In-flight annihilation during positron channeling

Energetic positrons propagating along low index directions in a crystal interact strongly with the periodic array of atoms via a process known as channeling. The trajectories of such channeled positrons can be manipulated to sample different spatial regions in the crystal. Here we report the first observations of channeling effects on in-flight annihilation radiation from positrons traversing a thin gold crystal. 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. Both classical and quantum calculation of annihilation yields are compared to the experimental data.