In flight positron annihilation in thin targets

We report on the first direct energy resolved measurements of two quantum a nnihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable k inetic energy up to 70 keV was guided onto thin aluminum targets. To clearl y identify in flight positron annihilation a two dimensional Doppler broade ning technique was employed utilizing two HPGe detectors. The method unique ly identifies TQAF by providing the total energy of the annihilating electr on-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 ar e possible. (C) 1999 Elsevier Science B.V. All rights reserved.