Observation of triple-photon decay in positron-electron pair   annihilation: a triple coincidence setup for the undergraduate laboratory
  The positron-electron pair annihilation in two photons is known for its numerous applications using PET scanners. The decay of Positronium (Ps) from a standard sealed source in more than two photons is less likely but can be observed with a relatively simple setup. The main goal of this experiment was to verify momentum and total angular momentum conservation principles at subatomic level through the Ps annihilation. The two spin configurations of Ps are produced with a beta+ source. The decay modes are identified using gamma-ray spectroscopy techniques in a double and triple coincidence setup. Three germanium detectors (or two HPGe and one NaI(Tl) detectors) and a digital electronic system are required. 
M. E. A. Elbasher, W. Erasmus, E. A. M. Khaleel, J. Ndayishimye, P.   Papka
physics.ed-ph physics.ins-det