\label{fig:doi-ctr} The two timing coincidence apparatuses. In (a) the standard coincidence apparatus, used in this paper to measure the timing performance with the scintillator crystal length, $L$ is shown. In (b) the depth of interaction (DOI) coincidence apparatus is seen. In this, the measured scintillator detector is rotated 90 degrees, with respect to the reference scintillator detector. In the standard coincidence apparatus the Na22 source is placed equidistant between the two scintillator detectors, whereas in the DOI coincidence apparatus the source is placed much closer to the measured scintillator detector. This leads to electronic collimation forming a confinement region within the measured scintillator detector. The confinement region is shown in grey surrounded by a red dashed line\cite{Moses_Ullisch_2006}.
