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\section{Depth Of Interaction Coincidence}  \subsection{Overview}  The standard coincidence apparatus, as shown in figures \ref{fig:doi-ctr} and \ref{fig:actualsetup}, is altered in two key respects. Firstly the right photodetector is placed within a 3D-printed clamp designed to hold the scintillator crystal which is held vertically with respect to the reference detector. Secondly the $^{22}$Na source is placed much closer to the vertically aligned scintillator detector than the reference. As in the standard apparatus both scintillator crystals are coupled to their respective photodetectors, Hamamatsu MPPC S10931-050P SiPMs, using Rhorosil Rhodorsil  47V optical grease. The size of the confinement region is primarily determined by the separation distances between scintillator detectors and the $^{22}$Na source. The source is placed 5mm from the scintillator crystal under investigation. The reference scintillator detector is a further 40mm on the opposite side from the source, unless otherwise stated. As the $^{22}$Na is not a point source, it's finite size of 1mm$^3$ gives a minimum to the confinement region. For a source much closer to the scintillator detector under interest than to the reference detector, the confinement region will tend to the width of the source.