US2544754A - Electron camera tube - Google Patents

Description

March 13, 1951 c. H. TowNEs ELECTRON CAMERA TUBE Filed Jan. 29, 1948 FIG.

ATTORNEL vcent atoms.

`latentecl Mar. 13, 1951 YUNITED STATES ATENT OFFICE LIR)N CAMERA TUBE CharlesH. Townes, Chatham, N. J.,-assignor t Bell Telephone Lab oratories,Incorporated,` New York, N. Y.,a corporation of New York Application-January 29, 1948,Serial No. 5,056

7 Claims.

. ductivity.

It is another object of this invention `toirnprove the response ofcertain types of electron camera tubes.

In thefcopendingapplications of D. E. Wooldridge, Serial No. 747,888, led May 14, 1947, which is now U. S. Patent.#2,537,388,-granted JanuaryQ, l951,.and'K. G. .McKay, Serial No. 789,667, filed `December 4, .1947, thereware disi closed vvarious materials Whichexhibit the property known as bombardment induced conductivity. Each of these materials (such as, for `exemple, diamond, zinc sulphide, magnesium oxide,

siliconcarbide and stibnite) is normally an insulator lbut When it is struck by electrons (or other particles,tsuchas alpha or .betaparticles for examplehit becomes conducting if-atthe time an electric field exists between opposite. surfaces voi the insulator. The .bombarding particles penetrate the insulator, causinga disruptive separation of .the positive and negative Vcharges specific to the atoms Whichare effected by the bombarding particles. V-'l`hesecharges are drawn toward the electrodes producing .the Velectric ileld and this motion of charges constitutes a conduction current which is in many cases greatly in excess of the current of the bombarding particles.

.Diamond is a favored .solid insulator for this work (although .other materials .such as,`for example, `others listed Ain the 0Wooldridge and McKay applications Vcan also be used) `because it can easilybeobtained Withoutsuicientimpurities or .imperfections to affect its highqinsulation resistance, or .its conducting properties underY bombardment. The carbon atom-s therein consist each of a Vnucleus exhibiting viixed units of positive charge to which two electrons -are tightly bound. This core is surrounded byfour valence electrons. The carbon atoms areheld together by electron pair bondsbetween adja- T-he insulation resistance is high because the electron bonds are very tight. As ra result of this tightness, very few electrons are displaced from their bonds by thermal agitation. This is not the case in,for example, metals,

Where a large number of electrons are conmetal, Athis, i under L normal aconditicns, constituting the tusual current inca metallicconducting medium.

When* electron bombardment removes -a iva.- lence electron-from its bonds in4 an insulating target, producing fa deciency of :one electron in the atomic `structure immediately f affected, `this localized velectron .decie'ncy -is called a .hole. Under an@ applied electrici eld, ythe'arrangement of the electrons is changed, andfthe `location of any given hole Willchange. As a 'consequence, the holev can berconveniently regarded as a positive particlewhich `is free tomove under .the inuence oflthe `field. Similarly, the-electron freed from the-bond in .question constitutes `a negative particle which is free to Vmove under theinuence ofthe electric eld. .If there isi no applied field, any `free electron or positivehole moves in accordancefwith Ythermal agitationrand consequently has a `completely randomJnotion. Under.anappliedrelectric eld, there is adirectional motion superimposed on the random one. The order of `mobility of .the `electrons indiamond is of the orderof 1,000 centimeters per second for `a-ield of one voltIper centimeter. For a field of llfvoltspercentimeter-the velocity therefore 4is 1-07 centirrreters` per second. Fora diamond `crystal one millimeter thick,the.tran sittim-e therefore is 1 Oa seconds. The mobility of the electrons is affected by -the number-of traps that is,-the presence of foreign atoms or .imperfections in the crystal. If `anelectron gets into Aa trap, it takes agreater or lessfamount offtime to get out, depending upon the Vthermal energy required. .Further `information on traps andiother characteristics `of `diamond crystals are given inthe Wooldridge .and McKay applications referred to above.

In accordance with the present invention there is lprovided an electron camera tube -including an electron targetembodying material which exhibits the property of electron bombardment induced conductivity. Diamond is -.the preferred material for reasons given above. `The camera tube includes a two-sided mosaic-target comprising a mosaic layer of diamond or :other crystals exhibiting the property of electron bombardment induced conductivity. The diamond .layer is ooatedon one side with -a thin conducting layer and on the-.other With-a multiplicity of islands oficonducting` material. The thin conductinglayeris struck by high velocity electrons `modulated by light from an objector eld of view while the conducting islands are scanned by Va high 'velocity electron beam.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawing forming a, part thereof in which:

Fig. 1 is a schematic representation of a cathode ray tube of this invention and certain of its associated circuits and auxiliary equipment;

Fig. 2 is a schematic view showing, in greatly enlarged form, a portion of the target and of the photol-:ctric cathode of the tube shown in Fig. 1; and

Fig. 3 is a graphical representation to aid in explaining the invention.

Referring more particularly to the drawing, Fig. 1 shows, by way of example to illustrate the invention, a cathode ray television transmitter tube i9 employing a two-sided mosaic target II containing material exhibiting the property of electron bombardment induced conductivity. The tube i9 comprises an evacuated container I2 enclosing the mosaic target II, an electron gun I3 for generating, focussing and accelerating a beam of high velocity electrons toward this target, a photocathode 43 on the side of the target Ii remote from the electron gun I3, and two sets of electrostatic deecting plates I6 and I? for causing the beam of electrons to scan every elemental area in turn of a eld of view on the mosaic target Il. Radiations from an object or field of view O are applied to the photocathode I4 by means of any suitable optical system represented schematically by the single lens I8.

The electron gun I3 preferably comprises a cathode 20, a control electrode or member 2l, a rst anode member 22 and a second and final anode comprising a cylindrical member 23. A coating 24 on the inside walls of the envelope I2 extending from the region of the cylinder 23 to the region of a mosaic target I I serves to collect electrons from the target. The conducting coating 24 serves as a collector for electrons from the target I I.

Referring now to Fig. 2 for details of the elements Il and I4, the member I4 includes a semitransparent coating 28 of photoelectric material on a suitable transparent support 25 such, for example, as glass. If desired, the coating 26 may be applied to the end wall of the cathode ray tube. Separated from the member I4 by a small space is the target Il which includes a layer 2T of material exhibiting the property of electron bombardment induced conductivity. Such a material may be a single diamond sheet or, more practically, it can comprise a composite structure formed of a layer (preferably one particle thick) of diamond chips or diamond dust. Alternatively, the layer 2l can be of any other suitable material exhibiting this desired property. The layer 2 is coated on the side nearer the photocathode i4 with a very thin conducting coating 28 of gold or other suitable material deposited in a continuous layer. On the other side of the layer 2 may be deposited a continuous layer 29 comprising a number of small islands of a suit-` able vconducting material such as gold, for example, the islands being insulated from one another. These islands can be produced, for example, by covering the layer 2'! with a line uniform meshof wire, evaporating a suitable conducting material over the surface, and then removing the wire mesh. For some types of layers 2l, a layer 29 thereon is not necessary although the beam velocity may have to be different from that which would be necessary if a layer 2S were present (due to the different secondary emitting characteristics of the materials making up the layers 2l and 29).

A high potential is maintained between the members I4 and I I. By way of example, a source 34 of the order of 10,000 volts is connected between members I4 and II so that the conduct" ing coating 28 of the member II is highly positive lwith respect to the member I4. If the members I4 and I I are spaced apart by any great distance, any well-known electrostatic or electromagnetic focussing arrangement can be used to keep the photoelectrons in paths parallel to the axis of the tube I0.

The control electrode 2I is placed at any suitable negative potential with respect to the cathode 20 by means of an adjustable source 30; and the first anode 22 and the final anode 23 are placed at appropriate positive potentials with respect to the cathode 2li by means of the source 3i and the source 32. The first anode 22 is'placed at an appropriate positive potential which is somewhere between the potential of the anode 23 and that of the cathode 2i).k The actual potentials applied to the various electrode members and their congurations and shapes are such that a beam of focussed, high velocity electrons strikes the target Il at a velocity which produces a secondary emitting ratio for the layer 29 equal to unity. Any suitable source 35 can be utilized to heat the cathode 20. The negative terminal of the source 3l is connected to the cathode 29 and the positive terminal thereof is connected to the first anode 22. The negative terminal of the source 32 is connected to the positive terminal of the source 3l and the positive terminal of the source 32 is connected to ground. The metallic coating 24 is connected to ground through the signal output resistor 36. Exemplary values for the Various potentials applied to the electron gun elements and to the target I I will be given below.

Any suitable amplifier 38 is connected to the signal resistor 36 and the amplifier is in turn connected to the other elements Vof the television transmitter circuit which prepares the video signal for transmission to the receiving station.

The electron beam is deflected over a suitable eld on the target II by means of appropriate potentials applied to the deiiecting plates I6 and I'I by electrostatic sweep circuits (not shown). As examples of satisfactory sweep circuits, reference is made to Patent 2,178,464 issued October 31, 1939, to M. W. Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable for this purpose. Connections can be made from the balanced sweep circuit to the pairs of plates I6 and II by means of coupling condensers 49, 4l, `42 and 43, respectively, of about one microfarad capacity each. Coupling resistances 44 and 45, of the order of many megohms each, are respectively connected across the pairs of plates I9 and Il. The mid-points of the resistances 44 andl 45 are connected to the positive terminal of the source 32 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potentials of the anodes 23 and 24. This relationship is maintained to avoid changes in the sensitivity of the deilecting system and the consequent distortion of the image which would otherwise result. For a more complete description of the advantages of balanced sweep circuits for use with cathode ray television tubes, reference is made to the abovementioned Baldwin patent and also to Patent 2,209,199 issued July 23, 1940, tov Frank Gray.

The operation of the arrangement shown in Fig. lis as 'followsgreference also being made to Fig. 2. Radiations .from 'the object .or field of View O are projected upon .the photocathode vlll by Ameans of the lens system i8. At the same time a high velocity Vbeam of electrons is generated by thegun I3.and1is deflected over a field of the mosaic target i l corresponding to the area covered by the radiations from the object O on the electrode member I4, this scanning being made possible because of appropriate sweep p0- tentials applied to the deflecting plates I6, I1. Initially the cath-ode 20 is placed at a potential of about 1,000 volts negative with respect to the electrode 23, the electrode 24 being at the same potential as. .the vmember.231111 `the absence oi .cur-

rent in the signalresistor .35. .The electrode 22 is placedV at a yproper .value .intermediate that of the member 2|] and 23, to produce sharp focussing. An exemplary value for this potential is 300 volts. The source 39, of the order of 1,000 volts, places the potential of the metal layer 28 at a value of about 2,000 volts positive With respect to the cathode 20. Before electrons from the gun I3 strike the layer 29, it is at the same potential as the layer 28 (that is, at about 2,000 volts positive with respect to the cathode). Referring to Fig. 3, which is a graph of (ratio of number of secondary electrons emitted from the surface 29 to the number of primary electrons striking it) versus the electron velocity of the beam, in volts, at the surface 29, it is seen that When the electron velocity is 2,000 volts, is much less than 1.0. This means that, when the beam strikes the surface 29, electrons are accumulated on the surface 29, driving its potential in a negative direction. At about a potential of 100 volts negative with respect to ground (about 900 volts positive with respect to cathode potential, for example) an equilibrium will be reached with 521.0. The exact value of this equilibrium potential of the surface 29 with respect to that of the electrode 23 (ground) can be adjusted by varying the voltages of the sources 32 and 3l. At a potential of -100 volts for the surface 29, electrons are attracted to the collector 24. The potential of the coating 28 is thus about 1,100 volts positive With respect to the discontinuous coating 29 which is suflicient for obtaining electron bombardment induced conductivity, (The potential of the member 28 does not eiect the focus of the beam because of the screening action of the target Il.) This equilibrium condition is stable since if the potential of the conducting coating 29 becomes more positive with respect to the cathode, the number of secondary electrons being emitted is decreased and this tends to make the potential of the coating 29 swing in a negative direction. Conversely, if the potential of the discontinuous coating 29 becomes more negative than this equilibrium potential, the number of secondary electrons is increased, tending to restore it to its original Value. Under these conditions, when a light image from the object O falls on the photoelectric surface 26, the emitted photoelectrons from the various elemental areas of the surface 26 are accelerated to a very high velocity by the source 34, bombard the conducting surface 28 of the layer 27 and tend to make the various elemental areas of the layer 2 conducting by amounts determined by the light intensity of the corresponding elemental areas of the object. The bombardment induced current thereby initiated produces sudden changes in the potentials of the corresponding islands of the surface 29 on the opposite side of the layer 2-1. The next 4sweep .auf .the electron beam from the gun .i3 .across these Aislands tends to restore their original equilibrium voltages and in this process the .electron current. from the. surface 25 to the collecting electrode 2.4 is .effected in the desired way to produce an .output signal current. This current sets up. a signal voltage across the resistor 36 and the voltage across this resistor is amplified by theamplier .3B inamanner well lmown in the art. The. video `voltage thus amplied is then caused to .modulate a .carrier for transmission to a .distant station by means `Well known. The tube .nf .this invention is capable of a very large Voutput .current due. to the Vfeature of electron bombardment induced conductivity.

Various modifications can be madein the embodiment described .abone without departing p from the spirit of the invention, the scope of which is indicated by the appended claims. The voltages given above are merely by way of example and can vary over a wide range depending on the type of electron gun structure, the spacing and arrangement of the various parts of the target structure with respect to the gun, and the material of the target;

What is claimed is:

1. An electron camera tube comprising a target for electrons including a thin, substantially continuous layer of an electrical insulator which has the property of becoming a conductor when bombarded with electrons, a conducting coating on one side of said layer, means for forming and directing to said coating image-modulated high velocity photoelectrons, and means for scanning the side of said target remote from said conducting coating with a beam of high Velocity electrons.

2. An electron camera tube comprising an evacuated enclosure having therein a photocathode adapted to have applied thereto a light image, a target aligned with said photocathode, said target comprising a thin, substantially continuous layer of an electrical insulator which has the property of becoming a conductor when bombarded with electrons, a conducting coating on the side of said layer near said photocathode, a multiplicity of conducting islands on the side of said layer remote from said photocathode, and an electron gun positioned in said tube so that the electron beam produced thereby strikes the side of said target remote from the photocathode.

3. An electron camera tube target comprising a metal base member, a layer thereon of a material exhibiting the property of changing from an insulator to a conductor when bombarded with electrons, and a multiplicity of small conducting islands on the surface of said crystal layer remote from said base member.

4. An electron camera tube target comprising a metal base member, a layer thereon of a material exhibiting the property of changing from an insulator to a conductor when bombarded with electrons, and a multiplicity of small conducting islands on the surface of said crystal layer remote from said base member, said layer comprising a thin continuous sheet of said material.

5. An electron camera tube target comprising a metal base member, a crystal structure thereon of a material exhibiting the property of changing from an insulator to a conductor when bombarded with electrons, and a multiplicity of small conducting islands on the surface of said crystal layer remote from said base member, said layer comprising a mosaic of small particles of'said material, a single particle thick.

`6. An electron camera tube target comprising a metal base member, a diamond layer thereon, and a multiplicity of small conducting islands on the surface of said diamond layer remote from said base member.

7. In an electron camera tube, an insulating member exhibiting the Vproperty of electron bombardment induced conductivity, means for applying a potential thereacross, photoelectronic means for generating photoelectrons, said photoelectronic means being aligned with said member so that said photoelectrons strike it, thereby rendering it conducting, and produce a charge on the surface thereof remote from said photoelectronic means, and electron beam means aligned with said member so that the beam strikes the latter thereby removing said charge, said insulating member having a conducting layer on the side thereof near the photoelectronc means and also a conducting layer on the side thereof adjacent said electron beam producing means.

CHARLES H. TOWNES. Y

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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