An experimental study of the 1800 backscattering yield enhancement

A large enhancement in the yields of MeV 4He ions backscattered near 1800 from amorphous and polycrystalline materials was reported by Pronko et al. and subsequently interpreted in terms of correlated scattering between the incoming and outgoing trajectories. We have developed a simple magnetic deflection system which detects all ions backscattered within a small (approx. equal to 0.040) angular cone centered at exactly 1800 to the incident beam direction. Using this system, we have investigated the enhancement in scattering yield from several thick amorphous (or polycrystalline) targets and also from Bi atoms implanted at various energies (i.e. depths) into C and Si targets. The maximum in the observed enhancement factor is almost independent of target material, provided the 1800 collision involves a heavy atom for which the kinetic backscattered energy factor is close to unity. In the Bi-implanted targets, the decrease in enhancement factor at larger is observed to scale well with x/E0 and is essentially independent of the host. (orig.)