Designing the scintillation module of a pixelated mini gamma camera: The spatial spreading behaviour of light

Designing the scintillation module of a mini gamma camera requests good understanding of how the scintillation light signal spreads within the crystal and how photons are finally collected on the PMT. For a given scintillation process, the key parameters are the geometry of the crystal, its optical coating and the interface between the crystal and the PMT. In order to optimize the design of our gamma camera TReCam, we studied the shape of the spatial distribution of the scintillation light for different values and combinations of these key parameters. Our approach was to produce computer simulations, for which the optical simulation transport software DETECT2000 was used. Three major parameters were investigated: optical coatings for the crystal (aluminum, Teflon, corner cube retro-reflectors), the thickness of the crystal (ranging from 1 mm up to 5 mm) and the interfaces (air, optical grease). In addition to the FWHM of the light spot distribution, specific figures of merit were implemented to further characterize photon spreading. In order to evaluate the influence of these parameters on the imaging performances of the TReCam, the whole scintillation module (including the PMT) was also simulated with GATE v4.0.0 based on GEANT 4.9.1.p02. It appears that the best configuration is a 5 mm thick LaBr₃:Ce crystal plate covered on the top with Teflon optical coating coupled optically with a PMT.