A cryogenic system for measuring in the VUV range the absolute quantum efficiency of light detectors with large sensitive area

We developed a system to measure changes in PMT photocathode's behavior from room to cryogenic temperatures and to determine the absolute quantum efficiency (Q.E.) by comparing the photocathode current with that of a calibrated photodiode. Preliminary test results on the 8-inch Hamamatsu R5912-MOD PMT coated with ≈200μg/cm2 of tetraphenyl butadiene do not show significant variations in Q.E. at cryogenic temperatures with respect to room temperature and are in agreement with other measurements.