  In para-positronium annihilation, exploration of the polarization
correlations of the emerging gamma quanta has gained interest, since it offers
a possibility to improve signal-to-background in medical imaging using positron
emission tomography. The annihilation quanta, which are predicted to be in an
entangled state, have orthogonal polarizations and this property may be
exploited to discriminate them from two uncorrelated gamma photons contributing
to the background. Recent experimental studies of polarization correlations of
the annihilation quanta after a decoherence process induced by a prior Compton
scattering of one of them, had rather different conclusions regarding the
strength of the correlation after the decoherence, showing its puzzling nature.
In the present work, we perform for the first time, a study of the polarization
correlations of annihilation quanta after decoherence via Compton scattering in
the angular range $0^\circ-50^\circ$ using single-layer gamma ray polarimeters.
In addition, we compare the measured polarization correlations after Compton
scattering at $30^\circ$ with an active and a passive scatterer element. The
results indicate that the correlation, expressed in terms of the polarimetric
modulation factor, shows no significant difference at small scattering angles
($0^\circ-30^\circ$) compared to the correlation measured for direct photons,
while lower modulation was observed for $50^\circ$ scattering angle.
