US2231697A - Electron multiplier - Google Patents

Description

Feb. l,` 1941. v. K. zwoRYKlNv ETAL 2312697 ELECTRON MULTIPLIER Filed Jan. 29, 1958 2 Sheets-Sheet 1 :Summers Feb. 11, 1941.

V. K. ZWORYKIN ETAL ELECTRON MULTIPLIER Filed Jan. 29, 1938 2 Sheets-Sheet 2 Patented Feb. 11, 1941 UNITED STATES PATENT OFFICE ELECTRON MULTIPLIER poration of Delaware Application January 29, 193s, serial No. 187,634

7 Claims.

This invention relates to electron-multipliers (i. e., electron discharge tubes of the type wherein amplication of a primary electron stream, such, for example, as is emitted by a thermionic cathode or by a photo-sensitive surface exposed to light, is accomplished through utilization of the phenomenon of secondary emission), and has special reference tothe provision of improvements in electron-multipliers of the type operable without the use of auxiliary electron-lens systems. Discharge tubes of the class to which.

the invention particularly relates are well exemplied in U. S. Patents Nos. 2,125,750 an 2,147,173 to Edward G.v Ramberg. The electron-multipliers disclosed in the aboveidentied applications comprise a highly evacuated cylindrical envelope or tube containing a cathode, an anode, and a plurality of multiplying electrodes mounted in spaced relation on opposite sides of the long axis of the tube, intermediate the cathode and anode. The multiplying electrodes are treated with a substance which is the equivalent of caesium to enhance their ability to emit secondary-electrons. When the multiplier is a multi-stage device difculties may arise in activating the secondary-electron emitters. That is to say where, as is usually the case, the electrodes are activated after being mounted in the tube, the amount of caesium deposited upon the electrodes close to the point at which the caesium vapor is admitted intor the tube may be greater and perhaps more uniformly deposited than it is upon the more remote electrodes. 35 Accordingly, an object of the present invention is to provide an electron-multiplier which, by reason of the novel design and arrangement of the parts, permits of a more uniform activation of the emissive electrodes.

Another object of the invention is to provide an electron-multiplier which is smaller and more compact than those heretofore available and this, too, without any sacrifice in operating performance.

Another object of the invention is to provide a duplex electron-multiplier wherein the duplicate parts may be connected in parallel or in push-pull.

Another object of the invention is to provide 50 an electron-multiplier having a greater useful current output than prior art electron-multipliers of similar over-al1 dimensions.

Other objects and advantages, togetherwvith certain details of construction and mode of op.-A 55 eration, will be apparent'and the invention itself will be best understood by reference to the following specification and to the accompanying drawings, wherein Figure l is a View in perspective of a photosensitive electron multiplier constructed in accordance with the principle of the invention,

Figure 2 is an end plan view of the electron multiplier of Fig. 1, connected in circuit and exemplifying the manner in which the several electrodes are energized when the device is uti- 10 lized for certain of the purposes to which it is adapted,

Figure 3 is'an enlarged fragmentary end View of a controlled thermionic cathode which may be substituted for the photosensitive cathode of 15 Figs. l and 2,

Figure 4 is an end View, with the mounting discs removed, of a duplexfelectron multiplier device comprising two grid controlled electron multipliers utilizing a common cathode, a common control grid, and a common accelerating grid,

Figure 5 is a diagrammatic view of the device of Fig. 4 in circuit, here, the grids,v multiplying electrodes and anodes are connected, respectively, in parallel,

Figure 6 `is an enlarged end view of a cathode grid assembly which may be substituted for that shown in Fig. 4 when the device of that drawing is to be used in a Push-pull circuit, and

Figure 7 is a diagrammatic View of the duplex electron multiplier device of Fig. 4, but employing the split or bipart control grid of Fig. 6, showing the grids and anodes connected in pushpull.

, Like reference characters designate the same or corresponding parts in allr figures.

In Fig. 1, I designates a highly evacuated cylindrical envelope or tube within which two spaced insulating, electrode-supporting discs 2 and 3 are mounted, preferably with their outer edges in contact with the inner surface of the tube wall, in order to obviate microphonic vibrations such as might'occur if the electrode assembly was supported solely by the electrode leads 4. The electrodes supported by discs 2 and 3 comprise a photosensitive cathode 5, an anode or collecting electrode 6 and a pair of sets of multiplying electrodes mounted in spaced staggered relation on opposite sides of an endless cylindricalsurface a section through which comprises a curved arcuate median, line 1 (Fig.v 2) which extends around the central axis 8 of the tube between the cathode 5 and anode E. 55

Small anges 9 secure the electrodes to the discs 2 and 3.

In Figs. 1 and 2 the multiplying electrodes constituting the outer set are numbered I0, I2, I4, I6, and I8, respectively, and those constituting the inner set are numbered II, I3, I5, I'I and I9, respectively. All of the electrodes in a given set are preferably of duplicate construction and have concave surfaces facing the curved median line ,'I. The concave or dished" surfaces need not constitute true arcs of a circle but may, as shown more clearly in Fig. 2, be of modified L-shape contour. As indicated by the dotted lines, 20, Fig. 1, the generatrices of all of the multiplying electrode surfaces are parallel with'each other and with the central axis 8 of the tube. These emissive surfaces may also be described as being normal to the plane of the curved median line 'i which extends between the cathode 5 and anode 6.

The spacing between the two sets of multiplying electrodes is preferably, but not necessarily, substantially equal to three-fourths the distance between corresponding points (say, the midpoints) on adjacent electrodes of the outer set. The concave surfaces of the multiplying electrodes of the outer set are not directly opposite those of the inner set but are preferably somewhat off-set, with respect thereto, in the anode direction. Thus, the electrons leaving a given multiplying electrode advance in the anode direction in traveling to the next higher-numbered electrode. The cathode is provided with an apertured extension 5a which extends toward, but does not touch, the first multiplying electrode I0. Such construction ensures a desired distribution of the electrostatic eld adjacent these electrodes (5 and I) when the device is energized. The same result is achieved adjacent the output of the multiplier by mounting the anode 6 with its electron-collecting surface presented to, but not touching, the terminal edges of the multiplying electrodes I8 and I 9. The operation of the device is improved when the surface of the anode is of the modified S-shape contour shown in Figs. 1 and 2. A strip 2I of mica or the like, supported on the discs 2 and 3, in the space adjacent the side of the anode which is nearer thev cathode, serves to prevent photoelectrons from being drawn directly to the anode.

Referring particularly to Fig. l, a hollow rod 22 is provided for admitting caesium or the like activating vapor into the tube during its manufacture. This hollow rod or conduit extends through and is supported by the press 23 of the tube I; it extends along the axis 8 through a suitable orice in disc 3 and is provided with a. number of perforations 24 on that part of its surface which lies between the discs 2 and 3. The perforations 24 are so arranged that the activating material passing through the hollow rod is directed outwardly at numerous points throughout the length of the electrode assembly so that no one portion of an electrode receives more or less of the activating material than another portion.

In manufacturing the electron-multiplier device of Fig. 1, the electrode-plates supported between the discs 2 and 3 are first suitably mounted in the container I which is then heated and evacuated. After evacuation, oxygen is introduced into the container at a pressure in the neighborhood of 1/2 or 1 m. m, of mercury. A11 of the electrode plates of each set are then tied together and a source of high frequency cur-v rent connected therebetween to cause a glow discharge which oxidizes the silver surfaces of the emitter electrodes. The oxidization is continued until the electrode surfaces acquire a bluish-green tinge.

After the electrodes are oxidized, the residual oxygen is pumped out of the container and an alkali metal is distilled into it. Sodium, caesium, rubidium or potassium may be utilized for this purpose, caesium, however, is preferred. The container is next baked for about ten minutes at a temperature, of say, 210 C., which causes the alkali metal to combine with the silver oxide, thus rendering the electrode surfaces capable of copious electron-emission. During the baking step, the excess caesium is pumped out of the container.

During the heating process above mentioned, any caesium or other alkali metal which is deposited upon the container walls or upon the electroden leads is driven off and whatever caesium is not removed by the pumping operation may be taken up if desired by a small amount of lead or tin oxide which, though not shown in the drawings, may be introduced into the container at the time the electrode assembly is mounted there- .in The lead or tin oxide forms a relatively stable compound with the excess caesium and prevents it from being redeposited on the press of the container where it would provide leakage paths between the electrodes.

As in the case of the Ramberg electron multiplier, the multipliers of the present invention are operable without the use of an auxiliary magnetic field. To ensure optimum performance, the potential distribution must be substantially that expressed by the mathematical series IV, 2V, 3V. 4V, 5V, 6V, etc., where IV is the potential drop between the primary electron source and the first target or multiplying electrode, and 2V, 3V, 4V, etc., represent the potential drop between the respective succeeding electrodes, in point of electron travel, and said source.

Referring to Fig. 2: For the purpose of providing such a potential distribution, the cathode 5 may be connected to the negative terminal V of a direct current source exemplified in the drawing by a resistor R, and the first multiplying electrode, i. e., electrode I0, connected to a point IV somewhat more positive. The other electrodes Ill to I9, inclusive, in the order of their numbers, and the anode 6, are shown connected to successively more positive points 2V to I IV on the resistor.

The reference characters IV, 2V, 3V, 4V, etc., given to the several points on resistor R will be understood to indicate .that the voltage drop between a given electrode and the cathode is the designated whole number multiple of the drop existing between the cathode 5 and the first multiplying electrode I0. Thus, in the tubes of both Figs. 1 and 2, where the potential drop between the first multiplying electrode IU and cathode 5 is 100 volts, the drop between electrodes II and 5 should preferably be 200 volts; that between electrodes I2 and 5, 300 volts.

If a beam of light (from a source exemplified in Fig. 2 by a lamp 25 and lens 25) say of varying intensity, is caused to impinge upon the cathode 5, photoelectrons will be emitted in aA quantity determined by the instantaneous intensity of the light beam. These photoelectrons will be accelerated toward the first outer electrode I and, because of the described design, relative arrangement, and voltage distribution, will impinge upon thisA firstmultiplying electrode.4 The photoelec trons striking electrode I I) will cause theemission of secondary electrons, the number of secondary electrons emitted being dependent, in part at least, upon the magnitude of the potential between it and the cathode.

The next electrode in point of electron travel is the ilrst inner multiplying electrode I I. The trajectory of secondary electrons from the multiplying electrode I0 is such that they impinge upon the curved or cupped surface of this second multiplying electrode I I. Here again, a multiplication, by reason of secondary'emission, is secured, and this is repeated in any number of stages until the amplified stream of secondary electrons is collected upon the output electrode or anode 6 and caused to vflow in a utilization circuit arnpliied in the drawings by the resistor 1* included between the output electrode 6 and the positive terminal I IV of the potential divider.

A controllable primary-electron source may be provided, if desired, instead of the photo-electric source of Figs. 1 and 2, for the purpose of rendering the multiplier capable of uses to which well-known thermionic tubes are put. In Fig. 3 is shown a cathode-grid assembly adapted to be supponted between the mica discs in the same relative position, with respect to the multiplying and output electrodes, as the photo-sensitive cathode of the earlier described embodiment of the invention. Here the electron source comprises a metallic shell 30, the outer surface of which has a layer 3| of electron-emissive oxides thereon. If desired, the emissive coating may be conned to that portion of the shell which faces the first multiplying electrode II). Heat from a filament 32, mounted within the shell serves to release thermionic electrons from the oxide layer.

As in the device of Figs. 1 and 2, the otherwisev open entrance of the multiplying electrode assembly is provided with a conductive shield, here numbered 33, which is connected electrically to the cathode to ensure a desired distribution of the electrostatic eld thereabouts.

This shield 33, it will be observed, partially surrounds the cathode assembly so that the primary-electrons leaving it must travel substantially curved paths, indicated by the dotted lines, in passing to the first multiplying electrode I0. This construction has been adopted because it has been found that unless the rst multiplying electrode is at least partly shielded from the physical (as distinguished from electrical) particles or by-products thrown off by the thermionic cathode when it is energized, the secondary-electron emissive surface of the target I0 may be damaged. These particles, as far as can be ascertained, possess no electrical charge and hence will ordinarily not follow the curved paths of the electrons, though, indeed, they may impinge upon the leading edge portion of electrode IIJ.

One or more grids 34, which may be supported between the mica discs by supporting rods 35, surround theemissive portion of the cathode. The grid structure of Fig. 3 when supplied with proper potentials, either direct or fluctuating, serves to control the emission from the thermionic cathode 3i) in the same way as emission from the photo-sensitive cathode 5 in the device shown in Figs. 1 and 2 is caused to be controlled by variations in the light impinging thereon.

The device of Fig. 4 may be designated a duplex type electron multiplier in that two complete multipliers are here shown contained in a single envelope. The cathode is of the indirectly heated type described` in` connection with Fig. 3 but is here shown mounted along the central axis of the-tube andi surrounded by both a control grid 34 and anaccelerating grid 36. Two collecting electrodes or anodes Iia and 61T are provided, one for each group or set of multiplying electrodes. The multiplying electrodes I 0a, Ila, etc., of one set ar-e mounted on opposite sides of a curved median line l@ which extends substantially 180 between the cathode 30 and the anode 6a. The multiplying electrodes Ib, IIb, etc., of the other set are mounted on opposite sides of a second curved median line 'Ib which extends substantially 180 in the opposite direction to anode 6b. The entrance to each set of multiplying electrodes is shielded by a plate 33a, 33h, respectively, connected electrically to the cathode. As shown diagrammatically in Fig. 5, the multiplying and output electrodes of the two devices are here connected in parallel. The connections, if desired, may be made within the tube by wires, not shown, traversing the insulating discs (2 and 3, Fig. 1) between which the electrodes are supported. Ordinarily, in operating the device of Figs. 4 and 5, the accelerating grid 36 is maintained at a potential 5 or 10 volts positive with respect to the cathode 30. The formula for the potential distribution among the other electrodes is the same as that described in connection with the circuit of Fig. 2, i. e., let IV represent the potential drop between the rst multiplying electrodes (of each set) and the cathode, and 2V, 3V, 4V, etc., represent the potential drop between the respectivesucceeding electrodes, in point of electron travel, and the cathode. Where, as in the instant case, the device is of the thermionic type, it is preferable to connect small by-pass condensers 31 between the electrode leads and ground in order to prevent high frequency voltages being developed upon the multiplying electrodes and influencing the control grid 34. The input circuit elements, which are exemplified in Fig. 5 by an input resistor 38, a grid biasing potential source 39 and a potential divider 40, as

well as the output circuit elements, which areexemplified by the resistor 4I and capacitor 42, may be of any suitable or convenient type.

If the duplex device of Fig. 4 is provided with a split or bipart control grid instead of a unitary one, and if the anode leads are unjoined, then these discrete terminal electrodes may be connected in push-pull. The required modification to the grid structure is shown in Fig. 6 wherein (as in Figs. 3, 4 and 5) 30 designates the oxide coated cathode andl 34a, 34h represent, respectively, two control grids, one of which is presented to the entrance of one multiplier section and the other, to the entrance of the other multiplier section. 'I'hese control grids 34a and 34D are surrounded by an accelerating grid 36. Fig. 7 shows the tube of Fig. 4 as modified by the substitution of the grid assembly of Fig. 6, connected in a push-pull circuit. Here the grids 31|a and 34h are connected, respectively, to opposite terminals of an input resistor 38, the midpoint of which is connected to the cathode 30 through a suitable biasing source exemplified in the drawings by the adjustable direct current source 39, 40. As in the earlier described embodiments of the invention, the shields 33a, 33b are connected to the cathode and the accelerating grid 36 is maintained a few volts positive with respect to the cathode. The multiplying electrodes Illa, Ile, lub, IIb, etc., are connected in parallel, either within the tube or externally, and

are maintained at the relative potentials previously described. The anodes 6a, 6b are separately connected to the opposite terminals of an output resistor. A lead 6W connected between the positive side of source R and the midpoint of the output resistor 4l supplies energizing current to the separate anodes.

One advantage of the duplex tubes and circuits exemplified in Figs. 4 to '7, inclusive, is that the useful output current may be substantially twice that of a device of similar dimensions employing but a single output electrode. That this is so will be apparent when the factors limiting the current output in any electron multiplier are recalled. These factors are v(l) current density, i. e., the number of electrons impinging per unit of surface area. If the number of impinging electrons is too great, defocusing or spreading of the electron beam may occur. Further, negative or space charge caused by too great a concentration of the impinging electrons, and released secondary electrons, inhibits copious secondary electro-n emission. (2) The second limiting factor -is the radiating area available to dissipate the heat generated by the impact of the multiplied stream of electrons upon the last multiplying electrode and upon the anode.

Since the target area at any given distance from the cathode in the duplex devices is substantially twice that available in the usual single tube of k similar overall dimensions, it follows that the current density at any given point in the duplex device is less likely to exceed the optimum value required for efcient operation. It also follows that z with twice the target area, heat will be dissipated more rapidly.

While several embodiments of the invention have been shown and described, it is to be understood that the disclosure is tobe interpreted as merely illustrative of the invention and not in a limiting sense, except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

l. An electron-multiplier comprising an evacuated envelope having a longitudinal axis, a cathode mountedv substantially parallel to said axis, an anode having an electron-collecting surface which extends radially outwardly with respect to said axis, and a. plurality of sets of multiplying electrodes defining an electron path between said cathode and anode, said sets of multiplying electrodes being mounted in substantialy circular array at different radial distances from said longitudinal axis.

2. An electron-discharge device comprising an envelope containing a plurality of electrodes mounted in substantially circular array along and about a common axis, and a hollow rod constituted of insulating material extending along' said axis, said rod being provided with an opening communicating the interior of said envelope and through which an electron-emissive coating material may be applied to said electrodes.

3. An electron-discharge device comprising an envelope containing a plurality of electrodes mounted in substantially circular array along and about a common axis, and a hollow rod constituted of insulating material extending along said axis, said rod being provided with a plurality of perforations arranged circumferentially therealong and communicating with the interior of said envelope whereby substantially all points on the electrode surface area of each electrode are equally accessible to an activating substance entering said envelope through said hollow insulating rod.

4. An electron multiplier comprising an evacuated envelope having a longitudinal axis, a cathode mounted along said axis, a plurality of sets of secondary-electron emissive electrodes mounted on opposite sides of said cathode with the electrodes of each set defining a curved electron conduit partially surrounding said cathode and having an open end accessible to electronsy therefrom, and an anode mounted at that end of each conduit which is remote from the point of electron entry.

5. An electron multiplier comprising an eva-cuated envelope having a longitudinal axis, a cathode mounted along said axis, a plurality of sets of secondary-electron emissive electrodes mounted on opposite sides of said cathode with the electrodes of each set deining a curved electron conduit partially surrounding said cathode and havingan open end accessible to electrons therefrom, an anode mounted at that end of each conduit which is remote from the point of electron entry, a control grid mounted between said cathode and the entrance to each of said electron conduits, and a separate lead for each of said grids whereby they may -be connected in push-pull.

6. An electron multiplier comprising an envelope containing a cathode and an anode, there being an endless imaginary cylindrical surface entirely within said envelope, a part of which cylindrical surface lies between said cathode and anode, and a plurality of dished multiplying electrodes mounted in staggered relation on opposite sides of said cylindrical surface.

7. An electron multiplier comprising a cathode, an anode mounted adjacent said cathode, a shield between said cathode and anode, and a plurality of multiplying electrodes having dished electronemissive surfaces mounted in staggered relationl i on opposite sides of an arcuate median line which lies between said cathode and anode and extends around an end of said shield.

VLADIMIR K. ZWORYKIN. RICHARD L. SNYDER, JR.

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