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. In this work, the effects of crystal intrinsic and extrinsic properties on light propagation and light collection efficiency were studied. Avalanche photodiode (APD) and photomultiplier tube (PMT) readouts were used to study the light output at different wavelengths. Experiments and simulations 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. Simulations indicate that ground surfaces and high reflectivity diffuse reflectors maximise light output, but measurements tend 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 LSO. This effect is more significant with the PMT photodetector, which has its maximum sensitivity in the 400-nm range. Inconsistent results observed for LSO and YSO are shown to be compatible with the presence of light scattering within these crystals.