Charge State Dependence of Stopping Power for Oxygen Ions Channeled in Silver

Abstract

In previous work [1] it was shown that electron capture and loss cross sections for 30–40 MeV oxygen ions in Au channels were small enough (~10^-19 cm²) to preclude charge state equilibrium in crystals 1 μm thick. In fact, a large fraction of charges 8+ and 7+ passed through the crystal without any charge exchange. In this work we show that similar conditions obtain in Ag and we have measured the charge state dependence of the stopping power for 27.8 and 40 MeV 0 ions in the [110] axial channel. We find that for those ions which do not undergo charge exchange S = kq² for charges 6+, 7+ and 8+. Other features observed in the energy spectra of the emergent ions are attributable to charge exchange in the crystal. Screening by Fermi electrons is expected to be the same for channeled and randomly penetrating ions. The results indicate that this screening is not effective in reducing the energy loss of the ion.

Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation.

† Department of Physics, Kansas State University.