Very high quantum efficiency fast light sensors for cosmic ray detectors

Low level light (LLL) sensors are widely used in cosmic ray detectors. Common LLL sensors are the classical photomultiplier tubes (PMT), which provide a peak quantum efficiency (QE) of 20-25% in the wavelength range ∼ 320 - 420 nm. Usually one tries to achieve a high detection efficiency and a low energy threshold for a cosmic ray detector. This is the case in experiments measuring fluorescence or Cherenkov light emission of relativistic particles in a given medium. A very cost-effective method to enhance the efficiency and to lower the threshold is the use of light sensors with higher QE. Here we discuss the recent progress in the field of hybrid PMTs applying GaAsP photocathodes, which provide a QE of ∼ 45 % in the wavelength range 500-600 nm and a significantly better amplitude resolution compared to classical PMTs. By application of wavelength shifting technique (WLS) their QE can be enhanced close to the maximum level also in the blue and near ultraviolet parts of the spectrum