A scintillator is a material that exhibits scintillation - the property of luminescence (sparkles of light), when excited by ionising radiation.
Scintillation detectors are usually water clear crystalline materials and work better if they contain heavy elements, which are more likely to intercept a gamma ray within the material and absorb its energy. Sodium iodide doped with thallium is one of the oldest known scintillation crystals, and is still in common use today. Its high sensitivity is very desirable.
After absorbing a gamma ray, a scintillation crystal emits a pulse of light, usually in the visible spectrum. Various types of sensitive photo-detectors are closely coupled to the crystal so the tiny sparkles produced can be fed to the optical sensing part. For lower sensitivities, there are quite a few semiconductor light sensors that can be used. But for maximum sensitivity, nothing can beat the photomultiplier tube (PMT) for performance. Despite being fragile, bulky & needing very high voltages, PMTs are still found in the highest sensitivity scintillation detectors.
Modern developments in PMTs have led to fairly small tubes with excellent light to current gain, much better than semiconductor optical detectors.
The RadEye PRD from Thermo Scientific is an excellent example. It's easily held in your palm, yet it contains a sodium iodide crystal with a miniature photomultiplier tube.
Despite being popular, sodium iodide is very hygroscopic and can absorb humidity from the air and turn from a water clear crystal into a sloppy yellow sludge. To avoid that problem, there are some exotic new and expensive scintillation crystals available, Bismuth Germinate, Cadmium Tungstate, Lutetium Yttrium Oxyorthosilicate, Lanthanum Bromide and Caesium Lithium Yttrium Chloride (CLYC).
The CLYC crystal has been incorporated into the Thermo Scientific RIIDEye X-GN isotopic identifier.
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In some materials, the energy of the incoming X-rays is converted into visible light. Detectors are designed using these materials by attaching photomultiplier tubes to read the flash of visible light.
Scintillators work by converting X-ray energy into visible light. The energy of the incoming X-ray is completely absorbed by the material, exciting a molecule of the detector material. When the molecule de-excites, it emits a pulse of light in the optical region of the electromagnetic spectrum. So the X-ray gets converted to optical light, which is much easier to detect.
The best materials to use for X-ray detection are those that can be made into large crystals, have good X-ray stopping power, and are efficient light producers. So far the alkali halides NaI and CsI, "activated" by a small amount of either thallium or sodium impurity, have been the scintillators of choice.
In X-ray astronomy, we distinguish scintillators from phosphors by defining bulk crystalline materials such as NaI and CsI as scintillators and thin granular layers of rare earth oxysulphides as phosphors.
The physics of X-ray conversion for phosphors is essentially the same as described for scintillators. Also, the key functional elements of any phosphor-based detector are the same as for a scintillator. For astronomy, the similarities end there. While large area single crystals of NaI and CsI are valued for their stopping power in hard X-ray and gamma-ray astronomy, thin layers of phosphors are used to do high resolution, soft X-ray imaging.
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