US2256460A - Electron discharge device - Google Patents

Description

P 1941' H. G. LUBSZYNSKI 2,256,460

ELECTRON DISCHARGE DEVICE Filed Dec. 20, 1939 INVENTOR HANSG. LUBSZYNSKI ATTORNEY Patented Sept. 16, 1941 ELECTRON DISCHARGE DEVICE Hans Gerhard Lubszynski, Hillingdon, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application December 20, 1939, Serial No. 310,098 In Great Britain December 20, 1938 4 Claims. 250165) The present invention relates to an electron discharge device in which radiation from an ob ect is made incident on an electron emissive surface and an electron image corresponding in intensity to the radiation from the object is thereby formed, the image finally obtained being increased in intensity by the initial electron image producing further radiation which is also made incident on the electron emissive surface. It has previously been proposed to increase the intensity of an image formed from the electrons emitted from an electron emissive surface when radiation is incident thereon by causing the emitted electrons to impinge on a fluorescent screen, the light radiation thereby emitted being focussed also upon the electron emissive surface by an optical mirror and optical lens arrangement to cause the emission of further electrons from said electron emissive surface and thus increase the intensity of the resultant image.

The use of an optical lens or of an optical mirror is however, disadvantageous, and it is the object of the present invention to provide improved means for increasing the intensity of the finally formed image.

According to one feature of the present invention there is provided a method of intensifying the resultant image formed by an electron stream emitted from a source when radiation is incident thereon which comprises causing said emitted electron stream to move towards the rear of said source and there to impinge on a luminescent layer positioned adjacent said source whereby further radiation is caused to be incident on said source. 7 According to another feature of the present invention there is provided an electron discharge device comprising a source adapted to emit electrons when radiation is incident thereon, wherein there is provided electron focussing means so constructed and arranged as to be capable of causing the electrons emitted from said source to move from said source towards the rear of said source where they impinge on a screen adapted to be rendered luminescent byincident electrons, the radiation emitted when said screen is rendered luminescent being caused to impinge on said source so as to emit further electrons so intensifying the resultant image.

Reference is now made to the accompanying drawing in which,

Figure 1 illustrates one embodiment of the present invention and Figure 2 illustrates another embodiment Of the present invention,

Similar parts in the drawing being indicated by the same reference letters.

In Figure 1, there is shown an evacuating tube T containing a mica disc M which carrieson one side a photo-cathodeP and on the other side a transparent coating B of a metal such as platinum on which transparent coating is deposited a layer of fluorescent material S, the construction being such that electrons incident on the layer S'cause the emission of light radiation therefrom, some of which light radiation is able to penetrate the layer B and travel through the mica to the photo-cathode P and thereby cause the emission of photo-electrons from cathode P.

Radiation is made incident on a lens system indicated generally by L and thereafter passes through aha'lf-silvered mirror D'to impinge on the photo-cathode P, the half-'silvered mirror D allowing the image finally formed on S to be viewed from the same side of the photo-cathode P as is the lens L'. Electrons are thereby emitted from the cathode P and areaccelerated towards the anode A which 'is maintained at an accelcrating potential by a suitable source (not shown). The tube T enclosing the mica disc Mand the anode A is of substantially circular form and has wound around it a toroidal coil C. Current is caused to fiow through the coil 0 and the'value of this current is so adjusted that the magnetic field set up is of a suificient strength so thatthe electronsemitted from the cathode P and acceleratedtowards the anode A movein curved helical paths, the radii of these paths in'a direction transverse to the axes of the helical paths being relatively small. Thus if there were no obstacles in its path an electron emitted from the cathode P would be accelerated towards the anode A and travel in a helical path of small radius transverse to the axis of the helix the axis of the helix being curved as shown and would be broughtto a focus at a point on the-other side of the cathode P substantially opposite the point from which it was emitted.

The magnetic lines of force areindicated at F and it will be noted that they project out wardly adjacentto the cathode P, owing to the coil C not being wound uniformly in this region. The-glass wall of the tube T is shaped to conform with the magnetic lines of force, and has a bulge in the region of the cathode P so that the electrons will not be intercepted by the glass wall in their paths along the magnetic lines of force;

The electrons emitted from the cathode P are thus caused to follow substantially circular paths along the lines of magnetic force F and are brought to fool along points on the luminescent layer S substantially opposite the points along the cathode P from which they are emitted. The luminescent layer S is maintained at a suitable voltage such that the electrons from the photocathode P are incident on the layer S with sufficient velocity to cause a certain amount of light to be emitted from the screen S, a portion of which light causes a further emission of elec-.

trons from the photo-cathode P. The incidence of the electrons on the layer S thus results in an optical feedback to the cathode P corresponding to the initial radiation incident on said cathode. This optical feedback causes further electrons to be emitted from the cathode P and these furtherelectrons enhance the light emitted by the screen S, resulting in a further optical feedback cycle to the photo-cathode P. This process of optical feedback to the photo-cathode P may be allowed to proceed for as many cycles as required.

In some cases the optical feedback may result in a less number of electrons being emitted from the photo-cathode after each cycle than was emitted at the commencement of the cycle, and so the luminescent energy radiated by the screen S will increase to a limiting value after a great number of cycles has been completed. In such cases, in order to obtain a high value for the amplification factor, by which is meant the ratio of the total light emitted by the fluorescent screen S to the light which would be emitted by the screen S due to one cycle of electrons rom P impinging thereon, a larg number of cycles would be required, so resulting in loss of definition. In order to reduce the number of cycles required, so as to improve the definition, it is contemplated that high potential differences of the order of 20 kilovolts between the photocathode P and the luminescent layer S will have to be employed. High voltage differences of this order are undesirable, particularly when caesium is employed in the tube since with such high voltages additional difficulties arise due to electrical leakage in the tube and cold emission. Furthermore, with high voltages the thickness of the mica disc'M must be increased in order to withstand the high potential difference across it. It is desirable, however, to keep the thickness of the mica disc M as low as possible, as a thick mica disk will result in a spread of the light on its way from the side S to the side P, with consequent loss of definition.

In order to obtain a high amplification factor and utilise only a relatively small number of cycles of the emitted electrons it is necessary to arrange that a greater number of electrons are emitted from the photo-cathod after each cycle than are emitted at the commencement of the cycle. An arrangement whereby this may be obtained is illustrated in Figure 2, wherein G1, G2 and G3 are multiplying grids arranged between the photo-cathode P and the screen S, the arrangement being otherwise substantially the same as that illustrated in Figure 1. On the electrons emitted from P being incident upon the grid G1 a multiplication in the number of electrons takes place, whereby th number of electrons thereafter proceeding from the grid G1 to the grid G2 is greater than th number of electrons initially incident upon G. This multiplying action is repeated at G and Ge, and the electrons finally emitted from G3 are caused to impinge on the fluorescent screen S. The surfaces of the grids G1, G2 and G3 are suitably constructed and they are maintained at suitable potentials for the multiplication process to occur at each of them in known manner. With such an arrangement as shown in Figure 2 a value of about 3 may be obtained for the ratio of the electron current emitted from the photo-cathode P after each cycle to the electron current emitted from the photo-cathode P at the commencement of the cycle, an accelerating potential of about 5 kilovolts being employed between the grid G3 and the photo-cathode P. The grids G1, G2 and Ge act so as to multiply the number of electrons emerging from each of them compared with the number incident upon each of them by a factor of about 3, so that the thre grids G1, G2 and G3 would be effective to multiply the number of electrons by a'factor of about 27, this electron multiplication factor being reduced to the ratio of about 3 finally obtained due to the low efficiency of transformation at the screen and photo-cathode.

When the arrangement is such that the amplification factor obtained after each cycle of emitted'electrons is greater than unity, the arrangement would tend to oscillate if a larg number of cycles were permitted. In such a case it may become necessary to interrupt the operations when the necessary amplification has been accomplished. This could be effected, for example, by applying a periodic negative pulse to one of the multiplying grids G1, G2 or G3 or to an auxiliary grid or other electrode, which may be suitably arranged in the tube. The grids G1, G2 and G3 are advantageously arranged near the screen S, and it is desirable to arrange that the electrons emitted-from P move slowly over most of their paths in order that the frequency of the negative pulses necessary to prevent oscillations occurring may be low and the electrons closely follow the lines of magnetic force.

The present invention may b carried out by using a tube for enclosing the electrodes having a form other than a substantially ring-like form, and the present invention may be applied to radiation other than in the visible region. The intensified image appearing on the luminescent screen S may be utilised to obtain an amplified optical image of the incident radiation; alternatively the intensified electron image may be utilised to provide an amplified electron image .I claim:

1. .An electron image tube comprising an evacuated envelope whose principal portion is a toric segment, a light sensitive electron emissive electrode within said envelope extending radially of said toric segment portion and intercepting substantially circular closed paths within said envelope to emit electrons in response to light, a substantially transparent foundation supporting said electrode, a fluorescent screen closely adjacent and coplanar with the side of said foundation opposite said light sensitive electrode, and means comprising a magnetic coil surrounding said toric segment portion to generate a magnetic field having substantially circular lines of force through said toric segment whereby electrons emitted from said light sensitive electrode are directed around said toric segment and impinge said fluorescent screen at points in substantial registry with the points of origin of said electrons.

2. An electron image tube comprising an annular evacuated envelope having a toric segment and an outwardly bulging portion which is transparent for the passage of light, a photoelectron emissive electrode therein facing the bulging portion of said envelope and substantially intercepting concentric circles through said toric segment to liberate electrons responsive to light projected through said bulging portion, a fluorescent screen coextensive with and closely adjacent the side of said electrode opposite said bulging portion, an annular anode in said envelope adjacent the wall of said toric segment to accelerate electrons liberated from said electrode and a magnetic electron focusing coil surrounding said toric segment to direct said electrons along substantially circular paths within said toric segment and upon said fluorescent screen.

3. An image tube as claimed in claim 2 including an electron permeable secondary electron emitting electrode within said envelope toric segment adapted to be maintained at a higher potential than said anode and to be impinged by said electrons to increase the number of electrons reaching said fluorescent screen.

4. An electron image tube comprising an annular evacuated envelope a portion of which is a toric segment and another window transparent tolight closing said toric segment, a semi-transparent photo-emissive cathode opposite said Window portion and within said envelope to receive light representative of an optical image from without said envelope, an annular anode in said envelope adjacent said cathode extending Within said toric segment to accelerate electrons emitted by said cathode, a fluorescent screen closely adjacent and coextensive with the side of said cathode opposite said window portion to receive said electrons and a magnetic focusing coil surrounding said toric segment to focus electrons over substantially closed paths within said envelope and upon points on said fluorescent screen directly opposite their points of origin on the cathode whereby said cathode emits additional electrons in response to light generated by said electrons.

HANS GERHARD LUBSZYNSKI portion having a

Images