Low-energy photon irradiation in an ultra-high vacuum, as well as low-energy electron irradiation, was found to damage single-walled carbon nanotubes, meaning that electronic excitations are solely responsible for the defect formation. The formation and healing of the defects were found to strongly depend on nanotube diameter; that is, thinner nanotubes are more easily damaged and hardly recovered. The results mean that the irradiation damage is prominent in a rolled up graphene sheet, but not in a planar one. The curvature-induced strain energy seems to be essentially important for the damage. (c) 2006 Elsevier B.V. All rights reserved.