DEVIATIONS OF O-16-INDUCED K-X-RAY YIELDS FROM Z(1)(2) DEPENDENCE

A careful comparison of K X-ray yields produced by 64 MeV O-16 ions an d 4 MeV protons of identical velocity has been carried out with +/-2% precision, using thin (similar to 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 o ur target thickness, the effective O-16 charge state is essentially eq ual to its incident value, For the highest-Z(2) target (Ag), our O-6+, O-7+ and 8+ yields were identical (to +/-2%) but were only 24 times th e proton yield. This ratio is considerably smaller than the Z(1)(2) pr ediction 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(1) mu ch less than Z(2). For the lower-Z(2) targets, a strong dependence on charge state is observed, Moreover, the measured O8+ enhancement facto r 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 O7+ and O8+ y ields in Ti and Cr is electron capture into K-shell vacancies of the o xygen ion. Residual discrepancies between the X-ray yield data and the ECPSSR ionization calculations are attributed to enhancement of fluor escence yield when Z(1)/Z(2) is large and the emergence of molecular o rbital effects when the projectile velocity is small relative to the t arget's K-shell electron orbital velocity.