US2544755A - Electron camera tube - Google Patents

Description

ELECTRON CAMERA TUBE Filed Jan. 29, 1948 3 AME I36 I Ill L I 34 I W F/GJ UL/LIGHT FROM OBJECT LAYER or HATER/AL. EXHIBIT/N6 PROPERTY 0F aouamomwr H 5 /{VDUCED CONDUCT/WT) (a./. c.) hlill I' 42 5 THIN CONDUCT/YE 33 mu i R C T THIN b y FIG. 4 NQN-CONDUCT/VE T 2 LAYER or PHOTO- b sums/v5 HATER/Al D sum-4c: or

a 5.1.c. CRYSTAL V I 34 R 6 I J. B. JOHNSON Patented Mar. 13, 1951 ELECTRON CAMERA TUBE John B. Johnson, Maplewood, and Kenneth G.

McKay, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 29, 1948, Serial No. 5,084

Claims.

This invention relates to electro-optical devices and more particularly to electron camera tubes for television.

It is an object of this invention to utilize in electron camera tubes materials exhibiting the property of electron bombardment induced conductivity.

In the copending applications of D. E. Wooldridge, Serial No. 747,888, filed May 14, 1947, which is now U. S. Patent #2537388 granted January 9, 1951, and K. G. McKay, Serial No. 789,667, filed December 4, 1947, there are disclosed various materials which exhibit the property known as bombardment induced conductivity. Each of these materials (such as, for example, diamond, zinc sulphide, magnesium oxide, silicon carbide and stibnite) is normally an insulator but, when it is struck by electrons (or other particles, such as alpha or beta particles, for example), it becomes conducting if at the time an electric field exists between opposite surfaces of the insulator. The bombarding particles penetrate the insulator, causing a disruptive separation of the positive and ne ative charges specific to the atoms which are afiected by the bombarding particles. These charges are drawn toward the electrodes producing the electric field 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 in the Wooldridge and McKay applications can also be used) because it can easily be obtained without sufficient impurities or imperfections to affect its high insulation resistance, or its conducting properties under bombardment. The carbon atoms therein consist each of a nucleus exhibiting six units of positive charge, to which two electrons are tightly bound. This core is surrounded by four valence electrons. The carbon atoms are held together by electron pair bonds between adjacent atoms. The insulation resistance is high because the electron bonds are very tight. -As a 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 continuously being displaced by thermal agitation and are relatively free to wander through the metal, this, under normal conditions, constituting the usual current in a metallic conducting medium.

7 When electron bombardment removes a valence electron from its bonds in an insulating target, producing a deficiency of one electron in the atomic structure immediately affected, this localized electron deficiency is called a hole. Under an applied electric field the arrangement of the electrons is changed, and the location of any given hole will change. As a consequence, the hole can be conveniently regarded as a positive particle which is free to move under the infiuence of the field. Similarly, the electron freed from the bond in question constitutes a negative particle which is free to move under the influence of the electric field. If there is no applied field, any-free electron or positive hole moves in accord-ance with thermal agitation and consequently has a completely randornmotion Under an applied electric field, there is a directional motion superimposed on the; random. one. The mobility of the electrons in diamond is of the order of 1,000 centimeters persecond for a field of one volt per centimeter. For a field of 10 volts per centimeter the velocity therefore is 10 centimeters per second. For a diamond crystal one millimeter thick, the transit time therefore is 10- 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 an electron gets into. a trap, it takes a greater or less. amount of time to get out, depending upon .thethermal. energy required. Further information on traps-and othercharacteristics of diamond crystals are given .in the Wooldridge and McKay applications referred to above.

In accordance with thepresent invention, there is provided an electron camera, tube including an electron target embodying material which .exhibits the property of electron bombardment induced conductivity. Diamond is thepreferred material for reasons given above. More specifically, the camera tube includes a two-sided mosaic target comprisinga mosaic layer of diamond or other crystals exhibiting the property.

of electron bombardmentinduced conductivity. The diamond layer is coated onone side. with a thin conducting layer and the otherside is covered with a very thin layer of a non-conducting material which is .photoemissive. vAn image of an object or field of view is applied to the photoemissive side of the target while the other side is scanned by a beam of electrons generated in the camera tube. Mesh grid members are mounted on opposite sides of theitargetand are spaced a short distance therefrom. Externally produced secondary electrons are collected on the grid nearer the conducting layer side of the target while photoelectrons are collected by the other grid member.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawings forming apartthereotin which:

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

Fig. 2 is a schematic view showing, in. greatly, enlarged form, a portion of the target and of the electron collecting electrodes in the; tubevshown" in Fig. 1; and

Figs. 3 and 4 are diagrammatic representations *1; to aid in understanding the invention.

Referring more particularlyyto the drawing;; Fig. 1 shows, by way of example to illustrate the invention, a cathode ray television transmitter tube 18 employing a two-sided mosaic target ll 0 containing crystals exhibiting the property of electron bombardment" inducedconductivity? The tube 1 9 comprises an evacuated container 1 2 enclosing the =mosaic target I l an electron gun l3 *for generating, focussing and accelerating a 25. beam-of-'high' velocity electrons toward-this target, electron-collecting electrode 94 on the side of the targetl I hear thegun 3, a photoelectron' collecting-electrodeon the side of the target ll remote-from the electron gun 13, and two sets of-'electrostatic deflecting-plates l6 and H for causing the beanrof electrons to scan every elemental area in-turn of a" field *of view 1 on" the mosaic -target; H. Radiations from'an object =orfield-of yiew Oiare applied to "the 1 side" or the mosaic target '11 l remote from the electron gun l3jby means of any suitable optical system representedschematicall'y by the single lens 18."

The electron gun l3 preferably comprises a cathode 2Il,la control electrode or member 2!, a first anode member 22,.and asecondand final anodecomprising a cylindrical member ZBJ-and a coating of .conducting material on the inside wallsjiof .the envelope. l2 extending from the region of the cylinder 23 to the region of the mosaic tar et I l. The collecting electrode It for the sec onda'ryelectrons emitted from the target .I I whenitis struckby the beam of highvelocity electrons preferably. con ists of mesh 7 material. The photoelectron collecting electrode E5 on the other 1 side of.'the target H. is alsopreferablyof mesh material.

The control electrode 2! is placed at any suitablene ative potential withrespect to the potential ofsthepathode 2!! by means of an adjustablesource,,3 .andthe first anode 22 .and the final anode .24. aro placed jat appropriate positive; potentialsswith' .respectjothecathode 2!! by; means of. the source 3t and the source 32., As .an example.,the .final1anode-23. glean be in the ran efrom 1.0.00 'to' 10,000 volts or more positive with respectatolthecathode.and thefir'st anode 22..can be-.; for example, 300 volts positive withrespect,to.-,the cathode; Any suitable source 35 can be utilized to heat the cathode" 2!}. The negative terminalgoithe source, 31. is connected to the catli'odeifl and-,the positive vterminal thereof is. connectedto, thefirst anode .22,; while the negatiiiete'rminalofvthesource, 22 is connected to the ositive terminaloi the source 3|. and the posi-. tive terminalgof gsourcejz is connected to ,the second anode 23, 2 Preferably the positive terminalgof -the source 32 is connected to groundthrough a source 3'! whichis used to make the electrode member l4 "positive with respect to the target I i for a purpose which will be pointed out hereinafter. The voltage of this source 3'! can be, for example, 20 to 50 volts. Batteries have been shown only for convenience in the drawing and it is to be understood that any other means for producing direct voltages can be used instead.

Reference willinow bermade" to Fig; 2 which shows in enlarged scale a portion of the two-sided mosaic target H and its associated electrode members is and i5. Fig. 2 has not been drawn tor-scale. The target H comprises a thin layer Q0 of insulating material which exhibits the property of electron-bombardment induced conductivity; For-example, the layer All is a very thin cutfofdiamond'or -a-; simulated sheet of diamond formed bya layer (preferably one particle thick) of: diamondschipsondiamond dust. Alternatively, any other suitable material exhibiting the property of electron bombardment induced conductivity can be used instead of diamond. The sidesof rthe layerrllt mear the e1ectron'gun2i3 "is coated with: a verythin fcon'ductirig film-4 i 10f a metal such as gold or platinum while 'the oppo-'- site side ofthe layer 513 :is. either left uncovered or (preferably) is 'covered'by a very'thin'layer of a= laterally -non conducting"material which yields a larger photoresponse than diamond 'to' light "of the-"desired wavelength? Such alayer, for examplegmay be no more than a single moleculn: thicknesss The-layer 42 may be a compound-prone of the alkaline metals, suchas, for

eXample,- caesium oxide; or it maybe a mixture such as -caesium oxide silver as in certain well known-commercial electron camera "tubes.-- The 1 layer =4! is-connected through a'-resistor'--33 to' ground." The electrode I 5 connected to ground through a'source 34. Any suitable "amp1ifie'r 36 is connecte'd to the signal'resistor- 33 through a suitable coupling condenser 38 and is in turn connected totheother elements of the television appropriate potentials applied to the deflectingplates [6 and H by'electrostaticsweep-circuits (not shown); As examples of satisfactory sweep circuits. reference is made to Patent 2.178.464 issued-"Octoberiil. 1939, to 'M'Wi Baldwin, Jr., which discloses balanced electrostatic sweep circuits suitable for this purpo e. Connections can be made from the balanced sweep circuits to the pairs of {plates 55 and I! by means of coupling condensers-'43, 44, t5 and 45. respectively. of

about one microfa'rad -capacityeach: Coupling resistances Al' and 48 of theorder of many meg ohms each-are respectively connected across the pairs of plates 16 and H; The mid-points of the resistances 4! and '48 are connected to the posi-' tive terminal of the source 32 so that the average of the potentials of the deflecting plates does not deviate more than slightly from the potential of t e anode z3y2 i. This relationship is maintained to avoid changes in the sensitivity of the deflecting system and the'conseouent distortion of the image 'jffhlchi would otherwise result. For a more complete description of the advantages ofbalanced sweep circuits for use with cathode ray television tubes-reference is made to the abovementioned Baldwinpatent and'also to Patent 2.209.199 issued July 23. 1940. to Frank'Gra-y.

The operation ot-the device *shown' in Fig.- l

will now be described, reference also being made to Figs. 3 and 4 which are diagrammatic representations to aid in understanding the invention. Radiations from the object or field of view are projected upon the left-hand side of the tube I0 and strike the photoemissive layer 42 of the target ll. Photoelectrons are emitted from this surface in direct proportion to the brightnesses of the various parts of the object or field of view to be televised. These photoelectrons are attracted to the photoelectron collecting electrode l5 since this electrode is positively biased with respect to the target II by means of the source 34. Since the layer 42 is thin enough to be non-conductive laterally, a series of discrete charges will be built up thereon proportional in each case to the intensity of the corresponding elemental area of the object or field of view.

At the same time that a charge pattern is being formed on the left-hand side of the target H, a beam of electrons produced by the gun l3 and deflected over a rectangular field thereof by the electrostatic deflectin fields applied between the sets of plates l6 and I! scans the righthand side of the target I I through the collecting electrode I4. Any externally produced secondary electrons are collected by the collecting electrode l4 which is at a positive potential sufiicient to collect all the secondaries produced. If the bombarding voltage is such as to make the target secondary emission coefi'icient 6 1, then no net current flows through the resistor 33. This is a desirable although not a necessary condition. As the beam traverses the diamond sheet or layer 40, it produces many electrons in the conduction band but where there is no voltage across the diamond these merely recombine with the positive holes. However, when the beam strikes a spot opposite one on the left-hand side of the target I I from which photoelectrons have been removed and which is at a certain potential, conductivity takes place across the diamond sheet 4|] and the charge produced by the removal of the photoelectrons is neutralized by electrons which flow from ground through the resistor 33 and through the diamond. Thus a pulse is produced across the resistor 33 which is applied to the amplifier 36 wherein it is amplified and then applied to other elements of the television transmitter circuit.

Reference will now be made to Fig. 3 which shows an element of the target 40 positioned near a portion of the collecting electrode l5, the collector [5 being connected to the back plate 4! of the target through source 34 and resistor 33 as in Fig. 1. Now assume that the surface 42 of plate 49 has been irradiated by light for a short time (duration of a frame). Photoelectrons have left the surface 42 for the collector l5 leaving behind a positive charge at the surface as indicated in Fig. 3 and a corresponding negative charge at the base. During this process there has been a net flow of electrons upward or a current downward in the load resistance 33. The magnitude of this current corres onds to the number of electrons leaving the whole surface of the irradiated plate 40 and is proportional to the average amount of light that falls on the plate in this interval. Since this average does not change appreciably during one frame, the current is sensibly constant and therefore carries no signal. Now a small area of the plate 40 is bombarded by primary electrons that penetrate the surface and produce local conductivity. The charges on the opposite Q faces of the plate in this elemental area then become neutralized through the conducting path produced thereby. The effect of this neutralization is the same as that of positive charge flowing downward through the plate 40 and through the resistor 33, the current being proportional to the charge built up in this area since the last scan, which in turn is proportional to the illumination of this area during time between scans. As the scanning beam moves over the surface the current in the resistance 33 due to the neutralized charge varies with the illumination from point to point on the surface constitutes a signal current.

There is still another way of looking at the operation of the arrangement of Fig. 1. In Fig. 4, the plate 4!] is represented by the condenser G1 with the base and top surface as electrodes, and the condenser C2 is formed by the surface and the collector electrode I4. A switch with contacts a and b connects either the resistance Ra across C1 or Rb across C2. The current through Rb represents photoelectrons charging the condenser 02 and that through Ra current flowing through the plate 40 by virtue of the induced conductivity which tends to discharge C2, the potential on C1 varying in very nearly the opposite sense. A mathematical analysis of this circuit yields an answer in very complicated terms but qualitatively the answer is that on closing the switch a transient signal current flows through the load resistance 33 as expected, depending in magnitude on the resistance Rb.

The arrangement shown in Fig. 1 is capable of very large output current due to the electron bombardment induced conductivity as much larger output current can be produced than the photoelectrons producin the individual charges: Moreover, due to the fact that the target is of the two-sided type, the photoelectrons and the beam are separated with consequent advantages in tubes of this type.

Various modifications can be made in the embodiment described above without departing from the spirit of the invention, the scope of which is indicated in the claims.

What is claimed is:

1. An electron camera tube comprising a target for electrons including an array of particles of an electrical insulating material which has the property of becoming conducting when bombarded with electrons.

2. An electron camera tube comprising a target for electrons including material which is normally insulating but which has the property of becoming conducting when bombarded with electrons.

3. An electron camera tube comprising a target for electrons including an array of crystals of an electrical insulating material which has the property of becoming conducting when bombarded with electrons.

4. An electron camera tube comprising a target for electrons'including a layer of electrical insulating material which has the property of becoming conducting when bombarded with electrons.

5. An electron camera tube comprising a target for electrons including a thin, continuous crystalline layer of an electrical insulating material which has the property of becoming conducting when bombarded with electrons.

6. An electron camera tube comprising a target for electrons including a thin, substantially continuous layer of crystals of an electrical insulator which has the property of becoming a conductor when bombarded with electrons.

7.. An electron camera tube comprising a target for electrons. including athin layer of an electricalinsulator which has the property of becoming. aconductor when bombarded with electrons, acontinuous metallic coatin on one side of said crystalline layer and a thin coating of photoemissive material on the other side thereof.

8. ;An' electron camera tube comprising means for generating a beam of electrons and a target positioned to have one side thereof impinged by said beam, said target comprising a thin layer of an. electrical insulating material which has the property of becoming conducting when bombardedv by electrons, and a continuous metallic coating on said layer on the sid thereof toward said beam generating means.

9. An electron camera tube comprising means for generating a beam of electrons and a target positioned to have one-side thereof impinged by said beam, said target comprisin a thin layer of an electrical insulating material which has the property of becoming conducting when bombarded by electrons, a continuousmetallic coating on said layer on the side thereof toward said beam generating means, and a photoemissive layer on said insulating material on the side thereof remote from said metallic coating.

10. An electron camera tube comprising means for generating a beam of electrons, a target positioned to have one side thereof impinged by said beam, said target. comprising a thin layer of an electrical insulating material which has the property of becoming conducting when bombarded by electrons, a continuous metallic coating on said layer on the side thereof toward said beam generating means, and a photoemissive layer on said insulating material on the side thereof remote from said metallic coating, and mesh collecting electrodes on respectively opposite sides of said target.

JOHN B. JOHNSON.

KENNETH G. McKAY.

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

. UNITED STATES PATENTS Number Name Date 2,251,992 Flory et a1. Aug. 12, 1941 2,256,462 Iams Sept. 16, 1941

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