US2147825A - Electron multiplier device - Google Patents

Description

b' 1939- v. K. zwoRYkm ET AL 2,147,825

ELECTRON MULTIPLIER DEVICE v Filed July 26, 1 'i'sheets-sheet 1 Feb. 21, 1939. v. K. ZWQRYKIN El AL ELECTRON MULTI'PLIER DEVICE Filed July 26,, I935 Ill 5 Sheets-Sheet 3 a5 9.5 as 1'15 1'25 1.551; 1.55

FOCUS/N6 COIL 60/?RE/V7.

Patented Fee. 21, 1939 UNETED STATES PATENT OFFICE anacrnon MULTIPIJER DEVICE Delaware Application July 26, 1935, Serial No. 33,266

'1 Claims.

Our invention relates to electric discharge devices and more particularly to devices of the type commonly designated electron multipliers".

In general, our invention contemplates the provision of methods of and means for utilizing an electron multiplier of the type disclosed in French Patent 582,428 or a device of the type disclosed in the copending'application of Louis Malter, Serial No. 4,049, filed January 30, 1935 and assigned to Radio Corporation of America.

In the Malter application there is disclosed a muiti-stage electron multiplier constituted by a plurality of pairs of opposed electrodes axially disposed in an evacuated container with their faces lying in parallel planes. For convenience, the electrodes may be designated, respectively, upper" or accelerating and lower or multiplying. Preferably, the faces of the multiplying electrodes are specially treated, as by photosensitization, to enhance their ability to emit secondary electrons and a primary electron source, photo-electric or thermionic in character, is provided. In addition, electromagnetic means are provided for the purpose of causing the electrons to describe approximately trochoidal paths from each electrode to the next adjacent electrode, thus permitting the use of high accelerating potentials on the upper electrodes without the disadvantages resulting from the space charges encountered in a device constructed according to the French patent.

The ratio of the number of secondary electrons to the number of primary electrons depends, in part, upon the character of the surface and upon the potential diiference between the surface and the source of the electrons. This ratio can be made considerably greater than unity. For example, a ratio of three or more secondary electrons to one impinging electron is readily obobtainable with metallic surfaces treated in known ways and subjected to discharges at potentials of 300 to 400 volts.

If the secondary electron current, in turn, is caused to impinge with sumcient velocity upon a furtherelectrode with a suitably treated surface, the ratio of secondary emission from the second multiplying electrode may also be greater than unity. Hence, one is able to obtain with n multiplying electrodes in cascade, for example, an amplification of the original or primary modulated or unmodulated electron current equivalent to the amplification per electrode raised to the nth power. A million-fold amplification has been readily obtained in a single device.

During the course of our experiments with electron multipliers of the Malter type we have discovered certain perculiar characteristics thereof, and the principal object of our present invention is to provide methods of and means for so making use of those characteristics that a 5 multiplier may be utilized for all purposes to which usual thermionic tubes are adapted. Among the peculiar characteristics under discussion may be mentioned the following: (a) the final output current is dependent upon the 0 strength of the focusing field; (b) the device, when the potential on one or more of the multiplying electrodes is properly adjusted, has the property of negative resistance; (0) the device may be made to generate oscillations if 15 "suitable back-couplings are provided, or if a tuned circuit is interposed in the potential-supply lead connected to one of the multiplying electrodes; (d) the tube may be caused to amplify linearly or non-linearly, dependent upon the po- 20 tential applied to one or more than one of the multiplying electrodes or upon the strength of the electron-focusing field.

It is, accordingly, another object of our invention to provide a method of and means for 25 utilizing an electron multiplier for the amplification of direct and alternating currents and voltages.

Another object is to provide means whereby an electron multiplier may be utilized as an oscil- 30 lator through the use of feed-back couplings.

Another object is to provide means whereby an electron multiplier may be employed as an oscillator through utilization. of certain negative resistance characteristics thereof. 35

Another object is to provide means whereby an electron multiplier may be employed as a detector of modulated carrier waves.

Another object is to provide means whereby an electron multiplier may be utilized as a com- 40 bined detector-amplifier.

A still further object of our invention is to provide means whereby an electron multiplier may be utilized as a modulator and as a combined modulator-oscillator.

In accordance with our invention, referring now to an electron multiplier of the general type exemplified by the disclosure in the aforesaid Malter application, we attain the foregoing objects and other objects ancillary thereto by 5 making use of those characteristics of the device which influence the output current therefrom, namely, (1) its response to grid control, (2) its negative resistance characteristic, (3) the linearity or non-linearity of output current, de- 5 pending upon the potentials applied to one or more than one of the multiplying electrodes, and (4) the further characteristic that the output current is dependent upon the strength of the magnetic field which is utilized to focus the secondary electrons and to cause them to impinge upon the electrode next succeeding the electrode from which they emanate. Characteristics 2, 3 and 4, it is to be kept in mind, result from the fact that the focus of the electrons is determined by electrode-potentials and by the field strength, and also that secondary emission is a function of electrode-potential.

The novel features which we believe to be characteristic of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a view in perspective of an electric discharge device of the type described in the Malter application, portions of the container wall and the associated magnetic field structure being broken away to more clearly illustrate the disposition and mounting of the electrodes,

Fig. 2 is an end view in elevation of the device shown in Fig. 1, exemplifying the manner in which modulating potentials may be applied to the principal field coilassociated therewith,

Fig. 3 is a sectional view in elevation of a cathode-grid assembly that may be substituted into the device illustrated in Fig. 1 in lieu of the photosensitive cathode,

Fig. 4 is a graph to which reference will be made in explaining the negative resistance characteristic of the electron multiplier,

Fig. 5 is a fragmentary diagrammatic view exemplifying certain circuit connections whereby, according to our invention, an electron multiplier may be caused to generate oscillations,

Fig. 6 is a graph exemplifying the relation between output current from an electron multiplier of the type provided with a grid, and the potential applied to the said grid,

Fig. '7 is a fragmentary diagrammatic view exemplifying several alternative methods whereby, according to our invention, an electron multiplier may be caused to generate oscillations,

Fig. 8 is a diagrammatic view exemplifying the manner in which we prefer to obtain the functions of amplification of a carrier wave and modulation thereof in a single device,

Fig. 9 is a graph exemplifying the relation be tween final output current and focusing-coil current.

In all figures of the drawings equivalent elements are similarly designated.

Referring now to Figs. 1 and 2 of the drawings, an electron multiplier of the Malter type to which our invention is applicable is constituted by a cylindrical, evacuated container I of glass or Pyrex" wherein is disposed a plurality of electron-emissive lower electrodes 3, lying in the same plane and spaced apart along the long axis of the container, and a plurality of upper or accelerating electrodes 5 disposed in a plane parallel to and spaced from the plane in which lie the first mentioned electrodes. The lower electrodes are made of a material having high secondary emission, such as silver having a surface coating of caesiated silver oxide, and the upper electrodes of molybdenum, tantalum, nickel, or any other metal which is easily de-gassed.

The upper and lower electrodes may be grouped, vertically, in pairs. All of the electrodes may have substantially the same dimensions and, preferably though not necessarily, they may be spaced apart substantially the same distance along the long axis of the container. Such configuration is desirable in that it permits accurate electronpath-control by means of a single magnetic field.

As will be apparent from an inspection of the drawings, the upper electrode assembly is constituted by a sheet of mica 1, extending longitudinally of the container, from which the upper electrodes 5 are supported and which serves to keep them accurately spaced apart. Each electrode is provided with two straps 9 which lie in corresponding openings I I in the mica sheet, straddling a small cross-rod or wire l3 disposed against the upper surface thereof. During the operation of welding the straps to an electrode, the rod i3 is forcibly biased against the mica sheet, thus locking the associated-electrode firmly in position. Each cross-rod has a lead l5 welded thereto, which lead extends through the wall of the container to the exterior thereof. Each lower crossrod (not shown) is also provided with an output lead II.

An output electrode I9 is mounted in one end 7 of the tube, preferably fairly close to the sets of electrodes and in a plane transverse to the tube axis. The output electrode is provided with a terminal connection 2| which extends through the wall of the container to the outside thereof.

In manufacturing the device, as explained in the Malter application, the several sets of electrode sub-assemblies and the output electrode are first suitably mounted in the container which is then heated and evacuated. After evacuation, oxygen is introduced into the container at a pressure in the neighborhood of 1 mm. of mercury. One of the lower accelerating electrodes is then made substantially 500 volts positive with respect to its corresponding lower electrode to cause a glow discharge which oxidizes the surfaces of the lower electrode. The oxidation is carried on until the lower surface acquires a bluish-green tinge. Each pair of electrodes is treated in the same manner and the process is continued until all of the lower electrodes have acquired an oxide surface. This oxidation process is not at all critical as the oxygen pressure, the applied potential and the degree of oxidation-(as evidenced by the 'color obtained) may be varied within very wide limits without affecting the operativeness of the device.

After the lower electrodes are oxidized the residual oxygen is pumped out of the container and an alkali metal is distilled into'it For this.

purpose sodium, caesium, rubidium, or potassium may be utilized, although we prefer caesium. The container is next baked for aboutten minutes at a'temperature of 210 C. which causes the alkali metal to combine with the silver oxide, thus giving rise to a highly photosensitive surface. During the baking step the excess caesium is pumped out of the device.

During the heating process just mentioned any caesium or other alkali metal which is deposited upon the upper electrodes or upon the container walls is driven off and whatever caesium is not removed by the pumping may be taken up if desired by a small amount of lead oxide, which, though not shown in the drawings, may be introduced into the container at'the time the sets of electrodes are mounted therein. The lead oxide forms a relatively stable compound with the excess caesium and prevents it from being redeposited on the inner walls of the container here it would provide leakage paths between the electrodes.

Any convenient means may be utilized for establishing a magnetic field parallel to the electrode surfaces, such, for example, as the device partially shown in perspective in Fig. 1 and in end elevation in Fig. 2. Preferably, this device is constituted by an U-shape element 23 oi magnetically permeable material on which is mounted an energizing coil 25 and to each upstanding portion of which is amxed a plate 21, also of permeable material. The tube is disposed between these plates in such position that a substantially uniform magnetic field is set up parallel to the opposed surfaces of the sets of electrodes. Any convenient source 29 of unidirectional potential may be used to supply field current to the coilv 25 and,.since the device is somewhat critical in its response to the magnetic fiux, a variable resistor 3| preferably is included in the current supply connections. Obviously, a permanent magnet may be substituted for the electro-magnet shown, or the tube may be supported within a coil of wire carrying an electric current.

It is to be understood, of course, that a thermionic primary source of electrons may be substituted for the photosensitive cathode 3, shown in Fig. 1 as farthest removed from the output electrode l9. For that purpose, the container may be provided with a depending neck portion (not shown) in which is sealed a press of the usual type which supports a virtual source of electrons in the plane of the multiplying electrodes.

Referring to Fig. 3 of the drawings, the electron source may be constituted by a metallic thimble 33, the upper end of which carries a layer 35 of electron emissive oxides. The thimble is completely surrounded and shielded by a cylindrical metallic grid structure 31 which terminates in a perforated cap 39. The upper face of the cap lies in the plane of the multiplying electrodes 3 and, preferably, the perforation therein is coaxial with the emitting portion of the cathode and is covered by a fine screen 4! to which it is electrically connected. The grid structure, when supplied with proper potentials, either direct or fluctuating, serves to control the emission from the thermionic cathode in the same way as emission from the first photosensitive lower electrode 3 in the device shown in Fig. 1 may be caused to be controlled by variations in the light impinging thereon. A cathode heater 43, of any suitable and well known type, is supported within the thimble 33. 1

Attention should now be directed to Fig. 4 of the drawings which exemplifies the relation between output current from an electron multiplier, of either the thermionic or photosensitive cathode type, and the potential applied to a multiplying electrode with respect to the electrode immediately preceding it in the series. From that figure it will be noted that, in the particular device under discussion, potentials from zero to approximately 320 volts positive cause an increase n the output current and potentials in excess of approximately 320 volts cause a decrease in the utput current. We are not at all prepared to state the exact reason why the device should Function in the manner described but it is obvious hat, in the region from 320 to 640 volts, it exiibits the phenomenon of negative resistance. We are of the opinion, however, that both secndary emission from the electrode and the focusing of the emitted electrons upon the next succeeding electrode are influenced by the electrode-potential.

In the case of a tube of the grid-controlled type the output current is reasonably proportional to the input potential applied to the grid, although, as will be noted from Fig. 6 of the drawings, the characteristic curve has a rather decided bend in the region between -2 volts and 7 volts. When utilized as an amplifier, therefore, it is desirable to so bias the grid that the device, provided other potentials hereinafter to be referred to are properly adjusted, will function linearly.-

Another of the peculiar characteristics of the electron multiplier to which reference has previously been made is exemplified by Fig. 9 of the drawings. That is to say, the tube is quite critical toward focusing field strength. The characteristic curve comprises a number of minor peaks and valleys with a principal peak occurring when the focusing coil current is of the order of 65 milliamperes. This characteristic is also important and, although it may be utilized for many purposes, we prefer to make use of it for the purpose of modulation. We are not entirely prepared at this time to advance any theory with regard to the erratic shape of the curve but it may be caused in part by secondary electrons missing the next adjacent target at certain field strengths and by their impinging upon a further target as the field strength is altered. Irrespective of theory, however, if the main focusing coil current or the current in an auxiliary focusing coil (not shown), for example, be so adjusted that the final output current from the tube has an amplitude corresponding to any straight portion of the curve, modulation may be obtained by varying either field current above and below the said value.

Referring once more to Fig. 2 of the drawings, if the device is to be utilized as a combined amplifier-modulator through use of the field-current characteristic under discussion, the modulating potentials may be impressed across the field coil through a plurality of blocking condensers from any given source 41. i

It is also feasible to serially interpose a source 49 of modulating potentials in the current supply connections to the field coil. This latter procedure we do not believe to be quite so satisfactory as the parallel connection and, for that reason, it has not been illustrated in more detail.

Again, we may make use of the characteristic exemplified by Fig. 4 of the drawings for the purpose of so introducing modulating potentials into the device that it serves as a combined amplifiermodulator. Fig. 3 of the drawings, it is desirable to maintain each of the multiplying and accelerating electrodes positive with respect to the primary electron source and with respect to any electrode between it and the said source. Accordingly, the cathode 33 may be connected to the negative terminal of a source of unidirectional potential, exemplified in the drawings by a resistor 5i, and the grid or control element 39 may be connected to the same terminal through an input impedance devic such as a resistor 53 and a source of controllable bias exemplified in the drawings by a battery 55 shunted by a potentiometer 51. The output electrode i 9 may be connected to the positive terminal of the source through an output device such as a resistor 59 or the like.

The accelerating electrodes preferably are connected to suitable intermediate points on the resistor, as shown in the drawings, for the purpose For this purpose, referring now to of drawing ofl the electrons emitted from the primary source, or cathode, and from the several multiplying electrodes. Also, it might be said in passing,- that the number of pairs of electrodes, each pair constituting an amplifying stage, may be increased indefinitely, depending upon the degree of amplification desired or upon the number of different functions it is intended that the device shall have.

When the device is connected as described, if potentials, for example at radio frequency, are impressed across the input resistor 53, greatly amplified output potentials will appear across the output resistor. For the purpose of modulating the output current advantage may be taken of the fact that the amplitude of the final output current is a function of the potential of the first multiplying electrode with respect to the electron source, as exemplified by Fig. 4. We prefer to maintain such potential at a point corresponding to one of the straight portions of the curve shown in Fig. 4 and to cause it to vary above and below such potential by the modulating current. For this purpose we may introduce the modulating potentials across a resistor 6| connected between the first multiplying electrode and the direct current source. It is also possible to introduce such potentials across similar resistors (not shown) connected between one or more than oneof the succeeding multiplyingelectrodes and the source.

Modulation may also'be obtained by providing an auxiliary magnetic field.which affects the electron stream between any one multiplying electrode and the adjacent electrode. That modification also is deemed to be so clear to one skilled in the art as to not require illustration.

For the purpose of causing an electron multiplier to generate oscillations, the output electrode may be coupled back in any desired manner to the input electrode. Such coupling, as shown in Fig. 5, may be had by replacing the input and output resistors by suitable inductors 61 and 69 coupled to each other magnetically.

Also, as illustrated in Fig. 'I, the output electrode may be connected back to the control electrode or a preceding multiplying electrode or to one of the multiplying electrodes through a series tuned circuit ll-13 and a blocking condenser 15 in a manner somewhat analogous to the well-known ultra audion circuit which is often used in connection with thermionic tubes. Furthermore, feed-back may be obtained by causing the output current to affect the focusing, as by including a portion of the focusing coil in the output circuit or by utilizing an auxiliary focusing coil connected directly into the output circuit.

In utilizing an electron multiplier as a combined demodulator-amplifier the potential on the first multiplying electrode 3 may be adjusted to such value that the stage including that electrode will function on the curved portion of the graph shown in Fig. 4. In the specific example given, such potential might be of the order of 560 to 640 volts positive with respect to the cathode. Under such circumstances an incoming modulated carrier wave impressed across the input electrodes would be demodulated in the first stage and greatly amplified in the succeeding stages.

Also, in accordance with our invention, a single electron multiplier maybe utilized to amplify the incoming modulated signal impressed across the input circuit between the grid and cathode and to mix the amplified signal with oscillations produced within the device in order to provide a beat frequency which may be demodulated in the manner previously described in one of the stages and thereafter amplified in succeeding stages to appear across the output device. In fact, through suitable choice of the number of stages and of the potentials applied thereto, we have found it possible to utilize a single electron multiplier for a very large number of purposes, the complete illustration of which at this time would be almost impossible. In order, however, that our invention shall be clearly apparent to those skilled in the art it may be recapitulated as follows:

An electron multiplier of the type under discussion may be utilized as an amplifier by impressing the potential or potentials to be amplified across its input circuit. It may be utilized as a modulator by suitably choosing the potential applied to one of the multiplying electrodes and by causing this potential to vary up and down in accordance with the modulating signals. It may be utilized as a demodulator by so choosing the potential applied-to one of the multiplying electrodes that the current to such electrode is non-linear. We may also cause an electron multiplier to generate oscillations by suitably coupling the input circuit to the output circuit or by intercoupling one of the multiplying electrodes with a succeeding multiplying electrode in ways well-known to those familiar with oscillation generators of the type utilizing thermionic tubes.

From the foregoing it might be inferred that our invention is limited to an electron multiplier of the type wherein the primary electron source is thermionic in character. Such is not the case, however, since a device of the type shown in Fig. 1 is capable of any of the hereinbefore mentioned functions. That is to say, if the cathode or primary source is photo-electric in character a control-grid may be supplied or the photosensitive cathode may be excited at high frequency by a suitable light source, not shown, to provide an amplified carrier current and modulating potentials may be introduced, as exemplified by Fig. 8 of the drawings, or in any other of the ways to which we have referred.

It is also possible, if the light source is steady or non-fluctuating, to cause the device to generate oscillations by means of suitable couplings between the input circuit and the output elec trode, as for example by means of an inductor interposed between the grid and the D. C. potential source, which inductor is preferably associated with an inductor in the output circuit. Oscillations may also be generated by interposing a tuned circuit, exemplified in Fig. 8 by the inductor 63 and the shunting capacitor 65, between one of the multiplying electrodes and the direct current source.

It will be apparent from the foregoing that we have by our invention provided means whereby an electron multiplier may be utilized for substantially any purpose for which thermionic tubes are adapted.

Although we have shown and described only a few specific embodiments of our invention we are fully aware of many modifications that will at once be apparent to those skilled in the art to which it pertains. Our invention, therefore, is not to be limited except insofar as is necessitated enemas by the prior art and by' the spirit of the pended claims.

We claim as our invention:

1. A container having a major axis, a plurality of spaced apart sets of discrete electrodes disposed within the container, the electrodes of one set being paired with the electrodes of the other set and the electrodes of each set being substantially uniformly spaced apart along and parallel to the said major'axis, an output electrode disposed within the container adjacent the end electrodes of the several sets, at least one electrode of each of said pairs being capable of secondary emission upon bombardment by electrons emitted by an acfiacent electrode, means for applying suitable potentials to said electrode, whereby said device may be caused to exhibit the phenomenon of negative resistance, and a tuned circuit interposed between at least one of said emissive electrodes and the potential source, whereby the system is rendered capable of selfoscillation.

2. In combination, annelectric discharge device constituted by a container in which are disposed a set of secondary electron-emissive electrodes and a set of accelerating electrodes, the electrodes of one set being spaced apart from the electrodes of the other set, connections for impressing static potentials between the electrodes of one set and the electrodes of the other set, means for causing magnetic flux to bathe said electrodes, the magnetic field being substantially normal to the electric field whereby electrons from said emissive electrodes are prevented from traveling to the electrodes of the other set under the influence of said electric field, a source of signal modulated potential, and means for causing the magnetic field to vary in amplitude in accordance with variations in said modulated potential.

3. An electron multiplier comprising a source of electrons, a secondary electron emitter and an output electrode mounted in spaced relation and in the order named within a sealed envelope, means for causing the electrons to flow in arcuate paths from said source to said emitter and in similar paths from said emitter to said output electrode, a source of high frequency voltage coupled to said multiplier for varying said electron stream in accordance with the instantaneous amplitude of said high frequency voltage, a source of modulating potential, means for impressing said modulating potential upon said first mentioned means to vary the radii of curvature of the electron paths and thereby cause some of the electrons to impinge upon-and some tomiss the electrode toward which they are directed, whereby the output current from the device is modulated high frequency current.

4. An electron multiplier comprising a source of electrons, a secondary electron emitter and an output electrode mounted in spaced relation and in the order named within a sealed envelope, a

' source of constant potential, connections between the source and said electrodes, respectively,

means including the potential impressed upon said emitter for directing the electrons in arcuate paths from said source to said emitter and in similar paths from' said emitter to said output electrode, a source of high frequency voltage coupled to said multiplier for varying said electron stream in accordance with the instantaneous amplitude of said high frequency voltage, a source of modulating potential, and means for superimposing said modulating potential upon the constant potential applied to said emitter electrode to vary the radii of curvature of the said electron paths and thereby cause some of the electronsto impinge upon and some to miss jthe-electrode toward which they are directed, whereby the output current from the device is modulated high frequency current.

5. Method of operating an electron multiplier having a cathode, a plurality of secondary elec tron emitters and an anode mounted in spaced relation and in the order named within a sealed envelope, said method comprising so directing the electrons that they travel in curvilinear paths from their electrode of originto the next succeeding electrode, causing the primary stream to vary in amplitude at a high frequency simultaneously applying a varying potential to an electrode intermediate the cathode and anode to alter the radius of curvature of the electron paths whereby some of the electrons impinge upon the electrode toward which they are directed and some fall within the space between the electrodes, whereby a modulated output current is obtained having a frequency corresponding to the variation of the primary stream.

6. Method of operating an electron multiplier having a cathode, a secondary electron emitter and an anode mounted in spaced relation in the order named within a sealed envelope, said method comprising subjecting the device to a magnetic field whereby to cause the electrons to travel in arcuate paths from said cathode to said secondary-electron emitter and in similar paths from said emitter to said anode, causing the primary electron stream from said cathode to vary in amplitude at a high frequency, and simultaneously varying the intensity of said magnetic field at a desired modulating frequency whereby the radius of curvature of the electron paths is varied and some of the electrons impinge. upon the electrode toward which they are directed and some fall within the space betweensaid electrodes, whereby a modulated output current is obtained having a frequency corresponding to the variations of the primary electron stream.

7. Method of operating an electron multiplier having a cathode, a plurality of secondary electron emitters and an anode mounted in spaced VLADIMIR K zwomrxm. LOUIS HALTER.

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