US3407324A - Electron multiplier comprising wafer having secondary-emissive channels - Google Patents

Description

Oct. 22, 1968 ROME 3,407,324

ELECTRON MULTIPLIER COMPRISING WAFER HAVING SECONDARY-EMISS IVE CHANNELS 2 Sheets-Sheet 1 Filed June 21, 1967 POWER suppu 22 INVENTOR.

Martin Rome ATTORNEY Oct. 22, 1968 M. ROME 3,407,324

ELECTRON MULTIPLIER COMPRISING WAFER HAVING SECONDARY-EMI SS IVE CHANNELS 2 Sheets-Sheet 2 Filed June 21, 1967 FIG 4 United States Patent 3,407,324 ELECTRON MULTIPLIER COMPRISING WAFER HAVING SECONDARY-EMISSIVE CHANNELS Martin Rome, Princeton, N.J., assignor to Electra-Mechanical Research, Inc., Princeton, N.J., a corporation of Connecticut Filed June 21, 1967, Ser. No. 647,822 5 Claims. (Cl. 313-95) ABSTRACT OF THE DISCLOSURE An illustrative embodiment of the invention discloses a lano-concave wafer of channel electron multipliers in an electron-optical system that produces inverted and intensified images of faintly illuminated objects. The individual channels are relatively short at the center of the Wafer and gradually increase in length toward the periphery in order to produce the concave equipotential surface needed for electron image inversion. The respective channel diameters are varied in accordance with each channel length to maintain a generally constant ratio of length to diameter and the associated electron gain for all of the channels in the wafer.

BACKGROUND OF THE INVENTION Field of the invention Description of the prior art A need exists for a compact, lightweight, rugged and relatively inexpensive device that can invert and intensify images of poorly illuminated objects. In astronomy, for example, the study of stars of lesser brightness will benefit from equipment of this character through the improved visual image resulting from the intensification of the light emitted by the'body. Fluoroscopy is another field that requires equipment of this sort. The application of desirably low radiation dosages to the patient undergoing an examination produces a poor fluoroscopic image that must be intensified to bring out satisfactory contrasts.

In the past, this need has been satisfied, at least in part, by electrostatic focus image intensifiers. Intensifiers of this sort, however, are heavy and complex.

Channel electron multiplier wafers also have been suggested for use in image intensifiers. Basically, these wafers are comprised of thousands of passageways or channels so arranged in a thin disk that the axes of the channels are more or less perpendicular to the parallel sides of the disk. The diameter of the individual channels in disks of this sort typically are about .001 inch, while the wafer thickness usually is about .05 inch.

In order to operate a water of this sort, a potential of approximately 2000 volts is applied across the opposite sides of the disk. A photocathode that emits electrons in response to light stimulation is placed adjacent and parallel to the low potential side of the wafer. Upon exposure to light, the photocathode emits electrons in proportion to the intensity of the light. These electrons enter the adjacent wafer channels and, under the influence of the applied voltage, strike the walls of the channels and release secondary electrons. If the wafer is made of a substance that has a high coefiicient for secondary electron emission, such as lead or vanadium-phosphate glass, there is a net increase in thenumber of electrons in the channel after each collision, Consequently, subsequent 3,407,324 Patented Oct. 22, 1968 collisions with the walls produce an amplified electron output at the high potential side of the disk that is generally prooprtional to the electron input from the adjacent portion of the photocathode.

The electrons from the high voltage side of the disk are accelerated by an electrical field across a gap to an anode, or a luminescent screen, that emits light in proportion to the intensity of the electron bombardment. Accordingly, the screen provides an intensified visual image of the object that caused the initial electron emission from the photocathode.

The intensified images produced by these devices, however, are not inverted. This is a serious inadequacy when these intensifiers are used in systems that invert the optical image. In this circumstance the image on the luminescent screen is up-side down, an undesirable situation that leads to confusion and error.

It is an object of the invention to provide a channel electron multiplier water for an inverted image intensifier.

It is another object of the invention to provide an improved image intensifier.

It is still another object of the invention to provide a method for manufacturing an improved channel electron multiplier wafer.

It is still a further object of the invention to provide a channel electron multiplier wafer that has improved electron-optical characteristics.

SUMMARY In accordance with the invention, a channel electron multiplier wafer is provided With electron-optical characteristics in order to manipulate electrons in a way that is somewhat analogous to the control of light rays with lens system. Thus, by imparting a lens-like shape to a channel electron multiplier wafer, the electron output from the water can be converged, diverged, or otherwise controlled, to produce specific image effects; as, for example, by converging the electrons to produce an inverted image.

For a given applied voltage and electron input, the electron multiplication factor, or gain, of each channel generally is a function of the ratio of the channel length to the channel diameter. Consequently, a failure to provide a substantially uniform gain from all of the channels in a wafer will produce irregularities in the intensity of the light emitted by the luminescent viewing screen relative to the electron input and thereby cause a brightness variation of the visual image. Thus, for example, if a piano-concave wafer is produced without further consideration to the gain characteristics of the individual channels at a given voltage, the longer peripheral channels will have different gains than the shorter central channels because the length to channel diameter ratio is greatest at the wafer periphery.

This problem has been overcome through a further aspect of the invention. Accordingly, the diameters of the channels in the lens-shaped wafer are controlled relative to the respective channel lengths in order to establish a predetermined gain relationship between all of the channels. In this manner, a substantially uniform gain is established for all of the channels in a lenticular wafer in order to prevent the visual image from being distorted through gain irregularities.

Alternatively, in those applications for which controlled shading of image brightness is desirable, the channel length to diameter ratios can be varied to establish a predetermined relation. Controlled gain variations among the channels within a wafer thus produce the desired shading in an entirely novel manner.

More particularly, an illustrative embodiment of an image intensification and inversion device according to the present invention comprises a photocathode faceplate spaced about one millimeter or less from the low-potential side of a plano-concave channel electron multiplier wafer. Electrons are emitted from portions of the photocathode in proportion to the respective incident light stimulation. The electrons traverse the spacing and, under the influence of an applied electrical field, enter adjacent channels at the low-potential, planar side of the wafer,

The diameter of the channels increase as the wafer periphery is approached, to keep the length to diameter ratio for all of the channels at a constant value in the range from about 30 to 1 to about 100 to 1 so the electron gain from each channel in the Wafer is generally the same.

The concave nature of the high potential side of the wafer, and the uniform voltage applied to that side, establish an electrical field that focuses the electrons and causes the electron output from the wafer to converge. The focal effect of the electrical field established by the concave shape of the high potential side is enhanced further by a hollow cylinder that protrudes from the wafer toward the luminescent viewing screen. This cylinder is at the same voltage as the concave output side of the wafer and provides an electrode lens, the electrical field of which increases the converging influence of the wafer concavity. The converging electrons are accelerated toward a crossover point and the luminescent scrceen by a hollow conical anode. The apex of the anode is positioned at the electron crossover. An aperture is formed in the apex to enable the converging electrons to enter the interior of the conical anode and strike the phosphor on the luminescent viewing screen that forms the base of the cone.

In traversing the cross-over, the electrons diverge and reverse their relative spatial relation in order to establish an inverted image on the screen. When coupled to systems that invert the object, the intensified image produced in accordance with the present invention is, in effect, a reinverted image. Thus, the intensified image is not up-side down and is oriented correctly relative to the object under consideration. This feature of the invention eliminates the bulk and complexity of a subsequent inverter in conjunction with prior art intensifiers that failed to provide a properly oriented image.

As an alternative embodiment of the invention, the photocathode can be deposited directly on the planar and low-potential side of the wafer.

The method of manufacturing the wafer provides still a further aspect of the invention. Accordingly, a group of tubes of uniform inside diameters that are formed from an appropriate secondary electron emitting material, is organized as a central and hexagonal bundle. A circumferential layer of similar hexagonal tube bundles, in which the tubes have larger inner diameters greater than those of the central bundle, is placed around the periphery of the central bundle. Additional layers of bundles containing tubes of a larger inner diameter greater than the tubes in the adjacent innermost bundles are arranged concentrically about the central bundle so that the tube diameters increase progressively with distance from the center of the entire array. This array is heated and drawn or stretched out one or more times to fuse all of the bundles together and to reduce proportionately the inner diameters of the individual tubes,

Two transverse slices are made in the drawn and fused array of tubes in order to produce a thin disk in which the ratio of the length of the outermost concentric annulus of channels to the respective channel diameters is in the range from about 30 to 1 up to about 100 to 1. The concave surface of the wafer can subsequently be formed by grinding to establish the appropriate radius of curvature and the correct inner channel lengths required to maintain about the same length to diameter ratio as the outermost channels.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

Brief deseriptioiz of the drawing FIG. 1 is a perspective view of one embodiment of the invention, in which some portions of the structure have been broken away or shown in section. The view in FIG. 1 is not drawn to scale in order to illustrate better some of the novel features of the device;

FIG. 2 is a longitudinal section of the, embodiment of the invention shown in FIG. 1;

FIG. 3 is a perspective view in partial section of a typical channel electron multiplier wafer according to one embodiment of the invention, in which the longitudinal dimension of the device has been greatly exaggerated for illustrative purposes; and

FIG. 4 shoWS, in full section, an illustrative alternative wafer structure for the image intensifier of FIG. 2.

Description of the preferred embodiments The embodiment of the image intensifier shown in FIG. 1 provides a more complete appreciation of the principles of the invention. Accordingly, a faintly illuminated object 10, which may be a low intensity source of light or a poorly illuminated object, radiates, or re-radiates, the schematically shown light rays 11 and 12. The light rays 11 and 12 strike a transparent glass input faceplate 13.

A photocathode material 14 coating the surface of the faceplate 13 opposite from the object 10 responds to these light rays by emitting electrons from the stimulated portion of the photocathode material 14 in an abundance that is proportional to the intensity of the respective light rays 11 and 12 incident thereupon. These electrons, schematically shown by the lines 15 and 16, emitted in response to an image 17 formed on the input faceplate 13 by the light rays 11 and 12 traverse a short space of about one millimeter or less to a planar, lowpotential side 20 of a piano-concave channel electron multiplier wafer 21.

A power supply 22 applies a positive potential through conductors 18 and 19 to the low potential side 20 of the wafer 21 relative to the photocathode 14. This positive potential is uniformly distributed across the flat surface of the wafer 21 by an electrically conductive film (not shown) of gold or a similar conductive layer. This positive voltage gradient accelerates the electrons 15 and 16 directly across the gap from the photocathode 14 to the wafer 21.

The power supply 22 also applies a positive potential of about 2000 volts through a conductor 27 to a concave high potential side 23 of the wafer 21 relative to the longitudinally opposite low voltage side 20. The voltage applied to the concave surface 23 also is distributed uniformly through an electrically conductive film (not shown).

The electrons 15 and 16 from the photocathode 14 enter adjacent longitudinal channels 24 that are formed in and pass through the wafer 21. Under the influence of the positive voltage gradient established in the channels 24 by the potential applied to the wafer 21, the electrons 1S and 16 are accelerated through the channels 24 and strike the channel surfaces 28 with sufficient energy to produce secondary emission electrons. Because the wafer 21 is formed of a material, such as lead glass, that has a high secondary emission coefficient, each collision between an electron and a channel surface 28 produces several secondary electrons that, in turn, are accelerated further through the channel to strike the surface thereof and generate still more secondary electrons. In this manner, the channels 24 amplify or multiply the electron input to the wafer 21, in proportion to the input intensity.

As hereinbefore mentioned, the electron gain of each channel is, for a specific input electron intensity and applied voltage, a function of the channel length to diameter ratio. As shown in FIG. 3, to provide the planar surface 20 and the concave surface 23 of the wafer 21, the central channels 24A necessarily must be shorter than the peripheral channels 24B. The electron gain of the shorter channels 24A is maintained at about the same value as the gain of the longer channels 24B by reducing the shorter channel diameters to an extent that is commensurate with the respective channel lengths, in order to keep the length to diameter ratio of both groups of channels'approxirnately the same.

Turning. again to FIG. 1, ahollow cylindrical electrode lens 26 of electrically conductive materialis secured to and at the same potential as the concave side 23 of the wafer 21. The electrode lens 26 protrudes longitudinally away from the wafer 21 and is concentric with the concavity in the side 23. The lens 26, moreover, terminates in a radiallyinward directed flange. The equipotential electrical field established by the electrode lens 26 and the concave side 23 of the wafer 21 causes the electrons emitted from the wafer to converge at a cross-over point 30. The electrons emitted from the wafer 21, moreover,- are accelerated to. the crossover. point 30 by a positive potential gradient of to kilovolts (kv.) established between the high potential side 23 of the wafer 21 and the voltage applied to a conical anode 31 by the=power supply 22 through a conductor 32.

The electrons converging at the crossover point. enter the conical anode 31 through an aperture 33 formed in the apex of the cone. The apex of the anode 31 is enshrouded by the flanged portion of the electrode lens 26, and the aperture 33 is in substantial alignment with the crossover 30. The precise longitudinal relation between the crossover 30 and the aperture 33 ordinarily is determined through adjustment to produce the brightest and most clearly defined visual image, as described subsequently in more complete detail.

Electron beams 34 and 35 emitted from the wafer 21 traverse the inner-electrode distance between the wafer 21 and the anode 31 and converge at the crossover 30. Within the hollow anode 31, the electron beams 34 and 35 diverge from each other. This initial electron beam convergence and subsequent divergence reverses the spatial relation between the electron beams in order to produce an inverted electron image 40. Best shown in FIG. 2, the beams strike an aluminized luminescent phosphor 36 coated on the inner surface of a transparent glass output faceplate 37. The phosphor 36 is at the same electrical potential as the anode 31. The combined effect of the desired electron multiplication and inversion produces the intensified and inverted optical output image 40 shown in FIG. 1.

The image intensifier is maintained in a vacuum by an enclosure 43, as indicated schematically by a broken line (FIG. 1). Appropriate gas-tight seals (not shown) are provided in the enclosure 43 to accommodate the input and output faceplates 13 and 37.

Turning once more to FIG. 2, the vacuum enclosure shown in FIG. 1 may comprise a longitudinally disposed hollow cylinder 44, formed of a suitable nonconducting material such as a ceramic or the like. In order to eliminate stray magnetic fields that tend to deflect theelectrons within the image intensifier from appropriate paths, a sheath of material (not shown) of high mag netic permeability, such as ,u-metal, can be placed around the cylinder 44.

In accordance with a further aspect of the invention, a wafer of the sort shown in FIG. 3 is produced by assembling a central hexagonal bundle of perhaps fifty or more tubes formed of a material that exhibits a satisfactory c-oefficient of secondary electron emission. The central bundle is surrounded by other contiguous hexagonal bundles of tubes. The tubes in the surrounding bundles have a somewhat greater inside diameter than the tubes in the central bundle. An array of tubes thus is prepared by continuing the process of assembling concentric bundles of tubes in which the radially more distant bundles are comprised of tubes that have greater inner diameters than the tubes in the more centrally disposed bundles.

The assembled array subsequently is heated and stretched out, or drawn, in a vacuum to decrease proportionately the inner diameter of the channels. As a consequence of the drawing step, the outer diameter of the entire array also is reduced. Drawing the array may be accomplished in one or more steps and is discontinued when the individual diameters of the tubes in the outermost annulus are reduced to an acceptable size. Of course, the radii of the more centrally disposed tubes are proportionately smaller, as hereinbefore considered.

A small disk or wafer is formed from the drawn array by two transverse cuts so spaced that the thickness of the disk is slightly greater than or equal to a specific channel length for the outermost annulus of tubes. The channel length so chosen, when taken with the respective channel diameter establishes the predetermined ratio of channel length to diameter to which the more inwardly positioned tubes must conform. The planar and concave sides 20 .and 23 (FIG. 3) of the wafer 21 subsequently are formed, for example, by grinding. Care, however, must be taken to avoid plugging up the channels with grinding debris. The radius or radii of curvature for the concave side 23 may be established not only on the basis of the desired electron-optical focusing considerations, but also to maintain a substantially uniform ratio of channel length to diameter for the centrally disposed channels.

In practicing the foregoing method, it often is advisable to use tubes that have their respective passageways filled initially with a material that is different from the surrounding tube structure. The heating, drawing, cutting and grinding steps remain substantially unchanged. The channels, however, are formed in the wafer by applying an etching solution that dissolves the filling material within the individual tubes in preference to the surrounding wafer structure.

An alternative photocathode structure in accordance with the invention is shown in FIG. 4. The transverse end of the tube envelope 44 is closed by a transparent glass plate 46 that has a flange 47 formed on the periphery thereof to provide a suitable gas-tight seal with the tube envelope 44. A photocathode material 50 is deposited directly on the low potential side 20 of the wafer 21. Thus, light quanta traversing the transparent glass plate 46 and striking the photocathode 50 produces photoelectrons in an abundance that is proportionate to the intensity of the incident light quanta. The electrical field established between the low and high potential sides 20 and 23 of the wafer 21 through the power supply 22 (not shown in FIG. 4) and the conductors 19 and 27 apparently draws the photoelectrons into the channels in the wafer 21 where the image is intensified and subsequently inverted as hereinbefore described.

Consequently, there is provided in accordance with the invention channel electron multiplier wafers of varied configuration for intensified image manipulation by controlling the gain of the individual channels. Thus, other electron lens structures, for example, plano-convex, concavo-convex, or lenses that have aspherical surfaces, are now possible through an application of the principles of this invention. The channel gain, moreover, need not depend entirely on control of the channel length to diameter ratio. In this connection, gain can be controlled by selecting tube materials that have differing coefficients of secondary electron emission in order to establish a uniform gain for all of the channels in the entire wafer in spite of a more relaxed control over the aforementioned ratio. Further in this connection, the application of the invention is not limited to light intensification devices, but can be applied to other types of equipment, such as neutron and X-ray image intensifiers.

While there have been described what are at present considered to be preferred embodiments of this invention,

it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, in-

tended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron multiplier comprising, a substantially circular disk of secondary electron emitting material having a first group of substantially parallel channels formed therein, said channels having a predetermined length and a substantially constant ratio of length to channel diameter determined by the distance of said first group of channels from the center of said disk, said disk having at least a second group of channels formed therein at a different distance from said .disk center, said second channel group having channel lengths different from said first group lengths and having a substantially constant ratio of length to channel diameter, said ratios of lengths to diameters for both groups being selected to yield equal electron gain for all said channels, said first and second groups comprising at least a portion of the electron multiplier.

'2. A substantially circular wafer comprising, a secondary electron emitting material, said material having an array of substantially parallel channels formed therein and passing longitudinally through the water at varying distances from the center of said wafer, said water having a transverse planar surface for terminating said channels on one side and a generally concave transverse surface for terminating said channels on the other side thereof, the diameters of said channels increasing with distance from the center of said wafer, the ratios of said channel lengths to diameters being proportioned to yield substantially equal electron gain from each of said channels.

3. An electron-optical system comprising, a photocathode for emitting electrons in response to light stimulation, a substantially circular wafer for receiving said electrons comprising, a secondary electron emitting material, said material having an array of substantially parallel channels formed therein and passing longitudinally through the wafer at varying distances from the center of said wafer, said wafer having a transverse planar surface for terminating said channels on one side and a generally concave transverse surface for terminating said channels on the other side thereof, the diameters of said channels increasing with distance from the center of said wafer, the ratios of said channel lengths to diameters being proportioned to yield substantially equal electron gain from each of said channels, an anode spaced from said disk for accelerating electrons from said wafer, and a target responsive to said accelerated electrons.

4. An electron-optical system according to claim -3 wherein said anode comprises, a hollow cone having'an aperture formed in the apex thereof for passing said electrons therethrough, said luminescent material forming the base of said cone for responding to said accelerated electrons.

5. An electron-optical system according to claim 4 wherein said disk comprises, a peripheral portion having at least one group of said channels formed therein, and a central portion having another group of said channels formed therein, the ratio of said channel lengths and diameters in each group being substantially constant for both of said groups.

References Cited UNITED STATES PATENTS 3,176,178 3/1965 Goodrich et a1. 3l3104 3,265,916 8/1966 Vestal 313-405 3,280,356 10/1966 Stoudenheimer et al. 31394 3,329,856 7/1967 FoOte 313-95 X ROBERT SEGAL, Primary Examiner.

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