Photomultiplier tubes consist of an evacuated envelope with a photocathode that emits electrons when exposed to light. These electrons are accelerated by a positive electrostatic field and fall upon a metal surface where they emit secondary electrons that are again accelerated to generate more electrons at the next metal surface, and so on. The whole arrangement thus acts as a combination of a simple photocell with a high-gain amplifier in a self-contained unit. Photomultiplier tubes are used in applications where rapid detection of light or low light detection is necessary.
There are two main types of photomultiplier tubes, side-on photomultipliers and end-on Photomultipliers. Side-on detectors are more economical than end-on models, and have the faster rise times. They are ideal for photometry and spectrophotometry applications. Their vertical configuration takes up less space than the end-on versions and they mount in standard or pulsed housings. The main disadvantage of these photomultiplier tubes is their nonuniform sensitivity. End-on photomultiplier tubes, sometimes known as head-on photomultipliers, offer better spatial uniformity and photosensitive areas from tens of square millimeters to hundreds of square centimeters.
There are a number of factors that determine the performance level of photomultiplier tubes. These include spectral response range, sensitivity, rise time, applied voltage, and gain. The spectral response range of incident light the photomultiplier tube detects, also called wavelength range. Sensitivity is a measure of the effectiveness of a detector in producing an electrical signal at the peak sensitivity wavelength. Rise time equates to the time necessary for the photomultiplier tubes’ output to go from 10% to 90% of its final value. Applied voltage is the anode-to-cathode voltage of the photomultiplier tube. Gain is the factor by which the current generated by a photon is increased before the signal is detected, also known as current amplification.
Some types of photomultipliers are designed to filter out some of the noise, which would negatively impact upon their ability to detect light. Two types of filters include dark current protection and noise equivalent protection (NEP). Dark current is associated with a detector during operation in the dark with an applied voltage. Increased temperature and applied voltage will result in increased dark current. Also, larger active areas will generally have a higher dark current. NEP deals with the power of incident light, at a specific wavelength, required to produce a signal on the detector that is equal to the noise.