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![[School of Physics - Optics Group]](http://optics.ph.unimelb.edu.au/its1.gif)
Christopher T. Chantler, Associate Professor & Reader |
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Optics Group, School of Physics (Room 505) University of Melbourne, Victoria, 3010, AUSTRALIA Phone: +61 (0)3 8344 5437 (Office, Room 505, 5th floor) +61 (0)3 8344 0015 (X-ray Lab, Room 560, 5th floor) Fax: +61 (0)3 9347 4783 URL: http://optics.ph.unimelb.edu.au/~chantler
Hollywood Senior High School, Perth, Western Australia 1975 - 1979;
BSc (Hons 1) University of Western Australia, 1980 - 1984.
D. Phil. Exeter College, Oxford University 1985 - 1990.
Prizes:
Digby-Fitzhardinge Memorial Prize for Physics (UWA) 1982.
Lady James Prize (Physical Science, UWA) 1983 (shared)
Shell Australia Postgraduate Scholarship for Science and Engineering 1985-88
St Anne's College Drapers' Company Junior Research Fellowship October 1989-1991
Lindemann Fellowship of the English-Speaking Union of the Commonwealth 1991-1992
X-ray Optics and Atomic Physics: Theory and Experiment
XAFS and Solid State Physics: Theory and Experiment
Powder Diffraction and X-ray Crystallography
QED explains how light interacts with matter and is fundamental to most of the technology we use today.
I have pursued precision tests of Quantum Electrodynamics in atomic systems, and in a series of international collaborations have produced several high-precision measurements of QED in the medium-to-high Z regime. I have been involved in the development of X-ray specroscopy on the novel Electron Beam Ion Trap devices, in collaborations primarily at NIST. I have worked on few-electron physics for 20 years and have extensive experience with investigations at accelerators in Oxford, GSI, Lawrence Berkeley Laboratory and Argonne. We have performed the most precise measurements of the resonance lines of a helium-like ion in the Z=19-31 range, which allows sensitivity to two-electron QED effects and excited-state QED effects.
Investigation of new structure in atomic systems has continually developed our understanding of physics and quantum phenomena. One of the goals of much current research is to test Quantum Electro-Dynamics (QED) critically in new and important regimes. Some areas of parallel investigations include exotic atoms like muonium, and positronium, and some investigations have involved g-2 experiments in different systems. Most effort has been directed to Lamb shift measurements in hydrogenic and helium-like systems. A significant realisation of recent years is that these complementary endeavours are investigating different fundamental issues and making major contributions to different fields.
How can relativistic quantum mechanics predict absorption and scattering coefficients, and are the results accurate?
Some of our theoretical developments in the computation of form factors have resulted in significant improvements upon earlier work, which can be tested by suitable experiments. The computations have been confirmed in selected regions. Atomic form factors determine photoelectric cross-sections, elastic and inelastic scattering cross-sections and X-ray (Bragg-Laue) coherent diffraction profiles. Major discrepancies exist between theory and experiment. The Web database has been receiving over 10000 hits per month for over five years since itŐs electronic installation as one of the three major references for atomic form factors and attenuation coefficients. Reliable knowledge of these factors is required for conventional fields such as crystallography and radiography, and also for the new fields of X-ray Anomalous Fine Structure (XAFS) and Multiple-wavelength Anomalous Dispersion (MAD).
Here the atomic scattering factor is given for Uranium at medium X-ray energies (keV). Click the figure for the corresponding attenuation coefficients.
Our recent experiments are two orders of magnitude more accurate than earlier work and reveal new physics, new processes and new applications. If we understand how light interacts with matter, we can use this insight in further applications.<\P>
The way that X-rays interact with matter should be well understood. However, deviations between latest theoretical computations lies at the 10% level over much of the energy ranges, for most elements. Even for the most investigated elements such as Si, Cu, Ag, Au, the few experiments which claim 1% precision show variation of 5-30%. We are addressing this with synchrotron experiments and with state-of-the-art facilities at the University of Melbourne. Recent results have broken through this barrier to an unprecedented 0.01% precision and 0.02%-0.3% accuracy - an improvement of two orders of magnitude over previous work. We have a specialised 18kW CE high-frequency rotating anode, plus several other features suited to X-ray physical investigations.
X-ray Absorption Fine Structure (XAFS) is a complex structure seen in the absorption coefficient just above the absorption edge, where an incoming X-ray has enough energy to ionise an electron from a particular bound state. The oscillations seen are particularly due to an interference effect between the emitted photoelectron and its own reflected wave. This signature allows many investigations of local structural information for crystallographers, chemists, medical scientists and mining / engineering investigations.
Some third or more of Australian synchrotron research uses XAFS (and the related technique called XANES) to indentify band distances, chemical valence, nearest neighbour coordination and geometry, and local structure.
Our new experimental techniques allow XAFS determination with an accuracy increased by up to two orders of magnitude, which in turn challenges all available theory and modelling. Our analytical work puts these discrepancies on a firm foundation, and our theoretical development holds promise to develop new tools and methods of insightful analysis.
Powder Diffraction is often required for structural determination of biologically active molecules, viruses, proteins or enzymes as well as for small inorganic molecules, especially where the samples cannot be grown into large crystals.
Standards for powder diffraction are well-known and widely used; though not frequently used in local Australian research. These standards are dominated by pure silicon powder and lanthanum hexaboride powder, which are the two principal lattice (and intensity) standards used in the world today. These standards are maintained by NIST. They determine the lattice parameter of an unknown sample under investigation and are a critical tool in determining the synchrotron beam energy in an experiment. Additionally, they monitor and can reveal several types of systematic errors in typical experiments.
In recent work using the X-ray Extended Range Technique (XERT) we have redetermined the lattice spacing of the second standard (LaB6) compared to the primary standard (Si) and find several standard deviations of discrepancy. This proves (i) that synchrotron techniques can be used to determine such standards and (ii) is the most accurate determination of lattice spacing except for that of silicon itself. This opens up the way for the implementation of new standards and methods of analysis.
These issues impact upon X-ray diffraction theory. My diffraction theory is the first dynamical theory for non-ideally imperfect curved crystals (and simpler subclasses) and shows significantly greater agreement for perfect curved crystal profiles than previous work.
The X-ray interaction with photographic emulsions is an interesting application of ideas from basic physics. Active areas of interest and development include ion chamber optimisation, new detector technology, state-of-the-art spectrometry and 2-dimensional (backgammon) proportional counters.
Applications of these ideas have led to new calibration devices for radiography and mammography, now patented in the US as part of the Quantum Metrology Group effort in the Atomic Physics Division at the National Institute for Standards and Technology, USA.
