US2335637A - Cathode ray tube - Google Patents

Description

Nov. 3o, 1343.l H BOERSCH 2,335,637

CATHODE RAY TUBE Filed Aug. 23, '1940 Inventor: 1 Hans Boersch,

His Attorney.

Eatented Nov. 30, 1943 CATIIIODE RAY TUBE Hans Boersch, Berlin-Charlottenburg, Germany,

assignor to General Electric Company, a corporation of New York Application August 23, 1940. Serial No. 353,949

In Germany September 12, 1939 g claims. (c1. sis-1i) The present invention relates to a cathode ray tube useful for investigating objects by electron optical means. I

In a cathode ray device such as an electron microscope in which a. visibleimage is formed. by the action of an electron stream, it often happens that the image produced is of blurred character, thisblurring being due at least in part to the fact that the various components of the electron stream are of different velocities. Thus, in electron microscopes of the type in which the image is produced by projecting an electron stream "through the .object under in. vestigation, electrons which are slowed down in passing through the object fail to follow the same paths as the electrons of higher velocity 'and consequently produce blurring of the image. 'A similar effect may be noted if the primary electron source is itself of such character as to give rise to electrons of varying velocities.

According to the present invention the dimculties described lin the foregoing are minimized by providing in proximity to the imagereproducing surface a braking electrode, preferably in the form of a. foil or grating, for preventing slowly movingelectrons from reaching the surface. It is found that in the use of the invention in an electron microscope the shadow of the grating which is produced on the image surface has little effect in obscuring the greatly magnified image of the object under investigation. v

The features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself may best Vbe understood by reference to the following dev scriptiontaken in connection with the drawing in which Fig. l shows schematically an elemen tary form of electron microscope embodying the invention and Figs. 2 and 3-respectively indicate modications of the invention.

Referring particularly to Fig. 1, there is shown anode. The object to be investigated is posiltioned as indicated at 'l in proximity to the anode opening 5 and is arranged to be traversed by the electron stream. It is assumed that the object 'I is at least partially of electron-permeable character and that it represents some ele- -`nientiigch as a minute organism, which is desired to be viewed asan enlarged reproduction of the same.

It will be understood that with the arrangement specified above, the electron stream, afterv enlarged image or shadow of the object on a suitable reproducing surface; Such a surface, which may take the form of a fluorescent screen or a. photographic film, is positioned in the path y ffrom the electron stream all electrons which are of such low veIocity as to have a tendency to in dotted outline a sealed evacuated envelope l i Asmall opening 5 provided in the end wall of the produce objectionable blurring of the image reproduced on the surface i0. Due to the fact that the braking electrode'is in close proximity to the f surface I0 the shadow which it casts on the surface does not obscure the 'object image to an objectionable degree.

It is desirable to assure the presence of a reasonably uniform eld Abetween the electrode II and the extremity of the anode 3. YTo facilitate this result one may provide a-further grating,.

indicated at I2, as an electron-permeable diaphragm across the end of the anode 3. Thisr grating is maintained at the samepotential as the anode.

As an alternative means for improving the elements described in connection with Fig. l*V

they are identically numbered except for the expedient of priming the various index numerals.

A still further variation which may prove useful in certain cases is shown in Fig. 3. Inthis case the braking electrode comprises a pair of gratings I3 and I4 which are successively arranged and which are maintained at a `common potential. The space between the gratings may be closed up laterally'as by the provision of a circularly extending conducting wall as indicated at l5. In the use of apair of gratings as indicated in Fig, 3 it is preferable to mount the gratings in such fashion that their Wires cross A assaut? 2. A cathode ray tube for investigating objects by electron optical methods comprising means inan electron-pervious electrode in the form of a perpendicularly so that .the projection of the two gratings represents a screen or grid..

The use of a braking electrode in the manner specified in the foregoing requires certain precautions in order to prevent the accumulation of charges on the image-reproducing surface. Normally the reproducing surface does not accumulate charges because secondary electrons liberated from it fly to the high voltage anode and thus help to `maintain the surface in discharged condition. I'he presence of the braking electrode, however, tends to lessen this eil'ect so that provision should be made for imparting transverse conductivity to the image reproducing surface as, for example, by giving it a metal coating.

The invention described herein may be used to advantage in investigating thick objects which have previously been found impossible of investigation by electron optical means because of the blurring effects initially referred to. Numerous additional modifications may obviously be made in the invention and I aim in the appended claims tov cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure. v

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A cathode ray tube for investigating objects by electron optical methods comprising means including an electron lens system. for producing an electron stream having a cross-sectional pattern representative of the particular object under investigation, a surface adapted to be affected by the electrons in the said stream to produce .a visible image corresponding to the cross-sectional pattern of the stream, an electron-pervious electrode lying in a surface having a substantial component perpendicular to the direction of travel of the electrons and interposed between the electron lens system and the said surface and in proximity to the latter, and means for applying a braking potential to the said electrode so as to prevent slow electrons in the stream from reaching the surface.

conductive grating interposed between the said electron lens system and the said surface and in proximityto the latter; and means for applying a braking potential to the said electrode so as to prevent slow electrons in the stream from reaching the surface.

3. A cathode ray tube for investigating objects by electron optical methods comprising means for producing anelectron stream having a crosssec'tional pattern representative of the particular object under investigation, a generally tubular electrode arranged to be traversed by the said stream for accelerating and directing the same. an image-reproducing surface adapted to be affected by the stream after its traversal 'of the said electrode, and an electron-pervious braking electrode extending across the path of the electron stream in a region between the 'said tubular electrode and the said image-reproducing surface for preventing slow electrons in the stream from reaching the surface, the said braking electrode being of curvilinear contour'ln planes taken parallel to the path ofthe electron stream.

4. A cathode ray tube for .investigating objects by electron optical methods comprising means for producing an electron stream having a cross-sectional pattern corresponding to the particular object under investigation, a tubular electrode arranged to be traversed by the stream and serving to accelerate and direct th same, an image-reproducing surface adapted to be affected by the stream after its passage through the said tubular electrode, an electron-pervious braking electrode interposed between the said tubular .electrode and the image-reproducingA surface for preventing slow electrons. in the stream from reaching the said surface and electron pervious conductive means of planar character mounted across the end of the tubular electrode which is adjacent to the braking electrode for producing a uniform eld distribution between such electrodes.

5. A cathode ray tube. according to claim 1 vcharacterized by the fact that the electron-pervi..

ous electrode comprises a pair of successively arranged conductive gratings which are maintained at a common potential.

. HANS` BOERSCH.

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