Annihilation of fast channeled positrons

Energetic positrons propagating along low index directions in a crystal interact strongly with the periodic array of atoms via a process known as channeling. These channeled positrons are focused into the interstices of a crystal by a series of highly correlated small angle scattering events, thereby suppressing close nuclear collision processes and increasing interactions with valence electrons. Moreover, the positron trajectories can be manipulated to sample different spatial regions in the crystal, simply by changing the direction of the incident positron beam. As this direction deviates from that of the low index crystal direction the positron momentum transverse to this crystal direction increases, and the trajectories penetrate closer to the atomic nuclei of the crystal's atoms. Thus when observing the angular yield of close encounter events with the atomic nuclei, like wide angle Rutherford scattering, a characteristic channeling dip is obtained for positive ions and positrons traversing thin crystals[8, 102].;In this thesis I discuss the channeling effects on in-flight positron annihilation. Both a quantum and classical descriptions of the channeled positron flux within the crystal are developed and used to calculate in-flight annihilation yields. In order to carry out positron channeling experiments, a 3 MeV monoenergetic positron beam was constructed at Lawrence Livermore National Laboratory. Using this beam the dramatic channeling effects on in-flight annihilation were investigated for the first time. The channeling minimum yield for two photon annihilation is shown to be significantly enhanced compared to single photon annihilation due to the selective interaction of well channeled positrons with valence electrons in the interstitial regions of the crystal. Finally, the possibility of developing this technique into a probe of valence electrons is explored.