Fig. 1

a The photo of J-PET scanner with the front view (see text for description). b The cross section of J-PET with superimposed arrows depicting the interaction of 511 keV photons (black solid arrows) originating from positron-electron annihilation and prompt photon 1275 keV before (orange solid arrow) and after (orange dashed arrow) scattering inside the scintillator. Red dot in the center represents the 22Na source