Emission channeling

Hyperfine interaction techniques like Mossbauer effect or perturbed gamma gamma angular correlation are commonly applied to study the structure and properties of impurity-defect complexes in solids. It is often difficult to resolve a certain defect structure unambiguously with these techniques, because an absolute determination of the lattice site of the probe atoms is not straight-forward. The emission channeling technique allows the direct determination of lattice sites of radioactive impurity atoms, incorporated into single crystalline solids. The channeling effects of electrons, positrons or alpha particles, emitted from radioactive impurities are measured along different crystal axes and planes. From the measured anisotropic emission distributions the lattice sites of the emitting atoms can be determined. Emission channeling can be applied to a large variety of different probe atoms. Also, rather low impurity concentrations, comparable to those typically required for hyperfine interaction techniques, are sufficient. In this contribution, the principles of the emission channeling technique, the experimental requirements and the quantitative analysis of emission channeling spectra are reviewed. The capabilities and possibilities, which the emission channeling technique offers, are highlighted by three recent experimental studies. First, studies of the diffusion of Ag in CdTe using transmutation doping with the electron emitting isotopes Ag-107m and Ag-109m are described. Second, lattice location studies of As in diamond, which is a potential n-type dopant in this material, will be discussed. Third, an experiment is described to study the lattice location of oversized impurities after low dose implantation into Fe. In this experiment, the unique decay properties of Fr-221 and Ra-221 are utilized to determine the lattice sites of five different impurity atoms in a single alpha emission channeling measurement.