Angular response of photomultiplier tubes: Photon-to-electrou conversion efficiency and reflectance

Reflectance and photon-to-electron conversion efficiency have been measured for photomultiplier tubes (PMTs; Hamamatsu R878 and 7696) using three-color light-emitting diode (LED) emitting photons with central wavelengths of 470, 525, and 640 nm. The use of a light guide, with a refractive index similar to that of photomultiplier window enabled the measurement at the angle of incidence to a photocathode up to 75 degrees. An appreciably large reflectance and conversion efficiency were obtained at the angles of approximately 45 and 60 degrees, respectively. It was shown that these PMTs work with a higher photon-to-electron conversion efficiency than the quantum efficiency given by the manufacturer in many cases of scintillation counting. The reflectance measured in this study is consistent with that calculated using an optical model, where the photocathode was a thin film with a complex refractive index.