Scintillators are a group of materials that luminesce when exposed to ionizing radiation. In layman’s terms that means these materials emit light when they absorb particles or electromagnetic waves that create “free” electrons in the material. While this may be viewed as a seemingly dry and uninteresting definition, scintillators enable a plethora of exciting and very necessary applications ranging from personal radiation detection to the detection of dirty weapons, and medical applications such as treating cancer.

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Scintillation Crystals, Arrays and Detectors

In this series of posts, we’ll explore the world of scintillation, starting at the most basic – how scintillation materials respond to radiation. While scintillators come in many forms – gas, liquid, solid, and both organic and inorganic – we will focus on inorganic crystal forms, a sweet spot for us since Hilger Crystals has grown crystals since the mid ’s. Then, in successive posts, we’ll discuss how they’re produced, specific applications, popular materials types, and more. Let’s get started!

Ionizing Radiation

Ionizing radiation is caused by atoms shedding energy in the form of particles or rays (photons) to become molecularly stable. Typical particle radiation types are alpha, beta, or neutron while electromagnetic radiation types include x- and gamma-rays. Particles and rays interact with matter differently, but a common trait of both is that in order to have optimised sensitivity (and therefore luminesce) they need to absorb as many particles or photons as possible. In general, high density materials with high atomic numbers (Z) meet this requirement.

The University of Florida’s Radiation Safety Short Course, Chapter 2, Section VII offers excellent deeper information on the interaction of particles and ionizing radiation.

Scintillation in Three Stages

1. Once a scintillator crystal is exposed to ionized radiation, electrons within the crystal become excited and move from their locked position within the valence band to the conduction band where they are free to move around, leaving an associated hole behind it.
2. An electron-hole pair, called an exciton, will continue moving throughout the lattice until it is trapped by a defect within the crystal or by a deliberately-introduced dopant, e.g. thallium or cerium. These defects are called activators, they create special areas within the crystal called activator sites and luminescent centers.
3. In the final stage, electrons trapped within these sites decay, emitting a photon causing – yep, you guessed it – luminescence.

Put more succinctly, the absorbed energy produces electron-hole pairs that migrate to the activator sites in scintillators in order to luminesce.

Stages of Scintillation. Source: https://web.stanford.edu/group/scintillators/scintillators.html

One quick caveat: when ionized radiation in any form is absorbed by the host optical material, it does not always follow that the material will scintillate. Consider tungsten or lead for a moment – both deliver high density and high Z, but both are use as radiation shielding materials. While they absorb ionized radiation, this energy is lost within the material by a number of non-radiative processes. Scintillating materials must contain luminescent centers that convert ionized radiation into a form that can be detected.

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Light It Up

As we are focusing on solid scintillators, one obvious property is that the inorganic crystal needs to be transparent to the scintillating light colour (wavelength). If not transparent, then that energy is lost to non-radiative processes.

Luminescence from scintillators vary by material, but generally ranges from 0.3 – 0.8 microns, spanning the range of human eye sensitivity so it is possible we can see it with the naked eye – if there is high enough excitation flux or dose.

Once light has been generated in a scintillator crystal it needs to be detected and this is done by attaching a photosensor along with suitable optics to reflect and scatter the light to the photosensor. Photosensors can range from photomultiplier tubes (PMTs), PIN diodes, SiPMs, and more, depending on emission wavelength of the scintillator and operating environment. We’ll review different types of photodetectors and key factors in choosing the right one in a future post – stay tuned.

If you have questions about a specific scintillation principle or crystal, please reach out our experts at Hilger Crystals.

We also invite you to try our newly-developed “Crystal Compass” – an easy 4-step tool to help you determine the best scintillator material for your application.

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Scintillation Detector

A scintillation detector is often portable. The scintillation detector is not as versatile as the GM meter, although it can be used to look for contamination from some radioactive materials. The scintillation detector's active portion for detecting radioactivity is a solid crystal (that is the scintillator) with which the radioactive emission must interact. This essentially limits use of the detector to gamma rays and high-energy beta particles since medium- and low-energy beta particles cannot penetrate the crystal and, therefore, cannot interact.

Scintillators can be made in different sizes, and the thickness of the scintillator determines its ability to absorb and detect certain radiation emissions. A thin scintillator is an excellent choice for low-energy gamma rays and high-energy beta particles. The ray or particle will be absorbed within the thin scintillator and the light produced by this interaction will be able to pass through the remaining thickness to allow the gamma ray to be detected. A high-energy gamma ray is likely to pass right through the thin scintillator without interacting.