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:: New Method for High-resolution Images of Minerals
An x-ray, photon-counting imaging device smaller than a postage stamp is the key to a powerful new method of taking high-resolution images of minerals.
Medipix is an imaging detector built around a sensor chip just 14 square millimetres in size and only 0.3 millimetres thick. It was originally developed at the CERN laboratory in Switzerland by scientists who build particle-tracking detectors for high-energy physics experiments such as the Large Hadron Collider (LHC). In the 1990s an international collaboration formed to find real-world applications for the technology. These applications now include astronomy and medical imaging.
Dr Josef Uher started using Medipix in 2002 when he developed novel neutron detectors during his PhD at the Institute of Experimental and Applied Physics at the Czech Technical University, in his hometown of Prague. Now a research scientist for CSIRO's Minerals Down Under Flagship, Dr Uher leads a research project aimed at using Medipix to characterise mineral ores through x-ray imaging. Unlike conventional x-ray films and cameras, new x-ray imaging detectors such as Medipix can measure the energy of individual incoming x-rays and add colour to traditional black-and-white snapshots.
While x-rays have long been used to image mineral ores, with shades of grey depicting the density of different components, these images do not easily identify the individual minerals present.
It is akin to seeing a foreign object on an x-ray of a human body - knowing that it shouldn't be there, but not being able to identify exactly what it is. In the Medipix process, an x-ray tube fires beams through grains of ore. A sensor on the other side measures how x-rays of different energies are attenuated by the objects in the beam. "For example, gold changes the spectrum of transmitted x-rays differently to nickel or molybdenum," Dr Uher says.
Medipix then images the grains of ore in detail, producing pictures in which colours correspond to the elements contained in the sample. The detector collects information at more than 65,000 points simultaneously and multiple images can be obtained and joined together to increase the field of view. "In every single pixel of the detector, you gain information about what the x-ray spectrum looks like. If you analyse it properly, you can say whether the materials in the sample at that point were nickel, copper, zinc, gold or something else."
The new method could reduce to just minutes what is currently an arduous preparation process. "So far this has been done mostly using scanning electron microscopes. You take the ore sample, put it in resin, make a cylinder that contains the ore particles inside, cut the cylinder in half, polish the surface and then look at that surface with an electron microscope. It takes hours," Dr Uher says. "It gives you lots of information, but only from one layer. If you have a particle that is thin, but long, and sits perpendicular to the surface, you would consider it a small particle, but in reality it's long. You don't see the third dimension with the existing technique, but with x-rays you can reveal that. You can also do it faster."
Under Dr Uher's supervision, undergraduate student Jack Williams is developing techniques that will read images taken from different angles, to carry out tomography that reveals materials in the sample in three dimensions. "I personally believe these will be the standard x-ray imaging detectors of the future. Other devices can provide nice spatial resolution and good x-ray detection efficiency, but the information that tells you the energy of the x-rays is still missing," Dr Uher says.
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