B a c kg r o u n d
Materials
TU Delft material shines in medical detectors 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. Jessica Vermeer
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hilst 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. 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 24
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is specialized in the production of glass and crystals. “Those crystals are supplied to companies that build detectors. SaintGobain 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 A set of luminescent scintillating crystals. Credit: Delft University of Technology
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
