Charge exchange processes for semi-relativistic helium ions (β = 0.51) in solid gold

Abstract

Interactions of 150MeV/amu 3He++ projectiles with solid gold targets have been studied at the isochronous cyclotron of the RCNP in Osaka. The 3He+ 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 ratio of singly to doubly ionized helium ions emerging from thin gold foils, He+/He++, has been measured as a function of the foil thickness. Extrapolating the results to zero Au target thickness permits to determine the cross section values for electron stripping from 3He+ ions, σSTRIP=1.05×10−17cm2, and for electron capture to 3He++ ions, σCAP=1.12×10−25cm2. The results obtained extend significantly the existing systematics for both processes to high (semi-relativistic) velocities. The collision strength deduced from the stripping cross sections deviates strongly from the theoretical predictions of Gillespie in absolute values as well as in the velocity dependence. It can, however, be well approximated by the simple Bohr formula for mid Z atoms. Also the capture data indicate the need to improve the theoretical approximations. A more detailed treatment of electrons captured from different shells in a high Z target is presumably needed. The astrophysical interest in the data of this kind for very light ions (hydrogen, helium) is indicated.