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

Abstract

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- Z 2 target (Ag), our O 6+, 7+ and 8+ yields were identical (to ± 2%) but were only 24 times the proton yield. This ratio is considerably smaller than the Z 1 2 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 Z 2 ⪡ Z 2. For the lower- Z 2 targets, a strong dependence on charge state is observed. Moreover, the measured O 8+ enhancement factor relative to 4 MeV protons is 125 in Ti, i.e., twice the simple Z 1 2 scaling value. Again, these observations are well reproduced by the ECPSSR calculations and the major cause of the enhanced O 7+ and O 8+ 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 Z 1 Z 2 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.