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October 2002

The X-ray CT Lab
Unlocking the structures of materials with
X-rays and high-powered computers

The X-ray Computer Tomography Laboratory probes the structure of materials by taking a number of X-ray images of an object from a variety of angles. Computers then stitch the images together to build a 3D model of the structure.

It's the same process as medical CAT (Computerised Axial Tomography) scans used to image the body or the brain (although in medical applications the patient stays in the same position and the X-ray scanner moves around the body; in the CT lab the X-ray scanner stays stationary and the object is rotated).

ANU's X-ray CT lab is an amazingly powerful and flexible facility that is helping scientists unlock many of Nature's best kept secrets.

Who runs it: The X-ray CT lab is operated by the Department of Applied Maths, part of the Research School of Physical Sciences and Engineering. Applied Maths operates in the cross-disciplinary area where chemistry, physics, biology, earth science and materials science meet. It's involved in the fundamental studies of surface forces, liquid crystals and mesostructured materials. The X-ray CT lab was constructed as part of Applied Maths' continuing studies on the relationship between structure and property.

How does it work: The key to understanding the structure and function of many naturally occurring and synthetic materials is probing their structure on many scales of length simultaneously, thereby relating microstructural character with macroscopic performance. Further insight is gained if you can observe the structures as they develop over time.

However making such measurements is extraordinarily difficult. While it might be possible to take accurate measurements on one scale, say on a micron scale, you normally have to use a separate machine and sample preparation to take a measurement at a higher or lower scale. The restrictions are magnified if you want to watch a sample develop over time.

The X-ray CT lab, however, is one of the few facilities in the world that can measure over three scales of magnitude simultaneously - from microns to millimetres. Further, the facility was purpose designed to be as flexible as possible allowing a wide variety of samples to be analysed over a broad range of time scales.

The apparatus is identical in concept to standard medical tomography, with the exception that it is the sample which rotates within a fixed X-ray microscope. The X-ray camera then records 2D radiographic information over a variety of rotation angles. High-powered computer manipulations of the data then allows the construction of a three dimensional image of the sample's structure.

While X-ray tomography has been around for many years, the CT lab put together by Applied Maths has many unique attributes which give it incredible power and flexibility. The micro-focus X-ray source can produce a wide range of X-rays (30-130 Kv) suitable for scanning soft biological materials through to hard materials like stone .

The X-ray detector is an extremely sensitive 16-bit, scintillator-coupled 2048x2048 pixel CCD camera mounted on a linear rail allowing it be moved back and forth from the sample and X-ray source. Moving the camera gives a range of magnifications from 3x through to 100x. The sample is mounted on a precision rotation stage, and the entire apparatus sits on custom-built vibration isolators (allowing the sample, X-ray source and detector to be rock steady over long time periods). The facility is contained in a lead-lined room with safety interlocks.

Because the CCD camera is so sensitive, and the apparatus is so stable, the X-ray source can be used at low power for extended periods of days to weeks meaning structural changes can readily be monitored over time. Because the apparatus is simple and open (ie, not encased in restrictive lead shielding, the lead is in the surrounding walls) elaborate experimental rigs can be accommodated.

On top of this, the CT lab has access to the awesome computing power of the ANU Supercomputer Facility, though APAC, to allow rapid manipulation of staggeringly large data sets. A single raw dataset may be as large as 32 Gbyte.

Some of the research the CT lab will assist in includes the characterisation and modelling of a wide range of materials including soils, sedimentary rocks, minerals, wood, bone, polymer composites, foams, catalysts, coatings, gels, concretes and ceramics. The results of this materials science will include the development of improved biomaterials, achieving higher yields of oil extraction from rocks, and designing better paper for printing.

The secret of its flexibility is its design. While most of the hardware incorporated into the lab is off-the-shelf equipment, its the way it's been configured together that gives it its power. And the credit for the design and construction of the lab goes to a large team of scientists, software designers, and electronics and mechanical engineers from all over RSPhySE. Leading the way has been Stephen Hyde, Mark Knackstedt, Tim Senden, Tim Sawkins, Arthur Sakellariou, Adrian Sheppard, Rob Sok, Stuart Ramsden, Vanessa Robins and Christoph Arns.

More information: Dr Tim Senden