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

he stability of matrix graphite under neutron irradiation and in corrosive environments is crucial for thesafe operation of molten salt reactors (MSRs). Raman spectroscopy and a slow positron beam were employedto investigate the effects of He ion irradiation fluences and subsequent annealing on the microstructure anddefects of the matrix graphite. He ions with 500 keV energy and fluences ranging from 1.1 × 1015 ions/cm2to 3.5 × 1017 ions/cm2 were used to simulate neutron irradiation at 300K. The samples with an irradiationfluence of 3.5×1016 ions/cm2 were subjected to isochronal annealing at different temperatures (573K, 873 and1173K) for 3 h. The Raman results revealed that the D peak gradually increased, whereas the intrinsic G peakdecreased 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 analysisdemonstrated that the density or size of irradiation defects (vacancy type) increased with higher irradiationfluence, but decreased rapidly with increasing annealing temperature. The Raman spectral analysis of samplecross sections subjected to high irradiation fluences revealed the emergence of amorphization precisely at thedepth where ion damage was most pronounced, whereas the surface retained its crystalline structure. Ramanand positron annihilation analyses indicated that the matrix graphite exhibited good irradiation resistance to Heions at 300K. However, vacancy-type defects induced by He ion irradiation exhibit poor thermal stability andcan be easily removed during annealing.