Raman spectroscopy of single-wall carbon nanotubes and graphite irradiated by $γ$ rays

Graphite and single-wall carbon nanotubes irradiated by $γ$ rays of energy of 1.3 MeV were investigated by Raman spectroscopy. Irradiation generates defects in the lattice as confirmed by the increase of the intensity of the defect-induced $D$ line in both materials. On the other hand, the intensity of the radial breathing mode of nanotubes is lowered. The intensity of the $G$ line does not change for graphite but increases for carbon nanotubes. For the latter, this behavior cannot be explained by the defect-mediated double-resonance mechanism. Softening of the $q=0$ selection rule is suggested as a way to explain the results.