Scintillator

A scintillator is a device or substance that absorbs high energy (ionizing) electromagnetic or charged particle radiation then, in response, fluoresces photons at a characteristic Stokes-shifted (longer) wavelength, releasing the previously absorbed energy. See also scintillation. Scintillators are defined by their short fluorescence decay times and optical transparency at wavelengths of their own specific emission energy, characteristics which set them apart from phosphors. The lower the decay time of a scintillator, that is, the shorter the duration of its flashes of fluorescence are, the less so-called "dead time" the detector will have and the more ionizing events per unit of time it will be able to detect.

Scintillators are used in many physics research applications to detect electromagnetic waves or particles. There, a scintillator converts their energy to light of a wavelength which can be detected by inexpensive or easy to handle detectors such as photomultiplier tubes (PMTs).

Types of Scintillators

Common scintillators used for radiation detection include inorganic crystals, organic plastics and liquids. However, many materials scintillate at some level; scintillation of atmospheric nitrogen is used by some high-energy astrophysics experiments, and scintillation of liquid xenon and neon plays a role in some ultra-low-background experiments. Most scintillators for common use are inorganic crystals or plastics, the most common being thallium-doped sodium iodide crystals, which have a high radiation-to-light conversion efficiency. However, organic liquid scintillating fluids are well-suited for detecting very low energy particle radiation such as beta radiation from tritium by simply immersing the sample to be tested in the scintillation fluid, thereby negating detector absorption problems due to the very short mean free paths associated with low energy particles.

Organic crystals

These are organic molecules which have an aromatic ring; the ionising radiation excites it to a rotational or vibrational mode. They are characterized by a fast response, in the order of one nanosecond. When pure, their form crystals, which are difficult to shape.

Organic liquids

The organic crystal scintillator can be dissolved in a transparent liquid, for example in mineral oil, mantaining properties similar to the organic crystal, depending on purity and concentration.

For the specific use of this form of scintillator, see Liquid scintillation counting.

Organic plastics

The organic crystals can be also be dissolved in a transparent plastic that becomes solid at ambient temperature, like polystirene. The plastic can be easily shaped and tooled. The solid plastic matrix has often the effect of increasing the the relaxation time to 2-3 nanoseconds.

Inorganic crystals

Are usually composed of alkali halides, like NaI. They are characterized by a high stopping power, which makes them most appropriate to detect high energy radiation. But they have longer decay times, in the order of hundreds of nanoseconds.

NaI(Tl) (thallium doped sodium iodide) crystals
are used in gamma cameras used for nuclear medicine radioisotope imaging.
BaF₂ (Barium fluoride)
CsI (Cesium iodide)
BGO (bismuth germanate) has a higher stopping power, but lower yield than NaI(Tl)
It is often used in coincidence detectors for detecting back-to-back gamma rays emitted upon positron annihilation in positron emission tomography machines.
the yellowish-white cerium-doped yttrium aluminum garnet (Ce:YAG) coating on the chip in some "white" light-emitting diodes (LEDs). This is used as a phosphor but is also suitable for use as a scintillator when in pure single crystal form. This converts part of the visible blue light emitted by the LED chip to visible yellow light. The blue and yellow light together create the subjective impression of white light.
LaBr₃ (Lanthanum bromide)

External links

Particle detectors

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