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

• Published in: Nature (London) 1999-11, Vol.402 (6758), p.157-160
• Main Authors: Hunt, A. W., Cassidy, D. B., Selim, F. A., Haakenaasen, R., Cowan, T. E., Howell, R. H., Lynn, K. G., Golovchenko, J. A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Cell division; Channeling phenomena (blocking, energy loss, etc.); Condensed matter: structure, mechanical and thermal properties; Electrons; Exact sciences and technology; Humanities and Social Sciences; letter; Metals. Metallurgy; multidisciplinary; Physics; Science; Science (multidisciplinary); Structure of solids and liquids; crystallography
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/45990

Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle 9 , 10 , 11 , 12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 10 5 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 10 7 particles per second.