The sensitivity of a photocathode is usually quoted in terms of the quantum efficiency. In general, the term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device. The quantum efficiency of the photocathode is defined as the probability for the conversion of incident photons to an electrical signal and is defined as:
The quantum efficiency of any photosensitive device is a strong function of wavelength of the incident light, and an effort is made to match the spectral response of the photocathode to the emission spectrum of the scintillator in use. In the photomultiplier tube the quantum efficiency is limited to 20-30%, but an average quantum efficiency over the emission spectrum of a typical scintillator is about 15-20%.
The standard for quotation is the number of photoelectrons per keV energy loss by fast electrons in a NaI(Tl) scintillator. For the peak quantum efficiency, about 8 ~ 10 photoelectrons are produced per each keV energy loss. Therefore, the average energy loss required to create a single photoelectron is ~ 100 eV, which is much bigger than the values in gas-filled detectors or semiconductor detectors.
The PMT has been the main choice for photon detection ever since due to the fact that they have high quantum efficiency and high amplification. Lately however semiconductors have begun to compete with the PMT, the photodiode for example which has higher quantum efficiency in the visible range and above, lower power consumption and smaller size. The quantum efficiency for the photodiode is high (60-80%) compared to the PMT (20-30%) which gives a higher energy resolution.