Backstage Science: Neutrons and Nanoscience
14 Mar 2012



Sean Langridge and Sarah Rogers explore instruments and beamlines at ISIS.





​Sean Langridge: “Hello, I’m Sean Langridge. I work in the Large Scales Structures group here at ISIS, which is a source of neutrons. If you look around you, at this most amazing building, we are in the second target station. The neutrons are produced in the big cylindrical structure in the middle and then they move around to all the instruments that you see here.

Neutrons are incredibly useful particles. They are used in a wide range of problems, which is useful to academic researchers, industrialists and so on. What I would like to do today is take you into three of the instruments that you see here: the orange one on the end, the purple one here and the large green instruments. Maybe the first thing that you can learn from that is that you should not allow scientists to choose the colour of their instruments!

These instruments that you’re looking at now are actually instruments that are optimised for studying materials that are on the nanometre length scale.

We’re now down on the experimental floor and we are going to have a look at the first instrument, which is the first one on the end, imaginatively called Offspec, which stands for ‘Offspecular’. This is a nice instrument because it is an example of collaboration between us here at ISIS and our European partners from the technical university in Delft in the Netherlands. The first thing that we need to get out heads around is what we mean by a nanometre because it is kind of a difficult concept. Everyone has their favourite definition of what a nanometre is. A typical one is: the thickness of a sheet of paper is about ten thousand nanometres. But I like to think of it in terms of things like, for example, the diameter of the DNA double helix is about two nanometres. State of the art transistors in a chip are sort of thirty nanometres and so on. I think one of my favourites is that your fingernails are supposed to grow at about a nanometre a second, so that’s a feel for how small nanometres are.

What we’re looking at here is an instrument which is called a reflectrometre. That’s easy; we’re all familiar with reflection. The classic example is if you see oil or fuel on top of a liquid surface and you see all those beautiful colours, that’s basically reflection of light interfering, which gives all those colours and from that you can tell how thick the layers are. That’s with light, so the structures are quite big, around hundreds of nanometres, if not thousands. With neutrons, because of the actual properties of neutrons, we can actually shrink that length scale down to the nanometre. The kinds of structures that we study are actually structured to the one, to ten to a hundred nanometre length scale.

The way this instrument works is that the neutrons from the big blue cylinder, we cannot see it now, are produced over there. They come flying down the beamline and the first thing we do is to polarise them. One of the properties of the neutron is that it behaves a little bit like a bar magnet. So the first thing we do is take this disorganised beam of neutrons and we put them all into a single spin state. They then come into these regions of magnetic field and just in the same way as light goes into an optical prism, is splits, we can do that with neutrons but with a magnetic field. So they split them out, they travel along the beamline and then they interact with the sample. Here’s a typical sample, in this case it’s a mirror. What neutrons allow us to do is to study the thickness of those layers, what material they are made of and the nature of the interfaces and that’s really important. We can then recombine those neutrons, so some of them spin like this and some of them spin like this. We can recombine them and analyse them so we can work out what the structure is within our film. That’s been used in a whole range of systems, trying to understand how biological systems, membrane, organise themselves in raft, how polymeric materials organise themselves into little structures. That’s only possible on nanometre length scales with an instrument like the one we have here. This is the instrument that looks at layered structures which has some structure in the plane. The next instrument that we are going to look at is one that’s particular optimised at studying magnetic materials. So we are going to use some of the similar technology but on a slightly different instrument.  Let’s have a look at that, and then I shall try to explain to you what it does.

This is Polref, it is the polarised beam instrument. At the moment, you can see that the beam is on. It would not be good for us to be inside when the beam is on, so the first thing we need to do is to close the shutter and get access to the blockhouse. It’s quite straight forward, we have a control box here and we can set it off to ‘close’. So now, it is telling me that the shutter is closed. There’s no radiation in the hutch, I can release the key, which then allows me to open the door and we can go inside.

So this is Polref, the instrument that has been set up to look specifically at magnetic and superconductive materials. Magnetism actually underpins our lives in many ways, if we think about how we store information on out computer hard disks, it’s all stored, by large, at the moment, by magnetism. We need to have tools that allow us to study materials that are magnetic but on the nanoscale. The neutrons are produced over in the blue target station. They come down, they are polarised in the same way as we do on Offspec. Neutrons come in, reflect off the surface and we have a series of slits here which allow us to define the beam so we have a nice clean reflection. What the system does here is allow us to analyse the spin of the neutrons. The neutron has a dipole moment, a magnetic moment, and it can point either up or down. What this system does is, using a series of mirrors; it allows us to tell whether the neutron is pointing up or down. We then count the neutrons. You will recognise this from the Offspec instrument. By combining all of this information, we can work out what the magnetic structure is in that layer. So you can see that we do not have to cut the sample, polish it or do anything clever, we just use the neutron beam, which allows us to get out the layered structure, but also determine how magnetic the materials are and which direction the magnetism is pointing in. That’s exactly what you have in a hard-disk, where you store information, but also in the read head, which is the little device that actually read the information off your hard disk, so in all magnetic systems that we really want to understand. We can take samples where you take properties that are often antagonistic, properties like magnetism and superconductivity and we can test how they work.

We close the door and release the key. I can put the key in the press here. This is Sans2D and it’s a user technique known as the Small Angle Scattering of light, which is really straight forward. If you have ever looked at a car’s headlight on a foggy night and you see rings of scattering, that’s small angle scattering of light. But light has a much longer wavelength than neutrons so those particles are much bigger- you are looking at the water droplets in the vapour. We have neutrons so we can look at objects that are nanometre in length scale. What we are expected to see if we put a detector in the beam are patterns that look like this.

You may recognise the safety features that we have seen on the previous instruments. Neutrons arrive from the left and interact with the sample. Behind us is this huge vacuum tank, which at the moment is closed off with a safety shutter just to protect the window. When we take a measurement, all of this will be locked up, the shutter will open and we allow neutrons into our detectors. They’ll come along here, go through the sample, the samples are hidden inside here, scatter out and will be detected by two detectors in the tank.

So what I will do now is see if anybody can open the tank up for us. Actually most of the shielding is not to stop neutrons getting out; it is to stop naturally-occurring background getting in. Not only do we have all of the systems to protect us from the neutrons, we have a vacuum system in there so we can make sure that we cannot inadvertently open the door and pump it out while there’s somebody inside and so on. So we have a whole load of systems. Sarah is now essentially going through the process that we have established to allow us to open the tank”.

Sarah Rogers: “This tank is usually in the vacuum so all the air is removed from it. It takes about 20 minutes to do that. The neutrons come from the sapphire window at the end there, they travel this way. We have one detector trolley at the front, two baffles, which stop all the neutrons that are knocking around in the tank scattering randomly around. Here, is our main detector, at the rear. This is a metre by metre square detector. It allows us to look at samples from a couple of nanometres out to hundreds of nanometres. You can see those cavium circles in the middle, they are our beam stops. What happens is a lot of the neutrons, rather than being scattered, come straight at the sample. In fact, if that beam was to hit the detectors straight on, the detectors would be destroyed, so they’re sort of a form of protection. They’re made of cavium. Although sapphire, from the window, is really thick, sapphire is transparent to neutrons but cavium absorbs very strongly, so you only need a small amount of cavium to stop there being damage to the detectors.

It is about 12 metres back, so the minimum distance is about a couple of metres all the way up to 12 metres. So if you want to look at small stuff, you push the detectors forward and if you want to look at big stuff you push the detectors far away.

Sean Langridge:  “It’s pretty impressive isn’t it? It’s one of the things that always amazes me is that to study these materials on these ever- shrinking land scales , so we have talked about microns, we have shrunk that down to nanometres and now we are talking about nanometre-sized particles, which you will find in a wide range of applications. To get down to those small land scales, you need to make big machines like Sans2D that you can see here”.