Instrument example: The two axis diffractometer

For the sake of putting this, rather detailed, chapter into its right context, we start with presenting an overview of neutron instrumentation. In particular, we will sketch a very simple neutron instrument, one example of which is shown in Figure xx--CrossReference--fig:heidiinstrument--xx. The neutrons we consider start their travel at the left of the figure: at the reactor. Then they are thermalized in the moderator, described in Moderators. Subsequently, the neutrons travel towards the right. At some instruments (not the one in Figure xx--CrossReference--fig:heidiinstrument--xx, the neutrons are transported by use of a neutron guide, which is described further in Neutron guide systems. At the center of the figure, neutrons with a particular wavelength are scattered by a monochromator, the details of which is described in Determining the incoming neutron wavelength. After the beam is being tailored by use of beam optics in Beam optical components, it finally impinges on the sample under study. The task of the instrument is now to determine how many neutrons are scattered in different scattering angles. This is measured by a movable neutron detector, which is described in Neutron detectors.

Figure 1: Sketch of the hot-neutron two-axis neutron diffractometer HEIDI at the FRM-2 neutron source, Munich. Details are described in the text.

The instrument here shown is denoted a two-axis diffractometer. The two axes here represent the two places where the neutron changes direction (the monochromator and the sample), while the term diffractometer`` represent that the instrument deals with medium-to-high angle elastic scattering, known as diffraction. This was the first type of instrument constructed, and improved versions are still in use for the purpose of determining the atomic structure of crystals.

We now proceed with a much more detailed description of the individual components in neutron instrumentation.