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

Abstract

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.