How to catch a neutron
14 Mar 2012







Nigel Rhodes is head of the Detector Group at the ISIS Neutron and Muon Source and is responsible for all of the neutron and muon detector systems across the instrument suite. He describes how scintillator detectors for the Polaris instrument are used to convert neutrons into flashes of light and then into an electrical signal. From these signals the data picture from the experiment is built up revealing the atomic structure of the material being examined.

Nigel Rhodes: “My name is Nigel Rhodes and I am the ISIS Detector Group Leader. My group looks after all of the detectors for the instruments here at ISIS. This is a prototype detector for the polaris instrument. It is a scintillation detector and the scintillator that we use is a zinc sulphide scintillator. The zinc sulphide is mixed with lithium fluoride. The lithium fluoride is enriched with isotope 6 of the lithium. There are very few elements that are actually sensitive to neutrons. There are only three that generate enough charge when they interact to be useful to us: Lithium- 6, Boron -10 and Helium -3. In this case, the neutron comes in, it interacts with the Lithium-6 particle, its nuclear reaction- an N- alpha reaction- an alpha particle and a triton particle are generated from that reaction. They go through the zinc sulphide lattice causing ionisation. When the electrons decay, they emit light and in that way, we wield up a light flash from each neutron that has interacted with the particle.

The light is then conveyed from the scintillator to the photomultiplier tubes, which change that light into electrical charge via fibre optics. There are quite a lot of fibre optics used in these detectors. In Polaris, for example, there are about 450 km of fibre. The fibre optics very efficiently collect light to the photomultiplier tubes and the photomultiplier tubes itself are changing light into charge. What we have on the surface of the photomultiplier tube is a photocathode, that’s a bi-alkali cathode. When the photon comes in, it generates a photo-electron. The electron moves to the first dianode under an electric field. When it hits that dianode, you get four more electrons generated and so on, under a cascade of ten different dianodes. So, in fact, the amplification is four to ten, which works out to about one million gain in this little device and it does it with very little noise, so it is a very sophisticated device and still very hard to beat. Solid state devices cannot match that gain in terms of signal to noise.

This is not just a flat sheet of scintillator under the detector, what you have are strips of scintillator. They are not flat, they are arranged at angles of 70 degrees. You will notice that the scintillator is opaque, you can hardly see through it, which is really bad news for a scintillator, so you have to come up with another way of getting the light out of the scintillator. In fact, you could have the fibres arranged like this and collect the light directly from one face of the scintillator but then you would limit the efficiency of the scintillator. As you angle the scintillator, the neutron path link through the scintillator continues to increase, but the light output of the scintillator is just dependant on the thickness of the scintillator. So in this way, you increase your neutron sensitivity without decreasing your ability to collect the light from it. In these screens, we have angled the scintillator so that we have three times the neutron path link through the scintillator compared to the photon path link and that gives us enhanced neutron efficiency in the neutron detector. Each strip of scintillator is viewed by a row of fibre optics. The fibre optics in that particular row are coded so that half of the fibres go to photomultiplier tube one and the other half go to photomultiplier tube two. In the next strip, half go to photmultiplier one again and the next half go to photomultiplier three and so on. We build up a code: one-two, one-three, one-four, thereby, if you know which pair of photomultiplier tubes went off, you can know which strip on the scintillator saw the event. It is that position information compared with the timing which tells you about the structure and dynamics of the materials" .