Correlation between isotope fractionation and surface composition profile during prolonged bombardment

Under 1 keV Kr ion bombardment, both primary and secondary effects on B isotope sputtering have been investigated by means of a dynamic Monte Carlo program. It was found in these calculations that at both zero and high fluences the light isotope was preferentially ejected in the near-normal direction; between these two cases there was a low fluence (1.4 X 10(15) ions/cm2), at which the isotope fractionation was independent of the emission angle over nearly the whole angular range. The variation in the atomic ejection probabilities of B-10 and B-11 from beneath the topmost layer owing to surface composition changes may be a important cause of the above phenomena.