In-Flight Annihilation during Positron Channeling:
   Spatial Sampling of Valence Electrons

Dr. Alan Hunt, Idaho State University, Research Assistant Professor

Abstract

When energetic positive charged particles impinge along a low index crystal direction, they undergo a large number of highly correlated small angle scattering events which guide them through the lattice via a process known as channeling.  This process effectively focuses positive particles into the interstitial regions of the crystal, thereby suppressing close nuclear collision processes (e.g. Rutherford backscattering), and increasing interactions with valence electrons.  It has recently been suggested that by observing annihilation radiation from channeled positrons, a quantitative probe of electron charge and spin densities may be realized.  This new technique would use the channeling process to control which atomic regions are sampled by the positron trajectories, while the radiation from the two photon in-flight annihilation process would provide an experimental signal that reflects electron properties in the sampled region.  Practical implementation, however, requires an intense monoenergetic positron beam with ~10⁷ e+/s.  While such a beam is currently not available, a flux of ~10⁵ e+/s was sufficient to demonstrate the dramatic new annihilation effects expected for channeled positrons.  Our experiments focused on observing single and two photon annihilation during positron channeling. Comparison of these two annihilation modes demonstrates the ability of channeled positrons to selectively sample valence electrons in the crystal.