PURPOSE: To improve thermal conductivity of a welding part and eliminate the possibility that a radiation absorber is removed from a vacuum chamber.

CONSTITUTION: The radiation light absorber of a particle accelerator vacuum chamber is provided on the inner wall 22a of a vacuum chamber 22 to absorb radiation 25 generated from particles passing inside the vacuum chamber 22 and formed by means of build up welding and a cooling water passage part 26 through the inside of which cooling water passes is provided on the wall part of the vacuum chamber 22 adjacent to the radiation absorber 21. Thus, the radiation absorber is firmly fixed in its condition of close contact with the predetermined position of the inside wall of the vacuum chamber and effectively cooled by the cooling water passage part 26. Since the radiation absorber itself includes no pressure boundary, it is not subjected to thermal fatigue and can be miniaturized.