Investigation of the saturation effect in ion-atom excitation cross sections

Recent measurement on the projectile excitation cross sections of He like Fe-1 and Kr-2 ions colliding with a variety of targets have shown saturation with the target atomic number, contrary to the first Born approximation which predicts a Z(2) dependence. This phenomena could be explained theoretically using the distorted wave formalism and also in closed coupling atomic and molecular orbital approach. It is interesting to see the existence of such effect using lighter projectiles. We have therefore carried out investigations using 80 MeV He like silicon ions colliding with various targets like He, Ne, Ar, Kr, Xe under single collision conditions. The projectile X-rays were detected with a Si(ti) detector having a resolution of 150 eV at 5.9 keV. The various Lyman components were well resolved. The excitation cross sections have been extracted from the characteristic X-ray spectrum observed. The preliminary analysis indeed shows that individual. (1s(2) - 1s2p, 1s(2) - 1s3p, 1s(2) - 1s4p) as well as the total projectile excitation cross sections exhibits saturation as a function of target atomic number. Detailed analysis in the framework of various existing theories are in progress.