US2203225A - Electron discharge device utilizing electron multiplication - Google Patents

Description

June 4, 1940. KLEMPERER 2,203,225

ELECTRON DISCHARGE DEVICE UTILIZING ELECTRON MULTIPLICATION Filed Jan. 4, 1959 INVENTOR OTTO KLEMPERER ATTORNEY Patented June 4, 1940 PATENT OFFICE ELECTRON DISCHARGE DEVICE UTILIZING ELECTRON MULTIPLICATION Otto Klemperer, Iver, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application January 4, 1939, Serial No. 249,279 In Great Britain January 4, 1938 4 Claims.

The present invention relates to electron discharge devices and more especially, but not exclusively, to electron discharge devices of the kind known as electron multipliers, in which a 5 stream of primary electrons is caused to bombard a surface which is thereby caused to emit secondary electrons usually in greater number than the bombarding electrons. The invention is applicable both to devices in which mere multiplication of a primary electron beam is required and to devices in which the primary beam of electrons is a composite beam corresponding to a desired image electron primary electrons as in the case of an image electron multiplier device or picture transformer for use in television systems or in electron telescopic or microscopic devices.

The object of the present invention is to provide an electron optical system suitable for use in a picture multiplier in which a beam of primary electrons may be caused to bombard a target electrode which emits secondary electrons in the opposite direction to that from which the primary electrons are received, the secondary electrons being operated on selectively to focus them on a suitable utilizing or receiving element such as a further target electrode which emits secondary electrons, a mosaic electrode, or a fluorescent screen.

In accordance with the present invention an electron discharge device is provided comprising an electrode system including a source of electrons from which electrons are emitted in one direction and a receiving member arranged on the opposite side of said source to that from which electrons are emitted, and an electrode or electrode system arranged in the path of electrons from said source, said electrode or electrode system being adapted when suitably charged to constitute an electron mirror by which said electrons are reflected towards said receiving member, the arrangement being such that said electrons are directed onto said receiving electrode.

In an initial stage of an electron multiplier embodying the invention, said source of electrons may be a thermionic cathode or other source of electrons such as a photo-cathode adapted to emit a composite beam of primary electrons corresponding to a desired image. In this case the arrangement should be rotationally symmetrical.

However an arrangement according to the invention may comprise several stages each arranged in accordance therewith, the first stage including a primary emitting cathode and further stages of the device including secondary emitting cathodes each of which is bombarded by the beam of electrons produced in the receiving stage it being considered that a beam regarded as secondary in one stage is to be regarded as a primary beam when it reaches the next stage and causes the emission of secondary electrons therein.

In a further application of the invention more especially in the case where the invention is applied in conjunction with a primary emitting source, modulating potentials may be applied to the shaped metal electrode constituting the equi potential mirror surface arranged in front of the source whereby the stream of electrons reflected by the mirror may be modulated, due to the fact that owing to the Maxwellian distribution of electron velocities in the incident beam, a quantity of electrons may be absorbed at the mirror surface with whichquantity varies with small potential changes in dependence on the potential applied to said electrode.

The nature of the invention and the manner of carrying the invention into effect will be clearly understood from the following description in detail, reference being made to the accompanying drawing in which:

Figure 1 shows diagrammatically the arrangement of an image electron multiplier according to the invention, and

Figure 2 shows diagrammatically the arrangement of simple electron multiplier in accordance with the invention.

Referring toFigure 1, [is an optical lens which is arranged to project on a photo-sensitized cathode 2 an image which is intensified or multiplied. Surrounding the cathode 2 is a cylindrical shield 3 which is maintained at the same potential as the cathode 2 or slightly negative with respect thereto. The cathode 2 and shield 3 are together supported within a cylindrical anode 6 having a closed end which is carried on a pillar or stem 5 suitably arranged within the multiplier housing. Surrounding the assembly consisting of electrodes 2, 3 and 4 is a further cylindrical anode 6 of large diameter and preferably of length of I the order of three times its diameter or greater.

The electrode assembly 2, 3, 4 is located centrally of the anode 6, and at one end of electrode 6 in the direction of lens I is arranged the cylindrical electrode I of the same diameter as the anode E but relatively short. The electrodes 4 and 6 may be maintained at approximately the same electric potential of about 300 volts positive and the electrode 1 may be maintained at a negative potential, for example, of 200 volts.

It will be seen that from the potentials given that the electrodes 9 and l constitute an electron mirror as described in the specification of co-pending application Serial No. 181,289, filed December 23, 1937, which will reflect electrons emitted from the cathode 2 in the direction 6pposite to that from whence they come, the'beam of reflected electrons being brought to a focus as described in the aforesaid specification. At the point where the electrons are so brought to a focus is arranged a further cathode 9 which.

emits secondary electrons when bombarded by primary electrons. This cathode 9 is surrounded by an anode in similar to anode 6 surrounding cathode 2 but maintained at a higher positive electrode as being maintained at a potent l leaving cathode 2 is indicated in dotted stantially equal to that of the anode I. end of electrode i is arranged a shorter cylindrical electrode M, on the closed end iii of which is arranged a target such as a fluorescent screen. Electrode I9 is maintained at a higher positive potential than electrode 90, for example, at a potential of 1600 volts.

The assembly comprising cathode 9, electrode all and electrode i2 may be supp rted on a stem do similar to the stem 5. Suitable means for maintaining the electrodes 2, 3, 4, 6, 1, 9, Ill, H, I2 and i9 are described in the specification of copending application Serial No. 221,898, filed July 29, 1938, with reference to Fig. 1 of the drawing thereof.

Surrounding the gaps between electrodes 9 and i, 6 and I0, and I0 and i 4 are electro-magnetic coils i5, i6 and I1 respectively which are suitably energized to constitute magnetic lenses.

With this arrangement it will be seen that the anode it acts in conjunction with the anode 6 to produce an accelerating and focussing field for electrons passing from left to right along the electrode 6 and thus electrons emitted by the cathode 2 and reflected by the mirror formed between electrodes 9 and i will be acted on by the field set up in the anode I0 which is ar-- ranged to assist in focussing the electron beam on the cathode 9. A typical path for an electron lines at However, in respect of electrons leaving the cathode 9 the combination of anodes l9 and 6 charged in the manner indicated above serves as an electron mirror in respect of electrons passing from right to left along the electrode I 0, provided that the source of electrons is more positive than or has the same potential as the anode 6. Thus secondary electrons emitted by the cathode 9 will be accelerated away from that electrode in the direction of the left hand end of the anode ID by the electrode l2 but will be reflected in the said end due to an equipotential set up between the anodes l9 and 6 the reflected electrons passing on either side of the electrode assembly 9, H, I 2, to be focussed as in the case of the first stage. Electrons emitted by cathode 9 will follow paths such as the path I 9 shown dotted in the drawing. The electrons so focussed may be received on a secondary emitting cathode similar to the cathode 9 arranged in a further stage of picture focussing efiect of electrodes 6, Hi and and electrodes 4 and At themultiplication, or, as represented in the drawing, they may be focussed on an image utilizing element such as the fluorescent screen H which is arranged on the end of a cylindrical anode 14 as above described. The field produced by anode l4 serves to focus the electrons received from the preceding stage on the fluorescent screen l3. The magnetic lenses I 5, l6 and i7 enhance the I4 respectively so that the electrons may be focussed without being unduly accelerated, for example, to such an extent that they cannot cause a satisfactory emission oi. secondary electrons by bombarding a cathode.

In an arrangement such as that shown in Fig. 1, the electrodes 1. 6, l0 and I4 may be made of sheet metal tubes of,.for example, 3" in diameter l2 .enclosing the cathodes 2 and 9 respectively may be made of tubes,

the cross-section of the beam as reflected at the left hand end of the electrode 9 or electrode I0 being arranged to be about 1 /2 in diameter. The obstruction presented by the electrode assembly 2, 3, 4 or 9, II,' in this case may cause a loss of about of the reflected electrons 62 as. the case may be,

As already pointed out, electrode 6, and like- I wise electrode l0, should have a length of about three times that of its diameter so that the overall length of each of these electrodes should be about 9" in the case given. Electrode i and electrode l4 are made as regard to the requirements of the electron optical short as possible, having system of the arrangement. It is to be observed that if electrode 1 were very long then the lens I or an optical system equivalent thereto would have to be of low power to give the long focal length necessary, in viewof the distance between the lens and the image formed by it, so that the amount of light derived from the source by which lens system I is illuminated would be small.

It will be seen that the multiplier shown in Fig. 1 is somewhat similar to the arrangement described in the specification of co-pending application Serial No. 221,898 above referred to, except that in the arrangement described in the prior case primary electrons are reflected rather than secondary electrons. that the initial stage of a multiplier as described in the present case has very much the same elements as the first stage of a multiplier described in the aforesaid application with the difference that the electron mirror in the prior case is arranged on the opposite side of the cathode'2 to that on which the lens system such as i is arranged and the electron receiving element such as the emitting cathode for the next stage is nearer the electron mirror than the primary emitting cathode.

When the invention is applied to an ordinary electron multiplier, the arrangement can be made the end is closed by an equipotential surface constituted by an electrode 25 which may be made curved and encloses the end of anode 24 as is described ln the specification of the aforesaid co- It will also be seen pending application Serial No. 181,289. Electrode 25 is associated with an adjustable source of potential indicated by potentiometer 26 and battery 21. and modulating potentials may be applied to the electrodev 25 from terminals 23 through transformer 29. At the right hand end or anode 24 is arranged a further similar anode Bil having within it a secondary emitting cathode 3| arranged similarly to the cathode 22 and having a shield 32 similar to the shield 23 of cathode 522. Cathode 3| is adapted toreceive electrons emitted by cathode 22 reflected from equipotential surface 25. In the arrangement shown the stage including cathode 8i and anode as is followed by one further multiplier stage including tubular anode 3 3 similar to the electrode as and cathode 35 similar to the cathode 3! there being on the right hand end of electrode as a further tubular anode 3d terminated at one end by an apertured diaphragm 31 about which is arranged a targetor collecting electrode 38 to which potential is applied through a load impedance represented by resistance 3t, the ends of which are connected to terminals it forming the output terminals for the multiplier. Magnetic lenses 68, ti and d3 similar to the lenses it, it and ll of Fig. l are provided about the gaps between electrodes 2d and lit, til and and 3t and The arrangement of Mg. 2, operates as follows: Assuming that the cathode 22 is at zero potential, the cathodes 32 and 35 may be maintained at potentials of 300 and 600 volts respectively, the collecting electrode 38 being maintained at the potential of 750 volts whilst the tubular electrodes it, so, at and 3d are maintained at potentials of 150, 450, 750 and 450 volts respectively. The shields such as 23 and 32 surrounding the cathodes 22, Si and 35, may be maintained at the same potentials as the surrounding tubular electrodes 2d, 30 and M as the case may be. The electrode this preferably maintained at a potential slightly less than that of the cathode 22, is preferably coated with a I suitable substance, like soot, to avoid undesirable reflection of the electrons, and is preferably shaped so as to exert a iocussing action on the electrons. The electric current through the lens coils il, c2 and 63 is adjusted in order to focus the electron of the required velocities at the cathodes.

It will be seen that with the above arrangement electrons emitted by cathode 22 in the (llrection of electrode 25 pass along paths such as I M shown dotted in Fig. 2 and will be reflected due to the equipotential surface constituted by electrode 25, through tubular anode i into tubular anode 3b, which will exert a focussing action additional to that exerted due to the magnetic lens 5i on the electrons which is arranged to be such that normally the electrons are received by the cathode 3i causing the emission of secondary electrons which will move initially back along the tubular anode 38 along paths such as 45 shown dotted in the drawing. These secondary electrons are reflected by the mirror equipotential surface set up between anodes lid and 2t and will be focussed within anode t l so as to be received by the cathode 35. Secondary electrons, perhaps more appropriately described as tertiary electrons, emitted by the cathode 35 will pass along paths such as it shown dotted and will be reflected at the mirror equipotentlal surface set up between anodes Bi and so, through anode 3 3 into electrode 38 and will be directed into the collecting electrode 36. If modulating potentials are applied to the reflection electrode 25 the potential of the reflecting surface will vary slightly and, due to the Maxwellian distribution of electron velocities in the incident beam, more or less electrons will be absorbed by electrode 25 as its potential varies, as explained in U. S. Patent 2,175,697. Collecting electrode 38 is shielded by the diaphragm 31' and the load resistance 39 is arranged to be large. In this way, as explained in the specification of co-pending application Serial No. 95,744, filed August 13, 1936, the electrostatic fleld inside the multiplier will not be seriously disturbed or changed, when, due toalterations in the load, the potential at the upper end of load 39 and consequently on the electrode 38 fluctuates appreciably.

In the case of a mutliplier such as that shown in Fig. 2 the tubular electrodes 2%, and 35 might be made of. lffldiameter tube, the cathodes 22,.3i and 35 and the collecting electrode 38 being made of about A" diameter. arrangement may include any convenient number of multiplying stages.

It is believed that in a multiplier of the type shown in Fig. 2 there will not arise such serious limitation in respect of space she as arises in the case of the usual type multiplier, also the glass work requir ing an envelope for the multiplier 21. required in adjusting the electrodes be likely to be easy with a straight m the type shown.

I claim:

1. An electron discharge device comprising electrode structure including a plurality of electron emitters axially separated one from the other and facing in the same direction, one of said emitters having on one side a surf ace of high secondary electron emissivity, a target electrode mounted on the opposite side of the said emitter,

means to reflect electrons from the preceding emitter to said emitter and from said emitter to the target, and means to focus electrons from the preceding emitter to said emitter and from said emitter upon the target.

2. An electron multiplier comprising an electrode system including an electron source which emitsprimary electrons from one side only, an electron receiving electrode adapted to liberate secondary electrons when impinged by high velocity electrons mounted on the opposite side of said source and facing the non-emitting side of said source, an electron mirror electrode systern to generate an electrostatic field the path of electrons from said source and constituting an electron mirror by which said primary electrons are reflected toward one side of said electron receiving electrode, a target on the opposite side of said electron receiving electrode further re moved from said source than said electron receiving electrode, and means between source and said electron receiving electrode to direct secondary electrons liberated from said receiving electrode toward said target.

3. An electron discharge device including an elongated evacuated envelope, a plurality of separated electron emitting cathode axially aligned within said envelope, the emitting portions of said cathodes facing in the same direction, electron mirror means between the cathode facing one end of said envelope and the said end of the envelope to reflect electrons emitted by said cathode toward the next adjacent cathode,

means-between the cathode facing the one end of said envelope and the said one end. of the 4, An electron discharge device comprising an electrode structure including an electron source which emits electrons from one side only, an electron receiving member-having on one side a surface with secondary electron emissivity greater than unity mounted on the opposite side of said source with the'said surface facing the nonaaoaase emitting side of said source, an electron reflector exposed to the emitting side of said electron source to reflect toward said receiving member electrons emitted from said source, means to generate a magnetic field between said source and said electron reflector to direct the reflected electrons toward said receiving member, means between. said electron source and said receiving member to focus the stream of refiected electrons on said receiving member, a target electrode mounted on the opposite side of said electron receiving member, and an electrode exposed to said surface of said electron receiving membar to reflect secondary electrons emitted from said surface toward said target electrode and means between said receiving member and said target to focus the reflected secondary electrons on said target.

' O'I'I'O KLEMPERER.

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