US2748304A - Electric discharge tube for intensifying fluorescent images produced with the use ofchi-rays - Google Patents

Description

May'Z9, 1956 T. P. J. BOTDEN ET AL 2,748,304

ELECTRIC DISCHARGE TUBE FOR INTENSIFYING FLUORESCENT IMAGES PRODUCED WITH THE USE OF X-RAYS Filed March 1, 1951 LUMINESCENT MATERIAL S R o T N E V N THEODOOR P.J. BOTDEN FLORIS DE BOER MARTEN c. TEVES TAEKE TOL BY %w% I AGENT United States atent O ELECTRIC DISCHARGE TUBE FOR INTENSIFYING FLUORESCENT IMAGES PRODUCED WITH THE USE OF X-RAYS Application March 1, 1951, Serial No. 213,352

Claims priority, application Netherlands March 27, 1950 Claims. (Cl. 313-65) In theuse of fluorescent screens for medical or scientific investigation with the use of X-rays and for technical uses, the difliculty arises that the images observed are less sharp than is desirable in many cases. This is attributable to straying light in the luminescent layer. According as the thickness of this layer is greater, which is beneficial to the brightness, the lack of definition increases, and this can be maintained Within reasonable limits only at the expense of the brightness of the image. Consequently, such fluorescent images must, as a rule, be observed in a dark room.

' The use of fluorescent screens having a small thick ness of layer for the purpose of increasing the sharp-1 ness of definition is already known. Here the fluorescent image isobserved with the use of a so-called image intensifier in order to compensate for the loss of brightness brought about by the decrease in absorption of rays on account of the thinness of the screen.

1 Furthermore, an image intensifier has been constructed. in which the screen for the X-rays is arranged within the exhausted envelope of the tube. The photo-electric cathode must then be arranged as closely as possible to the screen in order to avoid the use of lenses within the tube. Tubes of this kind are already known, in which the luminescent material is provided on the wall of the tube and thereon the photo-electric cathode.

The luminescent layer in this case has a thickness even smaller than that necessary for the required sharpness of definition, since in the conventional method of manufacture the binder, adhering together the grains of material, vaporises during the process of degassing the screen. When use is made of a thick luminescent layer, there is a great possibility that the grains of material may loosen and expand in the tube. The luminescence produced by the X-rays is thus extremely weak, high re-. quirements being imposed upon the sensitivity of the photo-electric cathode and the image-intensifying power of the tube.

The invention relates to an image intensifier of the above-described kind in which use is made of means for increasing the thickness of the luminescent screen to a value at which sufiicient brightness is ensured and the lack of definition as a result of expansion of the luminescence remains within reasonable limits. According to the invention, the luminescent layer is enclosed between two parallel plates. Said plates are spaced apart a distance preferably equal to the thickness of the luminescent layer, which layer is supported along its edge against falling out.

The space between the walls may be completely filled up with luminescent material without care having to be taken about the relative binding and the adherence of the particles to the walls, it being sufficient that the support for the layer along the edge is free from seams through which the material may penetrate.

In such a construction the plates may be set between two metallic frames, one of which fits into an upright 2,748,304 Patented May 29, 1956 2 edge of the other. In another suitable form of construc tion, the two plates, together with a frame-like intermediate piece, may be enclosed between the bent edges of a metallic frame.

As an alternative, this junction may be made air-tight; and the process of degassing the luminescent material may take place outside the tube. In this case the space containing this material may at the same time be exhausted.

Said hermetic junction may be realised by shutting ed the seams with the use of a suitable material, such as glass-enamel powder.

The constitutive material of the plates must have a high transmittivity for the different rays. For the plate arranged on the side adjacent the source of X-rays use may be made of material having a low atomic number, for example beryllium. Transmission of the luminescent radiation requires the use of a diflerent material so that at least one of the plates consists of mica or glass.

In order that the invention may readily be carried into effect, an example will now be described indetail with reference to the accompanying drawings, in which:

Fig. 1 shows the image discharge tube according to; the invention with electro-magnetic focussing and Figs. 2, 3 and 4 show details of the luminescent screen for X-rays.

With the use of an electron discharge tube 2, which emits a beam of X-rays 3, a fluorescent image of the, object 1 is produced on a luminescent screen 4 arranged within the exhausted space of an image intensifier tube 5. The luminescence of the screen 4 strikes a photoelectric cathode 6 which is sensitive to this radiation and which is provided on the surface of the luminescent screen remote from the source of rays. It may alternatively be provided on a separate support, but this results in an increase of the distance between the screen and the photoelectric cathode, which distance must be as small as possible to ensure satisfactory transmission of the image. Under the action of the radiation produced by the screen, the photo-electric cathode emits electrons which are led to a screen 7 by applying a potential diiference with the use of a voltage source 8. The electrons are accelerated by the electric field. A magnetic field produced by a coil 9 causes electrons leaving the photo-electric cathode from a particular point to be brought together again at a point on the screen. The magnetic field may serve, not only to focus the electron rays but also to enlarge or reduce the reproduced image with respect to the cathode image.

The luminescent screen 4 for the X-rays is constituted by two parallel plates 10 and 11, between which a luminescent substance 12 in granular form and without binder is enclosed. The thickness of the luminescent layer is thus determined by the spacing between the plates, which is the same throughout. The plate adjacent the source of rays is required to transmit X-rays and the other to transmit a maximum portion of the luminescence produced by the screen. Each of the plates may be constituted by a mica plate of about 50 microns thick or be manufactured from glass. For the plate adjacent the source of rays, use may alternatively be made of a metal of low atomic number, for example beryllium. The plates are spaced apart a distance of 0.5 to 0.7 mm.

In Figs. 2 and 3, the plates are enclosed between two metallic frames 13 and 14, for example of aluminum, which have upright edges 15 and 16 and which are adapted to telescope. They are telescoped until the plates are clamped in position between the frames 13 and 14 and a metallic intermediate piece 17 and are subsequently interconnected, for example, by spot-welding.

In the embodiment shown in Fig. 3, this securing is eifected by pushing a narrow metal strip 18 of the edge 15 of the outer frame into a notch 19 provided in the edge 16 of the innerframe. The edge 15 may have secured to it supporting members by which the screen is arranged within the tube.

. So far the screen is ready before being arranged within the tube. During the process of exhausting thetube, the screen is degassed and raised to the temperature required therefor by heating from the exterior. Since the volume of the substance to be degassed is fairly considerable on account of the greater thickness of the screen, the exhausting process will require a longer time than in the conventional construction. However, the screen may alternatively be degassed before being arranged within the tube, whereafter the space containing the luminescent material is hermetically closed. For this purpose the screen may be constructed similarly as that shown in Fig. 4. Here. the, plates 10 and 11, together with the intermediate piece 17, are enclosed between a double-bent metallic frame 20, which at. its; edges 21 establishes a hermetic connection with the plates 10 and 11, for example with the use of a glass-enamel powder which is heated to melting temperature when the pumping process is finished.

After the screen has beenarranged within the tube, the. photo-electric cathode may be provided in known manner on the surface of the plate 10. For this purpose a thin, transparent conductive layer, for example of irridium or chromium, is preliminarily provided on the said surface.

The manner of securing the screen inside the tube has been disregarded, but for this purpose use may be made of any of the methods which are known.

The invention is not limited to electric discharge tubes with electro-magnetic focussing, but is also important for discharge tubes in which electrostatic fields are used for this purpose.

What we claim is:

1. An image amplifier tube for intensifying X-ray images, comprising, an evacuated envelope, a luminescent screen responsive to X-radiation and mounted within said envelope, said screen including a pair of spaced parallel plates and a member disposed around the periphery of the plates and united to the edges thereof to produce a closed space, a mass of granular luminescent material disposed within and substantially completely filling up said closed space, and a photo-electric cathode disposed on that side of the screen remote from the side on which X'-radiation impinges.

2. An image amplifier tube for intensifying X-ray images, comprising an evacuated envelope, a luminescent screen responsive to X-radiation. and mounted within said envelope, said screen including a pair of spaced parallel plates and a frame member completely enclosing the edges of said plate and hermetically sealed thereto to produce a closed space, a mass of granular luminescent material disposed within and substantially completely filling up said closed space, and a photo-electric cathode disposed on that side of the screen remote from the side on which X-radiation impinges.

3. An image, amplifier tube as claimed in claim 1 wherein the plate adjacent the photo-electric cathode consists of glass, and the other plate consists of beryllium.

4. A11 image amplifier tube for intensifying X-ray images as claimed in claim 2 in which the frame member is sealed to the plates with a glass-enamel material.

5. An image amplifier tube for intensifying X-ray images, comprising. an evacuated envelope, 9. luminescent screen responsive to X-radiation and mounted within said envelope, said screen including a pair of spaced parallel plates at least one of which is mica and a pair of telescoping frame members enclosing the edges of said plates and having interlocking bent edges securing them together to produce a closed space, a mass of granular luminescent. material disposed within and substantially completely filling up said closed space, and a photo-electric cathode disposed on that side .of the screen remote from the side on which X-radiation impinges.

References Cited in the file of this patent UNITED STATES PATENTS 1,401,006 Soderberg Dec. 20, 1921 2,151,785 Lubszynski et al Mar. 28, 1939 2,188,115 Kallrnannet al. Jan. 23, 1940 2,344,042 Kallmann et a1 Mar. 14, 1944 2,523,132 Mason'et a1. Sept. 19', 1950 2,555,423 Sheldon June 5, 1951' 2,663,801 Slavin et a1. Dec. 22, 1953 2,667,585 Gradstein Jan. 26, 1954-

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