Observation of intense two-beam positron diffraction and the precise determination of the positron band gap in rare-gas crystals

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Abstract
We have measured the energy dependence of the elastic specular reflection of positrons from the (111) surfaces of solid rare-gas crystals. For each sample we observe an intense first-order Bragg peak with a profile closely resembling the ideal "top-hat shaped" profile first derived by Darwin. The well-defined width and center of the peak enables us to determine the positronenergy gap and inner potential as a sensitive test of the positron-gas-atom interaction in the solid.