In-flight annihilation during positron channeling

Energetic positrons propagating along low index directions in a crystal int eract strongly with the periodic array of atoms via a process known as chan neling. The trajectories of such channeled positrons can be manipulated to sample different spatial regions in the crystal. Here we report the first o bservations 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 s ingle photon annihilation due to the selective interaction of well-channele d positrons with valence electrons in the interstitial regions of the cryst al. Both classical and quantum calculation of annihilation yields are compa red to the experimental data. (C) 2000 Elsevier Science B.V. All rights res erved.