Bialkali photocathode

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

Figure 4.10 shows the spectrum of a green LED read out with an Advanced Photonix HAPD with a bialkali photocathode. At least three individual photoelectron peaks are clearly visilde above the pedestal. The separation between the single photoelectron peak and the ADC pedestal is about 6.5a. 

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