Fig. 2.

Schematic view of a single layer of the J-PET detector with two (up) or three (down) gamma quanta annihilation. In presently built geometry the first layer consists of 48 plastic scintillators (green bars). In this pictorial representation, for clarity, a smaller number of strips is shown. Solid dark blue lines indicate annihilation quanta and dashed brown line indicates de-excitation gamma quantum e.g. from the ${}^{22}\text{Na}\to {}^{22}\text{Ne}{}^{\ast }+e^{+}+\mathit{\nu }\to {}^{22}\text{Ne}+\mathit{\gamma }+e^{+}+\mathit{\nu }$ decay chain. Due to the momentum conservation annihilation quanta are moving along the same line in the case of $e^{+}{e}^{-}\to 2\mathit{\gamma }$, while in the case of the $e^{+}{e}^{-}\to 3\mathit{\gamma }$ they are included in a single plane. The de-excitation photon (dashed line) is not correlated with the annihilation photons and is isotropically distributed with respect to the annihilation plane-of-response. Due to the fact that annihilation and de-excitation occur in a good approximation at the same place the photons from the $e^{+}{e}^{-}\to 2\mathit{\gamma }$ form a plane with the de-excitation photon