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Stimuli responsive polymersomes: Small Angle Neutron Scattering studies
May 27 14:30 - 15:30
- Diblock copolymers can self-assemble in solution in stable and robust polymersomes (vesicles),...
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To deuterate or not to deuterate? That is the question
June 03 14:30 - 15:30
- Accepted wisdom when performing diffraction experiments with neutrons is to deuterate the...
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Static and dynamic properties of a strong-leg spin-ladder
June 17 14:30 - 15:30
- The AF S = 1/2 Heisenberg spin ladder belongs to the simplest quantum magnets, yet disclosing the...
Why neutrons?
Neutrons are light. They can be seen as particles and waves.
Several other properties of the neutrons are very suitable for research in the origin and structrure of matter:
- Neutrons tell, where the atoms are located in an agent,
- how they move
- and what defines their inner magnetism.
- They deeply penetrate into substances and give information about the atomic structure, for example in the inner of big constructional elements.
- Scheme of magnetic lattice in manganese silicon. Only with neutrons, the structure of the little compass needles of the material was visible.
Magnetic moment
Using the magnetic moment of neutrons allows for analysing magnetic properties of materials at atomic scale. Controlling the assembly of magnetic moments in a material may involve developing new methods for using magnetism to process and store information.
Though being neutral, neutrons bear a magnetic moment: They behave like little compass needles who get diverted by an external magnetic field. Hitting a sample, the direction of the compass needle within the neutrons changes, if the atoms within the samples bear a magnetic moment. The change between the position of the compass needles with in the neutrons before and after the sample is a measure of the quantity of magnetism of the sample.
An example for research with neutrons at the instrument MIRA in magnetism:
Discovery of a new magnetic order: Skyrmion Lattice in a Chiral Magnet
(Skyrmion Lattice in a Chiral Magnet; S. Mühlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch,
A. Neubauer, R. Georgii, P. Böni
Science, February 13, 2009, Vol 323, Issue 5916 – DOI-Nr.: 10.1126/science.1166767)
- The Structure Powder Diffractometer SPODI defines the assembly of atoms in different materials. (Foto: W. Schürmann / FRM II, TU Muenchen)
Appropriate wave length
The wave lenght of free neutrons is comparable to the distance of neighboring atoms in solid matter. Like water waves at a barrier, neutrons are diffracted at the regularily ordered atoms of a sample. The diffraction angle is a sensitive measure for the distance of the atoms. Thus, neutrons tell us, where the atoms are located.
Example for the definition of the order of atoms:
At the structure powder diffractometer SPODI scientists of the TU Darmstadt and the research centre in Karlsruhe examine hydrogen storage materials. These are hydrides (e.g. NaAlH4) with low net weight, that are able to ligate large amounts of hydrogen. They are potential future hydrogen storage materials for vehicles. The ligation of hydrogen, however, has to be reversable. Therefore, the scientists study, how the structure of the hydride changes with and without the load of hydrogen.
- The scientists found significant mechanical stresses within the compressor impeller, which was analysed at the instrument STRESS-SPEC at the FRM II.
High penetration depth
Neutrons penetrate several centimetres into materials. For example, ten centimetres of aluminum can be passed by almost two thirds of a beam of slow neutrons, ten centimetres of lead even four fifth. On the contrary, X-rays only pentrate few micrometres or millimetres into metals. Thus, neutrons are able to measure the distance of atoms deep inside construction materials and show the internal stresses, as at the instrument STRESS-SPEC.
Example for the high penetration depth:
Physicists at the TU München prove existence of internal stresses in compressors
- The flow model developed in theory by E. Falck et al and affiirmed by the experiments of S. Busch at TOFTOF. (Click on the picture opens a you tube film about the flow model.)
How atoms move
Apart from the atomic order in a material, also the movement of atoms or particles is a field of interest. Colliding with moving atoms or molecules, neutrons not only change their direction, but also their velocity. The Time of Flight spectrometer TOFTOF measures the neutron intensity and the change of their velocity.
Example for an experiment analysing the movement of molecules:
Flowing together, instead of jumping alone
Molecules in a cell membrane were analysed and the findings were revolutionary: Contradicting old theories, the molecules move in a flowing motion instead of jumping to empty spaces.
