Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of similar to 1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids. (c) 2008 Elsevier B.V. All rights reserved.