Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion-irradiated YBa2Cu3O7-de lta single crystals, it is found that at fields and temperatures where stro ng single vortex Dinning by individual irradiation-induced amorphous column ar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the sc reening current only at fields comparable to or larger than the matching fi eld, at which the majority of vortices is not trapped by an ion track.