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The entanglement of photons originating from electron--positron annihilation has not been experimentally proven. owever, the independent experiments performed so far unanimously confirm that the correlation between the linear polarizations of back-to-back photons from electron--positron annihilation is consistent with the assumption that these photons are entangled in the polarization. Yet, unexpectedly, recent experiments differ as regards the correlation of polarization direction of back-to-back photons after the decoherence induced by the scattering of one of these photons on the electron in the scattering material. In one of the experiments, the correlation before and after the decoherence of the photon state is the same, and in the other experiment, the scattering of one photon leads to a significant decrease in this correlation. Here we discuss this puzzle. Decoherent states were ensured provided by forcing one of the annihilation photons to scatter earlier before measuring the polarization correlation based on Compton kinematics. The comparison is made between the experimental setups used for the different measurements, and the results obtained are briefly discussed, highlighting the parameters that are important in performing such measurements. Finally, the main features of the J-PET detector are presented, along with the schemes for performing similar studies, so that the conclusive results can be used as remarks to solve the puzzle in question. Solving the decoherence puzzle will have crucial consequences for basic studies of entanglement, as well as for the proposed application of the photon polarization in positron emission tomography. If the correlation of the polarization of back-to-back photons from the electron--positron annihilation is the same before and after the scattering of these photons, then it will not be useful for the reduction of scatter fraction in positron emission tomography diagnostics. [ABSTRACT FROM AUTHOR]

We developed a multichannel high time resolution data acquisition (DAQ) system for time-over-threshold (TOT) application specific integrated circuit (ASIC). The developed field-programmable gate array-based DAQ is composed of 144 channels per board with an Ethernet readout to measure energy and timing information simultaneously. The measured time precision of the developed DAQ board was reduced to 62.5 ps. The proposed DAQ with TOT-ASIC can potentially be used for several radiation detection applications, such as positron emission tomography (PET) and Compton imaging. The DAQ board is applied to PET and named PET readout board with a high-speed network (PETNET). In the first version developed, the PETNET had a time precision of 1 ns by implementing a double data rate method. This demonstrates that this method is useful for capturing electron–positron pairs of 22Na using two PETNETs. We obtained 2-D cross-sectional images placing two 8 × 8 detectors facing each other. In the second version developed, the PETNET had a time precision of 125 or 62.5 ps by implementing a delay-line method. Cable length measurements proved that PETNET has a position resolution of 4.5 cm at the speed of light. To synchronize the time of data on several PETNETs, a time-of-flight method was adopted, and all the data were compared with each other with accurate time precision. [ABSTRACT FROM AUTHOR]