%0 Journal Article
%T Raman Spectroscopy for Quantitative Analysis of Point Defects and Defect Clusters in Irradiated Graphite
%A Keisuke Niwase
%J International Journal of Spectroscopy
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/197609
%X We report the development of Raman spectroscopy as a powerful tool for quantitative analysis of point defect and defect clusters in irradiated graphite. Highly oriented pyrolytic graphite (HOPG) was irradiated by 25£¿keV He+ and 20£¿keV D+ ions. Raman spectroscopy and transmission electron microscopy revealed a transformation of irradiated graphite into amorphous state. Annealing experiment indicated a close relation between Raman intensity ratio and vacancy concentration. The change of Raman spectra under irradiation was empirically analyzed by ¡°disordered-region model,¡± which assumes the transformation from vacancy-contained region to disordered region. The model well explains the change of Raman spectra and predicts the critical dose of amorphization, but the nature of the disordered region is unclear. Then, we advanced the model into ¡°dislocation accumulation model,¡± assigning the disordered region to dislocation dipole. Dislocation accumulation model can simulate the irradiation time dependencies of Raman intensity ratio and the c-axis expansion under irradiation, giving a relation between the absolute concentration of vacancy and Raman intensity ratio, suggesting an existence of the barrier on the mutual annihilation of vacancy and interstitial. 1. Introduction Graphite is one of useful materials for thermal nuclear and fusion reactors due to their outstanding nature with respect to heat load, neutron activation, and so forth. Since the fire of Windscale nuclear reactor occurred in 1957 [1] due to a spontaneous release of stored energy from irradiated graphite, radiation damage has been recognized to be important problems to be solved for irradiated graphite. Also, dimensional changes of growth parallel to c-axis and contraction within the basal planes decrease in c-axis electrical resistivity and so forth occur as serious problems. In spite of a lot of investigations on irradiated graphite [2¨C33], the natures of point defects and defect clusters, and their reaction kinetics are obscure. This can be attributed to the anisotropy of the graphite lattice which requires a description of the motion of interstitials and vacancies in terms of activation energies parallel and perpendicular to the basal planes, and the instability of vacancy and interstitial clusters [3, 7]. Raman spectroscopy has been used for the characterization of graphite and investigations on the graphitization process and on the change of graphite structure under irradiation. The advantages of this method for the examination of graphite are a selective sensitiveness to the structural
%U http://www.hindawi.com/journals/ijs/2012/197609/