Neutron Instrumentation
The neutrons which are produced in the spallation process and thermalized in the moderator are guided towards the neutron instruments which are operated by the materials scientists. The neutron instruments are placed in the instrument hall, and each one has its own designated area where the sample can be inserted and where the neutrons are finally recorded in a detector. The beam pulse and wavelength band are manipulated by the choppers via an instrument control computer, but sometimes other types of beam tuning are needed via optical components close to the sample such as a slit or a collimator. We describe some of these optical components as well as the detector below.
Collimator
The only two parameters neutron detectors can detect are the position of the detected neutron and its arrival time at the detector. In scattering experiments where we analyze the change of direction of the neutron caused by the sample it is very important to know the direction the neutrons are coming from when they hit the sample. To be able to do this we need to know in what direction the neutron was traveling before it hit the sample. Here we use what is called a collimator. There are different kinds of collimators, the simplest being a pinhole collimator. It is simply two plates a distance apart with a hole in each of them. Only neutrons traveling in a specific direction will make it through both holes. If a neutron goes through the first hole at a skewed angle it will hit the second plate instead of the hole. Another type of collimator is where many plates of neutron-absorbing material are placed a short distance apart to form a grill. Only neutrons flying sufficiently straight to make it all the way between two plates without touching them will make it. This is very important for the resolution of a neutron experiment since less collimation will give smearing of the peak position at the detector. However, collimation is also very costly in neutron intensity, so it's a trade-off between how sharp a scattering pattern we need, and how much intensity is needed to perform the experiment.
Slit
Most of the time it is preferable that the neutron beam only hits the sample, or sometimes a specific part of the sample, and for this we use slits. Slits are adjustable openings where the sides are made from neutron-absorbing materials. A slit is usually placed just before the sample.
Analyser
Since we cannot directly detect the neutron wavelength, neutron analyzers (or monochromators as they are called when placed before the sample) are sometimes used. They usually crystals that filter out specific neutron wavelengths. When the analyzer crystal is placed at a specific angle with respect to the incoming neutron beam it will diffract neutrons with a corresponding specific wavelength according to Bragg's law.
Neutron detectors
An essential part of a neutron instrument is the detector since it enables us to detect and record in which way the sample affects the neutron beam. Without a detector, we wouldn’t have any data. Creating a neutron detector requires a few tricks since neutrons interact weakly with almost any material. However, there are a few exploits that are routinely used to make detection as efficient as possible. One trick is using the ionizing effect of neutrons in matter: when a neutron goes through a gas (say, for example, 3He) then a molecule of gas intercepts the neutron, absorbs it, and is ionized. This releases an electron, which generates a measurable electric signal.
A neutron detector is made up of several small individual elements, each of which can detect when it is struck by a neutron. These elements, known as detector pixels, are arranged precisely in a grid: this allows the neutron detectors to measure the position of the impact between the neutron and the detector, as well as the time of arrival at the detector. The spatial resolution of the detector is defined by the size of the pixels.
Beam Stop
In neutron experiments, the beam used can be very intense with billions of neutrons hitting the sample every second but since the interaction with the sample is very small, most of the beam will pass straight through the sample and hit the detector. This can damage the detector and therefore a plate of neutron absorbing material is placed right behind the sample to stop the transmitted direct beam so it does not hit the detector. This is of course not used in an imaging experiment since it is the attenuation of the direct beam that is of interest in such an experiment.