The photons interacting in a given detector can either deposit all their energy at a single location, the so-called localized events, or in two or more sites, the un-localized events. Pre-launch simulations predicted that the instrumental background would be dominated by localized decays in the energy range 200 keV - 1 MeV.
The PSD electronic sub-system was designed to determine whether interactions are localized or not. Unfortunately, the fraction of decays observed once the instrument was already in orbit turned out not to be large enough for a significant background reduction. Despite important efforts from SPI experts, it has not been possible to improve significantly the data analysis by using the information provided by the PSD sub-system.