[en] Resistivity as a function of ion-fluence has been measured in situ in thin films of oxide superconductor EuBa₂Cu₃O_y irradiated with various heavy ions in a wide energy range of 90 MeV-3.84GeV. From the resistivity measurements, the quantities characterizing the irradiation induced columnar defects, such as the diameter of the columnar defect and the inside-resistivity (the resistivity inside the columnar defect), have been estimated. It is found that they are not scaled with the electronic stopping power, while they are strongly correlated with the primary ionization rate which is the number of atoms primarily ionized by an incident ion per unit path length. This result suggests that the defect production is triggered by Coulomb repulsion of ionized atoms. Based on the results of thermal annealing of irradiated samples, structure of the columnar defects has also been discussed. (author)