Scintillator

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Extruded plastic scintillator material fluorescing under UV inspection lamp at Fermilab for the MINERνA project.
Extruded plastic scintillator material fluorescing under UV inspection lamp at Fermilab for the MINERνA project.

A scintillator is a 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 (physics). Scintillators are defined by their light output (number of emitted photons per unit absorbed energy), short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The latter two characteristics 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 the energy to light of a wavelength which can be detected by inexpensive or easy to handle detectors such as photomultiplier tubes (PMTs).

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[edit] Types of Scintillators

Preparing single crystal lead tungstate scintillators for the ECAL on the Compact Muon Solenoid.
Preparing single crystal lead tungstate scintillators for the ECAL on the Compact Muon Solenoid.

Common scintillators used for radiation detection include inorganic crystals, organic plastics and liquids. However, many materials scintillate at some level; scintillation of liquid xenon and neon plays a role in some ultra-low-background experiments. Most scintillators for common use are either 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.

[edit] Organic liquids

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

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

[edit] 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, on the order of one nanosecond. When pure, they form crystals, which are difficult to shape. One of the best known organic scintillators is anthracene.

[edit] Organic plastics

The organic crystals can be also be dissolved in a transparent plastic that becomes solid at ambient temperature, like polystyrene, these mixtures are called plastic scintillators. The plastic can be easily shaped and tooled. The solid plastic matrix has often the effect of increasing the relaxation time to 2-3 nanoseconds. The three most common bases for plastic scintillators are polyvinyl toluene, polystyrene, or acrylic. However acrylic, as it contains no aromatic structures, has very low scintillation efficiency of its own; it gains acceptable efficiency if e.g. naphthalene is dissolved in it in amount of 5-20 weight %. The plastics when used on their own typically emit ultraviolet photons; to convert them to less attenuated visible light, a suitable fluorophor is added in amount of about 1 wt.%.

Plastic scintillators are robust and reliable, but also quirky. They undergo aging, gradually losing light yield with time, with solvents, high temperatures, radiation, or mechanical load accelerating the process. The surface can be damaged by formation of microcracks which cause light loss by reflection. Plastic scintillators are also sensitive to airborne oxygen which lowers their yield; this is known as atmospheric quenching. Some plastics change their yield slightly when subjected to magnetic fields. Radiation damage leads to formation of color centers (F-Centers) which absorb in ultraviolet and blue part of spectrum, lowering the optical yield. [1]

Some polymers can scintillate on their own. A commonly used polymer scintillator is polyvinyl toluene (PVT).

[edit] 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.

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