslie E. Flory, a separate transparent cathode-supporting surface may be mounted within the envelope adjacent the window. In any event, the transparent cathode-supporting surface is provided with a conductive frame which in the illustrated embodiment of the invention comprises an annular metallic ribbon i constituted of silver or other metal which terminates in an external lead la. Other elements in the device comprise a series of ring-like electron lens elements 2, 2, 4, and a cylindrical anode 5 mounted in spaced coaxial relation with re spect to the end surfaces W and S of the tube. For a detailed description of the function and operation of a similar arrangement of focusing electrodes, reference may be made to U. 8. Patent No. 2,189,319, issued February 6, 19.40 to inverted light image impressed upon the cathode of the tube releases an electron imagefrom its (later described) sensitized inner surface and that the electrons comprising the image traverse the conduit 2-5 to eventually form a reinverted optically visible electron image upon the fluorescent target or screen 8.

Usually in image tubes of the. character described one of the focusing rings, in this case ring 4, adjacent the point at which the paths of the electrons cross, may have a smaller central opening than the other focusing elements without in any wise interfering with the transmission of the image electrons therethrough. The side face of this disc 4 therefore may aiford a convenient mount for a shield lb which in turn comprises a support for a filament l which carries, between its turns, one or more pellets I of the antimony-silver alloy employed in the formation of the underlying layer I. (Fig. 2). of the emissive surface ll-ll which is formed in accordance with the invention upon the inner face of the glass window W. The main body of the tube may be provided with a tubulation 8 containing a number of pellets I of caesium which are employed in the manner later \described in sensitizing the antimony silver film Ill.

Where, as in the instant case, the electrodes present a shadow-forming barrier to the caesium vapor in its passage to the surface which it sensitizes, it is preferable to provide an opening 4 in the lens element which is adjacent the tubulation so that the vapor may approach the said surface in a path which is substantially free from such obstructions.

The photoelectric cathode of the invention is formed on its supporting surface (in this case the inner'surface of the window W) after the tube T has been completely assembled and thoroughly evacuated. A heating current is then passed through the filament I to evaporate the antimony-silver alloy which then settles in the form of a thin film i upon the inner surface of the window W. For optimum results, where a transparent cathode is desired, the evaporation of the alloy is stopped when the film on the window transmits from 30 percent to 53 percent of light impressed (as by a bulb, not shown) upon its outer surface. The degree of transparency is not substantially altered by the later described process of activation.

Films of greater thickness than above described are less emcient in that less of the light arrives at the molecularly thin emissive region, whereas thinner films are highly resistive and impede the passage of electrons necessary for continuous operation of the cathode. Subsequently, and without oxidizing the said film or breaking the vacuum, slightly more caesium 'or other alkali metal than is necessary to sensitize the antimony-silver layer is distilled into the tube, after which the tube is baked at about 160 C. until the photosensitivity rises to about 15 microamperes per lumen when tested by a tungsten light source,

How well the objects of the invention have been achieved is indicated in the chart of Fig. 3 which shows a series of curves indicative of the color response characteristics of various types of photoelectric cathodes. Curve A indicates the color response of a standard opaque, and curve B a transparent, caesiated-silver oxide cathode,

and shows that the sensitivity (photo-emissivity) 1 of such cathodes is very low in the blue region of the visible spectrum and reaches its maximum in the near infra-redregion (about 7500 Angstrom units) where the human eye is not sensitive. Curve C indicates the color response of a transparent caesiated silver oxide cathode to which an additional film of silver has been added after it has been caesiated, and shows that while the response peak has been shifted into the visible region there is still considerable response to near infra-red region which results in a lack of contrast in the images reproduced by such cathodes.

region of the spectrum and much less sensitivity in the near infra-red region than any of the cathodes of curves A, B and C. This results in a more faithful reproduction of the image or scene impressed thereon.

Curve E indicates the colorresponse of a transparent photo-cathode constructed in accordance with the present invention of an alloy comprising 15 percent antimony and percent silver. by weight, and treated with caesium in the manner described. Here, it will be observed, the maximum response is in the blue region (1, e., 4200 Angstrom units) and falls off rapidly in the higher visible portion of the spectrum with substantially no response in the near infra-red, as is the case of the response curve of the human eye.

As heretofore described, the method of the present invention does not require any oxidation of the underlying or surface layers (Ht-l I) of which the cathode is comprised. However, it has been found that the admission and immediate removal of a bare trace (e. g., 10- mm. Hg pressure) of oxygen into the tube after the un derlying antimony-silver layer has been actlvat ed by caesium, will serve to shift the peak of the color response curve from that indicated bythe curve E to that indicated by the curve F. Such oxidation should not be confused with the standard procedure in manufacturing ordinary caesiated silver cathodes wherein a copious supa color response characteristic indicated by the curves A and B, whereas in the instant case the presence of the antimony apparently prevents such reaction and gives rise to an entirely different color response characteristic.

It has been found that where an alloy comprising substantially more than 15 percent of antimony is employed in the method of the invention there is asharp increasein the resistivity of the cathode surface which may seriously inhibit the achievement of the desired degree of sensitivity. Further, it becomes more difllcult to evacuate the tube since the antimony tends'to distill oil during the baking process. Accordingly, for optimumresults and simplicity of manufacture an alloy comprising substantially no -more than 17-percent antimony to 83 percent silver is recommended. The use of less than 13 percent of antimony results in a cathode having a response more nearly approaching that of curve C and is not recommended in cases where excellent contrast is desired.

While the invention has been described as applied to a transparent photosensitive cathode, it will be apparent that the invention is likewise applicable to opaque type cathodes as well as to socalled mosaic structures. In the case of the opaque structure, the antimony-silver layer may be made as thick as desired without materially changing the color response characteristic peculiar to cathodes of the invention. Since, in the case of a mosaic electrode, continuous emission is not required, the film may be so thin that it possesses extremely high resistance and in fact may comprise merely a multiplicity of spaced apart molecules of the alloy.

Other modifications and applications of the invention will suggest themselves to those skilled We claim as our invention: 1. A photoelectric cathode comprising a supparent supporting surface, a transparent layer 10 of a silver-antimony alloy on said surface, and a transparent layer of caesium on said layer of alloy.

4. The invention as set forth in claim 3 and wherein said transparent alloy layer comprises 5 substantially fifteen percent antimony and eighty-five percent silver. by weight.

5. The invention as set forth in claim 3 and wherein the overall thickness of .said layers is transparent to from thirty to fifty-three percent 20 of light impressed upon the opposite surface of said transparent supporting surface.

6. Method of making a photoelectric cathode which comprises depositing a layer of a silverantimony alloy upon a supporting surface, and 25 subsequently depositing a layer of photosensitive material upon said layer of alloy.

7. Method of making a photoelectric cathode which comprises depositing a layer of silver antimony alloy upon a mmrtlnssurface, and sub- 30 sequently depositing caesium upon said layer of .alloy.

8. Method of making a photoelectric cathode which comprises depositing an unoxidized layer of a silver antimony alloy upon a supporting surface, and subsequently depositing a layer of unoxidized caesium upon said unoxidized layer of alloy.

1 9. Method of making a-photoelectric cathode which comprises depositing a layer of a silverantimony alloy upon a supporting surface mounted inan evacuated container, and subsequently depositing a layer of an alkali metal upon said layer of alloy while maintaining the vacuum in said container.

- 10. Method of making a photoelectric cathode which comprises evaporating a silver antimony alloy in an evacuated vessel containing a transparent supporting surface, halting the evaporation of said alloy when the thickness of the deposit upon said supporting surface is such as to permit the passage therethrough of from forty to fifty percent of light impressed upon said transparent supporting surface, and subsequently depositing a transparent layer of caesium .upon said layer of alloy without breaking the vacuum in said container.

ERNEST A. MASSA- EUGENE W. PIKE.

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