In the process ofsetting up an ion irradiation experi­ment, a number of parameters that involve beam

characteristics (energy, current/dose) and beam — target interaction must be considered. ASTM E 521 provides standard practice for neutron radiation damage simulation by charged-particle irradiation49 and ASTM E 693 provides standard practice for characterizing neutron exposures in iron and low alloy steels in units of dpa.9 One of the most important considerations is the depth of penetration. Figure 35 shows the range versus particle energy for protons, helium ions, and nickel ions in stainless steel as calculated by SRIM.50 The difference in penetration depth between light and heavy ions is over an order of magnitude in this energy range. Figure 36 shows how several other parameters describing the target

behavior during proton irradiation vary with energy, dose rate, the time to reach 1 dpa, deposited energy, and the maximum permissible beam current (which will determine the dose rate and total dose), given a temperature limitation of 360 °C. With increasing energy, the dose rate at the surface decreases because of the drop in the elastic scattering cross-section (Figure 36(a)). Consequently, the time to reach a target dose level, and hence the length of an irradia­tion, increases rapidly (Figure 36(b)). Energy depo­sition scales linearly with the beam energy, raising the burden of removing the added heat in order to control the temperature of the irradiated region (Figure 36(c)). The need to remove the heat due to higher energies will limit the beam current at a specific target temperature (Figure 36(d)), and a limit on the beam current (or dose rate) will result in a longer irradiation to achieve the specified dose. Figure 37 summarizes how competing features of an irradiation vary with beam energy, creating trade­offs in the beam parameters. For example, while greater depth is generally favored in order to increase the volume of irradiated material, the higher energy required leads to lower dose rates near the surface and higher residual radioactivity. For proton irradia­tion, the optimum energy range, achieved by balanc­ing these factors, lies between 2 and 5 MeV as shown by the shaded region.