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.
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 1930’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 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
- 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.
- 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.
- In the final stage, electrons trapped within these sites decay, emitting a photon causing – yep, you guessed it – luminescence.
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.
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.
About Hilger Crystals
Founded in 1874, Hilger Crystals has a well-established history and proven reputation for producing high-quality, commercial-grade synthetic crystals used in infrared spectroscopy and state-of-the-art scintillation and detection solutions. Hilger Crystals’ ability to grow synthetic crystals in large volumes and to incredibly demanding specifications is further boosted by their close collaboration with customers — a practice that has proven successful from prototyping new research to wide-reaching commercial engagements. Hilger Crystals produces an extensive range of scintillation crystals carefully selected for their high density and brightness, excellent light output, and short decay constants. Crystals are available as linear and two-dimensional arrays in sizes from 5mm to 200mm, and can be coupled to a position sensitive PMT, CCD array, SiPM, or linear photodiode detectors to form a complete assembly.