Application Note

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Photon counting applications play an important role in advancements in many different areas such as astronomy, particle sizing, disease diagnosis, medical analysis and imaging methods as well as the most recent area of quantum cryptography. The most spectacular application is surely biomedical research. The use of confocal and STED microscopy led to major advancements in the fluorescence analysis of the structure and function of biological molecules. Primarily two types of technology are used to detect a small amount of photons:

Photomultiplier – PMT
A photomultiplier typically consists of a photocathode and a downstream secondary electron multiplier in an evacuated (10-6...10-5 Pa) glass tube.

The photons encounter a photocathode which, depending on the material used, emits an electron in a spectral range starting from 115 nm. The electrons that are set free are accelerated in an electrical field and encounter other electrodes – called dynodes. To ensure that the electrons remain free, the entire assembly is located in a vacuum tube; a high voltage supply of 1-2 kV is required. The electron that encounters the dynode then causes several electrons, so-called secondary electrons, to be emitted. These secondary electrons are accelerated toward the next dynode.

This process is repeated by several dynodes connected in series, producing an avalanche of electrons. This large number of electrons is finally absorbed by the anode, and an electrical pulse is generated that is detected by an electronic counter.