Study of light collection in multi-crystal detectors

Phoswich detectors are currently being considered by a number of investigators for depth-of-interaction measurement in PET and multi-modality emission/transmission imaging. Many parameters affect light collection and S/N ratio from such detectors, made of crystals having different emission wavelength, refractive index and light output. Experiments and simulation with DETECT97 have shown that light collection efficiency and energy resolution degrade steadily with distance from photodetector, mainly as a result of additional crystal interfaces. Simulation indicates that ground surfaces and high reflectivity diffuse reflector maximize light output. Measurements Lend to show that similar results are obtained with polished surfaces due to the presence of crystal defects. An excitation band at 400 nm in GSO is found to absorb a fraction of the light from scintillators emitting at short wavelengths, such as LSO. This effect is more significant for photodetectors such as PMTs that have their maximum sensitivity in the 400-nm range.