Success stories
Andy Edgar: X-Ray Images From Glass Ceramics
In the past few years Andy Edgar and his research group have been developing glass ceramics that display a rare x-ray storage property. These materials, when used to replace the traditional x-ray techniques will vastly improve the efficiency and convenience of x-ray radiography systems.
The Challenge:
Using traditional photographic films for X-ray imaging is a painstaking business. Although the resolution of images is very good the chemicals used are messy and toxic. The process is slow, the images hard to reproduce, the x-ray films need to be archived and there is no easy way to access modern image processing methods. A new improved X-ray imaging method is well overdue. X-ray storage phosphors (XRSP's) are a prime candidate. They store x-ray radiation so that an X-ray image is held latent, like invisible ink within the phosphor material. The image can then be revealed later when the image plate is exposed to red laser light.
This rare X-ray storage property, called Photostimulated Luminescence (PSL) is not a recent discovery. For a while now, X-ray storage phosphors in powder form have been used for radiography in hospitals. The problem with these powders is that the individual crystals cause scattering of the read-out light, which makes the image blurry.
The problem of Scattering off crystals in existing XRSP's
Andy Edgar set out to locate and develop a transparent glass or glass-ceramic with X-ray storage properties that, having very small crystals would have improved resolution. This was not as easy as it may sound. The number of crystals possessing the x-ray storage property could be counted on two hands. Finding a glass was an even harder task.
The Process:
With a lot of experiment and educated guesswork a glass ceramic material competitive with current methods was developed.
The problem was and still is that the smaller you make the crystals in the glass ceramic the weaker the PSL. So there is a trade off between resolution and X-ray storage.
The glass ceramic samples are made by heating all the ingredients in a furnace at around 750°C so that they all melt together. The sample is then quenched (cooled quickly) to produce a glass with no crystals so no scattering effects but no X-ray storage properties either. The following image is the result of a Molecular Dynamics (MD) computer simulation of atoms in such a glass after it has been quenched.
Structure of Zr-Ba-La-Na-F Glass (20% Cl). Green=Cl, Yellow=Fl, Blue = Ba, red Zr= white. Notice the chlorine rich fluorine deficient region.
The sample is then annealed (heated to just below its melting point). With the increase in temperature atoms become free to move. Regions of high chlorine concentration agglomerate eject any impurity atoms and form pure barium chloride crystals. The resulting glass-ceramics are PSL active. Crystals grow in a cannibalization' process. First a huge number of nano-crystals form, then larger crystals start feeding on smaller ones and the crystal size increases to micrometer size and beyond. If you keep the annealing time very short you get a nano-crystalline glass-ceramic with negligible scattering effects. The nano-scale size of the crystals is the key to the material's success. By adjusting the chemical composition, annealing time and temperature Andy and his team have created glass-ceramics with 5nm 50nm crystal size and PSL at least as good as the X-ray storage powders used before. A little more fine-tuning and the glass ceramics will be ready for wide spread use in medical radiography.
How Do XRSP's Store X-Rays?
When an X-ray hits a storage phosphor atom the impact sends electrons flying from their orbits. These energetic electrons scatter into other atoms releasing more electrons. Electron energies get lower until they reach the bottom of the conduction band. In most materials the electrons would then immediately fall back into the valence band emitting photons of energy but in a storage phosphor a fraction of the electrons get trapped in local electron traps (F-centres) making recombination impossible. The same is true for holes (electron vacancies that have an overall positive charge).
The Storage Phosphor mechanism. The right hand side is the read-out process.
The Read Out Process:
The image latent in the phosphor material is recovered when the image plate is exposed to red light. Photons of light nudge the electrons and holes out of their traps allowing them to recombine. Energy released on recombination is transfered to fluorescent Eu 2+ ions that emit photons of blue light. This radiation can be collected and what do you know, you've got your X-ray image back.
The Image on the left is a photograph of a transister on the imaging plate. The image on the right is the X-Ray image recorded of the device. The internal structure of the device can be clearly seen.
No one understands completely what and where the electron and hole traps actually are. There are still a lot of unanswered questions and plenty more work to do to refine the materials but the progress is steady and the discovery of PSL properties in glass ceramics an illuminating and a satisfying achievement.
Applications and Furhter Research:
The main goal is to replace photographic X-ray techniques in medical radiography but beyond that there are many other applications such as checking for cracks in pipelines and non-invasive luggage checking at airports.
Graham Appleby a PhD student working with Andy is developing materials that capture neutrons in a similar way. Andy also hopes to extend research to gamma rays, which are more penetrating and better than X-rays for detecting cracks in thick or dense materials such as steel pipes and shipping containers.
Extremely fine fibre-optic cables made of transparent XRSP glass ceramics could be inserted into the body during radiation cancer therapy to tell accurately where the radiation is hitting. Currently this process relies on calculation, which has a much larger degree of uncertainty.
Andy is also doing research into the permanent refractive index changes that occur in certain types of glass when it is exposed to ultraviolet radiation. This relatively new area of research opens up the possibility of making optical devices on a nanometer scale such as filters and Bragg reflection planes in fibre optic cable. These could be used in fibre optic cables, for example under the sea to amplify signals and permit Wavelength Division Multiplexing (WDM) which increases the amount of information that can be passed along a single fibre optic cable.
The glove box. Compounds used to make glass-ceramics react to air so all preparation is done in a nitrogen atmosphere. The sealed box has a perspex lid so you can see what you are doing. To avoid any leakage of air into the box the pressure inside is kept higher than outside when not in use causing the rubber gloves to reach ominously into the room.
