Backstage science: Muons
10 May 2011



Dr Philip King explains the mystery of that exotic particle, the muon. He explores how they are produced, what we do with them and why these 'spy particles' are useful!




Muons are exotic particles w​hich exist for millionths of a second

Yet scientists can harness them as a useful tool​

This is how they do it

Phillip King: “I’m Phillip King and I’m the leader of the muon group at ISIS. So we’re standing on top of the proton beam and it’s going under our feet as we speak. The proton beam from the ISIS accelerator goes down and where the pipe work is down there, that’s the neutron target. About 20 metres upstream, that’s where we have our muon target. We take a few percent of the protons that come from the accelerator and the few percent of protons that get taken from our target produce muons. There’s a thin piece of carbon that we stick into the proton beam. Most of the protons go straight through it and onto the neutron target. The collisions that the protons make with the target produce muons and then we use the muons and feed them into experimental areas.

We’re going to go a little bit closer to the muon target. The muon target is just here. What happens is we have our piece of carbon and protons hit that and actually the first thing they do is produce pyons. Pyons are particularly exotic, they’re really weird! They only live for twenty-six thousandths of a millionth of a second, so they’re kind of gone in a real blink of an eye. When they do go in a blink of an eye, they produce muons and that’s the thing that we’re actually interested in. Muons themselves are an exotic particle. They are also short-lived; they only live for two millionths of a second. But actually, two millionths of a second is ages. In those two millionths of a second, we can look at exactly what they do inside a sample. I can take you down and show you a bit more about that.

So what you’ve got there is the actual split of the beam line into three. You’ve got some blue magnets there that are bending the beam around the corner.

So now we’re standing by one of the muon instruments and we’ve got three of them on this side at ISIS. The muons are coming into the middle of this big green magnet because sometimes when we’re doing a muon experiment, we want to put a strong magnetic field onto our sample. What you can see is the big magnet and there’s a thing going in at the side. The sample is on the end of that. The thing on the side allows the sample to be cooled down to low temperatures. At this end, you can see the wires coming out that are attached to the detectors. So we’ve made our muons, they come out of the carbon target. We’ve sent them around the beam line and into some material that we want to know more about. We want to know what’s going on inside it. We want to know what its atoms are doing, how they’re moving. Muons are going in and I like to think of them as being like little spies. Normally, we cannot see what the atoms are doing, but if we send in the muons they get a muon’s eye view of what’s happening. They’re like spies going into a place that we cannot normally get to and telling us what’s going on. Muons go in and they sit amongst the atoms in the middle of a sample and they’ll get a feel for what the atoms are doing. As I said, muons are exotic because they only live for two millionths of a second. Although that sounds like a short time, it’s actually more than enough time for a muon to get a feel for what the atoms are doing. When the muon dies at the end of its life, it fires out another particle. We detect those other particle. The wires here are the detectors. Those particles tell us what the muon was doing, kind of like a post-mortem. They tell us what the muon was doing and hence about the atoms.

This is the HiFi muon instrument. HiFi stands for High Field. We’ve got three other beam lines.

This instrument is called the MuSR, which stands for Muon Spin Research. This was the first muon instrument built at ISIS. It produced muons over twenty years ago now, although we have upgraded it a bit since then.

The noises that can be heard are pulses and are for keeping things cold. This is the Emu instrument which is a Euopean Muon Instrument. This was funded by a large grant from Europe and so we have lots of European researchers that will come and use it. We don’t need a lot of shielding around the muon instruments and so you can see the kit, the magnets and the detectors.

Our sample mounts for muon experiments are not usually hugely exciting. But if you want to look at a liquid, then we’ll put out liquid into a cell. This is about the size of the muon beam. In this particular cell, you can put a liquid in and fire your muons in. This is a thin window; muons don’t go through too much stuff. They’ll only go through about half a millimetre of stuff.

We have a hairdryer because the samples are very cold and so before we can touch them and use them we want to warm them up so that we can handle them.

What actually is a muon? Muons are the heavy versions of an electron. They come in positive and negative varieties, a bit like electrons. Electrons have a positive other particle, an antiparticle, muons are the same. We mainly use positive muons at ISIS. They’re the next particle level up from the electron. They’re exotic because electrons are found in the stuff around us. All the atoms around us have protons and neutrons in the nucleas and electrons going around the nucleas. There are no muons in there. So where do muons come from? When the proton beam hits the nucleas of the carbon atom in the target, other particles are produced in the energy of the collision.

I say muons are exotic and they are, but in fact they’re not so uncommon. We all get hit by about a muon a second. They’re produced by collisions between protons that hit the earth high up in the earth’s atmosphere. These protons come from space. They hit the atoms at the top of the earth’s atmosphere and in exactly the same process by which we produce muons here at ISIS, those collisions will produce pyons that live for a really short time. The pyons decay down to muons and those muons make it all the way down to ground level.  We just make a lot more of them at ISIS. So we can’t use the ones that come from high up, we need them in far bigger quantities than that.

Muons are one way of building up a picture of what’s going on inside things at the small scale. If we have a material that’s got an unusual property, maybe it conducts electricity in a new way, maybe it’s extra strong, that’s great. But if we want to actually produce more of the stuff with that property, we’ve got to know what the atoms are doing and muons are one way of finding out”.