US2144239A - Electron multiplier device - Google Patents

Description

v. K. ZWORYKIN 2,144,239 ELECTRON MULTIPLIER DEVICE Filed Jan. 31, 1935 OUTPUT a 57 (car/401w) INVELIV'T'OH Vladimi rlfiZwory/ilin HT'TO Jan-1 7, 1939.

LIGHT 371 BEHM Patented Jan. 17, 1939 ELECTRON MULTIPLIER DEVICE.

Vladimir K. Zworykin, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1935, Serial No. 4,316

-1 Claim.

This invention relates to electron multiplier devices, and has particular reference to means for obtaining primary and secondary emission in an electron tube such that an initial electron stream of primary electrons may be used to control progressively augmented streams of secondary electrons from one electrode to another.

The phenomenon of secondary emission has been investigated by other experimenters in the past and it has for some time been appreciated by those skilled in the art that an electron tube may be constructed having a plurality of electrodes including a cathode, an anode, and one i or more intermediate electrodes, whereby it is possible to produce primary emission from the cathode to the next adjacent electrode and secondary emission between the electrode last mentioned and another that is maintained at a higher potential. If, then, successively higher potentials are applied to the progressively more remote electrodes starting from the cathode it has been observed that the electrons may be accelerated toward the most positive anode, thereby to obtain useful results in the way of 5 amplification of the exciting energy applied to the cathode.

Commercial applications of the above described device have thus far been limited by difiiculties which stood in the way of proper separation between the primary and secondary electrons. It was found that stray electrons from the cathode which failed to reach the next adjacent electrode or which shot beyond it tended to diminish the efiiciency of the device to such an extent that if it were to be used as an amplifier, for examf ple, it would show no characteristics superior to those of an ordinary electron discharge tube of the diode or triode type. I have found, however, that almost complete separation between the primary and the secondary electrons may be obtained by immersing the electron discharge device in a magnetic field and by arranging the several electrodes of the tube concentrically with respect to one another and coaxially with respect 5 to the lines of force of the magnetic field. With the arrangement as thus provided, the electrons emitted from the cathode are caused to describe arcuate paths in which the radius of curvature is approximately half the distance between the electron emissive surface and the surface of the screen-electrode against which the electrons impinge. The field intensity and the potential gradient between the electrodes is preferably so adjusted that the electrons will just graze the surface of a cylindrically shaped screen electrode.

If they strike this surface they tend to release a family of secondary electrons considerably more numerous than parent electrons of the primary emission stage. If, then, further electrodes are provided for successive stages of con- 5 trol of one electron stream by another, the cycle can be repeated as many times as may be desired.

Among the objects and advantages of my invention I should state at the outset that I desire to provide a device of the character above 10 suggested, in which the component parts of an electron discharge tube may be so arranged as to provide an electron multiplier, of superior characteristics and one which permits of efficient operation for the control of an output cir- 15 cuit having many times the amplitude of energy with respect to the amplitude of the input energy of such a device.

It is a further object of my invention to provide an electron multiplier of the type herein- 20 before suggested in which the cathode of the electron discharge tube may be activated in response to the control of a light beam or, if preferred, in response to thermionic control.

A still further object of my invention is to 25 provide an electron discharge tube having associated therewith means for producing a magnetic field in which the electrodes of the tube may be immersed, thereby to control the inter-electrode paths of electron streams, and thus to obtain 30 substantially complete separation between the electrons of one stage of emission with respect to the electrons of another stage.

The foregoing and other objects and advantages of my invention will be more fully appre- 35 ciated upon reference to the accompanying drawing, in which Figure 1 is a sectional View of an electron tube of my invention wherein the axis of the-electrode system is to be considered as standing perpen- 40 dicular to the plane of projection,

Fig. 2 is a View in elevation of one of my electron discharge tubes surrounded by a field coil,

Fig, 3 is a view of such a tube as that shown in Fig. 2, but indicating a means for modulating 45 the output from such a tube through the control of the magnetic flux of the field in which the tube operates,

Fig. 4 shows diagrammatically how my electron multiplier may be constructed and operated if the cathode is disposed exterior to the other electrodes,

Fig. 5 illustrates one manner in which the electron discharge tube of my invention may be controlled by a light beam, I

the battery sections ll.

Fig. 6 illustrates another mode of operation of the same tube when the light beam is caused to impinge upon a centrally disposed cathode, and

Fig. 7 shows a typical curve of field intensity in respect to the output from one of my electron multipliers.

Referring to Figs. 1 and 2, I show an electron discharge tube having an envelope l which will be understood to be suitably evacuated for purposes of obtaining an electron discharge therein. The electrodes of this tube comprise a filament cathode 3 which is preferably straight and is mounted along the axis of the tube. Connections for heating this filament are shown as a conductor 5 leading out of the tip of the tube and to the negative side of a battery 1. The other terminal of the filament cathode 3 is connected by means of a conductor 9 with the positive side of the battery I. Surroundingthe cathode is a plurality of other electrodes l3 and I5, each being cylindrical in formation and of a diameter progressively greater than that of the electrode which it surrounds. Each electrode is also con- .nected to one or another of the intermediate points or to the extremely positive terminal of The anode I5, however, is preferably so connected through a resistor Id.

The electron discharge tube is preferably mounted within a field coil 2 energy for exciting which may be derived from a suitable source of direct current 23. The intensity of the field excitation may be controlled by means of a rheostat 25.

In the embodiment shown in Figs. 1 and 2, it may be desirable to apply input energy across between the primary and secondary of a transformer 21. The secondary of this transformer is in circuit between the electrode and the positive pole of the particular battery section whose negative pole is connected with the positive filament lead 9. Thus the potential difference between the cathode and the electrode may be varied by modulation. The initial intensity of the stream of primary electrons may thus be amplified by successive stages in which secondary emission is produced.

As hereinbefore stated, the operation of the electron discharge tube in a magnetic field causes the electrons to describe substantially circular paths, as shown by the arrows 29 in Fig. 1. With the adjustments of field intensity, potential differences between the electrodes, and the length of the electron paths from electrode to electrode all suitably coordinated, it becomes possible to greatly multiply the successive generations of electrons from one to another of the electrodes. The energy derived from such electron multiplication may be caused to produce a potential drop across the resistor 9. If, then, the input energy as induced in the secondary of the transformer 21 is made variable for any purpose, like variations in the output circuit may be impressed as contraphasal charges upon the two capacitors 3| respectively, and then utilized in any desired manner.

It is apparent, upon reference to Fig. 3, that the output of such anelectron discharge tube as that shown in Figs. 1 and 2 may readily be controlled by varying the magnetic field intensity. Thus, the field coil 2| may be excited by means of the source 23 and a connection is made thereto through the rheostat 25 and also through a secondary coil 33 of a transformer 35. Input energy may then be impressed upon the primary of this transformer 35 so that potential variations are produced in the coil 2|, thereby to vary the'field intensity. The operation of the tube I as shown in Fig. 3 is thus seen to be substantially equivalent to that which has been described with respect to Figs. 1 and 2.

The modification of my invention shown in Fig. 4 consists merely in impressing the most negative potential upon the outermost electrode 31, thereby causing it to become the cathode. In this case, the cathode is preferably of open mesh screen formation and so sensitized as to render it photoelectric. The other electrodes which are mounted 'coaxially therewith but one within another are all surrounded by the cathode. They are maintained at potentials each greater than that of the one surrounding it. Hence the innermost elec trode 39 is made the anode of highest potential, as shown by the several connections to the battery 4|. The output circuit includes a resistor l9 and capacitors 3|, the same as shown in Fig. 2.

The control of an electron discharge tube such as that shown at 31 in Fig. 4 may be obtained according to either of the arrangements illustrated in Figs. 5 and 6, respectively. In Fig. 5, a light beam impinges upon the cathode through a space which intervenes between two field exciting coils 2| and 2h, respectively. It will be understood, however, that these two coils are to be placed sufiiciently close together so that the lines of force within the electron tube or la Will be maintained as straight as possible and coaxially with respect to the axis of the electrodes.

If either the tube or the tube la is to be operated as a photoelectric tube, it may be so disposed that a light beam will strike the internal cathode of the tube or the external cathode of the tube la at an angle as shown in Fig. 6. Thus the cathode will not be screened from the light beam by the opaque field coil 2|.

Fig. 7 is referred to for showing how the output current I may be varied with respect to variations in the field intensity H. If the field intensity is increased from zero to a certain value, the radius of curvature of the electrons is at the same time gradually diminished until the electrons just graze the inner surface of the screen electrodes, as shown in Fig. 1, or the outer sur face of the electrodes, as shown in Fig. 4. A further increase in the field intensity, however, causes the radius of curvature of the electron paths to be so shortened that the electrons do not impinge upon the electrode surfaces as they should. Hence, the efficiency of amplification becomes diminished, as shown by the downward slope of the curve 43 to the right of the peak.

It is essential to the satisfactory operation of my invention that the intereelectrode distances shall be adjusted to a nicety in order that for a given field strength the electron paths in one stage shall produce maximum secondary emission in a succeeding stage. It may well be seen that if these electrodes are too close together in one case, in comparison with their proximity in another case, inefiicient results would be ob tained, because a magnetic density if varied in one part of the field must be correspondingly varied in another part of the field. Furthermore, it is desirable that the mesh of the screen electrodes be sufiiciently open that when primary electrons strike one side thereof the secondary electrons may be pulled through the mesh and toward an electrode of higher potential.

Although I have shown several embodiments of my invention, it willbe understood that these are merely illustrative and that other modifications may be made without departing from the spirit and scope of the invention. Many such modifications will suggest themselves to those skilled in the art. My invention, therefore, is not to be limited except as is necessitated by the prior art and by the spirit of the appended claim.

I claim as my invention:

An electron multiplier comprising an envelope containing a primary electron emitter, a plurallo ity of secondary electron emitting electrodes and an anode mounted in substantially circular symmetry with more than two interelectrode columns therebetween, said primary electron emitter being of foraminous construction and comprising the outermost of said electrodes and a magnet exterior of said envelope for producing a magnetic field substantially parallel to the axis of symmetry of said electrodes.

VLADIMIR K. ZWORYKIN.

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