US2260613A - Electron multiplier - Google Patents

Description

P. 'r. FARNEQWORTH Oct. 28, 1941.

ELECTRON MULTIPLIER ori ina Filed May 18, 1956 2 Sheets-Sheet 1 INVENTORT PH/LQ T. FARNSWORTH.

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Oct. 28, 1941. P. T. FARNSWORTH ELECTRON MULTIPLIER Original Filed May 18, 1936 2 Sheets-Sheet 2 INVENTOR...

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- limitations must-be avoided.

Patented Oct. 28, 1941 I UNITED STATES PATENT 1 (o F'FICE.

, ELECTRON MULTIPLIER Philo. T. Farnswoithfhiladelphia, Pa., assignor to-Farnsworth Television &Radio Corporation, Nevv'York. N Y., a corporation of Delaware Original application 'May 18, 1936, Serial No.

80,194. Divided and thi 1939,, Serial No'. 269,895

8 Claims.

My invention relates to electron multipliers, and more, particularly totha-t type of electron multiplier where electrons are directed to. successively impact a series of surface elements toproduce a current augmented by secondaryemis sion at each impact. I

Electron multipliers may-be roughly divided into two general types: first," the so-called direct-current multiplier, where the generation of secondary electrons takes place by successive impact ofan electron stream with a series. of emitting lements energized to successively increasing positive potentials; and second, that type. of multiplier where repeated impacts are, made; between a pair of operatively opposed surface elements'such as hasbeen described by me in my prior United States Patents Nos. 2,071,515

and, 2,071,516. I The present application deals. with the first type of multiplier mentioned above,

and is a division of my copending-application Serial No. 80,194, filed- May 18, 1936, now United States Patent No. 2,204,479, issued June .11, 19.40.

.Whena series of surface elements are energized to successively increasing potentials and placed with relation to each other so. that electrons may be. drawn from one to. another until the entire series has been impacted, a number. of factors'must be taken into account to produce efiectiveelectron multiplication. First, the elec-, trons must b accelerated toward each surface element with such a velocity that. they will generate secondary electrons on impact; second, the

secondaries generated; must -be' reformed and usually changed in direction so that theylmay againbedirected and accelerated toward .a Sec- 0nd and similar surface, and, so on until a sufiicient number of impacts have occurred to producethe desired multiplication; third, electrons, during their passage from one element to another, must-be so under control that as many electrons as possible reach the next element; fourth, the urface elements themselves must be so sensitized as to produce secondary electrons withe maximum ratio of secondary electrons to primary electrons; and :fifth My present invention has for its main object th provision of a means and method. whereby electron multiplication may take place with high efficiency and in which electrons may be made to follow predetermined paths between impacts with minimum collection, and it also provides a means. and method of adding to the field produced by the elements themselves an additional field which ensures that electrons strike each. elew space-charge 5 application April 25,

ment, andwithf a velocity sufiicient to emit secondariesat each impact.

Among other objects are: To provid a means and method of segregating accelerating fields in a multistage. secondary emission multiplier; to provide a means and 'method of insuring that electrons reach the proper target in a multistage secondary emission multiplier; to-provide, in a,

. secondary emission multiplier, a means and therein; to provide a meansand method of remethod 'of focusing. electrons between electrodes and features of advantagasome of which, to-

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gethenwith the foregoing, will be set forth in the flfolljowing description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other'apparatus embodiments, utilizing the method, within the scope ofthe appended claims.

.Broadlyjas to method, myinvention comprises directing the electrons, in the type of multiplier described, between impacts in such a manner that they reachthe next surface, and my method may also provide the'step of creating an accelcrating field in addition to the fields created by thesurfac elements themselves, that is, the inter-element held; and in this respect I may preferto utilize the' potential of the next succeeding element to accelerate electrons between a pair of elements lower in potential.

Broadly as toapparatus, myinvention comprises a series of surface elements energized to successively increasing positive potentials,- and means for directing electrons emitted from one element onto the element of the next higher potential. In order to ensure electrons impacting the surfaces with the proper velocity, I may desireto' insert operatively between each pair of 5 elements an accelerating electrode energized to a potential higher than either of that specific pair, and I thus provide an accelerating field graded as the elements themselves are graded,

the accelerating field being higher at all points ments, and I may make use of a Faraday space between surface elements.

My invention may be specific better understood by reference to certain particular embodiment-s of my invention, these embodiments, however, being used only for purposes of illustration, other structures utilizing the method being deemed full equivalents.

Referring to the drawings:

Figure l is a diagrammatic longitudinal sectional view of a tube embodying one preferred successive surface elements being. limited only by the number of secondary-electron generating impacts desired.

I have found that the surface elements 9, 9a and 917 may be sensitized for secondary emission by the deposit thereon in any well known manner of a thin film of thorium, barium, potassium, caesium or other metals in the alkali and alkaline earth groups, and I have obtained a sensitivity with such surfaces so that a primary-to-secondary emission ratio of 1:1 can be obtained with an electron velocity of volts, maximum emission of a ratio of 1:? being obtained with caesium surfaces. However, any material capable of emitting secondary electrons on impact is satisfactory.

form of my invention, together with a schematic circuit showing how it may be utilized.

Figure 2 is a diagram showing how the device of Figure 1 may be built, when used as a multiplier of photoelectrons.

Figure 3 is a diagrammatic sectional view of another embodiment of my invention utilizing accelerating electrodes.

Figure 4 is a sectional view by the line 4-4 in Figure 3.

Figure 5 is a longitudinal sectional view and circuit of an embodiment utilizing a unipotential space.

Figure 6 is a view taken along a sectional plane, as indicated by the line B-6 in Figure 5.

Figure '7 is a diagrammatic representation, partly in section, of a device utilizing a coneshaped accelerating electrode, and a magnetic deflecting field.

Figure 8 is a sectional viewof the structure shown in Figure '7, taken as indicated by the line 88 in Figure '7. 7

Referring directly to the embodiment shown in Figure 1, an envelope l is provided at one end with an input stem 2 through which cathode leads 3 are sealed, supporting a filamentary cathode 4. A band 5 is placed around the stem, and supports by means of risers 6, an input cylinder I. A control grid 8 is placed in the opening to the cylinder 1. The cylinder I extends away from filament 4 and terminates at the opposite end in a surface element 9 which is sensitized to readily emit secondary electrons at a ratio greater than unity when impacted by primary electrons traveling at the proper velocity.

One side of the cylinder is cut away to provide an opening toward which is directed the open taken as indicated end of a second cylinder I I, also terminating in a secondarily emissive surface 90.. The second cylinder is supported by a lead 12 through the wall of the tube I. Cylinder II is also cut away adjacent the emissive surface 9a, and a third cylinder [3 has its open end opposed to the opening, cylinder l3 also terminating in an emissive surface 9b. Cylinder I3 is provided with an opening exactly as before, and an output cylinder l4 has its open end presented thereto so that electrons emitted from surface element Sb'may pass to the cylinder l4, there to be collected by an anode surface l5. Anode cylinder I4 is mounted on an output stem I6, a lead I! passing through the stem to provide exterior connection.

It is of course to be understood that while I.

have shown, for purposes of illustration, only.

three surfaces sensitized for secondary emission, namely, 9, 9a and 9b, the construction may be repeated as often as is desired, the number 'of The setup can be made operative by energizing the filament 4 from a filament source 20 and attaching a resistor 2| across a potential source 22, taps 24, 24a and 24b being taken off and leading respectively to cylinders I, H and I3, to

produce progressively increasing positive potentials thereon, the end of the resistor 2| being connected directly to anode cylinder l4 through lead I! and output impedance 25, the latter being so connected that Variations in current therethrough may be utilized.

Energized as described immediately above,'electrons leaving the filament 4 enter the interior of cylinder 1. Cylinder I, being energized at a positive potential higher than that of the filament from which the electrons were emitted, tends to focus the electrons onto the central portion of the first element 9 in the series of surface elements, and secondary electrons will be emitted therefrom. These secondary electrons, having random velocities, are acted upon by the field of cylinder II and are drawn into that cylinder and accelerated toward surface 9a where more secondary electrons are emitted, these latter secondary electrons being drawn into cylinder [3 to impact surface element 91) where still further secondary emission takes place, the final multiplied current thenbeing drawn into cylinder l4 and collected therein. Each cylinder is therefore a part of each emissive surface and is at the same potential as that surface.

The focusing is enhanced by the fact that the exits from the cylinders are close to the surface elements and the length of the cylinders toward the lower potential surface is such that electrons entering each cylinder are deflected a minimum amount by the field from the succeeding cylinder. 7

However, while my device is entirely operative to produce electron multiplication as described, space-charge effects occur which limit rather definitely the amplification which can, be drawn from a device of this sort, and it is desirable,

when cylinders alone are used, that a fairlyhigli voltage per stage be used, perhaps from 200 to 500 volts. I

I have also found that the multiplication effect may be greatly increased by the use of an accelerating field in addition to the field produced by the surface elementsthemselves, or their connected cylinders, and I produce this field in one manner as shown in Figure 3, where fine wire grids 26, 26a and 26b are positioned immediately in front of surface elements 9, 91a and 9b, respectively, so that electrons entering the cylinders are subjected to the fields of these electrodes.

It is preferable that the potential on these accelerating electrodes will be greater than that of the surface element to which the electrodes'are being accelerated thereby, and in order-that the various acceleratingelectrodes be conveniently energized, I may prefer to attach; for example, accelerating electrode 26 to cylinder l I, thereby maintaining it at the same potential as surface element 9a. In like manner accelerating electrode 26a is attached to cylinder l3 and at the potential of surface element 91); Accelerating electrode 261) is attached to output cylinder I4 and therefore is at the samepotential as anode I5. Of course these grids may beseparatelyand independently energized directly from a potential source if desired.

It is relatively immaterial as to the direction in which electrons approach the surface elements,

as they will pass through the accelerating screens to reach the emissivesurface. They will, however, leave the emissive surface perpendicular thereto and therefore will be directedinto the next succeeding cylinderto repeat the cyc e.

In this manner I have also provided, in addition to the successively energized surface elements, successively energized accelerating electrodes, and the potential of the accelerating electrodes is always higher than the surface to which the electrons are being accelerated thereby. I prefer to make the accelerating electrodes either of extremely fine wire so that few electrons will be picked up thereby, either approaching'or'leaving the surface elements, or to form the accelerating electrodes in any other well known manner so that the electrons approaching each surface will receive the benefit of the increased positive potentials thereon, with minimum pickup or collection.

g In the manner described, I have been able to obtain extremely high multiplication per stage. First, due to the focusing, a majority of electrons leaving one surface element reach the next without collection; second, by the use of accelerating electrodes I ensure that the electrons will impact the surfaces with a sufiicient velocity to create secondary electrons and prevent formation of limiting space charges; and third, by continual focusing of the entire path from one end of the device to the other, I prevent gradual dispersion.

As the device is capable of extreme amplifica-- tion, it is of course adapted to be used either as a straight amplifying device, as indicated in Figure 1, where a grid is used to control the-emission from the filament; or as a photoelectric device, as shown in Figure 2, Where the grid 8 has been coated with a photosensitive material and connected to the first cylinder 1. Light projected against this grid by optical system 38 causes the emission of electrons which are then accelerated toward the first surface 9. The output in this case will then be proportional to the light falling upon the grid 8.

It should also be pointed out that the device may be used in conjunction with any other. apparatus where electrons from any source whatsoever may be passed into the input end of cylinder I. It may be seen, therefore, from the foregoing description of this particular embodiment that I have completely surrounded the entire path of the electrons between successive surface elements, thus obtaining electron focusing to ensure impact at each stage. provided a means and method for reducing spacecharge limitations within the device.

It should also be pointed out that While I have shown the device in the form of cylinders, it is also operative with conduits of other cross-sec- In addition, I have tional contours, and tubular in this application is deemed to mean a conductive surrounding of the path to produce an electrostatic focusing field.

It will also be apparent that the device is a self oscillator if feedback from theoutput'to input be used.

'In the embodiment of my inventionshown in Figures 5 and 6, surface elements 9, 9a., 9b, 9c and 9d, capable of secondary emission, are arranged opposite an accelerating electrode 21 which in this case takes the form of a wire screen extending parallel to each set of opposed surfaces, the wire screen being arranged to form side walls and end walls 3! also being provided, the entire accelerating electrode structure being supported on leads 32, one of which is brought through the envelope wall for potential connection. The surface elements are connected to resistor 2| in exactly the same manner as in the device shown in Figure '1, so that there is a progressive increase in positive potential on each element.

As the electrons leave each element at right angles thereto, the opposite element is positioned in the path of the outgoing electrodes,this giving rise toth'e' divergence of the accelerating screens and the increasing distance between the elements orieach side of the'device. This arrangement provides certain advantages. The electrons, after they leave the accelerating screen on one side and until they pass through the accelerating screen on the other side, travel in a Faraday or equipotential space, and the electrons in one inter-element path are shielded, during their trav-v ersal of that path, from the voltages on the other surface elements, the voltage on the accelerating screens being,of course, at all times greater than any potential on any surface element. Furthermore, the arrangement provides. a continually lengthening path between surface'elements, the electrons, therefore, being given more space as they increase in number. This has the effect of limiting the development of space charges in the device, and it may be seen that by proper coordination of path lengths the space chargeefiects may be minimized. Electrons leaving the last surface element 9d will be collected either by the end wall 3| or by the ends of the side walls 21. In this embodiment also it is obvious that the entering electrons may be photoelectrons or those emanating from any source to be amplified.

The device shown in Figures 7 and 8 has the accelerating electrode 21 in the form of a truncated cone, all portions of which are preferably meshed. Here, the surface elements 9, 9a., 9b, 9c, and 9d are arranged in spiral formation around the accelerating electrode,.and a longitudinal magnetic field is provided to cause the electrons totake a curved path between one surface element and the next, being deflected thereto by the field. This field may be produced in any well known manner, such as by a solenoid 34 energized by a deflecting source 35 under the control of a variable resistor 36. Variation of the magnetic field, in combination with the placing of proper potentials upon each electrode, will ensure that the electrons between elements arrive on the next succeeding element, and if it is desired the multiplication may be controlled by varying the deflecting field so that more or less of the electron stream will impact the surface elements. In this way a steady electron source may be used and the output modulated by a variation of the magnetic field, or several frequencies may be imposed upon the device, one through the input end and the other through the magnetic field. The device, therefore, is adapted to wide Focusing the electrons from one surface onto the next may be accomplished in two manners: first, by the use of an electrostatic focusing system, as

described with the tube of Figure l; or, second,

by means of an electromagnetic deflecting system which is the full equivalent of a focusing system, inasmuch as it regulates the path through which the electrons must travel, as shown in Figures 7 and 8.

In all of the devices utilizing additional fields there is the common attribute that space-charge limitations are materially reduced, thus allowing greatly increased power outputs with a given number of impacts, and also reducing collection on route.

I claim:

1. In combination, a series of surfaces arranged in the formation of a conical helix and adapted to be progressively energized to increasing positive potentials, an electron source for supplying electrons to the surface of lowest potential, and means for collecting electrons leaving the surface of highest potential.

2. In combination, a series of surfaces arranged in the formation of a conical helix and adapted to be progressively energized to increasing positive potentials, an electron source for supplying electrons to one end of said series, means for collecting electrons at the other end of said series, and means for causing electrons to impact said surfaces successively.

3. An envelope containing means for producing electrons, an anode, a series of surface elements capable of producingelectrons by secondary emission and adapted to be progressively energized to increasing positive potentials, and

tubular conductive electrodes enclosing su'bstan- H tially the entire path of the electrons between the various elements of said series, said conductive electrodes being connected to an element of higher potential and extending toward an element of lower potential.

4. An envelope containing means for producing electrons, an anode, a series of surface elements capable of producing electrons by secondary emission and adapted to be progressively energized to increasing positive potentials, and

tubular conductive electrodes enclosing substantially the entire path of the electrons between the various elements of said series, said conductive electrodes being connected to an element of higher positive potential and extending toward an element of lower positive potential, and the axes of adjacent pairs of said conductive electrodes forming an acute angle.

5. An envelope containing means for producing electrons, an anode, a series of surface elements capable of producing electrons by secondary emission and adapted to be progressively energized to increasing positive potentials, and tubular conductive electrodes enclosing substantially the entire path of the electrons between the various elements of said series, said conductive electrodes being connected to an element of higher positive potential and extending toward an element of lower positive potential, and said surface elements respectively comprising integral parts of said conductive electrodes.

6. An electron multiplying system comprising a cathode, an anode, and a series of surface elements adapted to be progressively energized to increasing positive potentials and capable of emitting secondary electrons on impact, and means for directing electrons leaving one element onto the next element in said series, said means influencing the electron stream throughout substantially its entire path between said cathode and said anode.

'1. A series of opposed surfaces adapted to be energized to increasingly positive potentials to cause electrons contacting one surface to emit secondary electrons which impact the opposing surface at the next higher positive potential and thence impact the following surface at still higher positive potential to repeat the cycle, means for supplying electrons to the first surface in said series, and means for collecting electrons emitted from the last surface in said series, said surfaces being spaced apart from each other at distances progressively increasing in the direction of the electron flow.

8. A series of spaced opposed surfaces adapted to be energized to increasingly positive potentials to cause electrons contacting one surface to emit secondary electrons which impact the opposing surface at the next higher positive potential and thence impact the following surface at still higher positive potential to repeat the cycle, means for supplying electrons to the first surface in said series, means for collecting electrons emitted from the last surface in said series, said surfaces being spacedapart from each other at distances progressively increasing in the direction of the electron flow, and an accelerating Emode spaced an equal distance from each surace.

PI-IILO T. FARNSWORTH.

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