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

Abstract: 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 1.1 × 1015 ions/cm2
to 3.5 × 1017 ions/cm2 were used to simulate neutron irradiation at 300K. The samples with an irradiation
fluence of 3.5×1016 ions/cm2 were subjected to isochronal annealing at different temperatures (573K, 873 and
1173K) 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 300K. However, vacancy-type defects induced by He ion irradiation exhibit poor thermal stability and
can be easily removed during annealing.