US2868994A - Electron multiplier - Google Patents

Description

Jan. 13, 1959 R, H. ANDERSON ELECTRON MULTIPLIER 2 Sheets-Sheet 1 Filed 001:. 24, 1955 HT ORA/E) INVENTOR. H A/VJEASO/V Roy/er BY I R. H- ANDERSON ELECTRON MULTIPLIER Jan. 13, 1959 Filed Oct. 24, 1955 2 Sheets-Sheet 2 INVENTOR. fiwz/vr/i/llvomso/v United States. Patent ELECTRON MULTIPLIER Robert H. Anderson, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware The present invention relates to improvements in electron multiplier tubes.

An electron multiplier is a device utilizing secondary electron emission to amplify or multiply the electron current from a primary electron source, such as a photocathode or a thermionic cathode. The usual electron multiplier comprises a series or chain of secondary emitting elements, called dynodes, interposed between a primary electron source and an output collector or anode. The electrodes are constructed and arranged to form an electron optical system for directing primary electrons from the primary source onto the first dynode, releasing therefrom several secondary electrons for each primary electron. These secondaries are directed by the electron optical system onto the next dynode where each produces more secondaries. This process is repeated at each succeeding dynode or stage of the multiplier, thus producing a greatly multiplied electronic current from the final dynode to the collector. The number of dynodes or stages may be from one to twenty or more depending on the amount of amplification needed. Each succeeding dynode in the chain is maintained at a potential substantially higher, e. g. 100 volts, than the preceding dynode, to accelerate the secondaries from element to element, and the dynodes are preferably shaped to direct and focus the electrons emitted thereby to the next dynode.

Electron multipliers are particularly useful for amplifying electron currents produced by weak signals, such as light, nuclear radiations or radio waves. When used for detecting and/or counting rapidly recurrent signals such as nuclear particles, it is necessary that the multiplier have sufiicient speed and a resolving time low enough to distinguish between successive signals or particles.

In addition, it is desirable that an electron multiplier experience space charge saturation at a high current level so that a high maximum current signal may be obtained.

Accordingly, the objects of this invention concern the provision of an improved electron multiplier which operates with reduced transittime and reduced transit time spread and experiences space charge saturation at a high current level.

According to the present invention, an electron multiplier device includes a primary electron-emitting surface such as a photocathode, a transition dynode for receiving electrons emitted by the primary electron-emitting surface and an electron multiplying chain of dynodes for amplifying electrons emitted by the transition dynode. The multiplier chain is preferably of the zigzag type having two rows of dynodes with the dynodes staggered with respect to each other. Each stage of the multiplier chain includes a dynode and an auxiliary electrode which is positioned adjacent to and in advance of the dynode and which cooperates with the next auxiliary electrode in the opposite row to form, in effect, a cylindrical lens which serves to focus and accelerate 2' electrons onto successive dynodes and to accelerate electron transit through the multiplier chain.

In the drawing:

Fig. 1 is'a sectional elevational view of a device including an electron multiplier embodying the principles of the invention; and, i

Fig. 2 is an elevational view of the electron multiplier portion of the device of Fig. I removed from the envelope of Fig. 1 and enlargedin size and having a modified system of electrical connections.

Referring to the drawing and to Fig. 1, one type of device which may embody the principles of the invention is a photomultiplier tube 10. Such a tube includes a tubular envelope 12 having a base 13 at one end and a comparatively large-area transparent face plate 14 at the other end, on the inner surface of which plate is formed a photocathode 16 adapted to emit photoelectrons when exposed to external radiation represented by the arrows 18.

The photocathode 16 may be of the alkali-antimony type or of any other suitable type and may be formed by evaporation of the selected materials onto the face plate. A conductive film 20 of aluminum or silver or the like, extends along the inner wall of the envelope from the photocathode 16 toward the opposite end of the envelope and serves to provide electrical contact to the photocathode.

An electron optical system is provided within the envelope 12 adjacent to the photocathode 16 and is adapted to focus and accelerate the photoelectrons to an electron multiplier 22 which serves to multiply the photoelectrons. The electron optical system may be of any convenient type and may include, for example, a tubular focusing electrode 24, a pair of deflection plates 26, and a disk accelerating electrode 28 having an aperture 30 through which electrons pass to the electron multiplier 22.

According to the invention, the electron multiplier 22 includes a transition dynode 32 which is generally smoothly curved in the form of a section of a cylinder and has its concave surface facing the aperture 30 in the disk electrode 28. The transition dynode is tilted at an angle to the plane of the aperture so that it is inclined to face also toward the first dynode of a zig-zag multiplier dynode chain 33. The area of the transition dynode 32 is preferably intermediate between the area of the photocathode 16 and the first dynode of the multiplier chain.

' The multiplier chain 33 includes a plurality of electronemitting dynodes, 34, 36, 38, 40, 42, 44, 46, 48, and 50, and a collector electrode 52. The dynodes are arranged in two substantially straight rows, on opposite sides of any axial plane (not shown) extending perpendicularly to the plane of Fig. l. The dynodes are staggered in position with one dynode in one row being in advance of the next dynode in the other row so that, in effect, a zig-zag line 54 may be drawn between each dynode in one row and the next closest dynode in the other row. In elfect, this Zig-Zag line representsthe path followed by electrons as they travel between successive dynodes in the zig-zag multiplier chain. All of the dynodes of the chain 33, except the last, have the same area and shape, each being a section of a cylinder tilted to face the next closest dynode in the opposite row, to promote the focusing of electrons from one to the other. The last dynode 50 is preferably generally cylindrical in form and partially surrounds the collector elec trode 54 which is described in greater detail below.

According to the invention, auxiliary means are provided for focusing and accelerating electrons through the multiplier. The auxiliary means comprises curved non-emissive electrodes 56, 58, 60, 62, 64, 66, 68, 70 and 71 disposed ahead of the dynodes 34, 36, 38, 40, 42,

44, 46, 48, and 50, respectively. Each of the auxiliary electrodes follows the general contour of the dynode which it precedes and it is generally of shorter length than the dynode. Thus disposed, each pair of adjacent auxiliary electrodes, for example, electrodes 56 and 58, 58 and 60 and so forth, comprises a cylindrical electron lens.

The collector electrode 52 comprises a thin flat plate disposed substantially parallel to and just outside of the main path of electron flow from the dynode 48 to the last dynode 50. Thus,the collector does not intercept electrons traveling in this path. At the same time, the collector presents a favorably large surface to the last dynode which is so curved that the secondary electrons emitted therefrom are focused on the collector. Since the collector is a solid flat plate, electrons which arrive there are not subject to oscillation as they are in a gridtype collector.

Suitable arrangements for supporting the various electrodes of'the electron-optical lens system and the multiplier chain are well known in the art and any of these may be employed. As shown in Fig. l, the multiplier chain may be supported by a pair of fiat, parallel mica plates, one of which, 72, is shown, which may be secured to opposite ends of the electrodes and dynodes of the multiplier chain 22. The length of the transition dynode 32, each of the dynodes of the multiplier chain 33 and the collector 52 in the direction perpendicular to the plate of Fig. 1 may be several times the lengths of the wall portions shown in section in Fig. l.

The tube is provided with separate electric-a1 leads for each of the dynodes, auxiliary electrodes and collector electrode for applying separate D. C. potentials thereto. Perferably, these leads extend through the base 13 of the envelope 21. Only four leads 74 are shown for simplicity, the rest being shown schematically as arrowtipped lines. As typical examples of electrode operatingpotentials, the cathode 16 may be operated at zero potential, the electrodes 24, 26 and 28 of the electron optical system at about 100 volts, the transition dynode 32 may be maintained at 100 volts and each succeeding dynode may be 100 volts higher than the preceding dynode, in which case, with the number of dynodes shown, the last dynode carries a potential of 1000 volts. The collector may be operated at a potential about 1000 volts higher than that of the last dynode, or about 2000 volts, in the example shown.

Fig. 2 shows an alternative arrangement for making electrical connections to the multiplier portion 22 of the tube 10. The multiplier is shown enlarged and removed from the envelope. In this alternative arrangement, all of the auxiliary electrodes are connected within the tube envelope to points having the desired positive potential. Thus, a lead 76 may be connected between the auxiliary electrode 56 and the dynode 38, a lead 78 between the auxiliary electrode 58 and the dynode 40, a lead 80 between the auxiliary electrode 60 and the dynode 42, a lead 82 between the auxiliary electrode 62 and the dynode 44, a lead 84 between the auxiliary electrode 64 and the dynode 46, a lead 86 between the auxiliary electrode 66 and the dynode 48, a lead 88' between the auxiliary electrode 68 and the dynode 50, and a lead 90 between the auxiliary electrode 70 and the collector 52. The auxiliary electrode 71, in this arrangement, is provided with a separate external lead 92. In this alternative arrangement, external leads, shown as arrowtipped lines are required only for the transition dynode, the dynodes of the multiplier chain, the collector electrode and the last auxiliary electrode.

What is claimed is:

1. An electron multiplier comprising a plurality of electron-emitting dynodes and a non-emissive auxiliary electrode adjacent to and in advance of each dynode and constituting, in effect, a continuation of the surface thereof, and means connected to each auxiliary electrode for applying a separate potential thereto.

2. An electron multiplier comprising a plurality of curved dynodes and a curved non-emissive auxiliary electrode in advance of each dynode and constituting, in effect, a continuation of the surface thereof.

3. An electron multiplier chain including a plurality of dynode-electrode pairs, each dynode-electrode pair comprising a curved dynode and a curved electrode adjacent to and in advance of it, each electrode of each pair constituting, in effect, a smooth continuation of the surface of each dynode of each pair.

4. An electronrnultiplier comprising a plurality of curved dynodes arranged in zig-zag relationship on opposite sides of an axial plane, and a curved electrode adjacent to and in advance of each dynode and comprising, in effect, a continuation of each dynode.

5. An electron device comprising a source of elec trons, a collector of electrons, a plurality of electronemitting dynodes placed between said source and said collector, a plurality of two-part electron lenses each between two difterent successive dynodes, one part of each two part lens being common to a succeeding two part lens, and connection means to each lens part for applying a separate operating potential thereto.

6. An electron multiplier chain comprising a plurality of dynodes arranged in zig-zag relation in two rows oriented on opposite sides of an axial plane, generally cylindrical electron lenses between successive dynodes in said chain, and separate electrical connections to the lenses for applying separate operating potentials thereto.

7. The device defined in claim 6 wherein said primary source of electrons is a photocathode.

8. The device defined in claim 6 and including a collector electrode partially surrounded by one of said dynodes and comprising a flat plate disposed parallel to and out of the path of electrons between said one of said dynodes and another dynode.

References Cited in the file of this patent UNITED STATES PATENT S 2,159,519 Brauer et al. May 23, 1939 2,200,722 Pierce et al. May 14, 1940 2,702,865 Herzog -4 Feb. 22, 1955

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