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In this paper we explore the relationship between the DOI of 511keV gamma ray photons and the timing and energy performance of the scintillator detector. The DOI is a potential source of degradation to the timing and energy performance of the scintillator detector, due to photon time of flight and light loss from increased path lengths within the scintillator crystal itself.  We begin by outlining the method utilised in the standard and DOI coincidence measurements in section \ref{sec:method}. Following this the reference detector, a $2\times2\times5$mm$^3$ Ca-co-doped LSO:Ce, is characterised as well as varying scintillator crystal lengths in sections \ref{sec:reference} and \ref{sec:standardctr} respectively. Next we look at the DOI results for two identical $2\times2\times30$mm$^3$ Proteus LYSO:Ce followed by a single $2\times2\times20$mm$^3$ Agile  Ca-co-doped LSO:Ce in sections \ref{sec:30mm} and \ref{sec:20mm}. This is to determine the contribution of scintillator crystal material, geometry and wrapping. All scintillator crystals are polished. PTFE (Teflon) tape is used as the wrapping material due its diffusive properties. Finally we bring the results together in the discussion in section \ref{sec:discussion}.