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\section{Method}  The coincidence apparatus used in this paper is comprised of two Hamamatsu MPPC S10931-050P SiPM connected to CERN-developed NINO discriminators from which the energy and timing information of individual pulses are collected using a LeCroy DDA 735Zi high-bandwidth oscilloscope. Scintillator crystals are coupled to the SiPM photodetectors using Rhorosil 47A optical grease to improve coupling. Prior to coupling all scintillator crystals are cleaned using methanol. Wrapped scintillator crystals are tightly bound in many layers of Teflon to ensure good coupling between the scintillator crystal and wrap. All scintillator crystals are handled with carbon-tipped tweezers to prevent formation of surface defects which may degrade the scintillator crystal performance. The coincidence apparatus is held within a temperature stable chamber to maintain stability of photodetector performance. Further to this, the first 5 minutes of each measurement is discarded due to any potential contribution of temperature variation. The optimal threshold and bias values of the SiPMs were determined by parameter sweep and are given in table \ref{tab:optimumparam}.  There are three principal sources of systematic error we wish to correct for, before attempting to determine the time resolution of a given scintillator detector. Firstly low-energy ($\lesssim$1MeV) gamma ray photons may interact with matter in two ways. We wish to select events which interact solely by the photoelectric effect to ensure there is no additional time ambiguity introduced by scattering within the system. To do this we fit to the `photopeak'\footnote{As the energy resolution is also not perfect, the 0.511keV gamma ray photon will be absorbed generating a Gaussian distribution known as the photopeak in the energy specta of the scintillator detector} generated by the total absorption of the gamma ray photon within the scintillator detector and exclude events outside a $2\sigma$ window about the peak location. Secondly, the number of `edges' recorded by the oscilloscope per sampling period will vary due to overlapping independent interactions within the scintillator detector. As a given logical pulse will generate two such edges, an ambiguity in the arrival time and the energy of a given interaction if multiple events occur within the same sampling period. The time information of a given signal is encoded in the first edge of the logical pulse. The width of the pulse itself is related to the energy deposited within the scintillator detector. Thus by selecting for sampling periods containing only two edges we remove this error. Finally we reject events where the difference in arrival time between the two scintillator detectors is greater than a prechosen limit. In this way, we reduce the chance of random events triggering the scintillator detectors simultaneously. This limit is typically no greater than a nanosecond.     $$   \text{CTR} = 4\sqrt{\ln{2}}\sqrt{\sigma-\sigma_\textrm{ref}^2}   $$     where $\sigma_\text{ref}$ is the reference time resolution subtracted in quadrature. A value of $43\pm4$ps was measured using the standard coincidence apparatus as given in \cite{ch_Meyer_Pizzichemi_Lecoq_2013}