IRRADIATION WINDOW

PROBLEM TO BE SOLVED: To improve a charged particle-passing rate while maintaining cooling ability by making the widths of a plurality of sash bars supporting and cooling a window foil gradually thin toward the center of the longitudinal direction. SOLUTION: The width of each sash bar 8 of an irradiation window 4a is gradually made thin toward the center of the longitudinal direction Y of each sash bar 8. Since charged particles have distribution of high density in the center part, the high density part of the charged particles can be efficiently passed since the opening ratio of an opening part 9 between the sash bars 8 in the center part of the longitudinal direction Y of each sash bar 8. As the result, the utilization efficiency of the charged particles is improved, and the processing efficiency of an object to be processed is improved. In addition, since the more the flow of the longitudinal heat of the sash bar 8 goes to an end, the larger it is, lowering of heat conduction property is prevented, damage and life lowering due to overheat of a window foil is prevented since the cooling ability of the window foil is maintained. The sash bar 8 and a frame 10 may be integrally formed by a wire-cutting method and the like, and mutually combined by being formed of different members.