Anisotropy of characteristicK-shell x rays from heavy-ion—atom collisions

Angular distributions have been measured and polarization fractions determined for target and projectile x rays emitted in collisions of 3-MeV protons on argon and 33-MeV fluorine ions on argon and helium. The near isotropy in target x-ray emission that follows $K$-shell ionization is not unexpected; however, the anisotropic emission of projectile x rays contradicts the assumptions that are made in the analysis of x-ray cross sections. The polarization of ∼ 23% in this characteristic $K$-shell radiation can be understood qualitatively from the alignment of excited states populated by electron capture in atomic collisions.