Measurements of mechanical properties on irra­diated materials usually require bulk samples and therefore neutron irradiation. Ion beams, however, can be employed for some measurements, such as plastic deformation. Typically, these experiments employ high energy protons, E > « 2 MeV, or He ions, E > 7 MeV, as these particles can penetrate through thin foils, such as Fe or steel, that are greater than 15 pm in thickness. Moreover, displacement rates «10~5dpas-1 are obtainable without excessive beam heating.33 Deformation experiments have also been performed using GeV heavy ions, as these pen­etrate targets several microns in thickness. The dis­placement rates, however, are low as most ofthe beam energy is lost through electronic excitations. Heavy ions with lower energies, E « 1-4 MeV, have also been used in deformation studies; for these, however, specimen must be very thin, «200 nm, and effects of the surface must be taken into account.34,35

1.07.4.2.3 Multiple ion beams

One of the difficulties in using ion beams to simulate neutron irradiation damage is the potential for miss­ing certain synergistic behaviors in the damage evo­lution. For example, neutron irradiation leads to transmutation products and the generation of He and fission gases in addition to displacement damage. Generation of gas is particularly relevant to 14 MeV neutron irradiation for which large amounts of He and H are produced. Ion beams, however, offer the opportunity of using two or even three beams simultaneously and thus to tailor test irradiations to meet expected reactor conditions; see, for example, Serruys et al.36 This is often not possible in existing test reactor facilities, and the building of new test facilities for fusion machines has been formidably expensive. The application of multiple ion beams is illustrated in Figure 17 in a study of void swelling in vanadium. Here, the synergistic effects of simulta­neously implanting 350 keV H and 1 MeV He, while irradiating with 12 MeV Ni ions are shown. Without the He beam, swelling is negligible, even with the implantation of H, but with it, the H greatly enhances the swelling. H implantation, on the other hand, is seen to reduce the density of cavities.