When a positron annihilates, two gamma photons are created with orthogonal polarizations. It is possible to use coincidence measurements where both photons undergo Compton scattering to estimate their initial relative polarization orientation. This information is of great interest in gamma imaging systems, such as Positron Emission Tomography, where it may be used as an additional tool to distinguish true coincidence events from scatter and random background. The successful utilization of this principle critically depends on the detector’s angular and energy resolution, which determine its polarimetric performance. In this study, we use Monte Carlo simulations based on the Geant4 toolkit to model two multi-pixel detector configurations identified as prospective for the measurement of gamma-ray polarization in PET. One is based on 2 mm × 2 mm × 20 mm LYSO scintillators and the other is based on 3 mm × 3 mm × 20 mm GAGG scintillators. Each configuration has a pair of modules, each consisting of 64 crystals set up in a single 8 × 8 matrix, where both the recoil electron and the Compton-scattered photon are absorbed. We simulate positron annihilation by generating two back-to-back gamma photons of 511 keV with orthogonal polarizations. The Compton scattering is successfully identified and the modulation of the azimuthal angle difference is clearly observed. The configuration based on GAGG crystals demonstrates slightly better polarimetric performance than the one based on LYSO crystals, reflected in the more pronounced azimuthal modulation.

We investigated two detector modules, each consisting of a 8 × 8 matrix of GaGG:Ce crystals with a crystal size of 3 × 3 × 20 mm${}^3$ and a 3.2 mm pitch. The light is collected by a 8 × 8 silicon photomultiplier array, with one silicon photomultiplier matching one crystal. The signals are read out and processed using the TOFPET2 ASIC. Performed laboratory tests of the detectors were performed using a ${}^{22}$Na source, where energy and coincidence time resolution with different optical coupling were examined between the crystals and silicon photomultipliers, as well as under various operating voltages. The mean energy resolution of $9.8\pm 0.6\%$ at 511 keV was observed and it was shown that the coincidence time resolution of $384\pm 33$ ps could be achieved. The results reassure that the GaGG scintillator is a very promising candidate for the development of imaging systems, in particular, ones utilizing Compton scattering where the energy resolution plays a critical role and a moderate timing performance is acceptable.

We report on new H$(e,e^{\text{'}}p)\gamma$ measurements in the $\Delta (1232)$ resonance at $Q^2=0.06$ (GeV/$c$)${}^2$ carried out simultaneously with H$(e,e^{\text{'}}p){\pi }^0$. It is the lowest $Q^2$ for which the virtual Compton scattering (VCS) reaction has been studied in the first resonance region. The VCS measured cross sections are well described by dispersion-relation calculations in which the multipole amplitudes derived from H$(e,e^{\text{'}}p){\pi }^0$ data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude $M_{1+}^{3/2}=(40.60\pm 0.{70}_{\mathrm{stat}+\mathrm{sys}})({10}^{-3}/m_{{\pi }^{+}})$ at $W=1232$ MeV is in excellent agreement with the value extracted from H$(e,e^{\text{'}}p){\pi }^0$ measurements.