Doppler broadening of in-flight positron annihilation radiation due to electron momentum

We report the first observation of electron momentum contributions to the D oppler broadening of radiation produced by in-flight two-photon annihilatio n in solids. In these experiments an approximately 2.5 MeV positron beam im pinged on thin polyethylene, aluminum, and gold targets. Since energetic po sitrons easily penetrate the nuclear Coulomb potential and do not cause a s trong charge polarization. the experimental annihilation line shapes agree well with calculations based on a simple independent particle model. Moreov er, annihilations with the deepest cure electrons are greatly enhanced.