US2728014A - Electron lens for multiplier phototubes with very low spherical aberration - Google Patents

Description

R. G. STOUDENHEIMER ET AL ELECTRON LENS FOR MULTIPLIER PHOTOTUBES Dec. 20, 1955 WITH VERY LOW SPHERICAL ABERRATION Filed April 26, 1951 bez'mer a mm N d u A wfl E W M n Ra] BY 2 71 United States Patent O ELEC RDN LEN S F OF. MULTIPLIER PHOTOTUBES /3TH VERY LGW SPHERECAL ABERRATION Richard G. Stoudenheimer and Ralph W. Engstrom, Laneasier, Pa, assignors to Radio Corporation of America, a corporation of Delaware Appiicafion April 26, 1951, Serial No. 222,998

7 Claims. (Cl. 313-95) This invention relates to a phototube, and more specifically to a large area photoemissive cathode for use in phototubes, such as high vacuum photomultiplier tubes.

Photomultiplier tubes having large area photocathode surfaces wherein the photosurface is closely coupled with a phosphor layer, have been used as scintillation counters. The operation of the tube is such that radiations from radioactive materials are caused to fall upon a phosphor surface and activate it to luminescence, the light of which in turn causes photoemission from the photocathode of the tube. in such applications, it is necessary that all electrons emitted by the photocathode be directed into the multiplier section of the tube to contribute to the output signal. To achieve this, it is necessary for electrons emitted from any point on the photocathode surface to be directed or focussed into a restricted region or point. Normally, the larger the photocathode of such tubes, the more pronounced are the aberrations of the focussing system which increase the size of the focussed spot. Thus, electrons, particularly those originating from the edges of the photocathode, are scattered and not collected by the multiplier section. This results in a loss of signal.

It is thus an object of the invention to provide a phototube having a large area photocathode and wherein the electrons from the photocathode are directed into a restricted region.

it is another object of the invention to provide a pho totube having a large area photocathode in which the focusing of the photoemission from the photocathode is of low aberration.

It is a further object of the invention to provide a photomultiplier tube having a large area photocathode and adjacent tube portions designed to direct the photoemission into a restricted region.

The foregoing and related objects are achieved in accordance with the invention by providing-a tubular The tubular envelope is' envelope for said phototube. closed by an end wall having, at its center, a substantially concave spherical surface and, at its peripheral portions, annular concave surfaces of increasing radii with respect to the distance from the center. Also the peripheral surfaces of the face plate may be formed with a negative radius of curvature forming a convex inner surface.

Figure 1 is a sectional view of a photomultiplier tube large-area light sources, such as scintillation counters for' the detection and measurement of nuclear radiation.'

The tube comprises essentially ,'a glass envelope 1 0,

closed at one end with a transverse wall section 12, upon which is formed a transparent photocathode surface 14.

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In one tube of this type, end-wall portion 12 has a diameter of approximately two inches. The exposed portion of the photocathode film 14 is approximately 1 /2 inches. This provides a useful, large, substantially flat cathode area which permits good optical coupling between the photocathode and the surface of a phosphor screen, used in scintillation counters, for example.

Spaced from the photocathode and along the tube axis is an accelerating electrode or portion 16 formed as a disc and having an aperture 17 at its center. A metallic wall coating 18 is formed on the inner surface of the tube envelope and extends from the photocathode film 14 axially down the tube to a portion below the accelerating electrode portion 16. Wall coating 18 provides electrical contact between the photocathode 14 and a lead 20, connecting the photocathode and the metallic film 18 to a source of ground potential, as shown.

As indicated in Figure 1, a potential difference of 150 volts is maintained as an optimum value between accelerating electrode 16 and the photocathode 14. Photoelectrons emitted from cathode 14 are thus accelerated toward electrode 16. Wall coating 18 aids in directing the photoelectron toward the opening 17.

Photoelectrons passing through opening 17 are collected by an electron multiplier section 22, which consists of a plurality of dynode electrodes (shown in dotted lines) enclosed in a cylindrical metal shield 24. The photoelectrons will impinge upon an anode collector electrode which is also a first dynode electrode 26 and will initiate secondary emission therefrom having a ratio greater than unity. This secondary emission is accelerated and directed by a fixed electrostatic field along curved paths to successive dynodes. Each dynode provides an amplification of the electrons striking it to form an ever-increasing stream of electrons until those emitted by the last dynode are collected by a collector or anode electrode 28. The current collected by collector electrode 28 constitutes the current utilized in the output circuit of the tube. This type of electron multiplier is fully described in U. S. Patent 2,285,126, of Rajchman et al. The specified details of the multiplier do not com stitute my invention.

Opening 17 into the multiplier section 22 is covered by a mesh grid 30. This grid is connected electrically to the anode collector or first dynode 26 and tends to prevent secondary electrons, from dynode 26, from passing back toward the photocathode 14. The first dynode electrode portion 26 is fixed to the accelerating disc portion 16 and is, thus, tied electrically to it. In normal tube operation, apotential difference of about 75 volts is maintained between each of the succeeding dynode stages. I p

Commercial tubes, of the type described, have been made with a photosurface formed by putting down on the end wall 12 a semi-transparent film of manganese which is oxidized and then depositing thereon a thin film or layer of antimony which is sensitized by condensing a deposit of cesium metalthereon. This type of photoemissive surface is fully described in the copending application of J. J. Polkosky, Serial No. 219,997 filed April 9, 1951, now U. S. Patent 2,676,282 which issued April 20, 1954. Such a photocathode surface has a spectral response, which may be varied over a range between 3000 A. to 6400 A. The response of the material is peaked at around 4800 Angstroms.

In the operation of tubes of the type shown in Figure 1, it is desirable that photoelectrons from all portions of the photocathode film 14 may contribute to the signal. It is thus necessary that the photoelectrons originating at all points of the photocathode be directed or guided into the restricted region of aperture 17, so that they can pass into the multiplier section and strike the dynode electrode 26. To focus or direct an electron emission from a large surface to a common point, a theoretically ideal cathode surface would be that having a spherical configuration. Thus, apositive voltage at the center of curvature of the surface would guide the electronemission into a small region at the center of curvature.

However, in tubes of the type described, the photo 2, an additional effect of a negative wall coating 18a.

upon electrons leaving the photocathode 14a is to shorten the focal distance from the photocathode at which the electron paths cross. This effect is greatest in the region of the photocathode nearest the wall 10a. Electrons leaving the edge of the photocathode film 14a Will cross at points closer to the photocathode than will the paths of electrons leaving the center portions of the film. The paths of the electrons as shown in Figure 2 disclose considerable aberration in the electron optics composed of the anode 16a, the photocathode 14a, and wall coating 181:. Thus a tube made with a spherical end wall does not provide complete collection of photoelectrons from all points of the photocathode. Photoemission from the edge portions of the photocathode tend to miss the opening 17 into the multiplier section and strike portions of accelerating electrode 16, instead. These electrons do not contribute to the output signal of the tube.

In accordance with the invention, all portions of the photocathode 14 (Fig. l) are provided substantially with a common focal length by properly forming the shape of the wall portion 12 of the tube. As shown in Figure 3, the end wall 12 is formed with a central portion whose inner surface facing accelerating electrode 16 is concave and substantially spherical with a center of curvature 40 on the axis 42 of tubular portion 10 of the envelope. Between this center portion and the edge of the wall 12, the intermediate annular portion is also substantially spherical with a larger radis of curvature, R2 for example. The annular region at the periphery of the end wall 12 and contiguous to the tubular wall 10 may be flat in a plane perpendicular to axis 42, or may have a negative radius of curvature, Rs so that the peripheral annular region has a convex inner surface as seen from anode 16, and as shown in Figure 3.

The effect of a photosurface, of the type described for Figure 3, is one in which the electrons emitted from photocathode 14 follow paths toward accelerating electrode 16, which will converge to a common restricted region within or closely adjacent the opening 17 of the multiplier section and as indicated by the convergence of the dotted lines of Figure 4. Such a photosurface provides the minimum aberration effects of the focusing fields between cathode 14 and accelerating electrode 16. The proper curvature of the inner surface of end' wall 12 of Figure 3 can be determined on a rubber membrane mode for the desired electrode structures used. In one commercially operated tube of the type described, shown in Figure 3, the radius of curvature R1 of the center section is 3 inches. The radius of curvature R3 of the peripheral region is /2 an inch and the center of curvature 44 of this surface is substantially of an inch from the axis 42 of the tube. The diameter of the tubular portion 10 is two inches.

Figure discloses a modification of theinvention which the central portion of the end wall 12 of the tube is formed with aconcave spherical inner surface having a radis R1 and a center of curvature 46 on the axis 48 of the tubular envelope portion 50. However, the annular peripheral regions extending between the center portion of radius R1 to the'edge of the end plate 12 have also substantially spherical surfaces of decreasing curvature with distance from axis 46. That is the radius of curvature of the end plate 47 increases with respect to the distance from the axis 48 of the tube envelope, as indicated by R; and R5, for example. A photocathode film, deposited on the end wall 47 of Figure 5, will also provide an electron emission which will be directed or focused to a small confined region on the axis of tube. Such a photosurface also will provide less aberration effects than that provided by a spherical surface of single radius as shown in Figure 2.

From the foregoing, it will be apparent that the invention provides an improved phototube having a large photocathode surface characterized by its improved electron focusing and collecting characteristics as disclosed.

We claim:

1. A phototube comprising, a photoemissive cathode, and an anode electrode spaced from said cathode, said cathode having a concave surface facing said anode, said cathode surface having at its center portion a substantially spherical curvature of one radius and an adjacent annular spherical portion having a curvature of larger radius than said one radius.

2. A photoelectron discharge device comprising, an envelope, at photoemissive film on the inner surface of a portion of said envelope, an anode electrode mounted within said envelope and spaced from said photoemissive film, said envelope portion having a curved surface of increasing radius of curvature relative to the distance of the surface from its center.

3. A phototube comprising, a tubular envelope, an end wall closing one end of said envelope, a photocathode film formed on the inner surface of said end wall, an anode electrode spaced within said envelope from said photocathode, said cathode surface having at its center portion a substantially spherical curvature of one radius and an adjacent annular spherical portion having a curvature of larger radius than said one radius.

4. A phototube comprising, an envelope having a tubular portion, a transparent end wall closing one end of said tubular envelope portion, a photoemissive film on the inner surface of said end wall, and an anode electrode on the axis of said tubular envelope portion within said envelope and spaced from said photoemissive film, said inner surface of said end wall at its center portion being concave outwardly, the portions of said end wall respectively having curvatures with increasing radii relative to the distance from the center of said end wall.

5. A phototube comprising, an envelope having a tubular portion, a transparent end wall closing one end of said tubular envelope portion, a photoemissive film on the inncrsurface of end wall, and an anode electrode on the axis of said tubular envelope portion within said envelope and spaced from said photoemissive film, the center portion of said inner surface of said end wall, facing said anode electrode being a concave substantially spherical surface with a center of curvature on said axis of said tubular envelope portion, portions of said end wall between the wall of said tubular envelope portion and said center end wall portion having annular spherical surfaces concave relative to said anode electrode and of decreasing curvature relative to the distance from the center of said face plate.

6. A phototube comprising, an envelope having a tubular portion, a transparent end wall closing one end of said tubular envelope portion, a photoemissive film on the inner surface of said end wall, and an anode electrode on the axis of said tubular envelope portion within said envelope and spaced from. said photoemissive film, the center portion of-said inner surface of said end wall facing said anode electrode being a concave substantially spherical surface with a center of curvature on said axis of said tubular envelope portion, and peripherical portions of said end wall contiguous the wall of said tubular envelope portion having an annular surface of convex curvature relative to said anode electrode, said photoelectric film extending over said convex annular wall surface.

7. A phototube comprising, an envelope having a tubular portion, a transparent end wall closing one end of said tubular envelope portion, a photoemissive film on the inner surface of said end wall, and an anode electrode on the axis of said tubular envelope portion within said envelope and spaced from said photoemissive film, the inner surface of said end wall facing said anode electrode being concave and having an increasing radius of curvature relative to the distance of the surface from its center, and the peripheral portions of said end wall which contact the wall of said tubular envelope portion having an annular spherical surface of convex curvature relative to said anode electrode.

References Cited in the file of this patent UNITED STATES PATENTS 1,460,501 Ritter July 3, 1923 1,929,228 Wilhelm Oct. 3, 1933 2,054,884 Schlesinger Sept. 22, 1936 2,179,083 Bruche et al. Nov. 7, 1939 2,231,698 Zworyking et al. Feb. 11, 1941 2,238,634 Flechsig Apr. 15, 1941 2,244,720 Massa et a1 June 10, 1941

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