Patent Number: 
Section: claims

1. A radiation tube comprising:an enclosure having an opening portion;a cathode having an electron source configured to emit electrons disposed inside the enclosure;an anode having a target unit configured to generate radiation in response to irradiation by being bombarded with electrons emitted from the electron source and a front shield configured to collimate the radiation and form a fan beam radiation, the front shield being disposed on the opening portion and having a proximal end closer to the target unit and a distal end farther from which the target unit than the proximal end; andan insulating tube having a pair of tube ends being connected to the cathode and the anode, respectively and configured to form a vacuum envelope with the cathode and the anode,wherein the front shield has a slit-shaped opening having a wide opening width and a narrow opening width, andwherein the wide opening width at the proximal end is larger than a diameter of the target unit and the narrow opening width at the proximal end is smaller than the diameter of the target unit. 2. The radiation tube according to claim 1, wherein the target unit includes a base member and a target layer film formed on a surface of the base member faced to the electron source,wherein the narrow opening width of the slit-shaped opening at the proximal end is greater than a diameter of a focal point of the radiation formed on the target layer film. 3. The radiation tube according to claim 1, wherein at least part of the front shield protrudes from the anode enclosure to the outside. 4. The radiation tube according to claim 1, wherein the slit-shaped opening is tapered so that the narrow opening width of the slit-shaped opening increases from the proximal side to the distal side. 5. The radiation tube according to claim 1, further comprising:a rear shield disposed on the opposite side of the target unit from with respect to the front shield,wherein the rear shield has an electron passage hole that allows the electrons emerging from the electron source to pass through. 6. The radiation tube according to claim 5, wherein the electron passage hole is a cylindrical opening. 7. A radiation inspection apparatus comprising:the radiation tube according to claim 1;a conveyance unit configured to convey an inspection object article in a direction crossing a longitudinal direction of the slit-shaped opening; anda detection unit configured to detect radiation generated that is emitted from the radiation tube and transmitted through the inspection object that penetrates the article. 8. The radiation inspection apparatus according to claim 7, further comprising:a slit member having a slit portion disposed between the front shield and the conveyance unit article,wherein a longitudinal direction of a slit of formed in the slit portion is oriented in parallel to the same as the longitudinal direction of the slit-shaped opening. 9. The radiation tube according to claim 1, wherein the slit-shaped opening shows an aspect ratio no less than 2 and no greater than 50. 10. A radiation tube according to claim 9, wherein the aspect ratio of the slit-shaped opening is no less than 4 and no greater than 20. 11. The radiation tube according to claim 2, wherein the wide opening width and the narrow opening width of the slit-shaped opening at the proximal end is larger than the diameter of the target layer. 12. The radiation tube according to claim 1, wherein the slit-shaped opening defines a fan angle and a radiation angle narrower than the fan angle of the fan beam. 13. The radiation tube according to claim 1, wherein the proximal end of the front shield is connected to the target unit.