Neutron detectors

We will here briefly touch upon the way neutrons are detected. The field of neutron detectors is vast and we refer the reader to more specialized literature for details.

Detection processes

Neutrons are typically detected by use of one of a handful nuclear reactions, which destroy the neutron as a result. Most used and most efficient is neutron capture of helium-3:

$^{3}{\rm {He}}+n\rightarrow \,^{3}{\rm {H}}+{^{1}{\rm {H}}}+Q,$

where the released energy, \(Q\), here is as low as 0.764 MeV . However, also capture of \(^6\)Li and \(^{10}\)B is often being used.

The charged products from these nuclear reactions give rise to an electrical signal, which is subsequently amplified by charge amplification in an Ar gas under high voltage, as in a standard Geiger-Müller counter. The signal can then easily be detected.

Detectors may have just a single channel, or can be position sensitive in one or two dimensions. For particular applications, there exist area detectors with pixel sizes of around \(1 \times 1\) mm\(^2\) of sizes up to \(1 \times 1\) m\(^2\). Alternatively, one may use detector tubes with a diameter of 25 mm (one inch), being several meters long and linearly sensitive with a positioning accuracy of the order 5 mm. Helium-3 detectors of this type can detect up to \(10^5\) neutrons/second before saturating, depending on the speed of the amplifier electronics.

At pulsed neutron sources, the detector electronics can in addition record the detection time of the neutron with a precision of a few \(\mu\)s. This is crucial in order to utilize the time-of-flight information, as will be described later.

Monitors

For controlling the possibly varying intensities of the beam, monitors are used at all neutron instruments for normalization purposes. A monitor is a deliberately inefficient detector that interacts with only a small fraction of the neutron beam (of the order \(10^{-3}\) to \(10^{-4}\)). The counting efficiency is determined by the neutron absorption cross section and is hence proportional to \(\lambda\). Monitors are typically placed at the end of a guide, just before or after the sample.

Background and background levels

Background is the general notion for all neutron detector counts that do not arise from the physical process under investigation. One source of background comes from the sample itself, e.g.~incoherent scattering, and is difficult to discriminate against.

Another background source is the experimental environment, e.g.~neutrons from other experiments or fast neutrons from the source that penetrate a series of shielding barriers to be counted in the detector. This background can always be improved by additional shielding, by moving the instrument further away from the source, and by eliminating line-of-sight between moderator and sample. Fast-neutron background is of particular worry in spallation sources, due to the high energies in the spallation process itself.

At pulsed sources, the fast-neutron background can in some cases dominate even the strong elastical scattering from the sample. However, time-of-flight can be very efficiently used to discriminate these fast neutrons, since they will arrive almost immediately after the accelerator pulse has hit the target.

Example

At the end of a 30~m guide for the instrument RITA-II /CAMEA at the medium-flux source SINQ (PSI), the background count rates for a detector the size of a typical \(^3\)He detector tube, 150~mm high, 25~mm diameter, is 0.10~counts/minute with both the primary and secondary beam shutters closed . This background is mostly due to electronic noise. During an experiment on the same instrument (both shutters open), the level of background not originating from the sample is around 0.2~counts/minute in the best cases.