Scintillator Materials

For the photon detection efficiency of a scintillation detector (not the used photodetector), the key factor is the probability that the incoming radiation is absorbed in the scintillator volume:

• For photons (X-rays or γ-rays), the absorption efficiency increases with atomic number ($Z$) and density, but decreases with photon energy. Dense high-($Z$) materials are preferred for high-energy γ-rays.
• For neutrons, there is no simple ($Z$) dependence. Capture depends on specific isotopes like ⁶Li, ¹⁰B or ¹⁵⁷Gd, providing very short absorption lengths for thermal neutrons (tens of microns to millimeters). Gd is extremely efficient, but its γ emission after capture complicates spectroscopy. For fast neutrons, elastic scattering on light nuclei (H, C) is exploited, often combined with pulse shape discrimination.
• For α and β radiation, efficiency is not limiting — both are strongly absorbed. Thin scintillators can be designed to suppress sensitivity to γ background.

Most scintillators are sufficiently transparent that reabsorption losses are minor. Some amount of scattering may improve the uniformity of light collection, but excessive scattering or absorption degrades energy resolution.

Some trace impurities (Fe, Cu, OH⁻, etc.) create absorption bands that can affect performance, and should thus be minimized.

The emission peak and bandwidth should match the photosensor's sensitivity. Silicon photomultipliers, for example, are strongest in the ≈400–550 nm band; VUV emissions (in noble liquids) require wavelength converters (Stokes shifters), converting initially emitted VUV photons to more suitable wavelengths.

Ideally, photons are emitted in a spectral region with little reabsorption in the material due to a suitable Stokes shift. Although reabsorbed photons may be re-emitted, that can lead to longer decay times, thus degraded timing resolution, and extra absorption losses. Such problems can also lead to position-dependent light yield, which is bad for the energy resolution.

As Rayleigh scattering is strongly wavelength-dependent, it can substantially vary within the emission spectrum, and degrade the energy resolution.

Emission anisotropy and site-to-site variations of activators can broaden emission spectra, which can affect light transport in large crystals.

The light yield is defined as the number of optical photons created per deposited MeV. Generally, a high light yield (e.g. 50k ph/MeV) is preferable, as a higher number of photons can be measured more accurately, but energy resolution can also depend on other statistical properties.

Short rise and decay times are important for good timing resolution and high count rate capability. Afterglow (phosphorescence) from charge traps is undesirable. Time constants vary widely: For example, LYSO:Ce has ≈40 ns decay time, CsI:Tl about 1 µs, which is longer e.g. than typical dead times of silicon photomultipliers.

Ideally, a scintillator would generate photon numbers strictly proportional to the deposited energy. However, nonlinearities can arise, e.g. from quenching for heavy ionization (described by Birks' law). Non-proportionality is a key limit to room-temperature energy resolution, especially for α particles.

Various characteristics such as light yield, absorption and decay times, as well as the photon detection efficiency of the light sensor, can all shift with temperature.

Some scintillator materials exposed to high radiation doses can suffer from the formation of color centers (radiation-induced defect states) that absorb visible light and thus reduce transparency. Ionizing radiation typically creates trapped-charge centers, while neutrons and other hadrons can cause displacement damage in the lattice. These defects degrade light yield and energy resolution.

Some types of damage are reversible: Defects may be removed by annealing, either naturally at room temperature or accelerated by heating, but in many materials permanent degradation accumulates.

Some hosts and dopants contain naturally radioactive isotopes (e.g., rare-earths, ⁴⁰K, ¹³⁸La). This causes internal radiation backgrounds that matter for low-background experiments. Low-background experiments demand highly radiopure crystals and materials.

Hygroscopic crystals require hermetic sealing and robust optical windows, as moisture ingress reduces transparency.

Hardness, brittleness, thermal expansion and shock tolerance of materials determine manufacturability and can affect the lifetime.

Some materials (e.g. organic scintillators, CLYC) exhibit distinct fast and slow scintillation components, enabling discrimination of neutrons vs. γ-rays.

Growth methods (Czochralski, Bridgman, micro-pulling-down), boule size, and yield influence cost and feasible detector geometry.

Consistency of various properties matters as much as nominal performance.

Pure alkali halides like NaI, CsI or KI scintillate weakly in pure form, but have a very low light yield and long decay times, also a strong temperature dependence. Practical scintillators are made by doping such materials, often with thallium (Tl⁺), which creates activator levels inside the band gap:

• With NaI:Tl (commonly used e.g. in gamma spectroscopy), one typically obtains a high photon yield around 40k ph/MeV (photons per MeV), a decay time around 230 ns, and emission around 415 nm. Large crystals can be made, but the material is hygroscopic, thus requires hermetic housing. Unfortunately, thallium is quite toxic. Other dopants like Eu, Tm or other rare-earth ions are less frequently used.
• CsI:Tl has an even higher photon yield around 50k ph/MeV, with longer decay time around 1 µs and stronger afterglow. Emission is green around 540–560 nm. The material is mildly hygroscopic.

Single-crystal iodides are often used for best energy resolution, but polycrystalline materials are used e.g. for large screens in X-ray imaging, where area coverage and cost matter.

BGO (Bi₄Ge₃O₁₂) is very dense, providing strong X-ray absorption, as needed for compact detectors. It yields around 8–9k ph/MeV and moderate energy resolution, with ≈300 ns decay time. It emits around 480 nm and is non-hygroscopic.

This material is a quite dense silicate garnet, yielding ≈25–32k ph/MeV with only 40 ns decay time. It emits 420 nm and is non-hygroscopic. It is widely used for PET scanners, as it provides excellent timing resolution (sub-200 ps CTR with SiPMs). Downsides are high cost and some non-proportionality.

This material excels in energy resolution for spectroscopy (≈2.6–3% at 662 keV) and timing accuracy (≈ 16 ns), also has a high photon yield of ≈60–63k ph/MeV. It emits around 380–390 nm and is hygroscopic. The intrinsic radioactivity of (¹³⁸La) produces an internal background.

This is an efficient (≈45–55k ph/MeV) and reasonably fast (90 ns, some afterglow) non-hygroscopic and non-radioactive material with SiPM-suitable emission.

Some plastic materials are suitable for very large devices. They have a yield around 10k ph/MeV, high timing accuracy (2–4 ns), low energy resolution, and produce blue emission. The low density leads to longer X-ray absorption lengths, but this is less relevant for use in large shapes. Some materials are PSD-capable, i.e., can distinguish neutrons from γ rays.

Ar and Xe can be used as liquids at cryogenic temperatures. They emit VUV light (128 nm Ar, 175 nm Xe) with a high photon yield (around 40k ph/MeV) and both fast and slow components. They can be used in high purity for huge scientific detectors with excellent homogeneity.

Some materials are particularly suitable for neutron detection:

• Stilbene/ej-301/ej-276: This is an organic material, offering pulse shape discrimination for n/γ separation.
• ⁶Li glass is a lithium-containing silicate or phosphate glass, enriched in ⁶Li, which can capture thermal neutrons and then emit an α particle and a triton, which produce scintillation. It is suitable for thermal neutron spectroscopy, neutron imaging and mixed-field dosimetry.
• ⁶LiF:ZnS:Ag also works with neutron capture, but with higher light yield and a pronounced slow component which enables pulse shape discrimination against γ-rays. It is used for neutron monitors, neutron imaging plates and He-3 replacement counters.
• ¹⁰B is suitable for capture-based thermal neutron detection.
• Elpasolites (CLYC, CLLB) are suitable for thermal neutron detection and γ spectroscopy via ⁶Li(n,α). They are also PSD-capable.

Crystals such as NaI:Tl and LaBr₃:Ce must be hermetically sealed, as exposure to humidity rapidly degrades performance.

In many applications, scintillators need to be shielded against ambient light, which would otherwise create false detection events. For example, one can use thin aluminized mylar foil to block light while not having much effect on high-energy radiation.

For efficiently transferring generated photons to a photodetector, polished surfaces are best. However, roughened (scattering) surfaces and diffuse reflectors (e.g. made of PTFE) may be preferred to achieve better uniformity.

A common toxic material is thallium (Tl). It must be used in sealed units, which are properly disposed of at the end of their lifetime.

One often uses continuous scintillator pieces, typically with cubic or cylindrical form. They may be grown as single crystals, manufactured as ceramics, glasses or glass ceramics. Crystal sizes range from a few millimeters to many centimeters per side; larger blocks are possible for glasses, and particularly for plastics, where meter-scale dimensions are possible.

A pixelated scintillation array is a block of scintillator material that has been segmented into many small optically isolated pixels, each coupled to one or more photosensor channels. Instead of one large crystal producing light that spreads unpredictably, the pixelation localizes the scintillation signal and thus provides high spatial resolution — better than with a single block and a segmented detector.

Pixelated arrays can be fabricated as follows:

• Dicing or machining: A large scintillator block (e.g. NaI:Tl, LYSO:Ce, GAGG:Ce) is cut into an array of small elements (pixels) with typical dimensions around 0.5–5 mm.
• Reflective separation: The gaps between pixels are filled with diffuse reflective material (e.g. PTFE, ESR film, white epoxy). This optical isolation prevents most scintillation photons from crossing into neighboring pixels.
• Monolithic bonding: The array is held together with mechanical frames or optical cement, forming a rigid “pixelated block” detector.

Some designs encode both lateral and depth information by using angled cuts or light-sharing patterns.

Typical applications of pixelated scintillator arrays include:

• Positron Emission Tomography (PET): PET scanners use arrays of LYSO:Ce or GAGG:Ce pixels (often 2–4 mm wide, 20–30 mm long), coupled to silicon photomultipliers. Each pixel detects 511 keV annihilation photons with high timing resolution.
• Single-Photon Emission Computed Tomography (SPECT): NaI:Tl arrays are used in Anger cameras, where pixelation improves position encoding.
• X-ray and γ-ray imaging detectors: Pixelated CsI:Tl or GOS arrays are coupled to CCDs or photodiode arrays in security scanners, CT detectors and digital radiography.

There are various design trade-offs:

• Pixel size: Smaller pixels improve resolution but reduce light yield per pixel and increase complexity. Gaps for reflectors reduce the active scintillating volume; very fine pixelation can lower sensitivity.
• Light cross-talk: For optimizing the fill factors, small gaps are desirable, but some light may then still leak between pixels, limiting resolution.
• Manufacturing complexity and cost: Large, fine-pitch arrays require precise cutting, polishing, and alignment.

These are large-area layers of scintillating material, which are available in different forms:

• powder phosphor coatings (e.g. Gd₂O₂S:Tb)
• columnar films (e.g. CsI:Tl grown in needle-like structures)
• composite or ceramic sheets

Screens are used for radiography, digital X-ray flat panels and neutron imaging. They are often directly coupled to films or sensors. Compared with a photographic film alone, they can provide superior sensitivity, as the absorption efficiency of films is low. Best image sharpness is achieved with thin screens, but this compromises detection efficiency. Some screens are pixelated (see above).

Columnar CsI:Tl screens offer the best compromise of efficiency and resolution for digital flat-panel detectors.

Since single-crystal material cannot usually be used in large-area screens, the energy resolution is poor, making them unsuitable for spectroscopy.

There are thin scintillating fibers, made of plastic or crystalline material. They are often bundled. Such light guides are used in high-energy physics scintillating fiber trackers (e.g. LHCb and CMS hadronic calorimeter) and beam monitors. They provide good position resolution and flexible geometries, but the light output per fiber is modest.

Micrometer-scale scintillating layers can be deposited on substrates by evaporation, sputtering, sol–gel or other methods. They are used in micro-imaging and for coupling to CCD/CMOS sensors, for example. They can provide high resolution, but their limited thickness implies low detection efficiency for high-energy photons. They are particularly useful for α and β detection since their limited thickness discriminates against γ-rays.