Investigation of positron annihilation-in-flight using a digital coincidence Doppler broadening spectrometer

This work reports on precise measurements of two-quantum positron annihilation-in-flight using a digital coincidence Doppler broadening spectrometer. Annihilation-in-flight was measured for positrons emitted by Ge-68/Ga-68 and Na-22 radioisotopes and for various targets. Experimental data were compared with theoretical prediction by quantum electrodynamics. It was found that two-quantum positron annihilation-in-flight can be clearly recognized in two-dimensional coincidence Doppler broadening spectra as a hyperbolic curve with shape described well by the relativistic theory. The contribution of annihilation-in-flight is determined predominantly by the energy of incident positrons and is only weakly dependent on the target material. The profile of the positron annihilation-in-flight contribution for positron kinetic energies above 100 keV is well described by theory.