Long columnar defects with constant column size in 180-MeV Fe-irradiated Bi₂Sr₂CaCu₂O_x crystals

180-MeV Fe ion beams were used to bombard the Bi₂Sr₂CaCu₂O_x single crystals. The damage morphologies along the ion traces were investigated using cross-sectional transmission electron microscopy. Comparing with heavy-ion (Au or Pb) irradiation, the light Fe-ion irradiation shows several specific damage behaviors: the appearance of long columnar defects with constant column size, the disappearance of the peak of damage efficiency, and the much lower threshold of stopping power for generating columnar defects. An effective method has been developed for qualitatively analyzing the changing law of the damage size with the ion velocity. The applications on both Fe- and Au-irradiated Bi₂Sr₂CaCu₂O_x crystals and the obtained results indicate that the appearance of long columnar defects with constant column size is a specific phenomenon only occurring in light-ion irradiation. A qualitative analysis has been also done to prove that the threshold of stopping power for generating columnar defects is not a material constant but decreases as the mass of incident ion decreases, which further explains the lower threshold of stopping power in light Fe-ion irradiation.