K-shell ionization cross sections for Si, P, K, Ca, Zn, and Ga by protons and carbon ions in the energy range 1-6.4 MeV.

Abstract

Absolute K-shell ionization cross sections have been measured for thin targets of Si, P, S, K, Ca, Zn, and Ga using carbon ions between 1.0 and 6.4 MeV and protons of 1 and 2 MeV. The dependence of x-ray production cross sections on target thickness was determined. The experimental results are compared to the semiclassical approximation [Laegsgaard, Andersen, and Lund in 3 Proceedings of the Tenth International Conference on the Physics of Electron and Atomic Collisions, Paris, 1977, edited by G. Watel (North-Holland, Amsterdam 1977)], to the theory for direct Coulomb ionization of the 1s\ensuremath{\sigma} molecular orbital [Montenegro and Sigaud, J. Phys. B. 18, 299 (1985)], to the perturbed stationary-state approximation with energy-loss, Coulomb, and relativistic corrections (ECPSSR) [Brandt and Lapicki, Phys. Rev. A 23, 1717 (1981)], and to the modification of the ECPSSR approximation (MECPSSR) [Benka, Geretschl\"ager, and Paul, J. Phys. (Paris) Suppl. 12, C9-251 (1987)]. The results for carbon ions are also compared to the statistical molecular orbital theory of inner-shell ionization for symmetric or nearly symmetric atomic collisions [Mittelman and Wilets, Phys. Rev. 154, 12 (1967)].The proton results agree with empirical reference cross sections [Paul, Nucl. Instrum. Methods 169, 249 (1980)]. Also, MECPSSR and ECPSSR provide best overall agreement for protons. For carbon ions the MECPSSR theory predicts the experimental data best. For the lightest targets, however, the MECPSSR underestimates the measured ionization cross sections appreciably for scaled velocities 0.4\ensuremath{\le}\ensuremath{\xi}\ensuremath{\le}0.8. We found that in this \ensuremath{\xi} range Pauli excitation via interacting level crossing contributes strongly to the measured ionization cross section. The sum of Mittelman-Wilets predictions and MECPSSR predictions then reproduces the measured cross sections satisfactorily. This result indicates that the direct Coulomb ionization of atomic orbitals, the Pauli excitation, and the direct Coulomb ionization again, but now of molecular orbitals, are the significant K-shell ionization mechanisms for carbon ions (atomic number ${Z}_{1}$=6) on targets (atomic number ${Z}_{2}$) with ${Z}_{1}$/${Z}_{2}$\ensuremath{\sim}0.4, for high ion velocities, intermediate velocities, and the lowest velocities, respectively.