Application of ion implantation as a tool to study neutron induced morphological changes in HOPG and RBMK-1500 reactor graphite

Based on the RBMK-1500 (rus.: peak op o ݧo \Mo oc i ka a ݧߧ, P MK; reaktor bolshoy moshchnosti kanalnyy, "high-power channel-type reactor") reactor irradiation conditions, ion implantation was used as a tool to study neutron induced morphological changes in both highly oriented pyrolytic graphite (HOPG) and nuclear grade RBMK graphite. Graphite samples were implanted with 180 keV 14N+ ions at the fluences of 1.0 x 1016 ions/cm-2 and 2.5 x 1016 ions/cm-2. To study temperature effects on both ion migration and structural disorder level in the graphite matrix, the implantation procedures were carried out under different temperature condi-tions - room temperature (RT) and 500 degrees C. Subsequently, the distribution profiles of the implanted 14N+ ions were obtained by using secondary ion mass spectroscopy (SIMS) technique, while the microstructural properties of the graphite were evaluated by Raman spectroscopy. Implantation induced primary displacement damage profiles and implanted ion profiles were evaluated theoretically by using GEANT4 10.6 and SRIM-2013 codes. The profiles of implanted nitrogen obtained by SIMS technique were found to be in good agreement with theoretical ones. The surfaces of HOPG samples displayed macroscopic damage in form of fractures after 14N+ ion implantation at 500 degrees C; however, in case of RBMK graphite, the sample surfaces remained without visually observable changes. The Raman spectra showed an increase of sp3-related content and formation of amorphous carbon in both HOPG and RBMK graphite samples; the latter exhibits higher structural disorder at microscopic level and stronger amorphization than HOPG.