Experiment: Single-Crystal Diffractometer
A single-crystal diffractometer is an example of a neutron scattering experiment. As the name already suggests the experiment is studying the structure of such crystals (single-crystals consist of completely ordered atomic layers, which characterize the chosen material). The scheme of the experiment is depicted below. The waves of the incoming neutron beam (matter wave, blue) are scattered by the atoms of the crystal lattice (green). In this representation, we show the neutrons not as particles but as waves, which become refracted and reflected (see Dualism). By detecting the scattered neutrons, we see interference patterns (diffraction) of the reflected waves (red, orange), which are dependent on the distance of the layers in the crystal lattice. At the position of detector 1, the incoming red and orange waves become superposed in such a way that the two wave maxima merge and enhance each other (constructive interference). The detector 1 will register an enhanced number of neutrons. In contrast, the detector 2 will register no neutrons. At the position of detector 2 the red and orange waves become superposed in a way that the wave maximum of the one meets the minimum of the other and the two waves extinguish each other (destructive interference).
If there are some irregularities in the crystal structure, like missing atoms (shown on the left) or an atom of a different type on a regular lattice place (depicted red on the right) in an otherwise regular crystal, than new structures appear in the neutron interference pattern.
To describe the scattering on a crystal we used the conception of waves, but detecting the signal in a detector (see Detecting Neutrons), we must consider that neutrons are particles, because they are detected individually by a nuclear reaction. This is not a contradiction, because in a world of small particles, our classical concepts are not valid any longer. The concept of waves and the concept of particles are just two different descriptions of the same very small quantal objects. This is one of the fundamental statements of quantum.
Texts: Th. Wilpert , Ch. Schulz
