Scattering Experiments with Neutrons
For a better understanding of material properties, we study the arrangement and motion of the constituting atoms inside the solids. Besides the well known X-rays and electron beams, neutron beams as matter waves offer significant advantages: as a neutral probe neutrons are able to penetrate deeply into matter. When neutrons hit atoms they bounce of, the resulting scattering pattern of neutrons is characteristic for the positions of atoms in the material. Neutron beams offer three decisive advantages compared to X-rays or electron beams:
(i) | Neutrons can pass through massive material layers of several centimeters thickness, |
(ii) | They track down light elements (like hydrogen) in heavy metals, |
(iii) | They react differently with different isotopes of elements (these are the atomic nuclei of the same element but with different numbers of neutrons) and allow therefore the method of contrast variation. |
The pioneering neutron scattering experiments performed in the early 1950s by the Nobel Prize winners of 1994 in physics help to answer the questions "where the atoms are" (C.G. Shull) "what they do" (B.N. Brockhouse). Though nowadays (2002) the experiments are much more sophisticated, they are still based on the same principles that were introduced by Shull und Brockhouse. In a typical scattering experiment, neutrons originating from a reactor core are led in a beam tube onto a "monochromator". For a fixed deflection angle the monochromator (a large single-crystal) reflects only one wavelength of neutrons onto the sample. The outgoing neutrons have a specified velocity, which corresponds to a known energy or wavelength. These neutrons are used to analyze a sample. In analogy to optics, the neutrons having only one wavelength are called monochromatic.
The direction and the intensity distribution of the scattered neutrons are measured behind the sample with neutron detectors. The measured intensity distributions give information for example on the arrangement of the atoms in the sample. A scheme of a typical scattering experiment is shown in the figure above.
A more detailed description of the experiments at the BER II research reactor may be found at BER II.
Texts: Th. Wilpert , Ch. Schulz
