Spatial sampling of crystal electrons by in-flight annihilation of fast positrons

Energetic, positively charged particles travelling along a low-index crysta l direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or 'channel' (refs 1-8) the pa rticles through the lattice, Channelling effectively focuses positive parti cles into the interstitial regions of the crystal: nuclear collisional proc esses such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channell ed positrons with electrons produces annihilation radiation that can in pri nciple(9-12) serve as a quantitative, spatially selective probe of electron ic charge and spin densities within the crystal: in the interstitial region s, two-photon annihilation is enhanced relative to single-photon annihilati on, because the latter process requires a nuclear recoil to conserve moment um. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 10(5) particles p er second. Comparison of these two annihilation modes demonstrates the abil ity of channelled positrons to selectively sample valence electrons in a cr ystal. Useful practical implementation of the technique will require the de velopment of more intense positron beams with fluxes approaching 10(7) part icles per second.