US2249016A - Electron multiplying electrode - Google Patents

Description

Patented July 15, 1941 ELECTRONMULTIPLYING ELECTRD Hans Gerhard Lubszynski, Hillingdon, and James Dwyer McGee,-Ealing, London, England, assigners to ElectricA & Musical Industries Limited, Hayes, Middlesex, England, a `company of Great Britain ApplicationFebruary 8, 1939, Serial No. 255,206 In Great Britain November 24, 1937 1 (Cl. Z50-175) 6 Claims.

The present invention relates to electron multiplying electrodes in electric discharge apparatus. M-ore especially the invention relates to electron multiplying electrodes having surfaces adapted to emit secondary electrons when bombarded by primary electrons and from which the secondary electrons are drawn off in substantially the same direction as that in which the primary electrons arrive.

rlhis invention relates to an improvement of the grid structure within a tube of the type disclosed in British Patent No. 457,493, in which secondary electron multiplication of an electron image is utilized to generate television signals.

One object of the present invention is to provide an image electron multiplier utilising the phenomenon of secondary electron emission in which the stream of emitted electrons passes in substantially the same direction as the stream of incident primary electrons, the stream including the emitted electrons being constituted by electrons. all travelling with substantially the same velocity.

Another object of the invention is to provide a target electrodeffor use in van'fimage electron multiplier, said electrode'being4 eiectively impervious to primary electrons -incident upon it, whilst allowing the passage of electrons emitted within it.

A still further object of the invention is to provide a multi-grid target electrode for use in an image electron multiplier, the grid being so formed as to provide several stagesofY multiplication.

According to one feature of the invention an image electron multiplier is provided comprising a source of electrons, -atarget electrode of grid form adapted to emit secondary electronsy under impact by primary electrons, means for deriving from said source a composite beam of electrons and focussing said beam on to said target electrode with such velocity as to cause the emission of secondary electronsgmeans for withdrawing secondary electrons from said target electrode in the same general direction as that in which primary electrons arrive thereat, and means for focussing on a receiving memberthe secondary electrons emitted from-said target electrode, the arrangement being characterised by the arrangement of said targetv electrode in such manner that substantially all'the electrons from said source are intercepted by secondary electron emitting surfaces.

In one aspect, the invention includes a target electrode` for use in an image electron multiplier,

said electrode comprising a plurality of elements of material adapted to emit secondary electrons under impact by primaryV electrons,` and to alTord passage for the emitted'secondary electrons in vthe same vgeneral direction as that with' which the primary electrons arrive, the arrangement of elements being 'suchthat no rectilinear path is afforded through the grid, whereby incident primary electrons are substantially wholly intercepted by the emitting'elements. Thus, in one embodiment of the 'invention the target electrode may comprise ay rst'emittinggrid, and at least one additional grid similar to the iirst grid but arranged with `itsvelements inV staggered or transversely displaced'relaton'to the elements of the rst grid, and maintained at a higher positive potential, the arrangement being such that any primary electrons which pass through the interstices of the rst grid are intercepted by the elements of at least one of the additional grids', secondary electrons emitted by any of the grids except the last being intercepted by the succeeding grid. The arrangement may also include an electrode or withdrawing grid having its elements aligned with the elements of the -rear target electrode in the' shadow thereof, this further electrode being maintained at a positive potential with respect tothe said elements in order to draw the-relatively slow-speed electrons released from the target electrode through the interstices thereof. The electrons so drawn through the target electrode may be vdrawn into an accelerating and focussing eld for projection on to a further target electrode or onto a mosaic or uorescent screen or the like.

In accordance with other embodiments ofthe invention, a target electrode may be provided having a grid structure of Vwhich the'el'ements arel arranged so that their projections at right angles to the direction'of the incident primary electron beams overlap, so that the primary electron beam is thereby completely intercepted. In

some cases, according to the invention, a target electrode may be provided wherein elements of the electrode'are so formedfthat surfaces from which no useful secondaryemission takes place when the electrode'is in use, are constitutedv by coatings adapted to be charged or become charged, when the electrode is in use, to `such a potential as to prevent or tend to prevent the re-absorption of secondary electrons emitted from one-element at said surface of a further element.

Thev nature of the invention and-the manner in which the invention may be carried into veffeet, will be clearly understood from the following description, reference being made to the accompanying drawing in which:

Figures 1 to 3 show various constructions of target electrodes according to the invention, and Figure 4 shows a modification of the electrode shown in Figure 2.

Referring to the drawing, in particular Figure 1 thereof, it will be seen that the target electrode comprises three grids I, 2 and 3 respectively, arranged one behind the other, the wires of each grid being wider than the interstices between them, and the wires of grid 2 being located centrally with respect to the interstices between the wires of grid I and the wires of grid 3 being arranged directly behind the wires of grid I. Each of the grids I. 2 and 3 is formed of wires shaped in the manner described with reference to and shown in Figure 4 of British patent specification No. 457,493, each wire of the grids having for example a cross section approaching a parabola or a semiellipse or a triangle with its apex facing the incident primary electrons. Behind the last multiplying grid 3 is a withdrawing grid 4 which is preferably arranged in the shadow of the wires of grid 3. Grids 2, 3 and 4 are preferably maintained at successively increasing positive potentials with respect to grid I as indicated by the connections to the battery 5 shown at the bottom of the figure, the difference in potential between each grid and one on either side of it usually being of the order of 50 to 300 or more volts. If desired a cylindrical electrode is arranged behind -the last grid, which can be held at the same or preferably at a negative potential in respect to the last grid 4, in order to make the equipotential planes of the field in front of the grid substantially flat or converging, so as to concentrate the electron beam for example,V as indicated by the path commencing at the arrow 6 in the drawing, which arrow represents the direction of a primary electron incident on one of `the wires of grid I. Secondary electrons emitted in this way will pass through `the interstices between the wires of grid I and will be directed therethrough substantially on tol the 'crowns or apices of the wires of grid 2 with a velocity depending upon the difference of potential between the grids I and 2. This potential is chosen such that as a result of the bombardment of the wires of grid 2 by electrons emitted from grid I, a favourable emission of electrons which may be termed tertiary electrons is obtained, which electrons will bombard the Wires of grid 3 to cause the emission of what might be termed Quaternary electrons which are drawn through the interstices of grid 3 by the accelerating electrode 4 arranged behind grid 3, for example, as indicated by the arrows 'I shown in the drawing. In addition to the primary electrons passing along paths such as 6, intercepted by the wires of grid I, some of the incident primary electrons will reach the interstices between the wires of grid I, for eX- ample, along paths such as indicated by arrows B in the drawing, and will pass through without being intercepted until they strike the wires of grid 2 which effectively intercept primary electrons passing grid I. Any secondary electrons emitted by grid 2, due to bombardment by primary electrons so passing through grid I, will be drawn forward in the stream of tertiary electrons and will have substantially the same velocity as the tertiary electrons so that the presence of the second electrons in the tertiary electron stream will not be obnoxious. The emergent beam of electrons obtained from the arrangement of Figure 1 can be focused on to a further target electrode or mosaic screen or other electron image utilising element as desired. At the same time an amplification equivalent to that produced by several stages can be achieved without providing a focussing arrangement between each stage, due to the fact that successive secondary electron emitting grids are arranged immediately behind each other instead of being separated in each stage with an intervening focussing arrangement as described in British patent specification No. 457,493, for eX- ample. If desired, more or less than three grids, such as grids I to 3 may be provided, the arrangement being eifective when two grids are provided, and being more effective, particularly in respect of the amplification produced, as the number of emitting grids is increased.

If desired, any or all of the grids such as I or 2 of Figure 1, for example, in a composite target electrode as described with reference to Figure 1 might have an accelerating or withdrawing electrode similar to the grid 4 formed behind it to minimise the effect of the adverse field gradient due to the presence of the more negative grid on the secondary electrons emitted from a succeeding grid such as 2 or 3. For example, any or each grid might be formed as described in the specification of co-pending application Serial No. 236,876, led October 25, 1938, this arrangement also being applicable for the last emitting grid of the target electrode, such as grid 3 and grid 4. The withdrawing grid would preferably be held at substantially the same potential as the emitting grid behind it.

An `alternative arrangement of a target electrode according to .the invention is shown in Figure 2. In this arrangement a target electrode comprising a single grid of emitting wires III is provided, the wires I0 being in the form of flat strips which may be arranged in the manner of the strips or slats of a Venetian blind, that is to say to form louvres, as shown at Il in Figure 2. The projections -of the Slat-like wires IB perpendicular to the direction of the incident beam of primary electrons indicated by arrow II, are arranged to overlap so that substantially no i primary electrons can pass directly through the grid. With this arrangement a withdrawing grid may be provided by applying on the back of the wire I I), strips of insulating material I2 on which a continuously conductive surface of metal or other conductive material is coated as indicated at I3 for example by evaporation or sputtering.

It is also possible, according to the invention, to use a single grid of wires shaped for example in a manner simlar to 'those shown in Figure 1 this grid being tilted as shown at I4 in Figure 3. so that they completely obstruct the passage of primary electrons through the grid. A suitable withdrawing grid such as 4 may be arranged in any suitable way behind the grid I4. The withdrawing grid might also be arranged as described in aforesaid specification No. 236,876.

One further disadvantage of grid electrodes of the kind described with reference to Figure 4 of British patent specification No. 457,493 is that secondary electrons are emitted from the wires or elements of the grid in more or less random directions and secondary electrons emitted from x one element may fall on a neighbouring element and be reabsorbed thereby reducing the effective emission from the grid.

For example, as mentioned above, in Figure 4 of the aforesaid British patent specification No. 457,493, the secondary emitting wires or elements shown have a cross-section which is tapered somewhat towards the source of incident primary electrons. Now with such a grid it has been found that an appreciable number of the secondary electrons released from the wires or elements of the grid are arrested on elements in the neighbourhood of the emitting element. The electrons so arrested are lost as far as the multiplication process is concerned.

Now more especiahy in the case of a grid of the kind above described with reference to Figure 2 of the drawing itis readily possible to avoid loss of eective electron emission arising from the emission of electrons in random directions as described above. Thus, referring to Figure 4 of the accompanying drawing, the slat-like' emitting elements IG of the grid may each be formed on or have applied to them a backing slat or 'layer I5 of material having such work function that under operating conditions it sets itself at such negative potential with respect to the corresponding emitting element It, and accordingly, with respect to adjacent emitting elements IG, that electrons emitted in undesired directions from elements l@ are deflected in to the main electron stream as indicated by arrow s and are not reabscrbed, but are drawn into the main secondary emission stream under the inuence of the usual accelerating field set up by means of a positive grid as described in aforesaid specication No. 457,493, the elements of such a grid being shown at IB in the present drawing, or the accelerating eld may be produced by the next multiplying grid.

For example, if the emission surfaces i are formed of caesium or potassium which have a work function below 1 volt, the surfaces l5 may be formed of arsenic, gold, bismuth, carbon, platinum or tungsten or other materials having work functions up to 6.5 volts, so that the difference between the potentials of the elements I and l respectively under operation conditions would be of the order of, say, 5 volts. As the bulk of the secondary electrons is emitted at velocities below 3 volts, none of these could settle on or be re-absorbed in the back of a slat adjacent the slat from which emission takes place, and loss of electrons due to re-absorption may be at least largely if not wholly eliminated.

1f desired, the emissive material may be applied on one side of slats of material of high work function cr vice versa, or both could be deposited on opposite sides of slats of a further material before or after assembling the tube or before or after the activation process.

Another method of preventing undesired absorption of secondary electron-s is to cover the rear surface of the emitting slats or elementsy of the grid of Figure 4 .with a layer of highly insulating material. This would charge negatively with respect to the front surface of the slats under working conditions and so prevent any secondary electrons from being re-absorbed.

It will be appreciated that the method of reducing reabsorption of secondary electrons described with reference to Figure 4 is applicable to target electrodes any of the elements of which have surfaces from which no useful emission takes place and at which re-absorption occurs. Moreover, if desired, in some cases, a target electrode might be formed of elements of material adapted to acquire, when the electrode is in use, a potential such as to reduce re-absorption, said elements having layers of secondary electron emissive material applied on the surfaces from which emission of secondary electrons is required.

Also, in some cases, instead of providing surface coatings adapted to become charged as above described, conducting coatings insulated from the emissive elements and arranged to be charged by suitable means to a desired negative potential, might be used.

We claim:

1. An electrode system for a secondary electron multiplier tube comprising a planar grid electrode made up of spaced elements having interstices between them, said elements of which have each two major faces which are wider than the interstices thereof, said elements having the surfaces of one face thereof rendered secondary electron emissive, said grid being adapted to be bombarded by primary electrons on the secondary emissive surface and inclined to the normal path of said primary electrons, and a cooperating accelerating grid electrode adjacent to the other face of said first named electrode, each element of said accelerating grid being in register with a corresponding element of said rst named electrode.

2. An electrode system as claimed in claim 1 and wherein each element of said planar grid electrode has a parabolic cross-section.

3. An electrode system as claimed in claim l and comprising in addition, .an auxiliary cooperating conductive coating of high work function on said other face.

4. An electrode system for a secondary electron multiplier tube comprising a planar gridAV electrode made up of spaced elements having interstices between them, said elements of which 'have each two major faces which are wider than the interstices thereof, said elements having the t surfaces of one face thereof rendered secondary Vso Y wherein each element electron emissive, said grid being adapted to be bombarded by primary electrons on the secondary emissive surface and` inclined to the normal path of said primary electrons, Va layerV of insulating material upon the other face, and a layer of conducting material deposited upon said insulating material to accelerate secondary electrons emitted under the bombardment of the primary electrons.

5. An electrode system as claimed in claim 1,

Y of said planar grid electrode is planar.

6. An electrode system as wherein each element ofv said planar grid electrode 1s planar, and wherein each element of said cooperating accelerating electrode is of a circular cross-section.

claimed in claim 1,

HANS GERHARD LUBSZYNSKI. JAMES DWYER MCGEE.

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