Small-angle scattering instruments

A small-angle scattering instrument at a continuous source

Figure 1: The principles of a SANS instrument. From the moderator (left), the beam is monochromatized by a velocity selector. Then a pair of pinhole slits limits the beam divergence, while a position sensitive detector (red) inside a vacuum tank detects all neutrons scattered at small angles from the sample (green). A beamstop (black) prevents the direct beam from hitting the dtector.

The principle of an ordinary SANS instrument is rather simple. We will here present a SANS instrument for a continuous source, although a generalization to a time-of-flight instrument is straightforward. A sketch of a SANS instrument is shown in Figure xx--CrossReference--fig:sans_instr--xx.

• Source: A SANS instrument uses neutrons from a cold moderator and is situated at the end of a guide.

• Velocity Selector: The neutrons are being monochromatized by a rotating velocity selector, letting through a wavelength band of typically \(\Delta \lambda/\lambda \approx\) 10 %.

• Collimator: The divergence of the incident neutrons is limited by a pair of pinhole, that acts as a collimator. Often, one can control the pinhole diameter and their distance, the collimation length, \(L_{\rm c}\), by inserting different pinholes at a number of fixed positions. The smallest collimation length is typically 1 m, while the longest varies between 5 m and 20 m, depending on the particular instrument.

• Sample: The sample is often flat and mounted perpendicular to the beam direction, so that all neutrons scattered at small angles will penetrate the full sample thickness. Hence, samples are often thin to limit absorption and multiple scattering.

• Detector: The neutrons are detected by a position-sensitive detector (PSD), which can determine the position of an incident neutron, placed at a distance \(L_{\rm d}\) from the sample. A typical PSD is \(1 \times 1\) m\(^2\) with a precision in positioning of 1-5 mm. The PSD is placed within an evacuated tank to avoid air scattering (which is mainly due to nitrogen), and the sample-detector distance can be varied by moving the PSD within the tank. The minimum PSD distance is around 1 m, while the maximum distance varies in the range 5-20 m determined by the length of the tank. Typically, one would match the sample-detector distance to the collimation length.

• Beam stop: An absorbing beam-stop is placed in the direct beam just before the detector to limit the number of neutrons from the direct beam. This strong beam could otherwise saturate, or even damage, the detector. For correct placing of the beamstop, it is necessary to take into account how gravity affects neutrons of different wavelengths; see the problem The effect of gravity in the Simulation project: A small angle neutron scattering instrument.

A small-angle scattering instrument at a pulsed source

At a pulsed source, a small-angle scattering instrument is very much the same for a continuous source. The only difference is that there is no velocity selector. In stead, the neutron wavelengths are determined by their respective time of flight, as discussed in Determining the incoming neutron wavelength.