Energy-Resolved Positron Annihilation In Flight In Solid Targets

Energy-resolved two-quantum annihilation in flight of positrons with energies ranging from 10 to 71.6 keV was observed. An energy-dispersive two-detector coincidence system was used to observe the sum and difference energies of the gamma rays from annihilating positron-electron pairs. For positrons penetrating carbon foils the c/upsilon dependence of the annihilation cross section is confirmed. Spectra obtained from gold-coated carbon foils show evidence of in-flight annihilation with gold M-shell electrons.