Realisation of 50% quantum efficiency from photomultiplier cathodes

Abstract

Standard trialkali red sensitive photomultiplier (PM) tubes have cathode quantum efficiencies which typically fall from ∼25% at 400 nm to ∼1% at 800 nm, partly because the material has lower optical absorption coefficients at long wavelengths. Increasing cathode thickness benefits long wavelength response but overall reduces electron extraction efficiency. In the present work modelling of the interaction of light within the dielectric materials of the windows indicates that considerably greater quantum efficiency (QE) is feasible under a variety of conditions. Some of these possibilities have been explored with standard PM tubes giving data where the blue response is increased by a factor of 2, to ∼50%. Much higher improvement factors, of more than 10 times, are realised at longer wavelengths. Current work suggests that the enhancement methods can be exploited further, and at least 50% performance obtained across most of the range of spectral response. A secondary feature of the enhancement is that the useful operating range of the tube is extended further into the near infrared.