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.

Examples include

• thallium doped sodium iodide crystals in gamma cameras used for nuclear medicine radioisotope imaging.
• sodium iodide 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) 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.

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

In principle, scintillators can have any aggregate state: gaseous, liquid or solid state. Most scintillators for common use will come in solid state form with the commonest detectors being made of thallium-doped sodium iodide crystals; their popularity owing to a high impinging radiation to output 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.