['It', 'was', 'shown', 'that', 'small', 'He', 'interstitial', 'clusters', 'are', 'highly', 'mobile', 'but', 'become', 'effectively', 'pinned', 'with', 'the', 'emission', 'of', 'Fe', 'interstitials', 'when', 'the', 'clusters', 'contain', '5', 'or', 'more', 'He', 'atoms', '.']