In situ He implantation (ISHI) in mixed spectrum fission reactors is a very attractive approach to assess the effects ofHe-dpa synergisms in almost any mate­rial that avoids most of the confounding effects of doping. The basic idea is to use an implanter layer, containing Ni, Li, B, or a fissionable isotope, to inject high-energy a-particles into an adjacent sample simultaneously undergoing neutron-induced dis­placement damage. Early work proposed implanting He using the decay of a thin layer of a-emitting isotope adjacent to the target specimen.47 However,
the isotope decay technique produces few dpa at a very high He/dpa. The first proposal ISHI in a mixed fast (dpa)-thermal (He) spectrum proposed using 235U triple fission reactions to inject «16MeV a-particles uniformly in steel specimens up to 50 mm thick; the 50 mm thickness permits tensile and creep testing as well as microstructural characterization and mechanism studies at fusion relevant dpa rates and He/dpa ratios.31 The triple fission technique was applied to implanting ferritic steel tensile specimen, albeit without complete success.48 A much more prac­tical approach is to use thin Ni-bearing implanter foils to uniformly deposit He up to a depth of ~8 mm in Fe in a thick specimen at controlled He/dpa ratios.49

As illustrated in Figure 5(a)-5(c), there are at least three basic approaches to implanter design. Here we will refer to thin and thick, specifically meaning a specimen (ts) or implanter layer (ti) thick­ness that is less than or greater than the corre­sponding a-particle range, respectively. Ignoring easily treated difference in the a-particle range (Ra) and atom densities in the injector and specimens for simplicity, thick implanter layers on one side of a thick specimen produce linearly decreasing He con­centration (XHe) profiles, with the maximum concen­tration at the specimen surface that is one half the concentration in the bulk injector material, XHeo = XHei/2 (Figure 5(a)). If a thin specimen is implanted from both sides by thick layers, the He concentration

XHei Ri 2 Ra

У

X

R

(a)

(c)

is uniform and equal to one half that in the bulk injector material (Figure 5(b)). In contrast, a thin layer implants a uniform concentration of He to a depth of the Ra — ti. In this case, the He concentration in both the implantation layer and specimen is equal and lower than in the bulk (XHei) as XHes = tiXHei/ 2Ra (Figure 5(c)). Thus, the He/dpa ratio can be controlled by varying the concentration of the iso­tope that undergoes n, a reactions with thermal neu­trons, ti, and the thermal to fast flux ratio.

ISHI experiments were, and continue to be, carried out in HFIR using thin (0.8-4 gm) NiAl coating layers on TEM disks for a large matrix of Fe-based alloys for a wide range of dpa, He/dpa (<1-40 appm He/dpa), and irradiation temperatures. In this case, 4.8 MeV a-particles produce uniform He concentration to a depth of «5-8 gm (Figure 5(c)). Further details are given elsewhere.50 The first results of in situ implan­tation experiments in HFIR have been reported and are discussed in Section 1.06.6.23,51-53 The technique has also been used to implant SiC fibers irradiated in HFIR.50 More recently, the two-sided thick Ni implanter method was used to produce He/dpa ratios «25appm/dpa in thin areas of wedge-shaped specimen alloys irradiated in the advanced test reactor to «7 dpa over a range of high temperatures.54