A target assembly to produce radioisotopes for the synthesis of radiopharmaceuticals. The target assembly includes a target vessel with a target chamber adapted to receive a target material. A thin cover sheet of particle-permeable material covers the target chamber. In a bombardment process, a high-energy particle beam generated by a cyclotron or particle accelerator strikes the thin cover sheet, whereby at least some of the particles from the particle beam penetrate to the target chamber so as to interact with the target material, altering the nuclear makeup of some of the atoms in the target material to produce radioisotopes.