Fig. 6

The definition of the analyzed events with 3 hits is displayed. Of the 3 hits, the first two hits (1,2) were caused by counter-propagating 511 keV photons. The third hit (3) represents the interaction by a scattered 511 keV photon. Here we see the association of the scattered photon (3) with the 511 keV photon marked 1, which gives us access to the scattering angle. However, the scattered photons are associated with the primary 511 keV photons based on the scatter test S, which can be described as the difference between measured times and calculated times (S = \(\Delta \hbox {t}_{ij} - \hbox {D}_{ij}\)/c), where ij represents the pair of primary and scattered photon, \(\Delta\) \(\hbox {t}_{ij}\) is the measured time difference, \(\hbox {D}_{ij}\) is the distance between a pair of hits (31 or 32) and c is the speed of light