ABSOLUTE LIGHT YIELD MEASUREMENTS ON BAF2 CRYSTALS AND THE QUANTUM EFFICIENCY OF SEVERAL PHOTOMULTIPLIER TUBES

The quantum efficiency (QE) curves of five Philips XP2020Q photomultip lier tubes (PMTs), one Hamamatsu R2059 PMT, and a Na-Salicylate coated glass window were determined using a calibrated Thorn EMI 9426 PMT as a reference. The QE of XP2020Q PMTs at wavelengths smaller than 230 n m appears much better than the values specified by the manufacturer. C onsequently, the often reported photon yield of 2000 photons/MeV for t he fast component of the scintillation pulses from pure BaF2 crystals determined with this tube is overestimated. Our results, obtained by m eans of calibrated equipment and using 662 keV gamma-rays, yield a val ue of 1400 photons/MeV.