Contributed by Maurizio Biasini, ENEA, Italy
Figure 1 Momentum distribution in quartz obtained with the 2D-ACAR facility implemented by ENEA INN FIS DIAF. The integration direction is along the c axis of the hexagonal single crystal.
The narrow central peak is ascribed to the momentum distribution of parapositronium, the electron positron singlet bound state. The first and second series of satellite peaks are images at high momenta of the parapositronium peak due to the effect of the periodic crystal potential. The broad distribution beneath is ascribed to the annihilation of free positrons with valence electrons. One mrad is equivalent to 0.137 momentum a.u.
Figure 1b View of the corresponding spectra acquired with the FAST ComTec multiparameter system.