1. Field of the Invention
The present invention relates to a radiation generating tube which includes a transmission target. The present invention relates also to radiation generating apparatus in which the radiation generating tube is used.
2. Description of the Related Art
A transmission radiation generating tube is a vacuum tube including a cathode, an anode and an insulating tube. Electrons emitted from an electron source of the cathode are accelerated by high voltage applied between the cathode and the anode. The electrons collide with a transmission target on the anode and cause a ray to generate. The emitted ray is extracted outside through the transmission target. Thus, the transmission target also functions as a ray extraction window. Such a transmission radiation generating tube is used in radiation generating apparatus for medical and industrial use.
Such a transmission radiation generating tube and a reflective radiation generating tube have had a problem about how to improve their voltage withstanding capability. Japanese Patent Laid-Open No. 9-180660 describes a technique to improve voltage withstanding capability. In the described transmission radiation generating tube, a cathode-side end of an electron-focusing electrode is disposed between an insulating tube and a cathode and is fixed thereto. A gap (also known as “creepage distance”) is formed between the insulating tube and the focusing electrode. Creepage is the shortest path between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured along the surface of the insulation. A proper and adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of electric discharges on or close to an insulation surface. Since creepage distance of the insulating tube is thus elongated, voltage withstanding capability is improved. Japanese Patent application Laid-Open No. 2010-086861 and non-patent literature (NPL) article “Development of Portable X-ray Sources Using Carbon Nanostructures—A step toward X-ray nondestructive inspection and Rontgen examination using dry batteries as a power source” (Translation of AIST press release of Mar. 19, 2009) {http://www.aist.go.jp/aist_e/latest_research/2009/20090424/20090424.html} each describe a technique to improve voltage withstanding capability by providing an intermediate potential electrode (“intermediate electrode”) in a reflective radiation generating tube.
If, however, further improvement in voltage withstanding capability is desired in these techniques described above, the following problems are foreseen to potentially arise. In the technique described in Japanese Patent Laid-Open No. 9-180660, local potential of the insulating tube is determined in accordance with a dielectric constant (or volume resistivity in certain cases) of the insulating tube. There is, therefore, a possibility that electrical discharge occurs between the focusing electrode and an inner wall of the insulating tube in some situations depending on a distance from the focusing electrode to the inner wall of the insulating tube. For this reason, further improvement in the voltage withstanding capability in this technique has not been achieved. In the techniques described in Japanese Patent Laid-Open No. 2010-086861 and “Development of Portable X-ray Sources Using Carbon Nanostructures—A step toward X-ray nondestructive inspection and Rontgen examination using dry batteries as a power source”, since the intermediate electrode protrudes further toward an inner space than an inner wall surface of the insulating tube, electrons are emitted at an end portion of the intermediate electrode or from between a boundary of the intermediate electrode and the inner wall of the radiation generating tube. There is, therefore, a possibility that electrical discharge occurs between the intermediate electrode and the anode. For this reason, further improvement in the voltage withstanding capability in this technique has not been achieved.