Irradiation-induced defects in graphite and glassy carbon studied by positron annihilation

ACAR and positron lifetime measurements have been made on, HOPG, isotropic fine-grained graphites, glassy carbons and C60/C70. HOPG showed a marked bimodal ACAR distribution along the c-axis. By irradiation of 1.0 X 1019 fast neutrons/cm2 remarkable narrowing in the ACAR curves and disappearance of the bimodal distribution were observed. Lifetime in HOPG increased from 225 psec to 289 psec (positron-lifetime in vacancies and their small clusters) by the irradiation. The irradiation on isotropic graphites and glassy carbons, however, gave slight narrowing in ACAR curves and decrease in lifetimes (360 psec → 300psec). This suggests irradiation-induced vacancy trapping in crystallites. In C60/C70 powder two lifetime components were detected: τ1=177psec, τ2=403psec (I2=58%). The former is less than the bulk lifetime of HOPG, while the latter being very close to lifetimes in the isotropic graphites and glassy carbons. This and recent 2D-ACAR study of HOPG surface [15] strongly suggest free and defect surface states around ''soccer ball'' cages