S. L. Varghese

The electron capture cross section of 1.5–3 MeV protons from carbon☆

Abstract Since single collision conditions for electron capture processes cannot be obtained for solid carbon targets, gaseous carbon compounds (CH 4 , C 2 H 2 , C 2 H 4 , C 3 H 6 , C 3 H 6 , and C 4 H 8 ) were used as targets to determine the electron capture cross sections of protons from carbon. Although the statistical uncertainty of the measurement was less than 1%, the pressure determination limited our absolute accuracy to 10–15%. Within this uncertainty, the electron capture cross sections of protons from carbon was independent of the carbon compound used in the experiment. The measured cross sections were 8.6 × 10 −21 , 3.0 × 10 −21 7.4 × 10 −22 cm 2 /atom for proton enrgies 1.5, 2, and 3 MeV respectively.

Statistical scaling of C and O K-shell fluorescence yields

Abstract Relative C and O K-shell fluorescence yields, computed from statistically scaled radiative and nonradiative rates for various molecules, show good agreement with experimental fluorescence yields for CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CO, CO 2 , and CF 4 (C-bearing), and O 2 , CO, CO 2 , H 2 O (O-bearing) gases.

Charge Transfer of 0.8 MeV/u H+, He+ in Various Hydrocarbon Gases

Systematic variations in total single electron capture cross sections, ¿10(CmHn), for 0.8 MeV/u H+ and He+ on various hydrocarbon gases are interpreted in terms of a model that incorporates intramolecular post-collision-interactions to estimate the reduction in the number of exiting neutral projectiles.

Reduction of the Electron Capture Cross-Section for Protons in Hydrocarbon Cases

The total electron capture cross section of 0.8 - 3.0 MeV protons in various hydrocarbon gases (CHn - C4H , n = 2 - 8) has been measured. We find that the electron capture cross section per carbon atom decreases as the number of carbon atoms per molecule is increased. This reduction amounts to ~7% for 3 MeV protons and is ~ 25% for 0.8 MeV protons. We find no significant change in the electron capture cross section for carbon as the number of hydrogen atoms per molecule is increased. The observed variations in the cross section can be reproduced by assuming that post-collision-interactions destroy the required velocity match between projectile and electron.

K X-Ray Yields from C-, O-, and F-Bearing Gases

The K x-ray yields induced by 2 MeV proton bombardment of various C-, O-, and F-bearing gases show variations that depend in a systematic way on the effective charge of the atom in the molecule.

Recent Results on Radiative Electron Capture

We have three new results concerning REC since our last publication.1–3 (1) The measured REC cross-sections per K-vacancy scale according to the number of “free” electrons in the target atom at least for the cases of CI projectiles incident on C and Cu. “Free” electrons are those bound target electrons with average velocity ≪ than the projectile velocity. Excellent agreement with the Bethe-Salpeter free-electron theory4 is obtained if it is assumed that each “free” target electron contributes equally to the REC cross-section, and the effective Cl K-shell binding energy needed to compute the cross-section is taken as the measured thin target e.g. 10 µg/cm2 REC centroid energy less the mean kinetic energy of the captured electrons in the projectile reference frame. Our methods of determining the experimental and theoretical REC cross-section per K-vacancy may explain some discrepancies found by Lindskog et al5.

Angular Distribution of Pb L X-Rays Excited by 3 MeV Protons

We have measured the angular distribution of Pb Ll and L¿ x-rays in the case of lead where the targets were typically 100 ug/cm2 thick with a 400 ug/cM2 layer of silver evaporated on each side. Spectra were obtained at 25, 45, 65, 90, 115, 135 and 155 and ratios of Pb L x-rays to the isotropic Ag K x-rays were taken. The most readily interpretable case is Ll since this transition involves only the Lm subshell. Atter correcting for x-ray absorption in the target assuming it to be layered (i.e. neglecting diffision of the Ag in the Pb) we find a small deviation from isotropy, which is expected due to enhanced depopulation of M=0 substates at small impact parameter collisions. The velocity dependence of the alignment parameter A2 has been calculated using PWBA and screened as well as unscreened wave functions. Our results for A2 agree with these calculations. Further work is under consideration.