Investigation of Single-Layer GAGG: Ce and LYSO: Ce Multi-Pixel Detectors for Measurement of Polarization Correlations of Annihilation Quanta

Positron annihilation overwhelmingly results in two back-to-back gamma photons with orthogonal polarizations. This initial correlation of polarizations is reflected in the correlation of the azimuthal angles when both gamma photons undergo Comp-ton scattering. Using this information on polarization correlations may be of great value in medical imaging with Positron Emission Tomography (PET), where it can provide valuable information, independent of energy, to discriminate true coincidence events and thus contribute to higher image quality. In this work we investigate two detector configurations based on a single layer of scintillator pixels, that are capable of measuring the angular correlations of Compton scattered annihilation quanta. Detector modules consist of either GAGG: Ce or LYSO: Ce scintillators in 8 x 8 matrices with elements of 3 mm x 3 mm x 20 mm or 2 mm x 2 mm x 20 mm, respectively. A module is read out on the back side by a matching silicon photomultiplier array, hence the single detector layer acts as both the Compton scatterer and the absorber. The single-layer concept for polarimetric measurements is important since it could be applied in clinical size PET systems, without increasing the instrumental complexity of the apparatus. Pairs of detector modules of the same type were tested experimentally using a Na-22 source. The modulation of the azimuthal angle difference, linked with the initial polarizations of the annihilation quanta is clearly identified. We observe that the LYSO configuration generally measures larger modulation. Additionally we discuss the modulation dependence on the scattering kinematics and event topology. The same detector configurations are also simulated using the Geant4 toolkit and the results are compatible with the experimental findings.