Neutralization of helium ions backscattered from tungsten

Measurements and computer simulations of the energy spectra of helium ions backscattered from clean, polycrystalline tungsten at 138° have been combined to obtain estimates of the probability of survival against neutralization. For incident ion energies of 300, 1180 and 2600 eV, the estimated survival probabilities for ions backscattered near the surface are 0.05%, 0.2% and 1%, respectively. The estimated survival probability for subsurface backscattered ions has been found to range from a value of 1% at 2600 eV, comparable to that of near-surface backscattered ions, down to near zero at 300 eV incident ion energy. Evidence is presented that the final charge state of ions which backscatter from beneath the surface is not controlled by their emergence speed but by their previous history in the solid.