PHY6040 Particle Detectors Dr C N Booth

Scintillation Counters


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.

A scintillation counter is composed of four main components:  a sheet of scintillator, a light guide, a photomultiplier and the electronics in the p.m. base required to drive the photomultiplier and read out the signal.

Applications

Scintillation counters typically have a poor spatial resolution (equal to the size of the counter, which can be anywhere between a square cm and a square metre) but, at least for small counters, a very good time resolution.  They are also continuously sensitive, and are therefore often used as triggers for other types of detectors, which must have a high voltage pulse applied or a readout sequence initiated in order to observe a particle.  A pair of scintillation counters placed some distance apart can be used to measure the time of flight of the particles.  If other information is known, such as the momentum of the particles (e.g. from their curvature in a magnetic field), then the mass and hence identity of the particle can be determined.

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.


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