[en] In this study, CeO_2 was irradiated with 200 MeV Au ions at oblique incidence. Observation of as-irradiated samples by transmission electron microscopy (TEM) shows that hillocks are created not only at the wide surfaces, but also at the crack faces of the thin samples. Since the hillocks created at the crack faces can be imaged by TEM, their shape and crystallographic features can be revealed. From the images of hillocks created at the crack faces, many of the hillocks are found to be spherical. We present the first experimental evidence that hillocks created for CeO_2 irradiated with swift heavy ions have a crystal structure whose lattice spacing and orientation coincide with those of the matrix. The mechanism of spherical crystalline hillock formation is discussed based on the present results. (paper)