Design of an Instrument to Measure Optical Reflectance of Scintillating Crystal Surfaces

In order for a Monte Carlo simulation to be accurate in predicting and optimizing the light collection in scintillation detectors, the light reflectance off an internal surface within the scintillating crystal must be understood well. We present design studies for an instrument that will accurately measure the reflectance distribution within a scintillating crystal. A laser is aimed towards the center of a 50.8-mm diameter scintillating crystal hemisphere. The laser can be positioned at any arbitrary angle. The laser beam is reflected off the flat surface of the hemisphere and the light distribution is measured by a movable array of photodiodes that can measure the reflected light over the entire 2pi solid angle. Thirty-six PIN photodiodes, mounted in two rows and offset to each other by half the length of a photodiode, measure the reflected light. The currents from the photodiodes are switched through a multiplexer to a digital multimeter, where the current is recorded. The current measurements give a dynamic range of 105 : 1. A Lab VIEW program controls the motion of the laser and photodiodes, the switch, and the data collection. The mechanical set-up is placed inside of a light-tight glove box. By flowing dry nitrogen gas through the glove box we can control the water content in the atmosphere and so measure hydroscopic scintillators.