  A novel method is presented which allows the characterization of diameter
selective phenomena in SWCNTs. It is based on the transformation of fullerene
peapod materials into double-wall carbon nanotubes and studying the diameter
distribution of the latter. The method is demonstrated for the diameter
selective healing of nanotube defects and yield from C$_{70}$ peapod samples.
Openings on small diameter nanotubes are closed first. The yield of very small
diameter inner nanotubes from C$_{70}$ peapods is demonstrated. This challenges
the theoretical models of inner nanotube formation. An anomalous absence of
mid-diameter inner tubes is observed and explained by the suppressed amount of
C$_{70}$ peapods due to the competition of the two almost equally stable
standing and lying C$_{70}$ peapod configurations.
