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Fe-12 wt% Cr model alloy samples were implanted by 250 keV He 2+ ions to three different fluencies (3 × 10 17 , 9 × 10 17 and 1.5 × 10 18 cm −2 ) at T < 100 °C. In a depth profile manner, the implantation impact according to defined peak profile was investigated using variable energy slow positrons, with the primary focus on the 2–13 dpa region. The obtained data were compared to published data on Optifer IX steel samples [1] irradiated in the frame of a two-years irradiation program of the Swiss Spallation Neutron Source. Bi-modal defect distribution represented by two defect components in positron lifetime spectrum reveals two distinct helium bubbles growth mechanisms. While at the lower helium production rate of the spallation environment, the bubbles grow primarily by migration and coalescence, at the high production rates of helium in the implanted samples, the results indicate this growth is driven by Ostwald ripening mechanism. A competitive growth process via emission of interstitial atoms (clusters) is discussed in terms of low-temperature He implantations. [ABSTRACT FROM AUTHOR]