Projectile charge-state dependence ofKx-ray production by 1-4-MeV/amu heavy ions in gases

We report the observation of large projectile-structure effects in heavy-ion-induced $K$ x-ray yields from thin, light gas targets in a projectile energy region (∼ 1-4 MeV/amu) where molecular promotion effects are not expected to be significant. The $K$ x-ray yield from both projectile and target is found to be very sensitive to the ionic charge $q$ of the projectile. In some cases, notably for ${\mathrm{Ar}}^{q+}$ on Ne at 80 MeV and for ${\mathrm{Cl}}^{q+}$ on Ne at 42 MeV, the target yield increases exponentially with projectile charge, the projectile nuclear charge and velocity remaining constant. In all cases, the projectile $K$ x-ray yield increases faster than exponentially, even when the incident projectile has a full $K$ shell. It is evident that targets become highly ionized in these collisions, and already highly ionized projectiles are often further ionized in these targets of less-than-eqilibrium thickness. We discuss the limitations imposed on the determination of absolute x-ray cross sections in multiply ionizing collisions. These limitations cast serious doubt on the accuracy of $K$-shell ionization cross sections extracted from measured $K$ x-ray cross sections in this and other experiments. The projectile-target combination studied in the present experiment are ${\mathrm{Ar}}^{q+}$ on Ne (at 80 and 152 MeV), ${\mathrm{Cl}}^{q+}$ on Ne and S${\mathrm{F}}_6$ (at 42 and 50 MeV), and ${\mathrm{F}}^{q+}$ on two chlorine-bearing Freon compounds (at 43 MeV).