In flight positron annihilation in thin targets

We report on the first direct energy resolved measurements of two quantum annihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable kinetic energy up to 70 keV was guided onto thin aluminum targets. To clearly identify in flight positron annihilation a two dimensional Doppler broadening technique was employed utilizing two HPGe detectors. The method uniquely identifies TQAF by providing the total energy of the annihilating electron-positron pair. A basic understanding of the fundamental TQAF process has been obtained using this high resolution technique. With this fundamental understanding we believe several new avenues of experimental exploration are possible. (C) 1999 Elsevier Science B.V. All rights reserved.