Positron annihilation study of defect formation and evolution in matrix graphite under He ion irradiation

The stability of matrix graphite under neutron irradiation and in corrosive environments is crucial for the safe operation of molten salt reactors (MSRs). Raman spectroscopy and a slow positron beam were employed to investigate the effects of He ion irradiation fluences and subsequent annealing on the microstructure and defects of the matrix graphite. He ions with 500 keV energy and fluences ranging from 1.1x1015ions/cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${1.1}\times {10}<^>{15}\,{\text{ions}}/{\text{cm}}<^>{2}$$\end{document} to 3.5x1017ions/cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${3.5}\times {10}<^>{17}\,{\text{ions}}/{\text{cm}}<^>{2}$$\end{document} were used to simulate neutron irradiation at 300 K. The samples with an irradiation fluence of 3.5x1016ions/cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${3.5}\times {10}<^>{16}\,{\text{ions}}/{\text{cm}}<^>{2}$$\end{document} were subjected to isochronal annealing at different temperatures (573 K, 873 K and 1173 K) for 3 h. The Raman results revealed that the D peak gradually increased, whereas the intrinsic G peak decreased with increasing irradiation fluence. At the same irradiation fluence, the D peak gradually decreased, whereas the intrinsic G peak increased with increasing annealing temperature. Slow positron beam analysis demonstrated that the density or size of irradiation defects (vacancy type) increased with higher irradiation fluence, but decreased rapidly with increasing annealing temperature. The Raman spectral analysis of sample cross sections subjected to high irradiation fluences revealed the emergence of amorphization precisely at the depth where ion damage was most pronounced, whereas the surface retained its crystalline structure. Raman and positron annihilation analyses indicated that the matrix graphite exhibited good irradiation resistance to He ions at 300 K. However, vacancy-type defects induced by He ion irradiation exhibit poor thermal stability and can be easily removed during annealing.