Resistivity as a function of ion-fluence has been measured in situ in thin
films of oxide superconductor EuBa2Cu3Oy irradiated with various heavy ions
in a wide energy range of 90 MeV-3.84 GeV. From the resistivity measuremen
ts, the quantities characterizing the irradiation-induced columnar defects,
such as the diameter of the columnar defect and the inside-resistivity (th
e resistivity inside the columnar defect), have been estimated. It is found
that they are not scaled with the electronic stopping power, while they ar
e strongly correlated with the primary ionization rate which is the number
of atoms primarily ionixed by an incident ion per unit path length. This re
sult suggests that the defect production is triggered by Coulomb repulsion
of ionized atoms. Based on the results of thermal annealing of irradiated s
amples, structure of the columnar defects has also been discussed.