Thickness Variations and Lifetimes of Carbon Foils under Nitrogen-ion Bombardment

The deposition of carbon upon carbon foils and the lifetimes of foils during bombardment by nitrogen ions have been investigated under various experimental conditions. The rate of carbon deposition depends upon the energy and current density of the bombarding ions, as well as upon the condition of vacuum. For ion energies of 0.125-1.5 MeV/atom and current densities of 0.25-13.0 μA/cm2, increases in areal density of up to 40% for ≃ 10 μg/cm2foils have been observed under conditions typically employed in nuclear physics and beam-foil spectroscopy. The source of the deposited carbon appears to be carbon-containing molecules in the residual gas of the vacuum system. Elimination of carbon deposition has been achieved by surrounding the foil with a cooled baffle. The lifetime of the foil (time until breakage) is found to be, at a given energy per atom, the time required for a constant number of atoms to have bombarded the foil.