US2908840A - Photo-emissive device - Google Patents

Description

United States Patent O 2,908,840 PHOTO-EMISSIVE DEVICE Robert H. Anderson, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to a photo-emissive device and has special reference to improvements in electron-optical structures for tubes such as high vacuum photo-multiplier tubes.

Photo-multiplier tubes, having large area photo-cathodes wherein a photo-emissive cathode surface is exposed toa light source, have been used as scintillation counters. In the operation of such a tube, when radiations from radioactive materials are caused to fall upon a phosphor and activate it to luminescence, the light from the phosphorcauses photoemission of electrons from the photocathode of the tube. The photo-electrons are then directed, by-an electrostatic electron-optical eld, to a photomultiplier structure within the tube. The photo-multiplier structure usually comprises a staggered array of electrodes each adapted to produce secondary emission when subjected to electron bombardment. Amplification of the current represented by the photoelectrons from the cathode is thus achieved by secondary' emission phenomena.

It is often desirable to have a photo-multiplier tube which has a relatively small cross-sectional area in order that the tube be adapted for use in applications such as in oil well pipes for radioactive prospecting. When a staggered array of photo-multiplier electrodes are used in such a tube, successive multiplier electrodes are disposed at opposite sides of the multiplier structure in order that electrons emitted from each of the electrodes be directed to the succeeding one. Thus, the first of the multiplier electrodes is disposed at one side of the multiplier structure. However, electrons from a symmetrically disposed photo-cathode are ordinarily directed toward the center axis of the tube since the photo-cathode usually occupies an entire cross-sectional area at one end of the tube. Consequently, previous photo-multiplier tubes utilizing a staggered array of multiplier electrodes have required a 'relatively' large cross-sectional tube area in order to accommodate the iirst multiplier electrode on the tube axis and successive multiplier electrodes alternately to one side of and along the tube axis, the space on the opposite side of the tube axis being unused.

It isthus an object of the invention to provide an improved phototube having a relatively large area photocathode and a minimlun cross-sectional tube area.

It is another object of the invention to provide an improvedphototube comprising a relatively large area pho-` tocathode and a photo-multiplier structure having the first electrode disposed therein adjacent to one side of the tube.

Stated generally, the foregoing and related objects are achieved in accordance with the invention by the provision of a phototube having a photo-emissive cathode disposed symmetrically about an axis and a photo-multiplier structure having the rst electrode disposed at one side of the tube. An electron lens disposed between the photocathode and the irst electrode directs the electron emissionfrom the, photo'cathode to one side of the tube and toward the first electrode. The electron lens comprises apair of coaxial cylinders, the adjacent ends of which angle with respect to their common axis. The cylindersv thus create, between their adjacent ends, an electrostatic electron-optical eld which directs photo-electrons to the one side of the axis of the tube. The oi-axis arrangement of the rst electrode of the multiplier structure enables the multiplier structure to be mounted along the axis with alternate multiplier electrodes on opposite sides of the axis. This enables the tube to have a relatively large area photocathode and a relatively small crossv sectional tube area.

The invention is described in greater detail in connection with the accompanying single sheet of drawings wherein:

Fig. 1 is a partially broken away side view of a photomultiplier tube embodying the invention; and

Fig. 2 is a fragmentary sectional view of a portion of a photo-multiplier tube similar. to that shown in Fig. l and illustrating the potential distribution within a portion of an electron-optical structure therewithin.

Fig. 1 shows an embodiment of the invention as applied to a photo tube of standard form and having a relatively small cross-sectional area. Such a tube is used in applications involving light sources in restricted spaces, such as scintillation counters ,for the detection and measurement of nuclear radiation within an oil Well pipe. The tube comprises a tubular glass envelope 10, closed at one end thereof with ya wall section 11, transverse to a tube axis 12, and upon which is formed a transparent photocathode surface13 in the form-of an electrically continuous photo-emissive iilm. In one tube of this type the end-wall portion 11 has a diameter of approximately inch. y

The tube is provided with an accelerating electrode 14 which is spaced from the photocathode surface 13. The accelerating electrode 14 is in the `form of a metal disc and has an off-center aperture 15. A tubular shield 16 is xed to the acceleratingelectrode 14 around the portions dening the periphery of the aperture 15 and on the side of the electrode 14 adjacent to the photocathode 13. The shield 16 serves to protect electrode elements within the tube from contamination by material which is evaporated from the side of the accelerating electrode 14 adjacent to the photocathode 13 during a step in the manufacture of the latter. The accelerating electrode 14 also has a cylindrical extension portion which forms a focusing electrode 17 extending from the outer edge of the accelerating electrode 14 and toward the cathode 13. In accordance with this invention, the cathode end of the focusing electrode 17 is at an oblique angle with respect to planes normal to the tube axis 12 in order to aidin the establishing of an electron-optical field to be described below.

A metallic wall coating 18, which may for example be a relatively thin iilm or layer of aluminum, is formed on the inner surface of the tube envelope by well-known aluminizing techniques. The coating 18 extends from the photocathode surface 113 axially down the tube wall to a portion below the acceleratingelectrode 14 and thus forms a cylindrical, second focusing electrode coaxial with the rst focusing electrode 17. Electrical contact is made to the photocathode 13 by means of a lead 19 which connects the photocathode 13 through the electrode coating 18 to a lead pin 20 sealed through the tube base 21. The coating 18 is connected to the lead 19 by means of a metallic @nger 22 which is fixed to l the lead 19 and which is spring biased to contact the and into a photo-multiplier structure 23 disposed on the side of the accelerating electrode 14 remote from the cathode 13, in a manner described in detail below.

The photo-multipler structure'23 includes a plurality of electron multiplier electrodes or dynodes 24, 25, 26, 27, 28, 29, and 30. These are disposed in a staggered array, successive ones of the dynodes being disposed on opposite sides of the tube axis 12. Photoelectrons from the photocathode 13 impinge upon a first dynode or collector electrode (which is divided into two portions 24 and 25) and initiate secondary emission therefrom having a ratio greater than unity. This secondary emission is accelerated and directed by fixed electrostatic fields along curved paths to successive dynodes 26 through 30, the fields being formed by biasing each successive dynote at a predetermined positive potential, say 100 volts, with respect to the dynode previous to it. Each dynode provides an amplification of the current represented by the electrons striking it. There is thus formed an `ever increasing stream of electrons until those emitted by the last dynode 30 are collected by an anode electrode 31. The current collected by the anode electrode 31 constitutes the current in the output circuit of the tube.

In accordance with the invention, the first dynode 24 and is mounted eccentrically with respect to the tube axis 12. The first dynode is electrically connected to the accelerating electrode 14, has a larger electron receiving surface area than that of the other dynodes and, as mentioned above, is divided into two substantially fiat portions 24 and 25. The larger of the two portions of the first dynode, the one indicated by numeral 24, is the main portion of this dynode and is disposed on one side of the tube axis and serves as a collector of most of the photoelectrons emitted from the cathode 143. The smaller of the first dynode portions, the one indicated by numeral 25, is disposed adjacent to the tube axis and serves to intercept photoelectrons which for any reason pass through the accelerating electrode aperture 15 but fail to impinge upon the larger of the first dynode portions. Secondary emission electrons from the first dynode portions 24 and 25 are then accelerated toward the second dynode 26 of the multiplier structure by the second dynode which is positively biased with respect to the first dynode 24 and 25.

A rod 32, disposed between the first and second dynodes but adjacent to and electrically connected to the accelerating electrode 14, aids in directing electrons from the first dynode portions 24 and 25 toward the second dynode 26. This is accomplished since the rod 32 and the smaller first dynode portion 25 maintain the space potential between them at a lower potential than that between the rod 32 and the relatively highly positive third dynode 27. Consequently, electrons from the first dynode portions 24 and 25 are drawn into the space between the rod 32 and the third dynode 27 and impinge upon the lower portion (in the drawing) of the second dynode 26. Secondary emission electrons from the second dynode 26 are then directed by the electrostatic field between it and the third dynode 27 onto the third dynode where more secondary emission is produced. The ever increasing stream of electrons is similarly directed to successive dynodes 28, 29, and 3f) and, as indicated before, are finally collected by the anode electrode 31. The dynodes of the photo-multiplier structure other than the first dynode have the surfaces thereof in the form of portions of cylinders of circular cross-section which permits of the use of simple and inexpensive tools in their construction.

The electron-optical focusing structure of the invention which comprises the first focusing electrode 17 and the second focusing electrode 18, directs electrons from the photocathode 13 to one side of the tube axis and into the photo-multiplier structure described above. This is accomplished by providing, as indicated in Fig. l, a potential differencerof about 150 volts between the accelerating electrode 14 and photocathode 13. An electrostatic lens field is thus formed between the positively biased accelerating electrode 14 and the photocathode. Since, according to the invention, the end of the focusing electrode 17 is at an oblique angle with respect to planes normal to the tube axis 12 (and thus at an oblique angle with respect to the coating 18 which is at cathode potential) a skewed electrostatic electron-optical field or lens is formed between the electrode 17 and the coating 18. Since the vertex of the oblique angle, formed between the plane defining the end of the focusing electrode 17 adjacent to the cathode 13 and the other focusing electrode 18, is disposed adjacent to the side of the tube at which the main portion of the first dynode 24 is positioned, the electron-optical structure formed by these two coaxial electrodes 17 and 18 directs electrons from the cathode 13 to one side of the tube axis 12 and onto the main portion of the first dynode 24. This will be explained in greater detail in Fig. 2.

As shown in Fig. 2, where the coating 18 extends for only a relatively short axial distance from the photocathode 13, the potential distribution between the electron-optical field forming members (the focusing electrode 17 and the coating 18) directs photo-electrons toward the side of the tube adjacent to the vertex of the oblique angle between the end of the focusing electrode 17 and a plane 33 defined by the portion of the other focusing electrode, the coating 18, adjacent to the end of the first focusing electrode 17. In the tube described this plane 33 is normal to the tube axis 12. Thus electrons, the paths of which are indicated by dashed lines 34, are directed by the aforementioned electron-optical field from the photocathode 13 through the aperture 15 and into one side of a multiplier structure (not shown in Fig. 2) where the electrons impinge upon a relatively small area upon the first dynode thereof. In the same manner electrons from the cathode 13 of the tube of Fig. 1 are directed through the aperture 15 in the accelerating electrode 14, the electron-optical fields in the tubes of Figs. l and 2 being substantially the same.

The invention has been described with respect to an electron-optical structure comprising a pair of coaxial tubular electrodes one of which has an end adjacent to the other electrode which terminates in a plane normal to their common axis. It will lbe appreciated that the invention may instead take other forms. For example, the two focusing electrodes may both have their adjacent ends at an oblique angle with respect to the tube axis. In this case the ends may lie in parallel planes or in planes of an oblique angle with each other. Also, either of the two focusing electrodes may be in the form of a coating such as that described with respect to the second focusing electrode, coating 18 in the drawing; then, too, both of the focusing electrodes may be in the form of coatings or both may be in the form of sheet metal tubular elements such as the first focusing electrode 17.

From the foregoing it will be apparent that the improved phototube of the invention, which includes an electron-optical structure for directing electrons from a photo-emissive cathode to a collectorA electrode at one sideof the tube axis, enables the use of a relatively large area photocathode and at the same time a relatively small cross sectional tube area.

What is claimed is:

1. A photomultiplier tube comprising an elongated tubular envelope having a closed end and a longitudinal axis, a photoemissive area on said closed end and extending substantially normal to said axis, said photoemissive area being substantially coaxially arranged around said axis, an electron multiplier within said envelope and including a plurality of electrodes mounted in a staggered array on opposite sides of said axis, a hollow tubular electrode mounted coaxially around said axis and between said photoemissive Varea and said elec-A tron multiplier, said tubular electrode having one end closed except for an aperture adjacent to said electron multiplier, said aperture being positioned eccentrcally with respect to said axis and being aligned with the first electrode of said electron multiplier, the other end of said tubular electrode being open and deiining a plane oriented at an oblique langle with respect to said axis so that electrons from said coaxial photoemissive area are directed toward said eccentric aperture.

2. The tube defined in claim 1 wherein the portion of said plane lying closest to said photoemissive layer is on the same side of said longitudinal axis as Said aperture.

3. A phototube comprising an elongated envelope having a longitudinal axis, a photoemissive member within one end of -said envelope and normal to said axis, said photoemissive member being arranged substantially coaxially with respect to said axis, a plurality of electron multiplier electrodes mounted adjacent to the other end of said envelope and within said envelope, the electron multiplier electrode nearest said photoemissive member being mounted eecentrically with respect to said longi- References Cited in the iile of this patent UNITED STATES PATENTS 2,231,698 Zworykin et a1. Feb. 11, 1941 2,459,778 Larson Jan. 18, 1949 2,652,515 McGee Sept. 15, 1953 2,653,993 Schroeder et al. Sept. 15, 1953 2,728,014 Stoudenheimer et al Dec. 20, 1955 2,796,547 Nevin June 18, 1957

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