Tracer Studies on Radiation Damaged Graphite

The distribution of displaced carbon atoms in irradiated graphite has been studied with a radioactive tracer technique. Radiation-damaged graphite containing radioactive displaced carbon (C¹¹) atoms was prepared by cyclotron or betatron irradiation. After partial annealing, the distribution of the C¹¹ was determined by controlled oxidation and counting of CO₂ gas samples. Experiments on various types of weakly irradiated graphite indicate that a very small fraction of the displaced atoms are driven to particle surfaces during the annealing process, the fraction being higher for natural graphite than for artificial graphite and varying inversely with graphite particle size. Experimental conditions were varied to determine their effects on the distribution of the disaplaced atoms. Data obtained indicate that very little reintegration of displaced atoms occurs during short neutron bombardments at room temperature, that about 80% of the atoms reintegrate into vacancies during annealing below 400°C and that the remainder coalesce into complexes, and that large scale motion of the complexes begins at 400°C and ceases at approximately 1000°C. At this latter temperature the complexes appear to reach their final positions; however, they are relatively loosely bound and integrate progressively at these sites until they become indistinguishable from lattice atoms near 1700°C. Vacancies resulting from prior irradiation were found by tracer experiments to be effective traps for displaced atoms so that, during annealing of subsequent damage, the fraction that reaches the particle surfaces decreases rapidly with bombardment.