In Our Time - The Neutron

Episode Date: April 14, 2016

Melvyn Bragg and guests discuss the neutron, one of the particles found in an atom's nucleus. Building on the work of Ernest Rutherford, the British physicist James Chadwick won the Nobel Prize for Physics for his discovery of the neutron in 1932. Neutrons play a fundamental role in the universe and their discovery was at the heart of developments in nuclear physics in the first half of the 20th century. With Val Gibson Professor of High Energy Physics at the University of Cambridge and fellow of Trinity CollegeAndrew Harrison Chief Executive Officer of Diamond Light Source and Professor in Chemistry at the University of EdinburghAndFrank Close Professor Emeritus of Physics at the University of Oxford.

Thank you for downloading this episode of In Our Time. For more details about in our time, and for our terms of use, please go to BBC.co.com.uk slash radio 4. I hope you enjoy the program. Hello. In 1932 in a Cambridge laboratory, James Chadwick discovered the neutron, one of the building blocks of the atomic nucleus. It was a crucial stage in the development of nuclear physics.

Scientists quickly saw that neutrons were ideal for firing into an atom's nucleus. That made the nucleus disintegrate, and released huge amounts of energy. The popular term was splitting the atom, and it captured the public imagination. All the more so, when in the following decade, it led to the atomic bomb. In the last 70 years,

further study of the neutron has shown its applications in medicine, industry, energy, and technology. A deeper understanding of the neutrons reveals fascinating details of the origins of life, of all matter, and of the universe. With me to discuss the neutron are, Val Gibson, Professor of High End. Energy Physics at the University of Cambridge and Fellow of Trinity College.

Andrew Harrison, Chief Executive Officer of Diamond Light Source and Professor in Chemistry at the University of Edinburgh. And Frank Klaus, Professor Emeritus of Physics at the University of Oxford. To start with you, Frank Kloes, what is a neutron? Well, the neutron is one of the two basic constituents of the atomic nucleus, along with the proton. The proton is positively charged, and its electrical charge provides the electrical forces that holds atoms together

and ultimately enables chemistry and biology to happen. The neutron is electrically neutral, hence its name. So it doesn't affect chemistry, but in the nucleus it's an essential component. It gives the nucleus its structure, and to have a sound bite, it's the spark that lights the nuclear fire, in that by using neutrons,

you can liberate the energy that's latent within the atomic nucleus and do things with it. Can we just get our heads around this from the very beginning. We're talking about things that you're very, very, very tiny. Can you tell the listeners how tiny so that they, like me, can have a comprehension of it?

Well, if we all take a deep breath, we just breathed in a million, million, million, millions, millions atoms of oxygen. And that gives you an idea of how small the atom is. Now, if you could imagine one of those atoms being expanded to the size of, say, Wembley football stadium,

then the nucleus in the middle is about the size of a P. So that is the nucleus. nucleus made of neutrons and protons. It's incredibly small. We will absorb that as the day goes on. So is there anything more to say about the neutron per se, before we go to how it was apprehended rather than discovered by Rutherford at Cambridge in Wend? Can you develop that?

Well, the neutron and proton, apart from the electrical charge, are pretty well the same. And they each play a role in the atomic nucleus. I think for the moment that probably is it. We're going to see what the neutron does for us later on. So can you tell us how it was discovered and by whom and so on? Well, the idea of the neutron goes back to Ernest Rutherford about a century ago. Around 1911, 12, he'd discovered the idea of the atomic nucleus. He'd identified that in the centre of each atom, there's a lump of positive charge.

But initially, he didn't know how that was coming about. By 1919, I think, he'd discovered what we call the proton, which is the single seed at the heart of the hydrogen atom, the simplest of all of the atoms. And he then had the insight that as we move through the periodic table from hydrogen, helium all the way up to uranium, the amount of positive charge on the nucleus grows. And he had the idea that this is because the number of protons in the nucleus grows.

One gives you hydrogen, two gives you helium, all the way up to 92 protons, gives you uranium. So far so good. The problem is that they were able to measure the relative masses of each of these atomic elements. And helium, which has got two protons, turned out to be about four times as massive as hydrogen. And by the time you get to uranium, which has got 92 protons, is about 240 times the mass of hydrogen. So Rutherford realised there must be something else in there,

which was adding to the mass, but not affecting anything else. And so that was the idea of the neutron. and he had that idea, I think, around 1920, and it was not until 1932, that the discovery actually took place. Let's talk about this apprehension for this, this discovery without evidence for a time. It's so fascinating, isn't it? He was onto it. He called it neutral, and using the word neutral, there's something there.

I don't know what it is, I'm going to call it neutral, and just thought about it. Well, of course, the moment of insight and genius, there's no algorithm that enabled you to do that, because if there were, we'd all be doing it. But indeed, it is true. I think that the major dispute, if that's the right word at the time, was is the neutron a single particle in its own right,

analogous to a proton with the charge taken away? Or is it somehow a mixture of a proton and an electron? The electron with negative charge, the proton with positive charge, somehow fused together. So for a while it was in the laboratories of Cambridge with him and his colleague, junior colleague James Chadwick, who honed in on it and finally discovered,

although it had been there for 13 billion years, finally discovered for us the neutron. What equipment did they have there? So actually James Chadwick was working with Rutherford actually in Manchester when he discovered the atomic nucleus and they got a relationship of being really good experimentalists. Why have I said Cambridge then if it's working in Manchester? In 1920, Rutherford actually came to Cambridge as the Camerdish Professor

and he bought Chadwick with him as a PhD student. So they did a lot of experiments of nuclear disintegration trying to understand the nucleus. And then something happened sort of 1930-ish. There were other experiments going on in the world. So to set the scene, there was a Berlin group who were doing experiment by taking what we call alpha particles,

which are two protons and two neutrons so out of the, it's the helium nucleus and scattering off light elements so from natural radioactive sources and they found a new sort of neutral radiation. Now all that was known about at the time was gamma rays so photons natural light type radiation. Is it a link to x-rays?

Similar to x-rays but a different wavelength, yes. And they coined this neutral radiation gamma rays. In addition, a year later, in Paris, the Irene Giulio Curie and her husband, Frederick, they also saw the same thing, scattering alpha particles of light nucleus, light elements, and discovered the same radiation. However, they also thought it was gamma radiation. So back to Chadwick. So back to Chadwick, Rutherford thought that this gamma radiation could not be the reason, right? He was convinced, I don't believe it,

he was convinced that it was the neutron. So in a two-week period, actually Chadwick set up an experiment where he had a natural radioactive source on a beryllium target, and then he looked at the neutral radiation that was coming out. He set up an ionization chamber to actually, measure the energy loss in the collisions. And then the new technology of the time, actually, which is the electronic revolution, he actually connected his ionisation chamber to a first stage amplifier, which was being

developed in the Cavendish at the time, so he could record what was happening in the ionisation chamber. And he had enough information there to actually determine the mass of this neutral radiation, which just happened to come out approximately the mass of the proton. So there he had it. He had the neutron. He'd done it in two weeks. He'd repeated the experiment many times, with different targets. And he'd written a paper to the Royal Society,

a two-week period, discover the neutron, done, 1932. Terrific, isn't it? It's brilliant. And with a bit of equipment that looks like Heath Robinson, you've got it in your rucks at, but we haven't time to talk about it. But we have time to talk about it. We've got other things, if you might not mind. I don't know. I don't know what we'll do.

Put it on the website, will we? That's right. The producer's not it. We'll put it on the website. So that's all right. Thank you very much. So done, two weeks. We discovered the neutron or the basic...

Well, there we go. Andrew Harrison. So we have the discovery. What impact did it have? Well, Chad was famously interviewed by the New York Times two days after the publication of results in Nature. And one of his statements was, I'm sorry to tell you all readers,

but actually I can see no use whatsoever for this particle. But... Technically English, that is it? Despite that inauspicious start, it started to become used as a reminder. incredibly powerful tool to study further the structure of the atom. Now at the time, there were essentially two ways that you try to probe inside atoms. You either try to break them open, as we heard earlier, by bombarding them with alpha particles. And the problem there is the alpha particle is positive, the nucleus is positive, and light charges repel. So if you want to look at heavier and heavier atoms, which have more and more and more positive, it becomes harder and harder for the alpha particle to get past the repulsive force. Now, one way around that is to build machines which accelerate charged particles to higher our higher energies. So, for example, at the same time in Cambridge, Cockcroft and Walton, under the, I imagine,

the auspices of Rutherford at the time, were developing precisely these machines, these linear accelerators. I should say, by the way, that we've all probably had linear accelerators in our homes, in that what we regard now as the old-fashioned TV with a cathode ray tube accelerates electrons to make an image on the screen. Well, these were just bigger and beefier accelerators, which would accelerate charged particles to very high energies and smash them into atoms and break them apart.

So these atom smashes were also one way to look inside the atom. Now, in Rome at the time, there was a brilliant young Italian physicist called Enrico Fermi. Italian physics at the time was not very well funded, still not very well funded. And they couldn't afford these big atom smashing

accelerators, he didn't have one. But he figured that if you took a high energy neutron that had been produced as a result of one of these nuclear reactions, maybe that could infiltrate the nucleus because it's neutral, it doesn't get repelled by the positive charge, and maybe it could sneak in and disturb and perhaps explore the properties of the nucleus. And that's what he started to do. Now, we've heard about Chadwick's sort of sealing wax and string apparatus, and at that time And that's what physicists did. They had brilliant ideas, but they had to make the kit. It's fascinating, isn't it?

The march of technology with science, which we, it happened from Galileo, onwards, with telescopes and so on, and Royal Society with Hooks, wonderful instruments for me, we can go there. And it's still the same. When they get the technology, they just rush forward, this alliance is wonderful. Absolutely, you know, the brilliant idea, but then sometimes enabled or inspired by the technology. So what Fermi did was he made his own neutron source, he sealed up little amper, of radioactive radon gas and beryllium, which is what you find in Chadwick's device, and he exposed a variety of elements to them.

And he discovered that the neutron could be made to produce new elements, which were themselves often radioactive. One of the things he thought he'd done, we may come back to this later, is he thought he'd made new elements that were heavier than any that existed at the time, by adding neutrons to uranium. I just think we should tip the cap to both Rutherford and Chadwick. Both of them got Nobel Prizes, so. It needs to be said.

Andrew Harrison, tell us how a nuclear chain reaction works if you can. Well, of course you can't, but what am I saying if you can't for? Just keep quiet. Well, I'm not the nuclear chain reaction works. Anyway, so the idea of a chain reaction, this is sort of what Fermi started looking at or was one of the things he was involved in next.

If you have a nuclear process, which is stimulated by a neutron, so a neutron comes in, starts the nuclear reaction, but that nuclear reaction then gives off a neutron, that can go on and catalyze the next nuclear reaction. So in principle you can set up a chain of events where the link in the chain is the neutron, passed from reaction to reaction.

Now, here's the interesting next development and also a slightly scary development, the one that takes both into nuclear energy and perhaps nuclear weapons. If instead of the nuclear reaction releasing just one neutron, it releases more than one neutron, that then has the potential to catalyze two further nuclear reactions. Each of those two further nuclear actions can catalyze four and so forth.

So where your nuclear reaction releases more neutrons than are put in, you can start to set off a chain reaction which increases in speed. Now, every step in this chain also involves the release of a large amount of energy. So what you have is an increasingly fast reaction that releases increasingly large amounts of nuclear energy. And a number of people at that time saw the potential that that could be used as a source of energy, you know, what we now realize can be realized in a nuclear reactor, but also, and this was scary at the time, of course, Europe has became an increasingly frightening place with the rise of the Nazis in Germany. This could be something that, this is a process that could be put to military use in an explosive device.

Thank you. To retrieve something, I meant briefly, if you can, but I dropped briefly because it seemed an embarrassing thing to say. Right, Frank. Frank, Frank, close. So how finely balance are the neutrons and the protons when this is happening? Well, the nucleus itself, there's a sort of paradox there because all those protons, each of them with positive charge, somehow crammed together, and as Andrews said in a different context, like charges repel each other. So how does the nucleus stay together at all?

And it turns out that it's a very strong force that neutrons and protons feel, so long as they're touching each other. If you've got too many protons, the electrical force will still overwhelm that strong force, which is why the periodic table gives out it at uranium. But neutrons, being neutral, don't feel that electrical repulsion, but they can contribute to the overall strong attraction. So adding neutrons to a nucleus adds some of the glue that will hold it together.

That's the good news. The problem is if you add too many neutrons, the nucleus, sort of gets overweight and has to get rid of it. So you've got a sort of Goldilocks situation that at one extreme, too many protons is bad news because the nucleus will just blow itself apart electrically. The other extreme, too many neutrons is bad news because the nucleus is overweight.

There's some ideal place in the middle, like a shape of a letter U. You want to be down at the bottom of the valley somehow. So it's this balance of neutrons and protons each doing their bit that combines to make the stable situation, get too far away, too many neutrons or too many protons, it becomes unstable, and radioactivity happens.

Amazing, these trillions and trillions and trillions of things are happening in this room even, in this studio. I can't get over the sort of intricacy of the tininess of it. And you and I are radioactive as we speak, but let's not go there. Well, you can speak for yourself. Val, Val Gibson. How differently, there's some elements, let's take uranium, the heaviest, the fire in. How differently did they behave according to the number of neutrons they have?

Okay, so Mandelaev was the person who sort of categorised all of the elements in the periodic table. And you find that for most elements, there are other versions of the elements which have more neutrons in them. These are called isotones. For example, hydrogen has an isotone called Deuterium and a third one called Tritium. In fact, Rutherford predicted the existence. of Deuterium when he predicted the existence of the neutron. Other elements, for example, iron have about six isotones and they have one which is mostly stable.

The rest tend to be radioactive. Uranium has about six or seven isotones, of which all of them are unstable, but in natural uranium, the lifetime of the uranium isotones themselves, which are uranium 238 and uranium 235. The uranium 238 has a lifetime, which is the lifetime of the earth, so 4.5 billion years. The interesting one within uranium is the uranium 235, because that has a propensity to capture the neutrons and hence go through the nuclear fission process.

So what does that mean? I kept calling them isotopes, and you call them isotones. you must be right. Am I right? I vote isotopes. Sorry, I meant isotopes. So in terms of uranium, people are alerted to that and we've talked about chain reaction.

Andrews very graphically told us how we can blow ourselves up very easily. So just what can you know what specifically happens when this reaction starts with uranium? So if you have a neutron which is captured by uranium 235,

then it gives it enough energy and it doesn't need a lot actually. It can be a very slow neutron, a thermal neutron, so room temperature neutron. And it initiates the uranium to be so excited that it will then divide into two nuclei. So it would divide into two elements, which about half uranium.

And then we have all of these other neutrons produce, which then produce the chain reaction. Okay, back to Andrew Harrison. And it's here, really, that the confusion in the minds of the public, including myself, which led to all sorts of massive changes of policy and running down a nuclear power station, because of massive confusion between nuclear power for good nuclear power, fusion and fission. So I think a huge question is, after what you said about the chain reaction,

it is possible to control them, how is it possible? why isn't that more widely understood? So Val has already laid out the basic principles of the chain reaction that you use in many types of reactors. You need something that is split by neutrons to produce more neutrons and to propagate the chain reaction. You need a second ingredient, and as she's alluded, the chain reaction is propagated much more effectively when the neutrons are slow. Now, when they come out... What does that mean? Well, when they're produced in the nuclear reaction, they've got loads of energy.

They're taking with them a lot of the energy that's released. in the nuclear reaction. And what you need to do to make them more effectively captured by other isotopes to propagate the chain reaction most effectively is slow them down. So here's an analogy. If you run into a crowded room of people, you'll knock into the first few, but gradually you'll come to, you'll try to move through it at the same speed as the other people milling about. If you fire high-energy neutrons into a substance that contains lots of hydrogen, which is atoms that are very similar mass to the neutrons, the neutrons collide, rattle around, in the sample that contains all this hydrogen,

and they come out on the other end much more slowly. So you put together uranium 235, you put together a material that's got lots of light elements in it, and that slows them down, and that helps propagate the chain reaction. But what you also have to do, and this is crucial, is you need to add something else that allows you to slow the reaction right down if you need to.

So you also add, and there are many ways of doing this, but you essentially need to add something that absorbs neutrons very effectively. So can slow down the nuclear, chain reaction. So there's just on average more than, slightly more than one neutron being passed from reactant centre to the reaction centre. If you have more than one, you have a cascade which leads to an explosion, but provided you have in your reactor a strongly absorbing element,

gadolinium is one element, cadmium is another, and many elements out there. So you might have, for example, rods which can be moved in and out of the reactor to control the rate of absorption of the neutrons and therefore control the rate of propagating through the reactor. reaction. And if that, for example, for whatever reason, because the electricity fails or something goes wrong mechanically, you've also got other fail-saves whereby you can

flood the reactor with solutions that are rich in these absorbers and so forth. So the key thing about the nuclear reaction in a nuclear reactor is there have to be many mechanisms that can slow it to a manageable and then stop it dead if there are any concerns

about keeping it under control. It's extraordinary, isn't it? I mean, the precision of the engineering is fantastic. The fail-safe that you need to incorporate in these devices is arguably the most risk-controlled type of process that there is. Frank, Frank Lewis, why do some elements become radioactive when neutrons are fired at them? Well, earlier I gave the analogy of the letter U, where I said on one side of the letter U, there's too many neutrons, the other side, too many protons, and down in the valley you've got the Goldilocks mixture, which is just wrong, So you take some stable element where you're at the bottom of the valley. If you add neutrons to that, it's like moving up one side of the valley.

And you want to get back to the bottom. So it is taking something that is in its normal state stable and then destabilizing it by adding the neutron, that the nucleus then will rearrange to get back to the bottom of the valley and how does it do that? And that is what we call radioactivity. And there are three main ways that it gets rid of it. that energy, the three first letters of

the Greek alphabet, at alpha, beta, and gamma. Alpha decay is when a bit of the nucleus chips off, two neutrons and two protons, as Fal mentioned earlier. Beta decay is an interesting one where, for example, a proton inside

this cluster turns into a neutron. Now the proton starts off with positive charge, and ends up as a neutron with no charge at all, so where does that charge go? Why does it do that? Why does it do that? Why does water run downhill?

Because nature likes to get to the most stable situation. So if this proton is in a nucleus that is inherently unstable, whereas if that proton were a neutron, we would have a more stable situation, then nature will do that. So it's sort of survival really, isn't? It is. It is. It is.

Never mind what I said. Away you go. Sorry. So the proton turned into the neutron. And the positive charge that a moment ago was in the process, is carried off by a positive analogue of the electron called a positron. This is a particle of antimatter to make it exciting. But this is an example of how this process is used.

I mean, if any of the listeners here have ever had a PET scan, that's positron emission tomography. It's being used in medicine, and what has happened is that you have ingested an isotope that isotron emitter. Where did that isotope come from? somewhere back in an accelerator somebody has irradiated

some material to produce a positron emitting isotope which can then be used. Val, as if our minds hadn't been completely ruined by the smallness of things already trying to work out Frank's deep breath with millions of atoms going in. Neutrons and protons aren't the end of it. They are made of things.

What things are they made of? So go forward 100 years, where we are now. What do we know about the neutron? Well, the neutron is actually made of quarks. And these were discovered actually in the 1970s, in very large linear accelerators in Stanford, in California, where we're doing exactly the same experiments as Rutherford was doing, actually, but just at a higher energy. And with that higher energy meant you could probe deeper inside the nucleus and inside the neutrons and protons. and you could look at the distribution of charge within the neutrons and protons.

And it was discovered that in fact there's fractional charges inside the neutron and proton and these are the quarks. And the neutron is made of two up quarks, sorry, two down quarks and an up quark. The down quarks having minus one-third the electron charge and the up quark having plus two-thirds the electron charge. So if you add up-up the two-thirds, the electron charge.

So if you add up-up, charges of two down quarks and an up quark, you'll get zero charge. This was originally the concept of somebody called Murray Gil, Merrill Mann and Naiman, who were actually doing very similar things to Mandalayev. They were trying to put all the particles that were known about in some order. And it was the quarks, the idea of this quarks that gave them the order of all the particles that were known about at the time. So it was discovered exactly the same as Rutherford discovered the nucleus. I mean, trivial matter,

a trivial observation passing, it's one of the first words that doesn't come out of classical literature. Quark comes out of Finnegan's Wake, doesn't it? It does. It's, quark, quarks for Mr. Mark. Yes, that's right, because they were reading it at the time. Right. How many of these quarks?

Can we count the quarks? Well, actually, it's more complicated that. If you look inside the neutron, there are these three nice quarks, but there's a sea of other quarks and anti-quarks in there as well which all the energy inside the neutron is allowing us to make matter and antimatter all of the time and to understand it in depth is actually a big question even now is to try and understand the distribution of energy and charge within the neutron.

It must have been great in 1932 when they thought that everything was made by electrons, protons and that's the end of it and now we know that protons and neutrons at least are very complicated things. So we've got the quarks there. Right, back to Andrew Harrison. So I've been reading about neutron beams in what three of you have Britain. What value do they have and what are they? Maybe they're questions the other way around. What are they and what value do they have?

So after World War II nuclear reactors became more common and they provided very intense sources of neutrons. of in the nuclear reactor, you have the sea of neutrons which is propagating the reaction, and you can actually tap some of them off. What do you essentially do is you put a pipe into the nuclear reactor and you let some of the neutrons come out of the pipe. You slow them down by letting them pass through

what I mentioned earlier in the nuclear reactor as a substance with light atoms in it. It could be a tank of water. And what you get out at the other end is a very intense beam of neutrons whose energy is similar to the energy of the molecules in the tank of water. And we call these thermal neutrons.

Now, they have another number of particularly useful problems, but probably key, is that when you look at particles at very small length scales, they also can be viewed as having wave-like properties as well. And the wave length associated with a neutron at these sorts of energy is about the same as a spacing between atoms in crystals. So if you direct these beams of neutrons, neutron beams at crystalline materials, the way they're scattered tells you about the atomic structure of materials. Now we already have a very good technique to do that already. So after World War II, Crick and Watson determined the structure of DNA using X-ray photography,

x-ray methods using photographs taken by Rosal and Franklin, because X-rays also have a wavelength similar to the spacing between atoms. So why would we go to all the bother of using neutron beams when we've got a very good technique in the form of x-rays? And the reason is that x-rays and neutrons tell you complementing. things about materials. And there are many differences, but I'll give you one or two key ones. We all know from x-ray radiography

when we look at our photographs in hospital, when we look at things going through the airport scanner, that x-rays are absorbed and scattered much more strongly by heavy elements, gold, so for example, in rings and so forth, than light elements. And that means that when you try to look at

the structure of materials with x-rays, it's actually relatively difficult to look where all the light atoms, like hydrogen are, whereas neutrons are scattered to a similar extent by light and heavy elements. And that means that neutrons provide a really sensitive probe of where the light atoms are in materials. The other thing, or one of the other things that's very powerful about neutrons, is because they're neutral, they're very penetrating. And that means you can look deep inside the structure of material. So, for example, if you wanted to look at the structure of a big lump

of steel that's an important engineering component. You've welded together somehow and you want to look deep inside at how good the weld is. It would be very difficult to do that with x-rays because they be absorbed by the steel, but the neutrons can be fired right through and tell you in exquisite

detail about the structure deep within this engineering component. It's sort of magic really, isn't it? Frank, Frank Close. What does neutron decay tell us about the early history of the universe? That's a starter. Well, neutron decay.

Neutrons inside a nucleus can be stable, but if you've got a neutron on its own, it has a half-life of about 10 minutes. That means if you've got half-life, lots of neutrons, after about 10 minutes, half of them will have decayed away by radioactivity. So in the very early universe, after about a microsecond, the universe consisted mainly of electrons, protons, neutrons and neutrinos. Neutrinos are like a neutral version of the electron,

like the neutron is a neutral version of the proton. And so in this early moment of the universe, you had got electrons and protons bumping into each other, giving up their charges and becoming neutrinos and neutrons. This is the first two or three minutes. Well, this is the first microseconds. Wow. Right.

And neutrons. and neutrinos will be bumping into themselves, turning into electrons and protons. Now we come to after the first microsecond, but not yet the first three minutes. The universe has cooled down. And so whereas before that, these reactions were going left to right,

right to left all the time, now one of them starts giving out because the neutron, we haven't gone into this, is very slightly heavier than a proton. About one part in a thousand. And that's bad news. Every time you're making a neutron,

you're trying to go uphill in a way. And after a microsecond, the universe could no longer do that. So no new neutrons were being created. So those that had already been created were now either dying off with a half-life of 10 minutes or getting captured by bumping into protons and making the deuteron, which Val mentioned earlier, the isotope of heavy hydrogen, a proton and a neutron gripped together.

And that is the first stage of building up the, the light elements, because when deuterons join together, they can make helium. And you can build up the light elements this way. So in the early universe, the neutron was playing an essential role in building up the seeds of the lightest elements. And in stars today, like in the sun, in a sense, that is going on now. Talking about the stars, Val R. Gibson, there are neutron stars, aren't there? There are. Can you tell us about them? these are awesome, mysterious objects in our universe. In fact, there's 100 million of them

in our own Milky Way, approximately. And this is the most densest material that you could imagine. So imagine taking our own sun two times and putting it, sort of stuffing it in a radius of 10 miles, right? The sort of analogy that if you took a teaspoon of neutron star, then you would have a mass of 10 billion tonnes, which is about the same as a very large mountain. So it's the most densest object you can imagine. And it has incredible properties. It has huge magnetic fields.

It has a million, million times our own gravitational field. And it's just a ball of neutrons. And they were discovered actually by, because neutron stars tend to rotate many hundred times per second. And when they rotate, they give off electromagnetic radiation. And the pulses of those are something called pulsars, which were discovered by Jocelyn Belbenel and Tony Hewish at the Cavendish again in 1967. Andrew Harrison, we haven't talked about the magnetic properties of neutrons,

so could you tell us about them and why they're important? So coming back to the neutron beams and the way we can use neutron beams to explore the structure of matter, the fact that... So neutrons, as you stated, have magnetic properties. You can imagine them being a little bit like compass needles. They can point up and down. And that allows us to probe the structure of...

the magnetic character of materials. So the materials around us that we recognize as magnetic, are magnetic, first of all because they have atoms that are themselves magnetic. Again, you can think of them in terms of the little compass needles, north-south. And what's also important is a way in which those atomic magnets are connected together. So if in your substance, they're all pointing in the same direction, all the norths are pointing one way or the south are pointing the other way, the whole lump of material has a north-south polarisation,

which is what you'd see in your real life, your macroscopic compass needle, or your bar magnet. Alternatively, all the little atomic magnet, that's going to be pointing randomly or they can be pointing, one neighbour can be pointing in opposed to its other neighbours, they cancel out. And what you observe is the overall material had no overall magnetic properties. Now that matters because many of the materials that we find functionally useful

in magnetic devices, whether their compasses, whether they're in recording media, in hard drives and so forth, depend on the individual atomic moments adding up together to give an overall magnetic polarization to the lump of material. And you can look at the individual atomic moments and the way they're connected with neutrons. So just as you can use neutrons to tell us about where the nuclei are in atoms, the way in which the neutrons pointing up or the neutron pointing down are scattered by the atomic magnets in a magnetic material tells us about how all the atomic magnets are connected.

And that tells us basically about how the magnets we put into important functional devices, whether it's for magnetic recording, whether it's in next generation electronics, how they work and how we can design better magnets. Can we go back for a moment, Frank, to this neutron being slightly heavier than the proton, and that being the key to the entire carry on? Can you just develop that? It's quite fascinating, isn't it? Had it not been, we would not be here. imagine that, Frank.

I find it hard to imagine that we wouldn't have been here, but yes, you're completely right. When I started at the beginning, I said that the proton with its positive charge, its charge is the seed ultimately of chemistry, whereas the neutron being neutral drives nuclear physics. The proton is one part in a thousand or so lighter than a neutron, which means by Einstein's famous E, it was MC squared,

there's a little bit less energy buried in a proton than a neutron. And nature always likes to end up in the state of lowest energy. So the natural tendency is for neutrons to end up as protons. And the proton, as far as we know, is completely stable, which is a good thing because it's the proton that is the seed of hydrogen, the simplest of the elements. And it's the positive charge of the proton that then seeds the elements that give rise to chemistry and biology and so forth.

If the neutron, however, had been lighter than the proton, then the tendency would have been for the proton to run downhill and it was a neutron. So the lightest thing would have no electrical charge at all. So you would not have a charged seed for atoms. Atoms would not exist, certainly not as exist as we know them, probably not exist at all. So it's the fact that the proton is lighted by this one part in a thousand that enables the seed for atoms and life to exist.

That's the first bizarre thing. The second thing is that the difference in mass of the neutron and proton, although it's very, very tiny, is nonetheless big enough that beta decay can happen, whereby a neutron can turn into a proton emitting an electron. If that didn't happen, you wouldn't again have the processes that seed elements. So we're here as a result of this one in a thousand chance, among other things. It's curious and curiously, doesn't it? The start of things.

I mean, more and more things that we would say were accidents, but there can't be accidents. So what's going on and why is it going on? That's another program, Frank. We have to go. Val Gibson. James Chaddick was interested in the medical applications of neutrons. 80 years on, that's in full flow and in other areas.

We said at the beginning, in industry and all over. Let's stick to medicine for a start. What's happening because of the neutron? in medicine at the moment. Yeah. I think if James Chadwick had not been interrupted by the war, then he would have actually looked at the medical properties of neutrons. And currently there are two areas that are going on

in neutron sort of medical applications. One is neutron radiotherapy, where you just take a beam of neutrons and put them through tissue. And because neutrons deposit a large amount of energy as they go through the tissue, then they can actually. like a very powerful nuclear scalpel.

So there are developments going on to look at using neutrons for malignant tissue, for example, for cancers. That's one area that's going on. So that's called fast neutron therapy. Another area is neutron capture therapy. And this is rather interesting because it's a sort of a two-stage process where if you have a patient, you inject them with an isotope, for example, boron. And then you just radiate them with slow neutrons. And within the body itself, so within the tissue itself,

you have a little fission reaction producing energy and lighter particles, which have a distance which will over the cell, malignant cells. And that's another way of killing malignant cells. So there's research and development going on all over the world. Unfortunately not in this country at the moment, but over the world neutron therapy is being looked at. Andrew Harrison has come to the end of the program, really. What's imminent?

What would you like to know, what you nearly know, some touch of the future about the process of these discoveries? So, I mean, the applications of neutrons to explore matter are actually so diverse that we could, that's a whole new program. Perhaps I could mention something different. And that is that, you know, neutrons occur naturally. VALs told us that they're in astronomical body. is they impinge on the earth

in the form of high-energy cosmic rays all the time. When you take a flight to the states, you are bombarded by neutrons as you go, and so are the electronics of the aircraft. And one of the consequences of that is that occasionally the electronics malfunction because a high-energy neutrons come in and

frazzled it. So, one of the key things that we'd like to know now is how do we make electronic devices that are radiation hard with respect to neutrons. And one interesting line of neutron research is to look at electronics in high-energy beams and try to work out

how we counteract that possible disruption on future technology. Well, thank you very much, Andrew Harrison, Val Gibson and Frank Close. I feel as if I've had a sort of workout. Next week, we'll be talking about the global impact of the eruption of Mount Tambora in 1815,

one of the largest in the last 80,000 years. It changed the global weather. Thank you very much for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. So what did we miss out? I think neutrons in forensic science

and art for... Oh, you want to show your gadget? Oh, you're going to show you like... This is the moment. All you podcast persons are having something that are listening. So we'll unwrapped. Did this do it? Looks like a sort of kids sewing machine. Yeah.

One of the... Kirkor, 1948. One of the privileges of working at the Cowandist's time, is that you get access to the artefacts. And this is Chadwick's neutron source, if you like. Don't have Chadwick DNA on it?

Most likely somewhere. Probably stuck within the beeswaxer. But at this end would be the polonium source that he actually acquired from the... It's sort of two tubes. One is about nine inches long. One is about six inches long. And they have like big pennies, as it were, at the end of them.

And that's about it. It's about as big as my hand, yes. It's made of brass, and at one end would be the polonium source, which he actually got from the Curis in Paris. Somewhere in the middle here would be the beryllium, which would probably be stuck at the end of this insert, which was the thing that went inside. This try and evacuate the tube a little bit,

and pumping out the air. This looks like a mini, mini chimney. It does, yes. To me, this whole thing looks like a, a model of the Stevenson's rocket without the wheels. And out this end where we've got a little aluminium window would become the neutrons.

And it's really held together by ceiling wax. And it's held to the ceiling wax and... Not stream. Not string, no. And that's it. That's what he built to discover the neutron. And this is the real object. Take that to the LHC and say,

well, this is still being recorded? So what else should we have said? You have said? I think neutrons and forensic science fascinates me that by irradiating a sample, you can activate maybe just a handful of atoms and they reveal their presence. You know, you try doing art forgery now. It's not sufficient just to get the chemicals right. You've got to get the isotopic content of those things right. I mean, this is the famous work on Napoleon, isn't it, and whether he had arsenic about his person.

And you can detect trace levels of arsenic by irradiating the tissue. you creating another radioactive element and because you can detect radioactivity so sensitively you can say yes there was arsenic actually I'm not sure what the answer was did he have arsenic in his hair it was either yes or no it was yes he did but it wasn't it was the wallpaper it wasn't thought it was a fatal amount

yes it was the green wallpaper I think but I think you know for just picking up though handfuls literally handfuls of atoms in the sample the whole polonium 210 business the fact that when you ingest polonium 210 there are other isotopes

in there as well. There are other traces of things in there as well. And by neutron activation, you're able to get a barcode, if you like, to tell you what the sample consists of, and then you can go and say, so where did that come from? You're talking about the Russian

assassination. London. Yeah, it's Ludwig. Anyway, here's Simon with a big announcement. It's tea or coffee, but I'm slightly apprehensive that it's going to be radioactive. Well, it is. Everything is. Don't worry about it. There are many more science and discussion programs from Radio 4 to download for free. Find these on the website at BBC.co.com.uk slash Radio 4.