US2754428A - Electron discharge device - Google Patents

Description

July 10, l956 G. w. FRANKs UAL 2,754,428

ELECTRON DISCHARGE DEVICE Filed Sept. 17, 1951 2 Sheets-$heet l AJ R n n s E o wo m s o n Ew M www o u Wm m ,m .m w mm m mw 5f@ 4 mw a m m m 7 .f a f I- .wam M o m .m M w e N .AVul JM um md n 6 El i. E w w 1 m M m M 6 2 M um m ad A Lf ..1 M f., .A10 f f f m v mi w mm A d July 10,1956 G. w. FRANKs ET Al.

ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 2 Filed sept. 17, 1951 MPM T HZ mm z mf. m

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www mw m F m w mw m m m ATTORNTJ ELEC'rRoN DISCHARGE DEVICE George W. Franks, Lorton, and John N. Hamilton, Alexandria, Va.

Application September 17, 1951, Serial No. 247,012

3 Claims. (Cl. Z50-211) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without payment to us of any royalty thereon.

This invention relates to electron discharge devices generally and more particularly to a self-energizing, photosensitive electron discharge device and method of initiating and maintaining the discharge.

Electron discharge devices of the type contemplated herein usually embodying a source of electrons known as a cathode and an electron receiving or collecting electrode commonly known as an anode. The usual device requires some form of external energy to energize the emission of electrons from the cathode surface. In its simplest form, this type of electron discharge device has only two elements, i. e., the anode and cathode, and is applicable as a detector or rectilier. For exemplary purposes, the description of this invention will be limited to the simplest form of the device, which is a detector or rectiier.

A conventional electronic rectier generally consists of the cathode, which is the source of electrons, and an anode or plate which is the collector element, both of which are sealed within a housing or envelope containing either a gas or a high vacuum. A common type of conventional detector or rectier utilizes a heated element as the source of electrons. Thus, the conventional discharge device requires an external source of energy to provide a source of electrons.

It is an object of this invention to provide an electrical discharge device having an electron collector and a source of electrons that is activated by electro-magnetic energy produced within the device.

lt is a further object of this invention to provide a photosensitive source of electrons which may be activated to freely emit electrons by absorption of electro-magnetic energy radiated from a luminescent electron collector.

A still further object of this invention is to provide an electron discharge device having a photo-sensitive source of electrons which is activated by electro-magnetic energy radiated from a luminescent electron collector initially rendered luminescent by electron activation resulting from ambient thermal energy activating the photosensitive source without other excitation under normal operating conditions.

A still further object of this invention is to aid the emission of electrons from a photosensitive source to activate a luminescent electron collector by providing a radioactive material in proximity to the electron collector.

Briefly, in accordance with this invention, there is provided an electron discharge device having a radiation absorbing cathode which freely emits electrons in response to absorption of electro-magnetic radiated energy generated within the device and having an electron-excitable radiating anode which radiates electro-magnetic waves corresponding to the spectral response of the radiation absorbing cathode in response to bombardment by cathode electrons. The radiating anode may be initially excited to radiate electro-magnetic energy by bombard- 2,754,428 Patented July 10, 1956 fic ' 2 ment from electrons emitted from the cathode due to ambient thermal energy without other excitation under normal operating conditions. In the alternative, the catode electron emission may be aided in exciting the anode by introduction of a radio-active material. The radiating anode and electron emissive cathode are mounted in spaced relation within an envelope and are each connected to externally accessible electrical conducting means extending from the envelope for connecting the cathode and anode to an electrically energized circuit.

Fig. l illustrates the construction of the electron discharge device as embodied in a half-wave .rectiiier having a single anode and cathode;

Fig. 2 illustrates the electron discharge device as embodied in a full-wave rectifier having a pair of anodes and a corresponding cathode;

Figs. 3 and 4 are vertical sectional views taken along the lines 3 3 and 4--4 in Figs. l and 2;

Fig. 5 is a schematic illustration of the half-wave rectifier of Fig. l connected in an electrically energized circuit;

Fig. 6 is a further embodiment of the invention wherein the radio-active material is a separate element of the electron discharge device; and

Fig. 7 is a vertical sectional view taken on the line 7--7 of Fig. 6.

As hereinbefore indicated, an electron discharge device, when exemplarily embodied as a rectiiier, consists of a source of electrons in the form of a cathode and a collector of electrons in the form of an anode or plate which are both sealed within a housing containing either a gas or high vacuum. The cathode source of electrons in a conventional rectifier is usually provided with an element heated directly or indirectly to initiate the emission of electrons. This invention contemplates the substitution of a photo-emissive or radiation-absorbing element for the cathode of a conventional type discharge device which does not require external electrical energy to initiate electron emission. With this substitution, if the photo-emissive cathode were illuminated with external electro-magnetic radiation which matches the response characteristic of the photo-sensitive cathode surface, the device would function in a manner equivalent to a cold cathode rectifier. Thus, this invention further contemplates the elimination of the external electrical source of electro-magnetic radiation by providing an electron collector or anode which is coated with a phosphor and which is capable of being excited to luminescence by electron bombardment, the luminous energy output of which is properly selected to match the spectral characteristic of the photo-emissive cathode and which has suitable persistence characteristics. This method of excitation may be termed cathodeluminescenceg that is, excitation by electrons emitted from the photocathode.

The electron discharge device thus constructed is energized under normal conditions of temperature without the requirement of additional radio-active elements or isotopes to excite the luminescent anode material to radiation. This results from the fact that, even under normal operating conditions, there is an equilibrium condition of constant small emission due to ambient thermal energy acting on the cathode surface material and return of electrons from photosensitive cathodes of the type herein contemplated in accordance with Richardsons equation:

I :Emission current density A and a=Constants depending upon the emitting substance and the state of its surface T :Temperature in degrees Kelvin s KzBoltzmanns constant (1.3l7 10*16 erg per degree per electron) G=The Napierian base of logarithms This condition has given rise to the suggestion vof an autophotoelectric effect. Regardless of the theory, however, this small emission current due to ambient thermal energy is considered to be sucient under the influence of a suitable potential to cause initial excitation of the luminescent material of the anodes; theluminous energy emitted thereby produces photo-emission at the cathode resulting in the build-up of the current through the rectifier or other discharge device as hereinafter to be more fully described. v

There are numerous types of photo-emissive cathode surfaces which may be excited to emit photo-electrons in accordance with the foregoing requirements. The material used must meet, as a minimum, two basic requirements which are, rst, that the photocathode should have a high sensitivity, i. e., a high value of micro-amperes per lumen, and, secondly, the response curve ofthe surface should match the light output of the anode luminescent phosphor. The exact composition of the preferred photo-emissive surface is not known; however, it has been prepared by empirically mixing antimony, cesiurn, and sometimes oxygen, and is known to the trade asthe S-4 antimony-cesium surface and described, for example, in the text Photo-Electricity by Zworykin and Ramberg, published by Wiley in 1949 on pages 114-115. In accordance with the preferred embodiment of this invention, the photo-emissive surface may be formed on the inside of a glass, metal, or quartz envelope in any well-known manner of which the following is an example. The bulb is first exhausted in a conventional manner and baked at about 350 C. After the bulb is cooled, antimony is evaporated on to the inner bulb surface, usually from a heated tungsten coil inside the bulb which contains antimony. There is an optimum thickness of the antimony layer which is of the order of one micron. The bulb is then heated to approximately 150"Y C. and cesium vapor is liberated into the bulb until a maximum sensitivity is reached. The bulb is then allowed to cool, after which several cycles of liberating cesium, followed by short baking of the bulb, is accomplished. It is understood that increased sensitivity. may be achieved by liberating suitable quantities of oxygen into the bulb. In practice it has been found that the resulting surfaces quite often have too high an ohmic resistance for proper functioning. In order to alleviate this condition, a highly conducting surface, such as evaporated silver, may be used as a base for the photo-emissive cathode. A high degree'of sensitivity is obtainable by using various alloys of antimony, such as gold, silver, copper, and zinc.

As hereinbefore indicated, the selection of the luminescent anode material is determined so as to match the spectral response of the resultant high eiciency photocathode. It is generally understood that the highest degree of eiciency is obtainable in the ultra-violet end of the spectrum. However, in accordance with the preferred embodiment of this invention, it is preferred to use a spectral response in the visible range. Y

The emission of luminous energy from matter under the influence of an exciting agent when in the visible range, is termed uorescence. sists after the removal of the exciting agent, the action is termed phosphorescence. The method of excitation utilized in the preferred embodiment of thisrinvention has been previously termed cathode-luminescence, i. e., the excitation of the radiation-emitting surface by electrons emitted from the photo-emissive surface of a cathode. There are numerous substances which exhibit the proper degree of fluorescence, and the selection of the' anode luminescent material for the purposes of this invention is determined in accordance with two major requirements, namely, the high efficiency of fluorescence when excited When the emission peri electron bombardment, and the radiation spectra matching as nearly as possible the response characteristics of the photo-emissive cathode. Any phosphor having these characteristics may be used, however in practice, it has been found that the phosphor composition ZnS:Ag exhibits the correct conditions to meet these requirements .when used with an S-4 surface as previously described. Additional continuous emission of light from the phosphor may be obtained if necessary or desirable by combining it with any of several radioactive elements or isotopes, such as radium. The phosphor coatings or screens may be prepared by any of several well-known methods, the usual method involving preparation of a suspension in distilled water to which a binding agent is added, with subsequent removalV of the suspended particles by settling, centrifuging, electrostatic deposition and spraying.

Referring now to Fig. 1, there is illustrated the preferred embodiment of the electron discharge device of this invention as physically exemplified in a half-wave rectier construction. The device includes an insulating base 1 of suitable material such as porcelain or the like which base has a raised portion in the form of a boss 2 whichl has a central recess therein for receiving Vand supporting the anode 3. The anode 3 is provided with a phosphor coated surface 4, which may comprise radio-active material, as hereinbefore described, and which may be deposited on or applied to the anode surface in accordance with the several methods hereinbefore described or suggested. Although it is understood that the discharge ldevice will operate as a half-wave rectier in the construction of Fig. l without the inclusion of a screen, the preferred embodiment is showrnrprovided with Van electron controlling screen 5, which may be in the form of a suitablywoven wire mesh mounted in concentric relation aroundy the anode 3 and connected in the ultimate circuit so as to Vremain positive with respect to the cathode to reduce bombardment of the cathode by electrons when the cathode is positive with respect to the anode Von the reverse half cycle. Other modes of operation may be used to accomplish a similar result, such as connecting the screen directly to the anode toV lessen the extent of electron bombardment of the cathode by creating an electron free eld between Vthe screen and anode. The assembly of the insulating support, anode, and screen is inclosed within avacuum in a suitable glass, metal, or quartz envelope 6` which is coated on its-inner surface with a suitable photo-emissive or sensitized cathode material 7 in accordance with the methods hereinbefore described. The envelope 6 is provided with an opening in one side thereof having a connecting cap 8 which cap is connected to a lead-in conductor 9 leading into electrical contact with the photo-emissive cathode surface within the envelope. Suitable electrical conductors Mi and 11 are shownV connected to the anode and screen respectively and extending from the base of the envelope to be externallyv accessiblefor connection to a suitable electrically energized circuit as exemplarily illustrated in Fig.

5 of the drawings.

The embodiment illustrated in Fig. 2 exemplifies the physical embodiment of the electron discharge device of this invention inthe form of a full-wave rectifier having two anodes 12 and 13 mounted in spaced relation within Ythe envelope in place of the single anode of Fig. 1. In

be unnecessary since the potential of one or the other of the anodes would be equal to or positive with respect to `the cathode at all times. l

v,In lieu of providing radio-active material as a component of the anode coating, a separate electrode providing radio-active material may be used, as recited elsewhere in this specitication and illustrated in Figs. 6 and 7. The structure of this embodiment is similar to that shown in Fig. l with the addition of the electrode 14 which provides radio-active material.

Thus, the combination of a photosensitive cathode, which has a constant small electron emission under normal operating conditions due to ambient thermal energy, mounted in spaced relation from an anode which may be activated to luminescence by virtue of electron bombardment resulting from electron emission from the photo-emissive cathode or, as further aided by the interposition of a suitable radio-active material in the region of the luminescent anode, results in an electron dis- Wcharge devicernot requiring external electrical sources sidered preferred embodiments of this invention, it will be obvious to those skilled in the art that changes and modifications, particularly in respect to the number of electron controlling or activated elements and their arrangement and application in electrical circuits, may be made without departing from the scope of this invention as defined by the appended claims.

Having thus described our invention, what we claim as new and wish to secure by Letters Patent is:

1. An electron discharge device comprising in combination, an electron-excitable electro-magnetic radiating element, a radiation absorbing element which at low ambient temperatures freely emits electrons in response to such absorption, a radio-active element mounted in proximity to said electron-excitable electro-magnetic radiating element for exciting said radiating element, means mounting said radiating element, said radiation absorbing element and said radio-active element in mutually spaced relation, an envelope enclosing said mutually spaced elements, externally accessible electrical conductor means extending from the envelope for connecting said radiating and radiation absorbing elements to an electrically energized circuit.

2. A structure as recited in claim 1 including an electron controlling screen element concentrically arranged with said electron-excitable electro-magnetic radiating element and intermediate said radiating element and said radiation absorbing element, and externally accessible electrical conductor means extending from the envelope for connecting the screen to an electron attracting potential in an electrically energized circuit.

3. An electron discharge device comprising in combination, an anode having a phosphor coating capable of being excited by electron bombardment to luminescence of predetermined spectral response, a radio-active element mounted in mutually spaced relation from said anode, a woven wire mesh screen formed to concentrically surround said anode in spaced relation therefrom, an insulating support including means to support said anode, said radio-active element and said screen in mutually spaced relation, a glass envelope inclosing the support, anode, radio-active element, and screen in a vacuum, said glass envelope having an inner photo-emissive coating with an absorption characteristic matching the spectral response of said anode, said photo-emissive coating being activated by ambient thermal energy and exhibiting a constant electron emission under normal operating conditions without external electrical excitation, said raido-active element insuring excitation of said anode surface in the absence of ambient thermal energy sucient to activate said photo-emissive surface, a source of electrical potential, and externally accessible electrical conductors extending from the envelope connecting said anode and photo-emissive coating across said source of electrical potential and said screen to an electron attracting electrical potential.

References Cited in the tile of this patent UNITED STATES PATENTS 1,145,735 Ainsworth July 6, 1915 1,173,110 Junghans Feb. 22, 1916 1,775,588 Cohn Sept. 9, 1930 1,788,553 Thomas Jan. 13, 1931 1,917,854 Rentschler July 11, 1933 1,965,849 McIlvaine July l0, 1934 2,039,134 Waldschmidt Apr. 28, 1936 2,092,814 Schaiernicht Sept. 14, 1937 2,206,387 Bruche July 2, 1940 2,218,340 Maurer Oct. 15, 1940 2,258,294 Lubszynski et al Oct. 7, 1941 FOREIGN PATENTS 499,661 Great Britain Jan. 26, 1939

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