Photocathodes multialkali

SIT ISIT. Spectral response depends on the selection of the photocathode and can be optimized for the blue or the near IR. Typical photocathodes are S-20 and S-20R (extended red multialkali) which allow an adequate response up to 900 nm. Because the electrostatically focused image section requires a special optical-fiber faceplate window (curved inside to match the focusing field contours) made out of glass, the cut-off... 

The dark count rate of a PMT can increase dramatically after the photocathode has been exposed to daylight. For traditional cathodes the effect is reversible, but full recovery can take several hours. An example for a Flamamatsu F15773P-01 (multialkali cathode) photosensor module is shown in Fig. 6.19. To show the full size and duration of the recovery effect, the experiment was performed at an ambient temperature of 5° C. 

The sensitivity above 500 nm has been increased by the introduction of multialkali and extended red multialkali photocathodes, which provide good sensitivity to 700 or 800 nm (Figure 2.25). Red-sensitive PMTs typically have higher dark current, but with the current generation of multialkali photocathodes the dark current is not a problem. Sensitivity to still longer wavelengths can be obtained... 

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