  The precise measurements of the Compton scatterings of photons originating
from the decay of positronium atoms can reveal information about their
polarizations. J-PET detector is constructed of 192 plastic scintillators and
is unique to study the scattering correlations of the annihilation photons with
an angular precision of several degrees. In this work, we present the first
experimental evidence showing the feasibility of measuring the photons relative
polarization using the J-PET detector.
