US2757293A - Luminoscope - Google Patents

Description

July 31, 1956 M. C. TEVES ETAI- LUMINOSCOPE Filed Aug. 29, 1952 LUMINESCENT CONDUCTA/E,

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PHO-ro CATH@ IZonA-nom Qe lNvENrQRs MARTEN CORNELIS T EV E S TAEKE TOL AGENT United States LUMINOSCOPE Application August 29, 1952, Serial N0. 306,974

Claims priority, application Netherlands September 26, 1951 3 Claims. (Cl. Z50-213) This invention relates to luminoscopes, i. e., a device for converting an infra-red or X-ray image into a luminescent image having a different wavelength or a higher degree of brightness.

Conventional luminoscopes generally comprise an electron-optical system constituted by a photo-electric cathode in the form of an arcuate body and a thimbleshaped anode having an aperture for the passage of the electrons emitted by the cathode to a luminescent or collecting screen. Luminoscopes of the foregoing type have also been provided with a conductive surface electrically at the same potential as the cathode and enclosing the path of the electrons between the cathode and the anode. Such tubes, however, have the great disadvantage that in order to obtain a sharply dened image on the collecting or luminescent screen, the spacing between the cathode and the anode must be held to extremely close tolerances, not more than r0.1 mm. It is obvious that the manufacture of glass-envelope luminoscopes fulfilling the above requirement is extremely diliicult and time consuming.

The present invention is directed to an improvement in the luminoscope described above whereby the spacing between the anode and cathode is no longer critical and whereby other advantages over the luminoscopes heretofore known are obtainable as will be more fully explained hereinafter.

According to the invention, a luminoscope, comprising a photo-electric cathode shaped in the form of a sphere segment and a thimble-shaped anode having its axis arranged at right angles to a collecting or luminescent screen and to the concave cathode surface and having a central aperture in the path of the electron beam whose diameter is smaller than that of the collecting screen, is provided with a conductive surface which encloses the entire path of the electrons between the cathode and the anode and which is electrically insulated from the cathode. lMeans are also provided for maintaining this conductive surface at a variable potential which differs from the potential applied to the cathode. By altering the potential diierence between the conductive surface and the cathode, the quality of the image on the collecting screen may be varied.

The polarity and magnitude of the potential dilerence between the conductive screen and the cathode will depend upon whether the spacing between the anode and the cathode is greater or less than the critical spacing, i. e., the spacing at which, with zero potential dilerence between the conductive surface and the cathode, a sharply defined image is produced on the collecting screen. So, for example, if the spacing is greater than the critical spacing, the potential applied to the conductive screen must be positive with respect to the cathode. Where the spacing is less than the critical spacing, the surface must be negative relative to the cathode. The magnitude of the potential difference required to be applied for the production of a sharply defined image is a linear function of the difference between the actual atent Patented July 31, 1956 2 spacing and the critical spacing, and, with conventional size tubes is about 20 volts per mm. It is, preferable that the actual spacing be greater than the critical spacing.

In a preferred embodiment, the photo cathode is not integral with the wall of the tube but is arranged in the tube as a separate body and the conductive surface which surrounds the electron path is cylindrically shaped. This arrangement has the advantage that a reduction in the diameter of the tube is obtainable. Furthermore, the positive potential applied to the conduc-v tive surface relative to the cathode completely avoids', or at least greatly reduces, the reduction in definition which previously occurred at the edge of the image. A further advantage obtainable with a positive potential on the conductive surface is that a larger part of the cathode, per surface unit, is projected on to the collecting or luminescent screen so that both a larger image and a brighter image is observed.

The conductive surface may be formed by a metal wall portion of the glass envelope or by a conductive coating on the inner surface of the glass Wall. A suitable material for this conductive coating is aluminum.

The invention will now be described in detail with reference to the accompanying drawing in which the sole figure is a view, in cross section, of a luminoscope according to the invention for amplifying X-ray images.

The luminoscope shown in the ligure comprises a narrowed part 2 including a re-entrant part 3. This reentrant part 3 is closed by a transverse wall 4 which is coated on its inner side with a lm 5 of a substance which uoresces on being struck by electrons, the wall 4 and the ilm 5 constituting a luminescent collecting screen. This film is shown in the figure in lines which are of exaggerated thickness.

Suspended from an arcuate part 6 of the tube wall, which serves as a radiation-transparent window, by means of small metal wires 7 and 8 sealed in the glass is a metal bowl 9, for example, of aluminum. The hollow side of this bowl contains the photo-electric layer or cathode 11. However, since such a cathode absorbs very few X-rays and, consequently, under the influence of these rays alone would emit very few electrons, provision is made in the bowl 9 of a layer 10 of a substance which lluoresces under the influence of X-rays. The light of this primary luminescent screen 10 causes the photo-electric layer 11, which Yis arranged to coat the layer 10 with a current supplying substratum, to emit electrons.

A metal cylinder 12 is arranged to grip the re-entrant part 3. The portion of the cylinder 12 adjacent the cathode yis narrowed and terminates in a thimble-shaped top 13 which serves as the anode of the tube. This top 13 has formed in it a circular bore or aperture 14, the diameter of which is, for example, 5 mms. On the collecting screen 5, a circular surface having adiameter of from 15 to 20 mms. is occupied by the image. The narrowed part 2 of the tube wall is provided on its inner side with an annular coating 15 of silver or other suitable conducting material and a wire 16 sealed in the wall of the part 2 for making contact therewith. The cylinder 12 is provided externally with a pair of contact springs 17 which engage the coating 15. Current is supplied to theanode via the wire 16, the coating 15 and the springs 17 and is supplied to the photo cathode 11 by way of the wire 8.

By means of a suitable source of potential 18, the positive terminal of which is connected t0 the wire 16 and the negative terminal of which is connected to the wire 8 through a switch 24, the anode is given a sulficiently high positive potential in relation to the cathode screen. The beams of electrons intersect in the vicinity of the aperture 14. The extent to which the points of' the photo-electric cathode 11 emit electrons varies in accordance with the degree of brightness of the primary screen 10, which, in turn, depends on the intensity of the X-rays, denoted in the figure by arrows 19 penetrating through the front part of the tube wall 6 and the metal bowl 9 into the fluorescent layer 10. Thus, a true image of the radioscopic image is produced on the luminescent screen 5.

The inner surface of the wall 1 is coated with a conductive layer 20. A suitable substance for this layer is aluminum, because aluminum does not remove the caesium 'introduced into the tube by the manufacture of the photo-electric cathode 11. The layer 20 extends from the center of the cathode 11 slightly beyond the marginal surface 21 thereof and towards the anode side of the tube as far as almost the narrowed part 2.

The supply conductor for the coating 20 is constituted by a wire 22 sealed in the wall.

Connected between the negative terminal of the source of potential 18 and the supply wire 22 is a second source of potential 23 having a variable voltage. This source 23 gives the coating 29 a positive potential 'in relation to the cathode 11 which may be varied, for example, between and 200 v. The distance between the top of the anode 13 and the cathode 11 is made larger than that at which a sharply defined image would be formed without any voltage between the coating 20 and the cathode 11` Consequently, the application of a potential to the coating 20 permits sharp focussing of the image on `the collecting screen 5. Correct adjustment is obtained by varying the voltage provided by the source of potential 23 while viewing the collecting screen 5 through a magnifying lens (not shown) in the re-entrant part 3.

While we have thus described our invention with Specic examples and embodiments thereof, other modiications will be readily apparent to those skilled in the art without departing from the spirit and the ,scope of the invention as defined in the appended claims.

What we claim is:

l. A luminoscope comprising an envelope, a radiation-transparent window in said envelope, a photo-electric cathode having a spherical segmental shape disposed adjacent said window for forming beams of electrons corresponding to radiation impinging on said photocathode, a luminescent collecting screen having a given diameter disposed within said envelope and serving as a target for said electron beams, a thimble-shaped anode disposed between said cathode and said collecting -screen and having its axis arranged at right angles to the screen and to the cathode surface, said anode having a `centrally-disposed aperture having a diameter smaller than said given diameter, a conductive surface enclosing the entire path of the electron beams between the cathode and the anode, said conductive surface being electrically insulated from said cathode, means for applying a potential between said anode and said cathode, and means for applying a a potential between said cathode and said conductive surface.

2. A luminoscope comprising an envelope, a radiation-transparent window in said envelope, a photo electric cathode having a spherical segmental shape disposed adjacent said window for forming beams of electrons corresponding to radiation impinging on said photo-cathode, a luminescent collecting screen having a given diameter disposed within said envelope and serving as a target for said electron beams, a thimble-shaped anode disposed between said cathode and said collecting screen and having its axis arranged at right angles `to the screen and to the cathode surface, said anode having a centrally-disposed aperture having a diameter smaller tthan said given diameter, a single cylindrically-shapcd `conductive surface enclosing the entire path of the electron beams between the cathode .and the anode, said -conductive surface being electrically insulated from said cathode, means for applying a potential `between said `anode and said cathode, and means `for `applying a `positive potential between said cathode and said conductive` surface.

3. A luminoscope comprising a glass envelope, .a radiation-transparent window in said envelope, a photo electric cathode having a spherical segmental shape disposed adjacent said window and spaced therefrom for forming beams ,of electrons corresponding to radiation impinging on said photo-cathode, a luminescent collecting screen having a given diameter disposed Within said `envelope and serving as a target for said electron beams, a thimble shaped anode disposed between `said cathode Aand -said collecting screen and having its axis arranged ,at right angles to the screen and to the cathode surface, Vsaid anode having a centrally-disposed aperture having `a `diameter smaller `than said given diameter and being `spaced from said cathode a `distance exceeding .the distance at which a sharply defined image is produced .on the colf lecting screen, a single cylindrically-shaped conductive surface enclosing the entire path of the .electron lbeams between `the marginal surface of the cathode and anode, said conductive surface being electrically insulated from said cathode, means for applying a potential between said anode and `said cathode, and means for Vapplying a positive potential between said cathode and said oonductive surface.

References Cited in the rile of lthis patent UNITED STATES PATENTS 2,179,083 Bruche et al. Nov. 7, 1939 2,306,881 Heimann et al. Dec. 29, 1942 2,523,132 Mason et al. Sept. 19, 1950 2,613,330 Bruining et al. Oct. 7, 1952 2,681,420 Teves et al June 15 1954 2,683,816 Bouwers July 13, 1954

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