Photomultiplier tubes
An incoming photon will strike the photocathode surface of the PMT which will emit electrons. These electrons are then accelerated toward additional electrodes by a potential difference of about 90 volts. Upon striking this electrode, more electrons are emitted. This process repeats nine times, each time generating more and more electrons. This process can generate 106 to 107 electrons for each individual photon. These electrons are then collected at the anode where the current and voltage is then measured.