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\section{Introduction}   Determination of the depth of interaction (DOI) is of importance for PET to reduce the cross-section of each scintillator crystal without degrading the overall sensitivity\cite{Moses_2001}\cite{Humm_Rosenfeld_Del_Guerra_2003}. Within monolithic scintillator detectors the same property allows spatial confinement \cite{am_Borghi_Seifert_Schaart_2013}\cite{Maas_Bruyndonckx_Schaart_2012}. \cite{am_Borghi_Seifert_Schaart_2013}\cite{Maas_Bruyndonckx_Schaart_2012}.\cite{Payne_Choong_Hull_Reutter_2008}  In this paper we explore the relationship between DOI and the timing performance due to scintillator crystal geometry, material and wrapping. Three scintillator crystals are investigated; Namely two identical $2\times2\times30$mm$^3$ Proteus LYSO:Ce and a $2\times2\times20$mm$^3$ Ca-co-doped LSO:Ce. All scintillator crystals are polished. PTFE (Teflon) tape is used as the wrapping material.