Field of the Invention
The present invention relates to an anode that includes a target and is used in an X-ray generating tube configured to generate an X-ray which is applicable to, for example, medical equipment and non-destructive testing apparatus, and also relates to an X-ray generating tube that uses the anode, and an X-ray generating apparatus and a radiography system that use the X-ray generating tube.
Description of the Related Art
A general X-ray generating tube is configured to control the trajectory of electrons emitted from a cathode such as a filament with the use of a control electrode, and then accelerate the electrons toward an anode to which an electric potential higher than that of the cathode is applied. The accelerated electrons collide with a target layer formed in the anode, thereby generating an X-ray. The target layer is formed on a support substrate that transmits X-rays, and the X-ray generated in the target layer is emitted to the outside of the X-ray generating tube through the support substrate.
The X-ray generating tube has an envelope in which the cathode is mounted to one end of an insulating tube and the anode is mounted to the other end of the insulating tube in order to maintain a reduced pressure space where electrons can fly. The support substrate, through which the generated X-ray is emitted to the outside, is a part of the envelope, and is joined to the surrounding parts of the envelope in a manner that ensures vacuum sealing. An effective measure of vacuum sealing joining is brazing joining, and a method therefor is disclosed in Japanese Patent Application Laid-Open No. H09-180660. In Japanese Patent Application Laid-Open No. H09-180660, a target layer is formed from W, Ti, or the like by vapor deposition on a vacuum side inner surface of a support substrate (transmissive window), and the support substrate is joined around the target layer to a part of an envelope by brazing with the use of a brazing filler metal (that has Ag as a main component). The target layer also needs to be electrically connected to the anode by a brazing filler metal or a conductive member in order to define the electric potential of the target layer during driving.
In the manufacture of the X-ray generating tube, melting a brazing filler metal by heating the brazing filler metal to 780° C. to 900° C. is required to join, by brazing, in vacuum, the support substrate on which the target layer has been formed. The melted filler metal sometimes accidentally flows over to the target layer. The metal surface of the target layer on which W or Ti is deposited as a target, in particular, is high in affinity to a brazing filler metal, which allows the fluid brazing filler metal to cover even an electron collision portion of the target layer in some cases. When electrons collide with the covered target layer, the metal component of the brazing filler metal covering the target layer, such as Ag or Cu, emits its characteristic X-ray, which is radiation unwanted in the X-ray generating tube, with the result that an X-ray spectrum that is actually needed in the X-ray generating tube cannot be obtained. Japanese Patent Application Laid-Open No. 2013-109937 deals with this problem by providing a barrier that blocks the overflowing brazing filler metal around the target and thus preventing the generation of the unwanted X-ray. The barrier is conductive so that the target layer is electrically connected to a joining member.
The structure disclosed in Japanese Patent Application Laid-Open No. 2013-109937 is, although capable of reducing the flowing over of the brazing filler metal to the target layer in the manufacture of the X-ray generating tube, not effective enough to prevent a drop in the quality of the emitted X-ray which is observed after the X-ray emitting operation is repeated a number of times.