1. FIELD OF THE INVENTION
The present invention relates to a vacuum jacket or envelope for an X-ray image intensifier tube. Vacuum jackets for X-ray image intensifier tubes essentially comprise a central body of revolution, whose ends are terminated by an inlet port or window for the passage of the radiation to be intensified and by an outlet port or window for the visible radiation.
2. DESCRIPTION OF THE PRIOR ART
Up to recent times, the inlet ports were conventionally made from glass, which led to few sealing problems with respect to the central body, even when the latter was partly made from a ferrous metal, because glass-metal seals are well known in the art. However, the use of glass for the inlet ports leads to a certain number of problems. Thus, the absorption of radiation, particularly X-radiation, as well as the diffusion of radiation are very great and increase with the tube size. The use of an inlet glass port consequently leads to a considerable limitation in the performance characteristics of the tube such as the contrast, resolution, etc.
To obviate these disadvantages, it has been proposed to make the inlet ports from a metal which is permeable to the radiation to be intensified. Thus, it has been proposed to produce concave inlet ports from titanium or steel. This inlet port configuration leads to limited metal thicknesses and consequently to ports which are not highly absorbent, but which are still strong enough to withstand atmospheric pressure. A titanium thickness of 250 micrometers permits the transmission of approximately 88% of the X-radiation flux and a stainless steel thickness of 100 micrometers permits the transmission of approximately 88% of the X-radiation flux.
However, the concave shape of these ports leads to various disadvantages when placing under vacuum. As the input screen of the tube is convex for electronic optical requirements, on using a concave port it is necessary to elongate the tube by a quantity equal to the sag or deflection of the input port. However, this sag increases with the size of the input field of the IIT.
The input plane of the tube moves away from the input screen. Due to the conical projection from the focus of the X-ray generator tube, the real input field of the tube, measured in the input plane, is reduced compared with the useful field of the input screen. Finally, due to the projection on to a concave surface, the distortion increases for an equal input field.
It has also been proposed to make the ports from aluminium or an aluminium alloy and with a convex shape. This shape permits a good mechanical strength of the part exposed to atmospheric pressure. For a diameter of 230 mm, its thickness need only be 0.8 mm. Diffusion is then very small and 94% of the X-rays are transmitted. In this case, various procedures have been used for bringing about the sealing of the window or port on to the central body.
Sealing between the port and central body can be brought about by thermocompression welding. Diffusion takes place in the solid state of the aluminium of the port and a metal coating deposited on the ferrous metal of the central body at a temperature below that of their fusion or melting. It is necessary for the contact surfaces to be planar, so that the cylinder-on-cylinder geometry is consequently excluded. In this case, the aluminium alloy or aluminium convex window has an annular peripheral flange and assembly between the port and the body either requires the body to have an annular flange perpendicular to the tube axis, or for a L or S-shaped connecting ring to be used.
Thus, although this technology makes it possible to obtain tubes with an optimized length, it suffers the disadvantage of considerably increasing the overall diameter of the tube. Another disadvantage of this technology is that it is necessary to adjust various parameters, such as the temperature, the mechanical pressure exerted and the contacting time of the parts. This requires time and energy and makes the process expensive to realize and operate on an industrial scale.
Another prior art solution consists of using a convex port with a copper coating applied to an aluminium coating, in which the copper coating is removed in that part subject to the radiation and the aluminium coating is removed from the periphery of a flat part surrounding the convex cup or cap, whilst retaining a local overlap of the two coatings. The copper is then welded by electric arc welding along a lip formed on the central metal body, which can be of stainless steel.
The same problems of the overall diameter of the tube occur here as in the case of thermocompression welding. Moreover, it is difficult to obtain an industrially produced material with two coatings and which still has the same reciprocal adhesion quality with vacuum tightness. Moreover, it is necessary to remove the metal before welding is possible.