Devices and accessories for the photon counting

How to choose a photomultiplier

In the market there are several kinds of photomultiplier tubes, each of which has been designed and optimized for a specific application: it is therefore necessary to pay close attention to the choice of the most appropriate.
To select the photomultiplier that best meets your needs, you should first consider:

• The range of wavelengths that the photomultiplier be able to detect:
  this range must be between 100 and 1,000 nm (ie between the UV-C and IR-A), since the photomultiplier are not able to detect X-ray emissions over and beyond the far infrared.
• What should be the shape and size of the sensitive surface to light: it is used in many applications, a circular window with a size between 3/4 of inch up to 11 inches, but for special needs you can use a square surface (for example, for application with spectrophotometers) or a hemispherical and at "2pi" one, and so on.
• If the detection must be done in the presence of light and / or at a low temperature: the photomultiplier tubes are generally used in the absence of detectable light (the only detectable source of light is made from the emission that you want to measure) and environment temperature, but otherwise there are models designed for the measure in this conditions.
• What should be the dark current (ie in the absence of emission): of course this value depends on many factors (including the sensitive area of the surface, the temperature and the needed gain), but it is important to know this datum.
• The gain range to be achieved: typically it is a range from 10,000 up to 100,000,000, and must choose according to the level of incident light.
• If it is necessary or not using photomultipliers with a very low natural radioactive emissions: of course all the photomultiplier tubes emit low amounts of particles (such as any material existing in nature), but they can be significantly reduced if you use quartz instead of a simple glass.
• What dynode array you prefer to use: venetian-blind, box and grid, linear or circular focusing. The selection of the structure depends on, among other things, the size, gain and linearity to be obtained.