On the growth of segregated C layers on top of Fe films on pyrolytic graphite samples during high-influence D+ irradiation at elevated temperature

On Fe films evaporated on pyrolytic graphite, thick C layers segregate during high-temperature (above about 800 K) light ion irradiation if the penetrating ions are energetic enough to reach the Fe-graphite interface. The thickness of the C segregated layer and the C depth distribution in the Fe film have been determined with 2-MeV 4He+ Rutherford backscattering. A steady-state carbon overlayer is reached at high fluences (above about 1019 particles/cm2), the thickness of which depends on the energy of the irradiating beam for a given thickness of the Fe evaporated film. The anisotropic structure of the pyrolytic graphite substrate influences the thickness of the steady-state C overlayer, thicker C layers being measured for edge-oriented C substrates. Using the Monte Carlo code trIm, the production of defects in the graphite substrate has been calculated for different thicknesses of the C overlayer. The total amount of defects produced in the graphite substrate has been identified as the parameter regulating the growth and the steady-state value of the C overlayer. With the depth distributions of defect production generated by trIm as source functions, the diffusion of C interstitials in graphite under the influence of recombination with vacancies has been modeled. The segregating C fluxes are identified with the fluxes of interstitials arriving at the Fe/graphite substrate interface for a suitable choice of the parameters in the diffusion equation