Experimental impact-parameter-dependent probabilities forK-shell vacancy production by fast heavy-ion projectiles

The impact-parameter dependence of the probability for production of target $K$ x rays has been measured for oxygen projectiles on copper and for carbon and fluorine projectiles on argon at scaled velocities near 0.5. The O-on-Cu data were taken for 1.56-, 1.88-, and 2.69-MeV/amu O beams incident upon thin Cu foils. A thin Ar-gas target was used for 1.56-MeV/amu C and F beams, permitting measurements to be made for charge-pure ${\mathrm{C}}^{+4\mathrm{}}$, ${\mathrm{C}}^{+6\mathrm{}}$, ${\mathrm{F}}^{+9\mathrm{}}$ and ${\mathrm{F}}^{+5\mathrm{}}$ projectiles. Ar and Cu $K$ x rays were observed with a Si(Li) detector and scattered projectiles with a collimated surface-barrier detector. Comparison of the shapes of the measured $K$-vacancy-production probability curves with predictions of the semiclassical Coulomb approximation (SCA) shows adequate agreement for the O-on-Cu system. For the higher ratio of projectile-to-target nuclear charge ($\frac{Z_1}{Z_2}$) characterizing the C-on-Ar and F-on-Ar systems, the SCA predictions are entirely inadequate in describing the observed impact-parameter dependence. In particular, they cannot account for large probabilities found at large impact parameters. Furthermore, the dependence of the shapes on the projectile charge state is found to become pronounced at larger $\frac{Z_1}{Z_2}$. Attempts to account for this behavior in terms of alternative vacancy-production processes are discussed.