A closure (10) for shielding, and selectively providing access to, the targeting assembly of a particle accelerator of a radioisotope production system. The closure (10) includes at least one, and in one embodiment, first and second doors (44, 46), for selectively covering an opening in the housing of the particle accelerator which provides access to the targeting assembly. A door mounting assembly is also provided for mounting the first and second doors (44, 46) on the housing of the particle accelerator. In one embodiment the door mounting assembly includes a frame (30) for being secured about the opening in the particle accelerator accessing the targeting assembly. Further, in one embodiment the frame (30) and first and second doors (44, 46) are fabricated of copper.