Inner shell ionization of intermediate- and high-z elements produced by collisions with fast heavy charged particles

Abstract

A review of the theoretical approaches to inner-shell ionization in fast ion-atom collisions has been presented. K x-ray yields have been measured for ten elements ranging from Fe (Z = 26) to Au (Z = 79) using 30-100 MeV alpha particles and for Fe, Cu and Sn using 28.2-196.4 MeV carbon ions. The alpha particle data were combined with some earlier measurements and a smooth curve fit was performed on the x-ray production cross sections plotted as a function of target atomic number. The "smoothed" cross sections were used in comparisons with the predictions of the binary encounter approximation (BEA), the plane wave Born approximation (PWBA) and the binding corrected PWBA calculations and resulted in the observation of a systematic atomic-number dependent deviation between experiment and theory. By comparing the measured cross sections to the universal BEA curve at velocity ratios in which the mean electron velocity was calculated "semi"-relativistically, overall agreement within ±16% was obtained for all elements studied. The theoretically predicted z²-dependence of the PWBA and BEA was tested by measuring the x-ray yield ratio of Cu using identical velocity alpha particles and deuterons and alpha particles and carbon ions. The alpha-deuteron results were compared with other theoretical calculations which predict a high energy crossing of the simple z²-dependent prediction. The alpha-carbon ion data exhibited deviations of as much as 34% above the z² prediction. Cross sections from the ionization of the L-shell in seven elements ranging from Sn (Z = 50) to Pb (Z = 82) and for the L-subshells of Tb, Tm, Au and Pb were measured using 30-80 MeV alpha particles. Subshell ionization cross sections were deduced from the Lα and Lγ x-ray lines and were compared to the predictions of the BEA, PWBA and the SCA. The resulting subshell cross sections were used to obtain average values of the total L-shell fluorescence yield for Tb, Tm, Au and Pb from which fluorescence yields for Sn, Te and Nd were extrapolated. In this way, ionization cross sections for these elements were obtained. Ratios of the σ[subscript L[subscript III]]/σ[subscript L[subscript II]] values were found to be in good agreement with previous data and the PWBA prediction. A computer program (ORION), written to compute theoretical cross sections based on the BEA, PWBA and the SCA, and to calculate the average depth of x-ray production and emission for a given target thickness and incident projectile, is also described.