US2285126A - Electron multiplier - Google Patents

Description

June 2, 1942. J. A. nkml-ifiAN'rAL 2,235,126 ELECTRQN MULTIPLIER M1 41 July 28.,1939

Patented June 2, 1942 ELECTRON MULTIPLIER Jan A. Rajchman, Philadelphia, Pa., and Richard L. Snyder, Jr., Glassboro, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application July 28, 1939, Serial No. 287,020

8 Claims.

This invention relates to improvements in electron multipliers.

The electron multiplier is a current amplifying device. However, in most of its applications a voltage variation such as is provided by a resistance in the circuit of the collector electrode, is required. It is desirable that the current to the collecting electrode be independent of potential changes occasioned by the very presence of such resistance, since if such independence is achieved, distortion is minimized and the maximum voltage variation across the said resistance is ensured.

Ernest A. Massa, Jr., in U. S. Patent 2,135,039 describes an electron multiplier of the magnetically focussed type wherein a screen grid is employed to obtain the above described, desired plate characteristic. In the Massa arrangement a grid-like electrode is located in front. of the collector electrode and shields the multiplying electrode assembly from the variations in the field due to potential variations on the collector. The electron current from the last multiplying electrode flows through the interstices of the screen and impinges the collector plate no matter what the instantaneous potential of the latter electrode is.

While electron-multipliers constructed and operated in accordance with the teachings of the Massa patent have a high factor of merit as compared with similar but unshielded tubes of the prior art, the Massa tubes nevertheless are subject to two drawbacks: First, an additional voltage is required for the screen, that is to say, the overall voltage on the multiplier has to be greater, for the same overall gain, than it would be if the potential applied thereto could be the same as that applied to either the collector (anode) or last multiplying electrode. Second, when the anode potential of the Massa device becomes negative with respect to the screen grid, the electrons striking the anode may release secondary electrons which are attracted to the screen grid, thus decreasing the current in the anode circuit, so that in this event the plate characteristic is not perfectly fiat.

' Accordingly, the principal object of the present invention is to obviate the above described and other less apparent objections to the so-called screen-grid electron multipliers of the'prior art.

The objects of the invention are simply achieved by so shaping the last multiplying electrode that it substantially surrounds the collector anode whereby it constitutes an effective electrostatic shield therefor.

The novel features characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages, will be best understood from the following description of certain specific embodiments thereof, when read in connection with the accompanying drawing, in which:

Figure 1 is an end plan view of a photoactuated electrostatically focussed electron multiplier wherein the intermediate multiplying electrodes are arranged about a common axis as in Zworykin and Snyder Application Serial No. 187,634, filed January 29, 1938 and including an output electrode assembly designed, positioned and arranged in accordance with the present invention;

Figure 2 is a View in perspective of the output electrode assembly of Fig. l; and

Figure 3 is an end plan view of an alternate embodiment of the invention showing a modified form'of collector-anode. v

In Fig. l,'T designates a highly evacuated cylindrical envelope or tube which is preferably provided with a radial bafile B which extends from a point adjacent or beyond the central long axis A of the tube to the inner wall thereof and serves to bar the passage of stray ions from the high voltage side to the low voltage side of the device.

Where,'as in the illustration, the device is provided with a photosensitive cathode C, the envelope is preferably constituted, in whole or in part, of glass/Pyrex? or other suitable transparent material to permit light (exemplified by the line a) from an external source, not shown, to be shined upon the cathode. A grid G, which is preferably connected to the photosensitive cathode C and is supported between it and the inner wall of the glass envelope T, serves to prevent the accumulation of a positive charge upon the inner surface of the envelope. In addition to the grid G and cathode C, the tube contains a plurality of intermediate electron multiplying electrodes, designated 1 to 8 inclusive, which are mounted in staggered relation on opposite sides,

of a curved substantially arcuate median line m, which may be said to bisect the electron path which extends from the cathode C to and beyond the last-mentioned multiplying electrode 8. Another electrode 9 which may extend to or across the median line 112 comprises the last multiplying stage of the device. This final multiplying electrode 9 preferably comprises a C-shaped or inflated U-shaped plate or sheet-metal structure having the open space between its inwardly directed terminal or marginal edges directed toward the source 8.

Mounted within the oval defined by the inner surface of the last multiplying electrode 9 is a collector anode It which, as more clearly shown in Fig. 2, may be of grid-like or other openwork construction to permit the electrons from the source 8 to pass therethrough and to impinge the secondarily emissive inner surface of electrode 9 without substantial numerical diminution. Alternatively, and as shown in Fig. 3, the collector anode may comprise one or more rod-like electrodes Illa, lab, which are mounted adjacent but outside the direct path of the electrons from the source 8. In either event, the collector anode (Hi-Figs 1-2, Illa-lllb-Fig. 3) is maintained at a potential higher than the last multiplying electrode 9 so that the secondary electrons which are emitted from the inner surface of that electrode when electrons from the source 8 impinge thereon are drawn to the collector anode It, as indicated by the arrows on the dotted lines e in both Figs. 1 and 3.

The spacing between the collector anode Hi and the emissive surface of the last multiplying electrode 9 is by no means critical nor is the exact contour of the surface of the last multiplying electrode important, provided it comprises a neck portion of reduced diameter which is arranged to accept the electrons from electrode 3.

As indicated by the arrows on the dotted lines extending between the several electrodes, the electrons leaving the photosensitive cathode advance in the anode direction in travelling to the next higher numbered electrode by reason of the electrostatic fields which are generated therebetween when each is maintained at a fixed potential which is higher than that of its preceding (in point of electron travel) electrode. Since electrode 9 more nearly approaches electrode 8 than does the anode It, the focussing field between electrodes 8 and 9 is determined by the constant potential difference therebetween and is not substantially affected by such potential variations as may occur by reason of the presence of a resistance in the output circuit.

While the invention has been described as applied to an electron multiplier wherein the electrodes are mounted on opposite sides of an arouate median line which extends between the cathode and anode, it will be apparent that the invention is none the less applicable to tubes wherein the electrodes are arranged along a straight axis. It is understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense except a required by the spirit of the appended claims.

What is claimed is:

1. An electron discharge device comprising a source of electrons, a bent plate having a secondary-electron emissive inner surface circumscribing a space toward which electrons from said source are directed, said plate having a pair of inwardly directed marginal portions defining an opening which faces said source and through which electrons from said source pass in traveling to said emissive inner surface, and a collector electrode for said secondary electrons supported within said enclosed space behind the said marginal portions of said plate.

2. The invention as set forth in claim 1 and wherein said co-liector electrode is of openwork construction and is coextensive with the opening between said marginal portions of said bent plate.

3. The invention as set forth in claim 1 and wherein said collector electrode is mounted adjacent but outside of the direct path of electrons from said source.

4. An electron discharge device comprising a source of electrons, an electrode comprising a sheet metal structure having a narrow open neck portion presented to said source and an expanded portion having an inner secondary emissive surface adapted to release secondary electrons by impact of electrons from said source, and an electrode mounted entirely within the volume circumscribed by said sheet metal structure for collecting said secondary electrons.

5. The invention as set forth in claim 4 and wherein said electron-collecting electrode comprises an openwork structure mounted intermediate the neck and emissive surface portions of said sheet metal structure.

6. An electron multiplier comprising a primary source of electrons, a multiplying electrode having a secondary-electron emissive surface portion facing said source, and a collector electrode for said secondary electrons mounted between said primary source and said emissive surface portion of said multiplying electrode, said multiplying electrode havin oppositely located integral shield portions which extend into the space between said collector electrode and said primary source.

'7. An electron discharge device comprising a source of electrons, an electrode surface comprising a, substantially C-shaped plate having open inwardly directed arms presented to said source and having an inner surface adapted to liberate secondary electrons by impact of electrons from said source, and an anode mounted entirely within the oval defined by the said inner surface of said C-shaped plate and adapted to receive the secondary electrons therefrom.

8. The invention as set forth in claim '7 wherein said anode comprises a pair of rod-like members mounted respectively behind the inwardly directed arms of said C-shaped plate out of the direct path of the electrons from said source.

JAN A. RAJCHMAN. RICHARD L. SNYDER, JR.

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