Damage microstructures are compared at a dose of 10 dpa in a high-purity Fe-Ni-Cr-Mo alloy after irradiation in EBR-II and after nickel ion bombardment at temperatures encompassing the peak swelling. The damage features are qualitatively similar, but there are less cavities in the ion bombarded specimens. Incorporation of helium during the ion bombardments improves but does not eliminate this discrepancy. The level of peak swelling is 1/3 that for the neutron irradiations. The upward shift in peak swelling temperature with four orders of magnitude increase in damage rate is 75 K when helium is absent during ion bombardment, and 125 K when helium is coimplanted. These effects are consistent with theoretical predictions. It is concluded that ion bombardments can match a required level of neutron swelling when the appropriate, empirical temperature shift and dose equivalency are applied, but such translations may not adequately reproduce the quantitative microstructural aspects.