Neutron Detectors

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Neutron Detectors

The most common neutron detectors are of the proportional gas type. Since neutrons themselves have no charge and are non-ionizing, they are not so easy to detect as X-rays. The method of detection relies on the absorption of the neutron by an atom with the simultaneous emission of a γ-ray photon, often referred to as an (n,γ) reaction. Since the absorbing material must absorb neutrons and be capable of existing in gaseous form, then the choice of materials is very limited. The most common is ³He gas, which relies on the reaction:

This produces the stable isotope of helium together with a γ-ray of a specific energy. Another suitable gas is BF₃, which uses the absorption properties of the isotope ¹⁰B. For thermal wavelength neutrons the ³He gas detector is considered best, but for long wavelength neutrons BF₃ is actually better, but is rarely used now due to the technical difficulties of handling a corrosive and toxic gas. Because the capture cross-section of gases is very small, ³He detectors are usually filled above atmospheric pressure, e.g. 5 to 10 bar. In size, they are typically 10 to 15 cm long and 2 to 5 cm in diameter.

Recent developments by the detector group at the ILL, Grenoble, have led to the production of single-tube He-gas detectors with positional sensitivity. This is achieved by measuring the charge developed at both ends of a resistive anode wire, a larger magnitude of charge detected being related to a shorter distance travelled by the pulse along the wire. These will have huge potential for high-resolution angle-dispersive powder diffractometers (such as D2B described later) since they will allow a greater solid angle to be measured without comprising instrumental resolution, the asymmetry due to the curved Debye-Scherrer diffractions being removed via software binning of the 2-dimensional data to the conventional 1-dimensional powder pattern.

Neutron multidetectors are also ³He gas based, though in addition they are also filled with a quenching gas such as a mixture of xenon (Xe) and methane (CH₄). The original multidetectors consisted of an array of vertical wires at fixed intervals apart, usually chosen so that the interval corresponded to a nice angle in degrees, e.g. 0.1°. They are very large, can be both one- and two-dimensional, and can cover a huge area of solid angle. A recent development to improve the spacing interval has been the development of microstrips which replace the conventional anode wires in these detectors.

Scintillation detectors have been developed using, for example, ⁶Li or Gd as the absorbing atoms. The latter may be used in the mixed metal oxide-sulphide Gd₂O₃.Gd₂S₃, which is able to convert the γ-ray photon directly into a UV/visible photon. The latter is counted by using a conventional photomultiplier tube. With scintillation detectors, the neutron is detected in a smaller region of physical space compared to a gas detector due to the higher density of absorbing material. This has advantages for some applications (see later page on time-of-flight methods). A potential disadvantage is that they may detect sample fluorescence due to (n,γ) reactions within the sample itself.

Thus one of the problems in detecting neutrons is the potential background from rogue γ-rays produced by neutrons from either the reactor or the pulsed source, plus those produced by reaction with a monochromator or even the sample. Therefore, the electronics controlling the detector must be capable of discriminating the energy (or emission time at pulsed sources) of the γ-ray so that only those produced in the detector are counted. In addition, the detector must be highly shielded from the general "cloud" of neutrons that exist around the diffractometer. The shielding is very thick: many centimetres of polythene are used to slow down any fast neutrons; the background neutrons then being absorbed by B₄C-doped plastic or rubber.

Monitors

In a laboratory, the source is assumed to produce a constant flux of X-ray photons that does not vary with time. As discussed earlier, this is certainly not the case for synchrotron X-rays since the beam decays with time. In principle, the flux of neutrons from a reactor is constant with time, though in some cases it may not be, e.g. if the reactor has to run for a period at reduced power due to pump failure. Also at a pulsed source the intensity of the neutron beam is not constant in a short time interval, although averaged over a long time period, it is effectively constant.

Powder neutron diffractometers need to monitor the incident intensity of the radiation illuminating the sample. This is done using a monitor, which is basically a detector that works at very low efficiency. Monitors transmit most of the neutrons passing through them while absorbing a very small percentage, e.g. 0.1%. Neutron monitors are often fabricated like ³He detectors, but with very thin aluminium windows, since the internal gas is at either at low pressure or is diluted to atmospheric with ⁴He.

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