Scintillation counters exploit the atomic or molecular excitation produced by a charged particle as it passes through matter.  Full details of these detectors are given in the handout, PostScript and PDF versions of which are available. This page contains a very brief summary of the key features of scintillation counters, with links to relevant sections of the Particle Detector BriefBook.

• There are two main types of scintillator, inorganic (such as sodium iodide) and organic (such as a plastic like polystyrene).  The passage of a charged particle through the material excites electrons, which can subsequently de-excite, emitting a photon.  However, this photon would normally be of exactly the correct energy to be reabsorbed by the material.  A small concentration of a wavelength shifter is therefore added.  This allows rapid non-radiative transitions of the excited electron, reducing its energy so that the photon subsequently emitted is of longer wavelength and insufficiently energetic to be reabsorbed.  The scintillator sheet is highly polished, and light is conducted along it by total internal reflection.

 
• Light must then be transmitted from the scintillator to a sensor.  This is done by a light guide.  A typical shape is that of the "fish tail", as illustrated above, which connects from the long thin edge of the scintillator to the circular end of a photomultiplier.  Other, more complicated geometries are also used, for example to connect a number of scintillator sheets to a common photomultiplier, e.g. in a calorimeter.

 
• The photomultiplier converts the optical signal to an electrical one, and provides a large degree of amplification.
  This consists of an evacuated glass envelope coated at one end with a photocathode made of alkali metals, which is maintained at a large negative potential.  Photons liberate electrons from the cathode, which are accelerated towards a (less negative) dynode, and here knock out a number of secondary electrons.  This process continues down the dynode chain, until eventually a large signal is collected at the anode.  Typical gains are of the order of 10⁷.  If the signal drives a 50 ohm load, a pulse of a few mV per detected photon is produced.



• Connections to the photomultiplier are made through the photomultiplier base, which contains a chain of resistors to provide the correct voltages for the cathode and dynodes.

Applications

Layers of crossed scintillation counters are also used to form a hodoscope, where the position of the particle can be determined from the coincidence between signals from counters in the different layers.

Another application of scintillators is within calorimeters.  Because of their short radiation length, inorganic scintillators make sensitive electromagnetic calorimeters, and are often used to detect medium energy gamma rays.  Sheets of plastic scintillator between metal plates are used in sampling calorimeters.  Here, the number of particles at a particular depth in a shower can be determined from the size of the pulse observed in the scintillator.

Detectors with a good spatial resolution can be made by forming layers of plastic optical fibres made out of scintillator material coated with a lower refractive index cladding.  These can typically have a diameter of 0.5 to 1 mm.  The small size of each independent scintillator means that many readout channels (typically tens of thousands) are required, and it is not practical to equip each one with its own photomultiplier.  One solution to this is to gather the fibres into a bundle and connect to an image intensifier.  This amplifies the light while maintaining an image, which can then be viewed with a CCD camera, and the position on the image associated with a particular fibre.  One price paid for this solution, however, is that the readout is now very slow (several ms), and the image intensifiers often have to be gated to ensure only interesting interactions are recorded.