Heavy ions enjoy the benefit of high dose rates resulting in the accumulation of high doses in short times. Also, because they are typically produced in the energy range of a few MeV, they are very efficient at producing dense cascades, similar to those pro­duced by neutrons. The disadvantage is that as with electrons, the high dose rates require large

Figure 32 Damage profiles for C, Al, and Ni irradiation of a nickel target at energies selected to result in the same penetration depth. From Whitley, J. B. Ph. D. Thesis, University of Wisconsin-Madison, Madison, WI, 1978.

1.2 1.0 0.8 0.6 0.4 0.2 0

Figure 33 (a) Subsurface swelling resulting from 5 MeV Ni+ ion irradiation of Fe-15Cr-35Ni at 625 °C and (b) displacement rate and ion deposition rate calculated for 5 MeV Ni2+ on nickel. Adapted from Garner, F. A. J. Nucl. Mater. 1983, 117, 177-197; Lee, E. H.; Mansur, L. K.; Yoo, M. H. J. Nucl. Mater. 1979, 85&86, 577-581.

temperature shifts so that irradiations must be con­ducted at temperatures of ^500 °C in order to create similar effects as neutron irradiation at ^300 °C. Clearly, there is not much margin for studying neu­tron irradiations at higher reactor temperature as higher ion irradiation temperatures will cause annealing. Another drawback is the short penetration depth and the continuously varying dose rate over the penetration depth. Figure 32 shows the damage profile for several heavy ions incident on nickel. Note that the damage rate varies continuously and
peaks sharply at only 2 pm below the surface. As a result, regions at a very well-defined depth from the surface must be isolated and sampled in order to avoid dose or dose rate variation effects from sample to sample. Small errors (500 nm) made in locating the volume to be characterized can result in a dose that varies by a factor of 2 from the target value.

A problem that is rather unique to nickel ion irra­diation of stainless steel or nickel-base alloys is that in addition to the damage they create, each bombarding Ni ion constitutes an interstitial. Figure 33(a) shows
that 5 MeV Ni2+ irradiation of a Fe-15Cr-35Ni alloy resulted in high swelling in the immediate subsurface region compared to that near the damage peak. As shown in Figure 33(b), the Ni2+ ions come to rest at a position just beyond the peak damage range. So even though the peak damage rate is about 3 x that at the surface, swelling at that location is suppressed by about a factor of 5 compared to that at the surface.46 The reason is that the bombarding Ni2+ ions consti­tute interstitials and the surplus of interstitials near the damage peak results in a reduction of the void growth rate.47,48 In the dose rate-temperature regime where recombination is the dominant point defect loss mechanism, interstitials injected by Ni2+ ion bombardment may never recombine as there is no corresponding vacancy production.