Figure 6

Sample volume effect. (a) An example pair of annihilation photons contributing to the coincidence sum peak. As the sample volume increases, there is an increasing likelihood that photons will escape from the hole at the top of the well (b). Counts lost from the coincidence sum peak due to reduced geometric efficiency become single-photon events. This mechanism tends to increase counts in the 511-keV peak, partially offsetting the loss of counts due to reduced geometric efficiency.