Radiative and non-radiative electron capture from carbon atoms by relativistic helium ions

Abstract

Interaction of radiation with helium atoms and/or ions is of interest in various astrophysical applications. The reverse reactions of fast 150 MeV/amu ³He⁺⁺ ions with solid C targets have been studied at the isochronous cyclotron of the RCNP in Osaka. The singly ionized helium ions resulting from capture of the target electrons to the projectile were observed with the use of large magnetic spectrograph, Grand Raiden, set at θ=0° with respect to the beam. The yield ratios of singly-to-doubly ionized helium ions emerging from thin carbon foils, He⁺/He⁺⁺, have been measured as a function of the foil thickness. Extrapolating the results to zero target thickness permits to determine the cross section values separately for electron stripping from ³He⁺ ions and for electron capture to ³He⁺⁺ ions. The radiative and non-radiative contributions to the capture cross section were determined in a separate experiment in which the 84 keV (C.M.) photons were observed in coincidence with the He⁺ ejectiles. The results are compared with theoretical predictions. Need for improved calculations is noted for all the three processes involved, i.e. for the radiative and non radiative electron capture as well as for the electron stripping.