Assessment of optical photon collection in a large plastic scintillator using Geant4-Gate code

The scintillating detectors are widely used in both X-ray and gamma-ray imaging systems. Optical photon collection has a significant impact on the scintillation gamma-ray spectrum. Besides, by increasing the size of scintillator, optical photon collection becomes more important. In this work, a suitable arrangement of photomultiplier tubes (PMTs) and an appropriate reflector for a large plastic scintillator whose dimensions were 122 cm × 18 cm × 6 cm were designed. For this aim, two types of reflectors (aluminum foil and Teflon tape) and three different arrangements of four PMTs were studied. All simulations were carried out by Geant4-Gate code. The simulations showed that the scintillator with the aluminum foil coverage has a better spatial uniformity response than the others. In fact, in this sample the counting efficiency and optical photon collection efficiency were relatively better than the other samples. The symmetrically arrangement of PMTs on 18 cm × 6 cm faces of the scintillator with the aluminum foil coverage was the best arrangement of PMTs and reflector for the large plastic scintillator. The spatial uniformity of total response at this sample is 70-fold better than the similar arrangement with Teflon tape coverage. Thus, this arrangement of PMTs on the scintillator surface and aluminum foil coverage can be used in an X-ray backscatter imaging system.