Isotope difference between hydrogen inventories in graphite under simultaneous H+ and D+ irradiation

The concentrations of H and D atoms retained in graphite by simultaneous dual implantation at room temperature with H+ and D+ ions, of which the peak depths in the target coincide with each other, have been measured as a function of time by means of an elastic recoil detection (ERD) technique. It has been observed that the relative concentrations of H and D increase in the same way in the beginning of the implantation, while thereafter separate from each other; and the relative concentration of H saturates to a lower level faster than that of D does to a higher level. The ratio of the saturation concentrations of D to H has been found to be 1.8 which is the so-called isotope effect in hydrogen retention by simultaneous dual irradiation. The experimental result on dual irradiation is shown to be very consistent with the theoretical value, 1.6±0.2, calculated by solving a set of the mass balance equations for H and D, which has been developed from a model for formation of mixed molecule HD and the mass balance equations successfully describing the retention and re-emission of hydrogen isotopes on single ion implantation. (orig.)