Chemical polishing of LSO crystals to increase light output

Surface treatment of scintillators is critical for light collection from narrow rectangular crystals. We investigated chemical polishing which is less labor intensive and expensive than hand and machine polishing. We used phosphoric acid to chemically polish LSO crystals at 110 degrees C, 150 degrees C and 190 degrees C, and compared these with unpolished and mechanically polished crystals. Groups of five 2x2x10 mm crystals were etched for different times at each temperature. Weight loss, light output and energy resolution were measured as a function of treatment time and temperature. We found that chemical polishing can increase light output by 250% relative to unpolished crystals and by 16% relative to mechanically polished crystals. The energy resolution was relatively independent of surface treatment, with values of between 14 and 16%. The rate of loss of LSO was 0.02%/min at 110 degrees C, 0.1%/min at 150 degrees C and 0.36%/min at 190 degrees C. Maximum light output occurred when 5-10% of the LSO was removed. The crystals were also imaged using scanning electron microscopy and the surface roughness quantitatively assessed by using a profilometer. These measurements helped to clarify the effect of acid polishing on the surface of the scintillator. In summary, chemical polishing appears to be a convenient and effective method for improving light output from small LSO crystals.