Optical properties of anisotropic polycrystalline Ce3+ activated LSO

Polycrystalline cerium activated lutetium oxyorthosilicate (LSO:Ce) is highly desirable technique to make cost effective and highly reproducible radiation detectors for medical imaging. In this article methods to improve transparency in polycrystalline LSO:Ce were explored. Two commercially available powders of different particulate sizes (average particle size 30 and 1500 nm) were evaluated for producing dense LSO:Ce by pressure assisted densification routes, such as hot pressing and hot isostatic pressing. Consolidation of the powders at optimum conditions produced three polycrystalline ceramics with average grain sizes of 500 nm, 700 nm and 2000 nm. Microstructural evolution studies showed that for grain sizes larger than 1 μm, anisotropy in thermal expansion coefficient and elastic constants of LSO, resulted in residual stress at grain boundaries and triple points that led to intragranular microcracking. However, reducing the grain size below 1 μm effectively avoids microcracking, leading to more favorable optical properties. The optical scattering profiles generated by a Stover scatterometer, measured by a He–Ne laser of wavelength 633 nm, showed that by reducing the grain size from 2 μm to 500 nm, the in-line transmission increased by a factor of 103. Although these values were encouraging and showed that small changes in grain size could increase transmission by almost three orders of magnitude, even smaller grain sizes need to be achieved in order to get truly transparent material with high in-line transmission.

► Powders of Ce3+ doped LSO were used to fabricate transparent samples with average grain sizes, 2000–500 nm.
► They produced microstructural variations due to the highly anisotropic properties of monoclinic LSO.
► The in-line transmittance in the near-UV region changes by 3 orders of magnitude by simply changing the grain size.
► A model using the Mie scatter theory compares these experimental findings to theoretical predictions for optical behavior.
► For truly transparent LSO:Ce with high in-line transmittance the model predicts material with grain size of about 100 nm.