US2231698A - Electron multiplier - Google Patents

Description

Fell 1l, 1941-. v. K. zwoRYKlN Erm. 2,231,698

ELECTRON MULTIPLIER Filed June 10, 1958 nvento'n Patented Feb. 11, 1941 UNITED STATES PATENT OFFICE Application June 10, 1938, Serial No. 212,890

1 Claim.

This invention relates-to electric discharge devices and particularly to electron multipliers and has for its principal object the provision of improVements in the construction and operation 5 of photo-actuated electron multipliers.

The present invention is predicated upon an appreciation of the fact that photoemission and secondary emission are not directly related. Photosensitive electrodes, especially those of high sensitivity, special spectral response, and low thermionic emission, are generally not good secondary emitters. The converse is also true, i. e., good secondary emitters are not always good photoemitters.

l5 In a photo-actuated electron multiplierthe preparation of a photoemissive surface necessitates the deposition of an alkali or other metal. the diffusion of which onto the multiplying electrodes is not always desirable. By way of example, if the multiplying electrodes are constituted of an alloy of gold or silver or copper and magnesium united in a thermodynamically stable alloy, as disclosed in the copending application Serial No. 197,994, filed March 25, 1938, of

Zworykin et al., their secondary emitting propertes would be considerably altered if coated with an alkali metal or other photosen'sitive deposit. Furthermore, the presence of an alkali metal in the multiplying part of the device may cause serious leakage and maylower the work function of the secondary emitting stages, thus giving rise to considerable undesired thermionic emission.

In this invention, the above-mentioned difliculties are obviated by dividing the tube into two separate compartments, one of which contains the photocathode and the other of which contains the multiplying and output electrodes. The sole communicating channel between the two compartments is an aperture in a tight partition or connecting tube. By means of a suitable electron optical arrangement, the electrons released from the surface of the photoemissive cathode are made to strike the first multiplying electrode after they have passed through the aperture in the partition dividing the two compartments. If the rate of diffusion of the cathode metal vapor is small, a permanent slit or a very narrow connecting tube will prevent all disturbing effects. Otherwise, the aperture or the connecting tube can be closed during the process of activation of the multiplier land later opened to permit v passage of the electrons. Various schemes for closing, covering or plugging the aperture in vacuo will be later described.

Other objects and advantages, together with certain details of construction, will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein 5 Figure 1 is a sectional elevation of a photosensitive electron multiplier constructed in accordance with the invention and wherein the cathode'and multiplying electrodes are in separate compartments, 10

Figure 2 is a fragmentary sectional elevation of a. modification of the device of Fig. 1, and

Figure 3 is a fragmentary sectional elevation of an electron lens system for focusing the photoelectrons upon the first multiplying electrode. 15

In Fig. 1, l designates an elongated highly evacuated preferably cylindrical envelope, the central longitudinal axis of which is marked :is-x. A photocathode 2 is brushed` sprayed, or otherwise depositedy on the inner surface of the 20 hemispherical end of and later activated in situ as by caesium vapor admitted through the side seal s. The emissive area of 2 may be limited to a circle viewed from the center of the sphere under an angle of and, in any event. 25 is accessible to light rays from an external source exemplied in the drawing by a lamp 3 and lens l.

A partition which may be of metal and may comprise three planes 5a, 5b and 5c, fits tightly 30 against the walls of the cylindrical envelope, as indicated at 6. The normal n-n to the central plane 5b of the partition preferably makes an angle of 60 with respect to the central axis :v-x of the main cylindrical portion ofthe en- 35 velope, The part 5b is provided with a circular or other shape orifice l i-n its center. The edge about I ls preferably rounded and thickened as indicated at 'la to minimize cold emission.

The rst multiplying electrode I0 has ay de- 40 pendent shield comprising two planes Illa and Illb forming a part thereof. A circular aperture I0c in the center of Ilbis the directrice of a portion of a focusing cylinder Illd whose generatrices are parallel to the axis n-n. curved cathode 2, the apertures l and lllc and the focusing rings 'la and Illd constitute an electron optical system having axial symmetry around the axis n n.

'Ihe succeeding multiplying electrodes ll to 50 i9, inclusive, are of the general L-shape type described by Rajchman and Pike in copending application Serial No. 171,916, filed October 30, 1937. The illustrated cylindrical output electrode or anode 2l is similar to that described by 65 The 45 Snyder in copending application Serial No. 205,- 208, iiled April 3i), 1938.

As above described, the photocathode 2 is painted on the glass wall of the envelope. This facilitates the usual alkali activation in that it permits a positive control of the surface under preparation. The alkali metal, which may be caesium, rubidium, potassium, or other substance which is the emissive equivalent of caesium, is introduced into the envelope in the form of a vapor through the side seal s. A door or gate l turning on hinges 8a may be used to close the aperture 1 during activation. Alternatively, and as described in connection with Fig. 2, the aperture between the compartments may be closed by a sphere (B, Fig. 2), which is afterwardsremoved from its seat in the aperture by tipping the tube. In this, or equivalent manner, the secondary electron-emitting electrodes |y| to I9 inclusive and the anode 20 are protected against the diilusion of caesium vapor from the compartment containing the photosensitive cathode.

The shield Ia, |b, about the rst multiplying electrode Ill, electrically separates the axially symmetrical electron optical system, which comprisas the cathode 2 and the small focusing cylinders or rings 1a and |0d, from the cylindrical system comprising electrodes to 20, inclusive. One is thus able to more easily calculate in advance the relative design position and arrangement of the several multiplying electrodes.

Fig. 2 shows a tube 30 in which the part 3| containing the photoemissive electrode 32 is connected through a narrow tubulation 33 to the part 34 which encloses the multiplying electrode assembly. y The part 3| has approximately a spherical shape. I'he inner surface of the base of the sphere 3| is metallized and constitutes a photocathode 32 whose shape is such as to focus the photoelectrons onto a multiplying electrode 35. The electrons from electrode 35 are focused onto electrode 40 by means of a cylinder or conduit 36 and a ring 31 in the shield 38 oi mult tiplying electrode 40. The electron optical system comprising the cathode 32, electrode 35, cylinder 36 and ring 31 has axial symmetry. The electrode system including the multiplying electrodes 4| to 41, etc., is cylindrical, i. e. the equipotential surfaces are cylinders.

Electrode 40 has a shape capable of focusing secondary electrons onto electrode 4|. The electrodes 4| to 41, inclusive, are similar to those described in connection with Fig. 1.

This embodiment (Fig. 2) of the invention is of particular utility when the activation of the photoand secondary-emissive surfaces necessitates, or is improved by, treatment at diii'erent temperatures. In this case, temperatures of compartments 3| and 34 can easily be controlled independently. A ball B seated in the opening of the conduit 33 provides an easy way ot separating the compartments 3| and 34 during the activation of the cathode.

Electrode 35 and the photocathode surfaces 32 are similarly activated. 'I'his gives to electrode 35 a relatively poor secondary emission ratio. However, when caesium is employed, the secondary emissive ratio will be greater than unity and sufficient to ensure an electronic current of an intensity to actuate electrode 40.

The original electron stream may be controlled by means oi' auxiliary electrodes inserted in the said electron conduit and suitably energized. 'Ihe control of the ultimate output current may thus be made much more sensitive than that resulting from control of the electron stream further along in the tube.

Fig. 3 shows such a modification of the invention. It represents in large scale the electron conduit of the device illustrated in Fig. 1 modliied by the addition of two rings 50 and 5|, the

' aperture 1 and ring |0d. 'I'he ring 5|) has a high positive potential with respect to the partition and may have the potential of any one of the early multiplying electrodes I0, or I2. The photocathode and the rings 1 and 50 constitute an electron lens system which focuses the photoelectrons through the electron conduit 1, 50, 5|, i3d. By varying the potential applied to the ring 5| with respect to that of the photocathode, electrons can be made either to strike the emissive surface of electrode I0 after passing through all of the lens elements, or else be made to turn back in the region ring 5| and be collected by and dissipated by the ring 50.

Other modiilcations of the invention Will Suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claim.

What is claimed is:

An electron multiplier comprising an envelope having two evacuated compartments, a photosensitive cathode in one compartment, a plurality of multiplying electrodes in the other compartment, a wall of one compartment having an aperture therein which communicates with the other compartment, means comprising a plurality of electron lens elements mounted adjacent said aperture for focusing electrons from said cathode through said aperture and thence against one of said multiplying electrodes, and means mounted within the compartment which contains said multiplying electrodes for electrically shielding said electron lens elements from said plurality of multiplying electrodes.

VLADIIKIR K. ZWORYKI'N. JAN A. RAJCHMAN.

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