Scintillation Counter

  Scintillation counters have been in use since the beginning of the century, making use of the property of certain chemical compounds to emit short light pulses after excitation by the passage of charged particles or by photons of high energy. Scintillation is characterized by the light yield, the absorption and emission spectrum ( Wavelength Shifter), signal linearity, and the pulse shape, viz. rise and decay times; the latter range from less than 1 ns (modern fast plastic scintillators) up to 5  (for RbCl). The numbers given in the literature vary considerably even for the same material, due to different surface treatments; ageing and radiation damage account for additional fluctuations.

Scintillating material can be organic (solid crystals, plastics, i.e. synthetic polymers, or liquids), or inorganic (crystals or glasses); also gaseous scintillators are in use. Examples of organic crystals are anthracene (C₁₄H₁₀), trans-stilbene (C₁₄H₁₂), or naphtalene (C₁₀H₈); organic liquids usually have brand names (PPO, POPOP, NE213, PBD, etc). Among plastics, two- or three-component scintillators are common, with a solid solvent, doped with aromatic compounds (TP, PPO or PBD) or with wavelength shifters; polysterene and polyvinyltuolene are most commonly used. Inorganic crystals include Na(Tl), CsI(Tl), BGO, and BaF₂; high-Z crystals make good high-energy physics scintillators, and are used in crystal calorimeters. As gaseous or liquid scintillators one uses Xe, Kr, Ar, He, or N.

In organic scintillators, ionizing particles provoke an excitation of molecular levels, which causes light in the UV region to be emitted. Added wavelength-shifting molecules absorb the UV photons and reemit visible light, in the blue region (around 400 nm wavelength). Inorganic materials are frequently doped with other materials acting as an activator centre by the capture of holes or electrons generated by ionization.

For historical reasons, anthracene (C₁₄H₁₀) is used as a standard for the light gain. The absolute scintillation efficiency of anthracene crystals is of the order of 0.05, and is discussed in [Brooks79] and [Birks64]. The most commonly used inorganic scintillator in nuclear physics is NaI(Tl) (NaI doped with Tl), because of its good energy resolution. As particle physics detector, NaI is not popular, being hygroscopic, difficult to machine, comparatively slow and expensive. All the same, inorganic scintillators are often compared to NaI(Tl), whose absolute scintillation efficiency is about 0.1 (or about 1 photon/25 eV, see [Heath79]).

For a good overview, see [Bicron93]). An introduction to (organic) scintillators can be found in [Zorn92]; [Doke91] discusses the scintillation of noble liquids.

In high-energy physics experiments, scintillation counters are used for timing (time-of-flight counters), or for fast event selection (trigger counters , or groups of counters connected by fast logic into hodoscopes ); they are also vital for measuring the energy of particles by total absorption in sampling calorimeters, which is possible due to the proportionality of light output to the energy loss of the particle. Inorganic materials are popular for high-precision calorimetry ( Crystal Calorimeter). In large scale calorimeters, wavelength shifters are also used as light collecting devices [Bourdinaud81].