ANSTO's research capabilities, led by the OPAL nuclear research reactor and associated instruments provide access to users investigating areas as diverse as materials, life sciences, climate change and mining/engineering.
Sika - Cold 3-Axis Spectrometer
| Instrument Scientists Dr Chun-Ming Wu Dr Guochu Deng | Instrument Cabin Phone 9717 3728 |
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More information on SIKA
3-axis (or triple-axis) spectrometers were originally developed by Bert Brockhouse (Nobel Prize in Physics, 1994) at Chalk River in Canada, and he was awarded the Nobel Prize in part for this invention, along with the constant-Q method of operation and for a series of seminal experiments performed at Chalk River in the fifties and sixties. His original spectrometer is now in the Canadian Museum of Science and Technology in Ottawa.
Double-axis axis spectrometers, whether of neutrons or X-rays, consist of the source, a monochromator crystal, the sample and a detector.
There are two rotation axes at monochromator and sample. In the triple-axis spectrometer, a further crystal 'analyser' is added after the sample. This allows one to measure the neutron energy after scattering, which may in general be different from the incident energy provided by the monochromator.
If the incident and scattered neutron energies are equal, there is no net energy transfer to the sample and this is called elastic scattering.
Elastic scattering typically dominates the scattering from solids, and in all the instruments we have reported on to date, we make the assumption that the scattering is elastic. If the incident and final energies are different there is a net transfer of energy to or from the sample and this is called inelastic scattering.
In solids, the transferred energy is in the form of quantised sound waves or phonons (by analogy to photons as quantised light), or magnons (quantised magnetic waves) or even as energy to transferred to individual electrons, as they jump from one quantum level to another. Typically, the inelastic scattering has an intensity one ten-thousandth of the equivalent elastic scattering.
Stated another way, double-axis spectrometers measure the total scattering from the sample, assume it to be elastic, and interpret it in terms of the structure of the material, while triple-axis spectrometers measure the energy spectrum of the solid, and interpret it in terms of dynamics or how the atoms move.
This in turn tells us about the forces between atoms, or between magnetic moments. This is particularly important in understanding how materials change structure (phase transitions), and in understanding other thermodynamic properties of solids (specific heat, magnetic susceptibility, bulk modulus, etc.).
Stated yet another way, double-axis diffractometers are the analogue of X-ray diffractometers, while three-axis spectrometers provide spectroscopic information like that obtained from optical spectroscopy methods like infra-red or Raman spectroscopy. The difference is that thermal neutrons from reactors also have wavelengths comparable with interatomic spacings so that inelastic neutron scattering provides additional information on the spatial nature of the modes or excitations.
Another triple-axis spectrometer, TAIPAN views thermal neutrons. TAIPAN is ideal for the study of phonons and magnons in normal materials, and in studying the physics of room-temperature transitions and processes. On the other hand SIKA (viewing the cold source) will be better suited to study problems in low-temperature physics, like understanding novel ground states of materials (superconductors, magnets, strange metallic states, etc.).
In fact the more exciting physics is likely to be done on SIKA, when looking at the types of articles that have appeared in recent years in leading journals like Science and Nature.
Much of the world's triple-axis capacity has recently been tied up in tackling the 'high-temperature-superconductor' problem, one for which neutron scattering might win a Nobel Prize, if our method provides definitive evidence regarding the mechanism for superconductivity.
Other important problems include understanding zero-temperature phase transitions, other unconventional superconductors besides high-Tc, model statistical-mechanics systems, metal-insulator transitions, ionic conductors and so on.
In all cases, it is crucial that large high-quality single crystals of the relevant materials are grown, and we are supporting a successful ARC-LIEF grant with Sydney University to get some state-of-the-art crystal growth in Australia for the first time.
TAIPAN as well as SIKA are both highly configurable and versatile, and will have the most intense thermal and cold beams in the whole facility, together with the lowest background levels. There will be a substantial number of diffraction experiments, particularly when looking for weak scattering effects, performed on both instruments.
In fact, the two triple-axis spectrometers are very well suited to real 'experiments' as opposed to the more routine 'measurements' for which our other instruments are typically better optimised.