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Materials Physics

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In materials physics research is carried out in the following fields: nanostructures, ion beam analysis, electronic structures, microtomography, computational materials physics, electronics and medical physics.

Laboratory for nanomaterials and ion beam analysis

Installation of the research equipment back into use started in February after the renovation of the accelerator building was completed. The 500-kV ion implanter KIIA was started by various implantation experiments until excessive corona problems turned up in mid-November. The principal cause of the corona was targeted at the main 500-kV power supply which is being prepared. The ion sources of KIIA were slightly modified to improve the quality of the ion beams. Because of the extensive updating and modifications at the 5-MV tandem accelerator (TAMIA) the first test runs could not be started before mid-December. The whole injector platform was redesigned and installed to allow improved ion optics and fast isotope switching in radiocarbon AMS. Facilities enabling direct feeding and measurement of gaseous samples into the AMS ion source were designed and installed. New control and automation systems for TAMIA were installed and tested together with new power supplies.

Due to the break caused by the renovation and facility updating the research activity was mainly focused on the study of structural, thermal, mechanical and magnetic properties of metallic nanoclusters and nanocrystalline films produced by using the reinstalled Facility for Nanostructure Deposition (FaNaDe). The development work concerning the low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) facilities were completed. The Cryogen-Free Dilution Refrigerator System is in use at the manufacturer's site until the new shielded room is being completed.

Laboratory of electronic structure and laboratory of microtomography

In the experimental research activities both synchrotron radiation facilities and the versatile instrumentation in the laboratory were used extensively. The experimental work was accompanied by computational activities which have played an important role in strengthening the fundamental understanding of electronic structure behind the properties of materials. Many of the research projects were multidisciplinary and carried out in international collaboration.

Spectroscopic investigations covered fields from Compton-profile measurements of ionic liquids to resonant and non-resonant inelastic x-ray scattering studies of high-TC superconductors. Highlights of the year involved a breakthrough publication describing a novel x-ray spectroscopic imaging technique, and an article giving new insights into the structure of water-ethanol mixtures. Small and wide angle x-ray scattering studies on the nanostructure of e.g. young aspen saplings, bio-inspired natural polymer composites, and surface layer proteins have been carried out using the setups of the laboratory. A highlight was studies on the cellulose structure of wood samples from the Swedish warship Vasa. Neutron scattering was utilized in studies of enzymatic hydrolysis of cellulose. At the laboratory of x-ray microtomography intensive work has continued strongly in multidisciplinary studies of evolutionary biology, tree physiology, paleontology, composite materials, food science and geology.

Computational materials science

In 2011 the very challenging project on modelling all aspects of electrical vacuum arcing started to bear fruit. A few years earlier we started to develop a multiscale simulation framework to examine this widely occurring, but poorly understood phenomenon. To tackle the issue, we use state-of-the-art atom-level computational methods, ranging from quantum-mechanical to large-scale classical molecular dynamics all the way to particle-in-cell plasma simulations. These are coupled together by sequential and in some cases even concurrent multiscale modelling. The modelling is done in close linking with experiments at CERN and our laboratory. In 2011, several publications and the first PhD thesis on this demanding topic was completed.  The results explained how the arc plasma produces craters on surfaces. The surprising conclusion was that the crater production is associated with keV ion irradiation producing multiple overlapping heat spikes that almost explodes to form massive craters.

Electronics

Novel measurement methods and sensors combining ultrasonics, optics, laser-acoustics, electronics, and advanced signal processing for industrial and scientific applications have been developed.

Medical physics

Boron neutron capture therapy (BNCT) is an experimental targeted radiotherapy method that has been actively researched in Finland since 1992, thus marking the year 2012 as the 20th anniversary of Finnish BNCT.  The clinical trials at the FiR 1 research reactor in Otaniemi (Espoo, Finland) started in May 1999 with primary glioblastoma patients. Since then, over 200 patients with malignant brain or head and neck tumors have been treated at the facility. This year, first two reports of clinical trials were published.

Occupational radiation exposures and related dosimetry practices were analyzed and the results including revised recommendations for exposure monitoring have been published.  One focus has been on developing and validating non-invasive techniques for preoperative evaluation of eloquent cortical areas before epilepsy and brain tumor surgery. During the year we have validated the navigated transcranial magnetic stimulation technique for motor cortical mappings with epilepsy patients.  The work with ultra-high resolution single-photon emission computed tomography demonstrated the feasibility of dopamine transporter tracer 123I β-CIT in preclinical research.

In August the research community hosted an IAEA regional training course in radiation protection, quality assurance and optimization in digital radiology.

Setting and revising the educational standards for qualification as a professional in medical physics has been as topical as ever: the news on recognizing medical physics as a profession in the upcoming version of the International Standard Classification of Occupations (ISCO) of the International Labour Organization (ILO) is regarded as a significant mile stone by the international community of medical physicists. 

Highlights of research

Novel synchroton technique for three-dimensional chemical imaging

With spectroscopic imaging methods it is possible to study the distribution of chemical compounds and elements within a given material. Using x-rays, for instance, images can be taken in a range of x-ray energies in the vicinity of the elements’ characteristic absorption edges. However, this is difficult when studying elements with a low atomic number, such as carbon and oxygen, since the corresponding x-rays have energies below 1 keV and probe matter only at a few micrometers depth. To overcome this problem, we have developed in collaboration with the European Synchrotron Radiation Facility (ESRF) a novel imaging method that uses hard x-rays (from 10 keV up) to study the chemistry and distribution of light elements in materials.

The new method is based on inelastic x-ray scattering (IXS) spectroscopy combined with imaging in a novel fashion. IXS probes electron excitations, for instance of a 1s electron of a carbon atom to the lowest unoccupied molecular orbitals, and is thus element-specific and sensitive to local structure and chemical state. Instead of the density variations that are measured in traditional x-ray imaging, IXS-based imaging reveals three-dimensional images of the distribution of electron excitations within the sample with a ~10-µm spatial resolution. The technique is enlisted in the methodology portfolio of the ESRF upgrade programme, in which also the Laboratory of Electronic Structure is strongly involved.

The technique can be used to determine for instance local molecular bond variations in carbon composite materials. In the figure below we show the application of the method to a carbon-fibre-reinforced silicon carbide sample.  From the IXS spectra of carbon 1s core electrons, different carbon bonds are easily discernible. Each measured voxel contains an IXS spectrum that reveals the average carbon bond type within it. We thus determined the carbon bond distribution in the sample, and found crystalline sp2 dominating. Since the sp2 bond is anisotropic, the spectra are also sensitive to its orientation, revealing a periodically alternating bond orientation.

Other examples of potential research objectives are in-situ studies of batteries, fuel cells, and rare geological samples. This opens up new applications in materials science, chemistry, physics and geology.

References

Huotari, S., Pylkkänen, T., Verbeni, R., Monaco, G., and Hämäläinen, K., 2011. Direct tomography with chemical bond contrast. Nature Mater. 10, 489-493.

Characterization of the local carbon bond variations within a carbon-fibre-reinforced silicon carbide material using IXS-spectroscopic imaging.Characterization of the local carbon bond variations within a carbon-fibre-reinforced silicon carbide material using IXS-spectroscopic imaging. Carbon IXS spectra can be measured inside a cm-sized samples. A 1×2×7 mm3 subvolume of the sample was selected for the measurement. The three-dimensional isosurface rendering shows the determined sp2 bond directional variation.

Cause of irradiation-induced instability in materials surfaces

A new discovery obtained as a collaboration between the University of Helsinki and Harvard University about the dynamic impact of individual energetic particles into a solid surface improves our ability to predict surface stability or instability of materials under irradiation over time.

The impacts, lasting a few trillionths of a second, are simulated using intensive computer calculations. The theory then "up-scales" the cumulative effect of individual energetic particle impacts to predict surface topography evolution over thousands of seconds or longer. The results illustrate how large-scale computer simulations can be combined with rigorous mathematical analysis to yield precise predictions of new phenomena on length and timescales that would otherwise be computationally impossible.

The researchers were surprised to discover that stability/instability is not determined by the atoms that are blasted away, but instead by the atoms that are knocked around and re-settle nearby.  The discovery overturns a long-held paradigm about what causes surfaces to erupt into patterns under energetic particle bombardment. The blasting away of individual atoms from energetic particle impacts has long been thought to determine whether a surface is stable or unstable. The results show, however, that the effect of atoms blasted away turns out to be so small that it is essentially irrelevant. The lion’s share of the responsibility of what makes a surface stable or unstable under irradiation comes from the cumulative effect of the much more numerous atoms that are just knocked to a different place but not blasted away.

The research further shows that the cumulative effect of these displacements can be either ultra-smoothening, which may be useful for the surface treatment of surgical tools, or topographic pattern-forming instabilities, which can degrade materials. The outcome depends on the type of material, energetic particle, and irradiation conditions. 

The discovery, while interesting in its own right, may also help to solve a mysterious degradation problem in tungsten plasma-facing reactor walls in prototype fusion reactors.

References

S. A. Norris, J. Samela, C. S. Madi, M. P. Brenner, L. Bukonte, M. Backman, F. Djurabekova, K. Nordlund, and M. J. Aziz, MD-Predicted Phase diagrams for Pattern Formation, Nature communications 2, 276 (2011).

Molecular dynamic simulations of ion impacts on surfaces combined with a new analytical theory allowed us to show that nanoripple formation on surfaces is due to atom flow and not erosion, as assumed before. Molecular dynamic simulations of ion impacts on surfaces combined with a new analytical theory allowed us to show that nanoripple formation on surfaces is due to atom flow and not erosion, as assumed before.