Jessica Vermeer
10 December 2019

You may not know it, but a type of material called scintillators was probably involved when you had your last medical scan. Working with the French company Saint-Gobain, scintillator expert Pieter Dorenbos at Delft University of Technology is currently developing the next generation of these materials.

Whilst rushing towards airport security, everyone will undoubtedly zigzag their way through a bunch of queue poles with belts that make out the waiting lines before the actual security check. Once you’ve passed these lines, you’re probably not aware that you’ve already been checked for radioactivity. Some of the queue poles in airports contain detectors for radioactivity. This is, of course, to prevent travelers from carrying radioactive material onto the plane. It’s one of many applications for a special type of material, called scintillators.

Scintillators are capable of absorbing ionizing radiation from a radioactive particle and convert its energy into a short pulse of visible light. This is why they can be used to build excellent radiation detectors: even a tiny bit of radiation produces a light pulse, which is easily detected using an electronic light sensor. The principle is not only useful at airports but also in hospitals: both PET and CT scanners always rely on scintillating materials.

A set of luminescent scintillating crystals. Credit: Delft University of Technology

Pieter Dorenbos of Delft University of Technology (TU Delft) has been researching scintillators for 30 years now. In fact, he was there when the research group was founded, not long after then research leader Carel van Eijk realized how useful these materials can be. This is aptly illustrated by the fact that Dorenbos’ Luminescence Materials research group in Delft has been collaborating with Saint-Gobain for over 20 years now. The French company is specialized in the production of glass and crystals. “Those crystals are supplied to companies that build detectors. Saint-Gobain also builds detectors themselves,” says Dorenbos. The partnership is currently focusing on developing crystals that outperform existing crystals.

Co-doping

In PET scans, radioactive material is injected into the patient and the emitted radiation is used to put together a scan of a body part. The job of the scintillator is to catch the radiation and transform it into a short flash of light. The shorter and stronger the flash, the better, because such a well-defined event is easier to work with than a weak and ‘stretched out’ light signal. The nature of the scintillator has a strong influence on the type of light flash.

A scintillator consists of a crystalline base material in which a radiation-absorbing and light-producing element is embedded. One of the better combinations is the base material CsBa2I5 (cesium, barium, iodine) with a little europium added. Europium absorbs the radioactive radiation quite well, but it also self-absorbs the light it emits, causing less light to leave the crystal. As the crystal becomes larger, this effect increases – and that’s no good when you want to build a detector.

Dorenbos’ latest research focused on the combination of europium (Eu) and samarium (Sm). Co-doping with samarium could effectively bend the disadvantage of europium into an advantage. This idea was applied to CsBa2I5. “Our idea was to add samarium, as it can absorb the light of europium and then emit it as a different color. In fact, samarium absorbs in the visible and emits in the invisible near-infrared, which makes the scintillator completely black.”

The europium-samarium combo works well: the best result so far shows that it has a resolution of 3.2 percent, which is quite high. The best resolution to date, 2.2 percent, was achieved with another material also discovered within the TU Delft research group of Dorenbos. “The europium-samarium material has the potential to beat that. It can go below 2 percent. It could take another five years but it does have that much potential.” Another advantage of samarium co-doped crystals is that the silicon-based infrared detectors aren’t affected by a magnetic field, which is important for 2-in-1 scanners, such as those that combine PET and MRI.

Collaboration with industry

The results in Delft are good news for Saint-Gobain. Several of the company’s successful products are based on the record-holding scintillator but that was invented 20 years ago and the patent is about to expire. Dorenbos: “The research on co-doping with samarium has potential, so they patented the material. By investing in research, Saint-Gobain can stay ahead of competitors. Whether the material actually ends up in a product is for them to decide. When the photon leaves the crystal, the work of our research group is done. What happens next is up to someone else.”

A cerium-doped lanthanum chloride scintillator in a quartz vial. Credit: Delft University of Technology

The combination of public and private funding is remarkable, especially for a topic that could be defined as fundamental research. “We do try to understand what’s happening, that’s fundamental. The interaction between high-energy radiation and the material, ionization, free electrons and holes: all of those aspects require complicated models to simulate. We need to know what’s happening to make the right choices in materials.” In the end, fundamental, empirical and applied research are combined to find an application.

There’s much to be improved upon the samarium co-doped scintillating material. Samarium could also be used in a material that could potentially generate more photons. Dorenbos hopes in a year a follow-up research project will be up and running with two PhD students.