Scintillator

  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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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 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.

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.

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, they form crystals, which are difficult to shape. One of the best known organic scintillators is anthracene.

Organic plastics

The organic crystals can be also be dissolved in a transparent plastic that becomes solid at ambient temperature, like polystyrene. 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).

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. NaI was the first known inorganic scintillator, discovered by Robert Hofstadter in the 1940's.

• CsI(Tl) (thallium doped cesium iodide) crystals are an alternative to NaI(Tl). They are more mechanically durable and have better resistance to moisture.
• BaF₂ (Barium fluoride)
• 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.

• Cerium-doped yttrium aluminium garnet (Ce:YAG), the yellowish-white 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(Ce) (cerium-doped lanthanum bromide)
• LuI₃ (lutetium iodide)
• Gd₂O₂S (terbium-doped gadolinium oxysulfide, GOS)
• CaWO₄ (calcium tungstate)
• CdWO₄ (cadmium tungstate), used in computer tomography and early fluoroscopes
• PbWO₄ (lead tungstate)
• ZnWO₄ (zinc tungstate)
• Lu₂SiO₅ (lutetium oxyorthosilicate), also known as LSO, used in positron emission tomography

See also

• Scintillation Counter
• Liquid scintillation counting
• Gamma spectroscopy