US2113378A - Electron multiplier - Google Patents

Description

April 5, 1938,

Mom/Laren 4 /GHT so uEcE.

E. A. MASSA, JR., ET AL ELECTRON MULTIPLIER Filed July 3l, 1955 Patented pr. 1938 UNITED STATES PATENT CFFICE ELECTRON MULTIPLIER Ware Application July 31, 1935, Serial N0. 33,996

4 Claims.

This invention relates to electric discharge de-` vices, particularly electron multipliers, and has special reference to improvements in construction of multi-electrode tubes of the type disclosed in copending application Serial No. 4,049 to Louis Malter, iiled January 30, 1935. Discharge devices of this type are adapted to be` used as amplifiers, oscillators, modulators and frequency doublers. They are especially useful as direct current amplifiers, radio frequency power amplifiers and as self contained, photo-actuated amplifiers.

The novel features characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, will be best understood by reference to the accompanying drawing wherein:

Figure l is a diagrammatic representation of a multi-electrode photo-sensitive electron multiplier, one of several disclosed in the above-mentioned Malter application.

Fig. 2 is a plan view of the lower electrodes of Fig. 1 showing the tendency of the electrons to spill-over the edges of the surfaces of the multiplying electrodes, a condition which the present invention is designed to obviate.

Fig. 3 is a perspective view of a multi-electrode photo-sensitive electron multiplier showing a unitary electrode and insulating assembly within the tube, with a portion of the tube envelope and of the external magnet broken away to show the elements more clearly.

Fig. 4 is an enlarged side elevation of a portion of the electrode assembly of Fig. 3 showing in detail the combined potential distributing and electrode supporting leads.

Fig. 5 is an end elevation of Fig. 4.

If electrons moving with considerable velocity strike an electrode surface, secondary electrons are emitted. The number depending upon the velocity of impact, the nature of the surface material and the field available to draw the secondary electrons away, there may for instance, be ten times as many secondary electrons as primary ones. If the newly-liberated electrons are accelerated and thrown against another electrode the number may once more be increased by the same or similar factor, again and again until the gain is, in fact, enormous.

While a wide variety of arrangements of electrodes is possible the general arrangement illustrated in Fig. l and described in detail in the above mentioned pending case has so far proven the most successful. Light entering the device and striking a photo-sensitive electrode 5I re- (Cl. Z-27.5)

leases primary electrons which are drawn upward by the electrostatic eld, provided by the upper or accelerating electrode 6I but are deected by a constant magnetic field perpendicular to the plane of the paper. When the voltages and Viield are properly adjusted the adjacent plate 52 is struck in the center by the group or bundle of primaryV electrons. As indicated in both Figs. 1 and 2 the secondary electrons from the point of impact encounter electrical conditions substantially identical to those met by the original electrons. The secondary electrons, however, become increasingly more numerous and spread laterally over a greater area of the electrode surfaces 52-56. After several such steps the output is taken from the collector plate 66a, at the opposite end of the tube.

As' previously set forth the gain per stage depends upon several factors.

(l) Upon the material of which the electrodes are made. Farnsworth once said that rhe has gotten an increase of 10 times with a surface of caesium on oxidized nickel. Malter has obtained 8.5 with rubidium on oxidized silver, 7 is a good figure for caesium on oxidized silver.

(2) Upon the voltages applied to the accelerating and to the multiplying electrodes.

v(3) The gain per stage also depends upon the ability to focus or otherwise coni-lne all of the secondary electrons from one electrode to the next succeeding one. This factor is determined to a large extent upon the adjustment of the magnetic construction provides an electron-multiplier device wherein, by reason of a novel insulating means, the electrons are conned within a predetermined path or area circumscribed by the several electrode surfaces.

'I'he invention further resides in the provision of an extremely rigid unitary assemblage of the several electrodes and the path defining insulating means whereby the efficiency of the device generally is increased and its manufacture is simpliiied. A

Another and important feature of the invention is the provision of an electrode assembly, including a translucent accelerating electrode designed, positioned and arranged to ensure optimum performance of electron multipliers of the photosensitive type.

Referring now to Fig. 3 of the drawing: The improved electrode assembly of the invention may conveniently 'be contained in an elongated evacuated tube T having `a preferably annular or cylindrical stem S around and through which the stem wires 3I-3'I are sealed. It is to the internally projecting terminals of the stem wires that the electrode leads 2I-2'I are respectively joined, as by welding. The stem wires are shown as terminating externally in the prongs III-41 Y y I blocked by the accelerating electrode.

of a seven plug socket P of conventional design. v

In the particular embodiment illustrated there are twelve electrodes; six accelerating or upper electrodes and six lower electrodes, five ofV which are multiplying electrodes and theother (the outermost) the primary electron emitter. 'I'he set of accelerating electrodes, numbered II,I6, are arranged in spaced relation in a single plane. The electrodes I-S of the lower set are paired with those of the upper set, i. e., theyare similarly arranged in a second, parallel plane. The upper electrode I6 (nearest the stem S) is the anode or output electrode, it has an extension IGa, which may conveniently be of wire cloth, bent downwardly towards but not touching the innermost lower electrode 6. This extension IGa to the plate I6 is designed to intercept electrons which, if it were not present, might pass through the open end of theassembly without being utilized. This bi-part electrode I6, I6a. is connected to a single lead wire 2'I.

In accordance with the earlier Malter disclosure and as indicated in Fig. l of theinstant case each of the lower electrodes, with'the exception of the primary emitter I may be operated at a potential corresponding to that of the next preceding upper electrode. When so operated each electrode lead, with the exception of lead 21 to the anode I6 and vlead `2I to the primary emitter I supplies the operating voltages for two electrodes. Thus lead 22A supplies electrodes 2 and II; lead 23, Velectrodes 3 and I2; lead 24, electrodes 4 and I3; lead 25, electrodes5 and I4; and lead 26, electrodes 6 and I5. As shown in detail in Figs. 4 Vand 5 the connections between electrodes of similar potential and their common lead may conveniently bemade by short terminal rods 2| 22', etc., anchored, as by welding to the back or outer surface of each electrode.

The electron-confining .and electrode-supporting assembly comprises a pair of outwardly extending, oppositely located, parallelly arranged strips A-B of mica or other insulating material. These insulating strips are preferably prefabricated and suitably oriced to accommodate the oppositely located bent-over lugs L of the several electrodes. Increased rigidity may be ensured by bending these lugs L over the edges of the insulating strips as indicated in Fig. 5. As clearly shown in Fig. 5, the insulating strips, together with the respective upper and lower sets of electrodes form a conduit for the electrons constituted by a series of box-like containers. The electron conning effect of this conduit is due not alone to the mechanical presence of the insulating side strips A-B but may be accounted for in part at least vby the fact that the inner walls become negatively charged during operation of the device and, being so charged, serve to exert an electrically repellent or space charge effect upon the electrons, directing them inwardly away from the side walls and towards the center of the electrodes.

A photo-sensitive surfacepermits of a more compact and small electrode assemblyi than that practical with the usual thermionic type primary emitter though either may be employed depending to some extent upon the use to which the tube is to be put. One drawback encountered in the operation of photo-sensitive electron multipliers is the difficulty in focusing light from the external modulated light source upon the photosensitive surface. Up to now it has been common practice to direct the light rays at an acute angle towards the tube so that they will not be (See Fig. 1). In so doing, however, the beam will usually fall upon the glass at such an angle as to diifuse it so that the rays will impinge upon the photosensitive surface with an intensity less than may be desiredinhibiting optimum performance of the device.

The above `and other disadvantages inherent in existing photo-tubes are obviated, in accordance with the present invention, by the provision `of a translucent accelerating electrode I I, Fig. 3. Since this electrode IIis electrically charged, it is preferably made of metal, suitably oriced as at Ila to permit passage of the light rays. To ensure the desired rigidity and potential distribution its foraminous surface is provided with a surrounding frame work I Ibi. With such aconstruction the light rays may now obviously be so directed 4as to fall'directly and at a desired angle upon the photo-sensitive primary electron emitter I.

In operation a single magnetic field is preferably employed for concentrating and directing all of the electron streams to the proper target or multiplying electrodes 2 6, whereby the major portion of each secondary electron stream, is utilized and the primary electrons are prevented from being drawn past the targets to impinge directly upon the output electrode. Any convenient means kmay beutilized for establishing a magnetic eld parallel tothe electrode surfaces, such for example, asthe device partially shown in perspective in Fig.` 3. Preferably this device is constituted `by -a U-shape element of magnetically permeable material on which is mounted an energizing coil M and to each upstandng portion of which Vis affixed a `plate N `also of permeable material. The tube T is disposed between these plates vin such position that a substantially uniform vmagnetic field is set up parallel to the opposed surfacesof the sets of electrodes. Obviously, a permanent magnet may be substituted forthe electro-magnet shown. or the tube may be disposed within acoil `of vwire carrying an electric current.

The single embodiment Aof ,the invention which has been illustrated'ior purposes of explaining the inventive concept is susceptible of various modications which will be apparent to others skilled in ytheart. Y

What is claimed istl. An electron multiplier device comprising `a sealed container, a plurality of separate electrodes mounted within said container `and spaced from the walls thereof, said'electrodes having surfaces adapted to liberate electrons by secondary emission, and insulating means for conning said electrons within a predetermined path dened by the boundaries of said surfaces, said insulating means constituting a support for said electrodes.

12. A multi-electrode electron multiplier device comprising an elongated container, a pair of -insulating strips fmountedin ,parallel relation on opposite sides of the long axis .of said container,

comprising an elongated container, a pair of insulating strips mounted in parallel relation on opposite sides of the long axis of said container, a plurality of sets of electrodes supported by said insulating strips, the electrodes of one set being paired with the electrodes of another set, one electrode of one pair being electrically connected yto another electrode of another pair, and a plurality of current carrying support Wires for said electrode assembly.

ERNEST A. MASSA, Jn. LOUIS MALTER.

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