Measuring the Optical Scattering Length of Scintillator Crystals

A novel method is presented to determine the optical scattering length in scintillator crystals by directly measuring the scattered light power. It enables to eliminate the large uncertainties in deriving scattering lengths from the measured transmittivity using literature data for the index of refraction ("indirect measurement"). Using the new method, we distinguish the fluorescent and non-fluorescent part of the optical response of the LYSO scintillators that are widely used in PET detectors. Below 420 nm the scattered intensity mainly stems from fluorescence while above this wavelength it is due to elastic or inelastic scattering. The latter is strongly dependent on the crystals' defect properties. Scattering lengths at 405 nm are in the range of 1 to 2 m and vary from sample to sample even from the same vendor. In the non-fluorescent regime, Rayleigh scattering of the studied sample measured at 532 nm shows strong orientation dependence. The average scattering length determined by the direct method is 29 +/- 3 m (as compared to 0.02 to 0.5 m derived from indirect measurement), i.e. orders of magnitude larger than the typical crystal dimensions. Therefore, in contrast to the widespread belief in the literature, scattering in LYSO crystal scintillators has potentially negligible effect on the operation of PET systems, at least in the non-fluorescent regime.