Scintillation (physics)

Scintillation, sometimes called radioluminescence, is a physical process where a material emits light. This material is known as a scintillator. The light is usually visible or ultraviolet. This happens when the scintillator is excited by high energy photons or energetic particles. These particles can include X-rays, gamma rays, electrons, alpha particles, neutrons, or ions.

Scintillation is a type of luminescence. This means the material emits a specific spectrum of light after absorbing radiation. Because the emitted light usually has less energy than the radiation that caused it, this is considered a down-conversion process.

The process occurs in three distinct stages: conversion, transport, and luminescence. First, the conversion stage begins when the scintillator absorbs incident radiation. This absorption creates highly energetic electrons and holes. These charge carriers interact with other particles like phonons or plasmons. This creates an "avalanche event" that produces many secondary electron-hole pairs. These carriers then undergo thermalization, where they lose some energy through Auger processes or phonons. This entire conversion and thermalization phase is extremely fast, occurring in about 1 picosecond.

During the second stage, called charge transport, the thermalized electrons and holes migrate through the material. They move toward specific luminescence centers. This stage is a critical part of the process because it is where efficiency is often lost. Defects in the crystal, such as grain boundaries, ionic vacancies, or impurities, can trap the charges. This is known as non-radiative recombination. If the charges get stuck, they do not produce light. This stage can also create a bottleneck for the timing of the scintillation event.

The final stage is luminescence. This happens when the electrons and holes reach the luminescence centers. At these centers, the electrons and holes are captured and recombine radiatively. This recombination is what actually releases the light. The specific details of this stage depend on the type of material being used. For example, inorganic crystals and organic molecules behave very differently during this final step.

Inorganic crystals are often used to detect gamma rays. A common example is thallium activated sodium iodide, or NaI(Tl). Another option is CsF crystals, which offer a faster response time, though they only provide 5% of the output. The way these crystals absorb energy depends on the energy of the incoming radiation. At low X-ray energies, below 60 keV, the photoelectric effect is the dominant process. In this effect, photons are fully absorbed by core electrons in the K- or L-shell. This leads to the ionization of the host atom. At higher energies, Compton scattering becomes the main process. This is the inelastic scattering of photons by bound electrons.

When gamma-ray energies exceed 1022 keV, a process called pair production begins. This is a relativistic phenomenon where photon energy converts into an electron-positron pair. These new particles then interact with the material to create more electron-hole pairs. Pair production becomes the most dominant process at energies above approximately 8 MeV. Therefore, at very high energies, absorption depends on both the density and the average atomic number of the scintillator. Other minor processes include Rayleigh scattering and photonuclear reactions, which only become relevant at very high energies.

Organic scintillators work through a different mechanism involving organic molecules. In these materials, scintillation is a product of pi-orbitals. These molecules form crystals held together by Van der Waals forces. When radiation hits these molecules, it can excite delocalized pi-electrons. The de-excitation of these electrons results in luminescence.

This can result in a "fast component" called fluorescence, which involves singlet states. It can also result in a "slow component" like phosphorescence, which involves triplet states. Scientists can use this to perform pulse shape discrimination. By looking at the shape of the light pulse, they can identify which specific particle was detected.