Deviations of 16O-induced K X-ray yields from Z12 dependence

A careful comparison of K X-ray yields produced by 64 MeV 16O ions and 4 MeV protons of identical velocity has been carried out with ±2% precision, using thin (∼ 10 nm) targets of Ti, Cr and Ag. The incident oxygen charge state was varied from 6+ to 8+. Since the path length for charge exchange in the K-shell of oxygen greatly exceeds our target thickness, the effective 16O charge state is essentially equal to its incident value. For the highest-Z2 target (Ag), our O6+, 7+ and 8+ yields were identical (to ± 2%) but were only 24 times the proton yield. This ratio is considerably smaller than the Z12 prediction of 64, but agrees well with the factor of 28 predicted by the ECPSSR theory. The lack of dependence on charge state indicates that electron capture into the K-shell of oxygen is negligible when Z2 ⪡ Z2. For the lower-Z2 targets, a strong dependence on charge state is observed. Moreover, the measured O8+ enhancement factor relative to 4 MeV protons is 125 in Ti, i.e., twice the simple Z12 scaling value. Again, these observations are well reproduced by the ECPSSR calculations and the major cause of the enhanced O7+ and O8+ yields in Ti and Cr is electron capture into K-shell vacancies of the oxygen ion. Residual discrepancies between the X-ray yield data and the ECPSSR ionization calculations are attributed to enhancement of fluorescence yield when Z1Z2 is large and the emergence of molecular orbital effects when the projectile velocity is small relative to the target's K-shell electron orbital velocity.