Scintillation counters exploit the atomic or molecular excitationproduced by a charged particle as it passes through matter.  Fulldetails of these detectors are given in the handout, PostScriptand PDF versions of which are available.This pagecontains a very brief summary of the key features of scintillation counters,with links to relevant sections of the ParticleDetector BriefBook.

• There are two main types of scintillator, inorganic (such as sodiumiodide) 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, thisphoton would normally be of exactly the correct energy to be reabsorbedby the material.  A small concentration of a wavelengthshifter is therefore added.  This allows rapid non-radiativetransitions of the excited electron, reducing its energy so that the photonsubsequently emitted is of longer wavelength and insufficiently energeticto be reabsorbed.  The scintillator sheet is highly polished, andlight is conducted along it by total internal reflection.

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

 
• The photomultiplierconverts the optical signal to an electrical one, and provides a largedegree of amplification.
  This consists of an evacuated glass envelope coated at one end witha photocathode made of alkali metals, which is maintained at a largenegative 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 processcontinues down the dynode chain, until eventually a large signal is collectedat 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 photonis produced.



• Connections to the photomultiplier are made through the photomultiplierbase, which contains a chain of resistors to provide the correctvoltages 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 coincidencebetween signals from counters in the different layers.

Another application of scintillators is within calorimeters. Because of their short radiationlength, inorganic scintillators make sensitive electromagneticcalorimeters, and are often used to detect medium energy gamma rays. Sheets of plastic scintillator between metal plates are used in samplingcalorimeters.  Here, the number of particles at a particular depthin a showercan be determined from the size of the pulse observed in the scintillator.

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