A scintillation counter or scintillation detector is a radiation detector that uses the effect known as scintillation. Scintillation is a flash of light produced in a transparent material by passing a particle (an electron, an alpha particle, an ion, or a high-energy photon). Scintillation occurs in the scintillator, a key part of a scintillation detector. In general, a scintillation detector consists of:
- Scintillator. A scintillator generates photons in response to incident radiation.
- Photodetector. A sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode) converts the light to an electrical signal, and electronics process this signal.
The basic principle of operation involves the radiation reacting with a scintillator, which produces a series of flashes of varying intensity. The intensity of the flashes is proportional to the energy of the radiation, and this feature is very important. These counters are suited to measure the energy of gamma radiation (gamma spectroscopy) and, therefore, can be used to identify gamma-emitting isotopes.
Scintillation Counter-Principle of Operation
The operation of scintillation counters is summarized in the following points:
Ionizing radiation enters the scintillator and interacts with the scintillator material, and this causes electrons to be raised to an excited state.
- For charged particles, the track is the path of the particle itself.
- For gamma rays (uncharged), their energy is converted to an energetic electron via either the photoelectric effect, Compton scattering, or pair production.
- The excited atoms of the scintillator material de-excite and rapidly emit a photon in the visible (or near-visible) light range. The quantity is proportional to the energy deposited by the ionizing particle, and the material is said to fluoresce.
- Three classes of phosphors are used:
- inorganic crystals,
- organic crystals,
- plastic phosphors.
- The light created in the scintillator strikes the photocathode of a photomultiplier tube, releasing at most one photoelectron per photon.
- Using a voltage potential, this group of primary electrons is electrostatically accelerated and focused so that they strike the first dynode with enough energy to release additional electrons.
- These secondary electrons are attracted and strike a second dynode releasing more electrons. This process occurs in the photomultiplier tube.
- Each subsequent dynode impact releases further electrons, so there is a current amplifying effect at each dynode stage. Each stage is at a higher potential than the previous to provide the accelerating field.
- The primary signal is multiplied, and this amplification continues through 10 to 12 stages.
- At the final dynode, sufficient electrons are available to produce a pulse of sufficient magnitude for further amplification. This pulse carries information about the energy of the original incident radiation, and the number of such pulses per unit of time also gives information about the intensity of the radiation.