ISIS - How Neutrons Help Understand the World
ISIS is a centre for neutron and muon science based in Oxfordshire and run by the Science and Technology Facilities Council. It's used for a vast range of different research as any scientist who has a good enough application can use the facility for free. To find out more about ISIS itself, I went to visit Martyn Bull who took me on a little tour...
Martyn - Right now, we're standing in the middle of target station 1. We call things target stations here because that's where we make neutrons which we use like beams of light to investigate the structure of materials. Neutrons are pretty common. Each person listening to the radio now is made of half neutrons, but they're locked away inside the nucleus, the centre of the atom. And so, we need to release them to do our experiments.
So what we have is a high power accelerator that fires a bunch of protons which have about the same mass as a neutron and that knocks them out of the metal target which we've got which is tungsten. Then we have a big cloud of neutrons that you can send to experiments to work with.
So the structure that we're standing in front of, it's pretty big and in the sensor is a big bunker. It's as high as a house and probably several houses wide. Connecting to that is a long tunnel made of big blocks of steel and concrete, and down that tunnel whistle the protons as they're on their way to smash into the target. The target is living inside the big bunker in the middle of the hall. We need all the shielding just to make sure that the proton beam doesn't get out. The neutrons are emerging from small holes in the big bunker into instruments which are clustered around them. So, it's a little village of experiments. At each, we've got about 20 of these experimental areas clustered around this targets. Each of these can operate independently of the others, so we're doing 20 different scientific experiments at any one time.
Ben - So, your accelerated protons speeds their way down a beam line, slam into a bit of tungsten that liberates a whole load of neutrons. What sort of quantities are we looking at?
Martyn - So, we've optimised the way we create neutrons. Firstly, the protons are traveling about 84% of the speed of light. That delivers a lot of energy into the target and each proton in the bunch is able to cause around 20 neutrons to be released, so that gives a bit of a multiplication. But we're still quite faint compared to what you might get out of a modern synchrotron x-ray source, so we have to use our neutrons very carefully.
Ben - The protons hit your tungsten target with a great deal of energy and scatter neutrons everywhere. How do you control them to actually corral them and get them to go past your sample rather than just out into these enormous concrete walls we can see?
Martyn - Herding neutrons is a delicate art. They have no electrical charge which means you can't use any of the normal tricks with electric fields or magnetic fields. But instead, we can use their properties to reflect them off mirrors or crystals so you can actually shape and control a beam in it in slightly different ways, the way you might do with a charged particle, but still very, very flexible.
Ben - So is it just as simple a case of putting whatever it is you want to look at in the way of the neutrons and then seeing what happens?
Martyn - It is very straightforward, yeah. You have hole, you lower your experiment down in the hole, so it's inside the beam of neutrons and the neutrons then scatter, you collect them in a big detector. So, it's a bit like having an ordinary camera on your mobile phone, but instead of using light scattering off your friend, you have an experiment and you're scattering neutrons off. So, you can think of neutrons you're going to swap for light and what we have here are very powerful neutron cameras that are able to image the molecular world.
The neutron source is used to look at the structure of materials. We like to say that we can see where atoms are and also what they're doing, so how they're vibrating and rotating, and joined to each other. Unlike a large accelerator like Large Hadron Collider which is interested in the very basic makeup of the universe, we're looking at a whole range of the science that affects everyday life. So we're looking at materials that you might commonly find in products around the home or in cars, or in power plants. Now we're trying to understand how the atomic structure, so where the atoms are located, how they're joined together, how that affects our real world experience of them that allows us to either change them to have different properties or to invent new materials that will solve problems that we're interested in.
Ben - What sort of things can we actually learn about that material?
Martyn - The basic experiments we're doing here is looking primary at the structure of materials. So we're finding out how in all sorts of different materials such as the hydrogen absorbing sponges for powering cars, on hydrogen instead of petrol or looking at the structure of fibres made by spiders. We're looking at the individual atomic structure and trying to understand how that is linked to properties that you experience. So what you get out is a funny picture which you can interpret and determine the actual spacing between layers of atoms which is quite phenomenal. So we have an enormous machine here that's the size of about 33 football fields and that allows you to see something that's about 100,000 times thinner than your hair. So, it's an incredible machine and it allows us to do an enormous range of science from engineering through physics, chemistry out into some more unusual areas such as biophysics and archaeology.
Ben - So is it just imaging that you're doing, seeing where each item is or are you also learning things about their properties, about their magnetic fields, about the way that different atoms interact?
Martyn - We're actually looking at, as well as the structure, you can treat your sample at different temperatures and pressures, magnetic fields, so that will tell you how it reacts to conditions it might experience. But also as well as measuring the structure, we're actually able to measure how atoms are bonded together. So, this gives you the access to all sorts of different information such as how a long polymer chain might be flexing and parts of it rotating, and how that affects the properties. It might also be looking at the magnetism of a material. Take iron as an example, if you take that down to its atomic structure, every atom of iron behaves like it is a magnet. Neutrons are actually also able to sense the magnetism, and that gives you a whole unique insight into how they're working, something you can't really do very well with many other techniques. So when you combine being able to measure the structure with also the motion and the magnetic information that can also come out, then you have a really very, very powerful tool.
Ben - Looking at other imaging techniques, there's usually a compromise. Electron microscopes are fantastic but you need to very carefully control the material you put in in the first place. The Diamond synchrotron, which is just a few meters away, is a very powerful x-ray source, but you often vaporise the things you put in. How do you need to prepare a sample to go into a neutron source?
Martyn - Neutrons, by having no electrical charge are actually a nice, delicate way to visualise things. You mentioned the Diamond Light Source and the x-rays it produces are an extremely powerful and successful way of measuring materials. However, some very delicate samples such as protein crystals can suffer damage from the x-ray beam. That's where you might start to be interested in using a neutron technique. It's when you want to look at a slightly different way of seeing the material. Because the neutron has no electrical charge, it doesn't cause any damage and it's also very penetrating. So you can see right inside things that you wouldn't normally be able to see into. Say, a piece of aircraft turbine with an x-ray beam, you might struggle to see right away through it, but with a neutron beam, that's pretty simple to do.