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To provide a basic understanding of the synergy effect of hydrogen and helium on structural materials for future advanced nuclear systems, the vacancy-type defects in SIMP steel induced by separate and sequential H and He implantation at room temperature was investigated using positron annihilation Doppler broadening spectroscopy (DBS) and transmission electron microscopy (TEM). The DBS results indicated that, when implanted by H and He separately, the ΔS parameter of the H-implanted sample was greater than that of He, although the damage level (that is, the displacement per atom) induced by H was lower than that induced by He. This could indicate that He is more easily trapped by vacancy than H and prefers to occupy the center of the vacancy. When sequentially co-implanted by He and H, whatever the implantation sequence, the ΔS parameter was lower than that of H-only implantation. In …