Radiant sensitivity photocathodes

Wavelength dependence of the radiant sensitivity of various photocathode materials. S-1 = AgOCs,... 

The most useful parameter for characterising the efficiency of a photoncounting detector is the quantum efficiency. The quantum efficiency, QE, of a photocathode is the probability of the emission of a photoelectron per incident photon. It is directly related to the radiant sensitivity, S ... 

The general wavelength dependence of the radiant sensitivity for some commonly used photocathodes is shown in Fig. 6.16. 

The radiant sensitivity of different PMT types of the same cathode material can vary considerably, especially for NIR-sensitive tubes. Reflection-type cathodes are usually a bit more efficient than transmission type photocathodes. Even tubes of the same type and cathode material differ noticeably in their radiant sensitivity. 

The Cathode Radiant Sensitivity" is the current of the photocathode divided by the power of the incident light at a given wavelength. Measuring the Cathode Radiant Sensitivity requires a lamp, a monochromator and a reference detector, e.g. a calibrated photodiode. The setup is difficult to calibrate due to the various error sources. 

Figure 5.15. Radiant sensitivity of a selection of photomultiplier photocathode types the dotted lines refer to a specific gain in the dynode chain.

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