\section{Reference Detector Measurements}
\label{sec:reference}
The reference scintillator crystal, shown on the left of figure \ref{fig:actualsetup}, is a $2\times2\times5$ mm$^3$ Agile Ca-co-doped LSO:Ce wrapped in PTFE tape. Using two such identical crystals the CTR was determined using both the standard and DOI coincidence apparatuses. The key values from the measurements are shown in table \ref{tab:referencevals} as time resolution and as CTR values. We note that the CTR from both measurements are in agreement. The CTR value from the standard coincidence measurement is in agreement with a prior measurement in [Table 2]\cite{uffray_Jarron_Meyer_Lecoq_2014} of 123$\pm$7 ps. In this paper the reference detector CTR value is taken as $131\pm$4 ps unless otherwise stated.
