Episode 35 - The nucleus

Tomlin Hawthorne

I have a jar of radioactive atoms. They're of a kind where, in any one hour, each atom has an even chance of falling apart. Fifty-fifty. So after an hour, about half of them are gone.

Tomlin Hawthorne

Now pick out one particular atom. It's a survivor. It's been in the jar for ten hours. It's come through ten coin-tosses in a row, a one-in-a-thousand run of luck.

Tomlin Hawthorne

What's its chance of falling apart in the next hour?

Tomlin Hawthorne

Fifty-fifty. Exactly what it was at the start.

Tomlin Hawthorne

The atom doesn't know it's been lucky. It isn't getting tired. It isn't overdue. An atom that has lasted a billion years is no closer to going than one made this morning. Atoms don't age. They have no history. Each one simply has a fixed chance, every moment, of ceasing to exist.

Tomlin Hawthorne

And from that total lawlessness, one atom at a time, comes the most reliable clock in nature. It's how we know the age of a cave painting, and of the Earth. Today we go inside the nucleus.

Tomlin Hawthorne

Welcome to Part Seven. Last time, I hope, you held a pair of polarised sunglasses in front of your phone and turned them until the screen went black. And if you could find a third filter and slip it in between at an angle, you saw the light come back. More obstacles, more light. A small piece of real quantum mechanics, on a sofa.

Tomlin Hawthorne

In Part Six we lived among the electrons, out in the body of the cathedral. Now we go to the fly. For five episodes we're going down, as far as anyone has been, to find out what everything is made of. And the first question is the obvious one. What holds the centre of the atom together? And what happens when it comes apart?

Tomlin Hawthorne

In the Bibliothèque nationale in Paris there's a collection of laboratory notebooks from the turn of the last century. If you want to consult them, you can. But you have to put on protective clothing, and you have to sign a form accepting the risk. They're kept in boxes lined with lead.

Tomlin Hawthorne

They belonged to Marie Curie. So did her cookbooks, which are in the same state. They'll be radioactive for another fifteen hundred years.

Tomlin Hawthorne

The story begins by accident, as so many of these do. In February 1896 a Paris physicist called Henri Becquerel was investigating whether certain minerals, after being left in strong sunlight, would give off the newly discovered X-rays. His method was to wrap a photographic plate in thick black paper, lay a crust of a uranium salt on top, and put it out in the sun. And the plate did fog, under the paper. He was pleased.

Tomlin Hawthorne

Then Paris clouded over for several days. Becquerel put his next plate, with the uranium on it, away in a drawer to wait for better weather. When the sun still hadn't come out, he developed the plate anyway. Nobody knows quite why. He expected, at most, a faint smudge.

Tomlin Hawthorne

The image was stronger than any he'd got in sunlight.

Tomlin Hawthorne

So the sun had nothing to do with it. The uranium was giving off something all by itself, in the dark, in a drawer, without being fed anything at all. And it went on doing it, week after week, with no sign of running down.

Tomlin Hawthorne

This was deeply strange. Remember episode six. Energy is never created. So where was it coming from?

Tomlin Hawthorne

The person who took the question up was a Polish student in Paris, looking for a subject for her doctorate.

Tomlin Hawthorne

Maria Skłodowska was born in Warsaw in 1867, when Poland was under Russian rule and women were barred from the university. She studied in secret, at an underground school that moved from flat to flat to avoid the police. Then she worked for six years as a governess to pay for her older sister to study medicine in Paris, on the understanding that the sister would then do the same for her. In 1891, at twenty-four, she took the train to France, travelling fourth class, on a folding stool she'd brought herself.

Tomlin Hawthorne

She lived in a garret, and by her own account sometimes fainted from hunger. She came first in the physics degree and second in mathematics. And she married a shy, gifted French physicist, Pierre Curie, who had the sense to see what she was.

Tomlin Hawthorne

Her first discovery was simple and profound. She tested every element she could lay hands on, and found that the strength of the rays depended on only one thing: how much uranium was in the sample. Not what it was combined with, not whether it was hot or cold, wet or dry, powder or lump. So this wasn't chemistry. Chemistry is about how atoms join up. This was coming from inside the atom. She invented the word for it: radioactivity.

Tomlin Hawthorne

Then she noticed that one uranium ore, a black mineral called pitchblende, was far more radioactive than its uranium could explain. There had to be something else in it. Something unknown, in tiny amounts, and fantastically active.

Tomlin Hawthorne

To find it, she and Pierre worked for four years in a shed with a leaking glass roof that the medical school had previously used for dissecting corpses. It had no proper ventilation. She processed tonnes of ore, boiling it down in great iron cauldrons, stirring with a rod nearly as tall as she was. At the end she had a tenth of a gram, a few grains, of a new element.

Tomlin Hawthorne

Actually, two. The first she named polonium, after a country that at the time wasn't on the map. The second was radium. It was a million times more radioactive than uranium. It glowed. She wrote of going back to the shed at night to look at the tubes and dishes on the shelves, shining faintly blue in the dark, like fairy lights.

Tomlin Hawthorne

She was the first woman to win a Nobel Prize, in 1903. The committee's original plan had been to give it to Becquerel and Pierre and leave her out. Pierre was tipped off and wrote to say that wouldn't do. Three years later he stepped into a rainy Paris street and was killed under a horse-drawn wagon. She took over his professorship, the first woman to teach at the Sorbonne, and began her first lecture at the exact sentence where he'd stopped. In 1911 she won a second Nobel, in chemistry. During the war she designed and drove mobile X-ray vans to the front.

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She died in 1934, of a disease of the blood, almost certainly caused by her work. Nobody then understood the danger. She used to carry tubes of radium in her pocket.

Tomlin Hawthorne

So what was going on inside the atom? This took another thirty years.

Tomlin Hawthorne

By 1932 the picture was this. The nucleus is made of two kinds of particle. Protons, which are positively charged. And neutrons, discovered that year, which are about the same weight and have no charge at all. The number of protons decides which element it is. One is hydrogen. Six is carbon. Seventy-nine is gold. Ninety-two is uranium.

Tomlin Hawthorne

And straight away there's a problem. Protons are all positive. Like charges repel. And you'll remember from episode sixteen how strong the electric force is. In a nucleus the protons are jammed together a hundred thousand times closer than the atoms in a solid. The repulsion between them is enormous. A nucleus ought to fly apart instantly.

Tomlin Hawthorne

So there must be another force. Something stronger than electricity, holding the protons and neutrons together. Physicists, with their usual poetry, named it the strong force.

Tomlin Hawthorne

And it has a peculiar character. It's about a hundred times stronger than the electric force. But it has almost no reach. It only works between particles that are practically touching. Think of two things coated in very powerful glue: nothing happens until they meet, and then they stick hard. The electric repulsion, though weaker, reaches right across the nucleus.

Tomlin Hawthorne

So a nucleus is a standoff. Glue between neighbours, holding it together. Electric repulsion among all the protons, pushing it apart.

Tomlin Hawthorne

In a small nucleus the glue wins easily. But make the nucleus bigger. Every proton you add repels every other proton, right across. But it's only glued to the handful beside it. So as nuclei get larger, the repulsion gains on the glue. Neutrons help: they add glue without adding charge, which is why big nuclei have extra. But there's a limit. By the time you get to the heaviest elements, uranium, radium, the truce is strained. Those nuclei are unstable. And sooner or later, they shed something.

Tomlin Hawthorne

That's radioactivity. It comes in three kinds, named by Rutherford after the first three letters of the Greek alphabet, before anyone knew what they were.

Tomlin Hawthorne

Alpha. The nucleus spits out a tight little bundle of two protons and two neutrons. Those were Rutherford's bullets, in episode twenty-eight.

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Beta. A neutron inside the nucleus turns into a proton, and a fast electron comes flying out. I'll come back to that one. It's stranger than it sounds.

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Gamma. The nucleus settles down and gives off a photon of extremely high energy. Light, a million times more energetic than what you see.

Tomlin Hawthorne

And where does the energy come from? You know now. Episode twenty-four. The pieces afterwards weigh slightly less than the nucleus did before. The missing mass, times the speed of light squared. Becquerel's drawer was a demonstration of an equation that wouldn't be written for another nine years.

Tomlin Hawthorne

Now, the jar. When does a particular nucleus decay?

Tomlin Hawthorne

There's no answer. It's the quantum randomness from Part Six, in its purest form. In fact alpha decay is tunnelling, the effect we met last time: the little bundle is trapped inside the nucleus behind a barrier it hasn't the energy to climb, and every so often it turns up outside. For each kind of nucleus the odds per second are fixed. And that's all that can be said.

Tomlin Hawthorne

But put a huge number of them together, and the randomness averages out into something exact. For any radioactive substance there's a definite time in which half the atoms will have decayed. It's called the half-life. And it can be anything. For some artificial elements it's a fraction of a second. For radium, sixteen hundred years. For the common kind of uranium, four and a half billion.

Tomlin Hawthorne

After one half-life, half are left. After two, a quarter. After three, an eighth. It never quite reaches zero.

Tomlin Hawthorne

And that makes a clock. A clock that can't be hurried or slowed by heat, or pressure, or chemistry, because it runs inside the nucleus where none of that can reach.

Tomlin Hawthorne

The best-known is carbon. Nearly all carbon is stable. But a tiny fraction, made in the upper air by rays from space, is a radioactive kind called carbon fourteen, with a half-life of five thousand seven hundred years. Every living thing takes it in, through the air or its food, and so every living thing has the same small proportion of it. Until it dies. Then the intake stops, and the carbon fourteen it already has ticks away. Measure how much is left in a piece of bone, or wood, or cloth, and you know how long ago it stopped living.

Tomlin Hawthorne

That's how we know the paintings in the cave at Lascaux are about seventeen thousand years old. It's how we know when the Dead Sea Scrolls were written. It turned archaeology from guesswork into a science with dates.

Tomlin Hawthorne

For older things, you need a slower clock. Uranium decays, through a long chain, into lead. In the 1950s an American geochemist, Clair Patterson, set out to measure the tiny amounts of uranium and lead in meteorites, which formed at the same time as the Earth. It took him seven years, because he kept finding lead everywhere. In his samples, in his laboratory, on his hands, in the air. He had to invent the ultra-clean laboratory to get rid of it.

Tomlin Hawthorne

In 1956 he got his answer. The Earth is four and a half billion years old. That figure has barely changed since. It's said that when he saw the result he was so agitated that he thought he was having a heart attack, and had his mother drive him to hospital.

Tomlin Hawthorne

And then he asked: why is there lead on everything? He traced it to petrol. Lead had been added to motor fuel since the 1920s and it was coming out of every exhaust pipe on Earth. Patterson spent the next twenty years fighting the oil and lead industries to have it removed, and eventually won. The level of lead in the blood of American children fell by something like eighty per cent. Very few scientists can claim two results on that scale. Hardly anyone has heard of him.

Tomlin Hawthorne

Now, two more things a nucleus can do, and you know what they led to.

Tomlin Hawthorne

Fission. A very heavy nucleus, like uranium, can split roughly in half. That's what Lise Meitner worked out on the log in the snow. When it splits, it releases a great deal of energy, and also two or three spare neutrons. And a neutron hitting another uranium nucleus can make that one split. Which releases more neutrons. Which split more nuclei.

Tomlin Hawthorne

That's a chain reaction. Let it run away and you have a bomb. Control it, by soaking up some of the neutrons so that each fission triggers exactly one more, and you have a steady source of heat. A reactor.

Tomlin Hawthorne

The first one was built in a squash court under the stands of a disused football stadium at the University of Chicago. It was a pile of graphite bricks and uranium, the size of a small house, assembled by hand. On the afternoon of the second of December 1942, Enrico Fermi, the man with the piano tuners, ordered the last control rod to be pulled out a few inches at a time. The counters clicked faster and faster and then merged into a roar. He let it run for about four and a half minutes. Then he shut it down, and someone produced a bottle of Chianti and paper cups.

Tomlin Hawthorne

The message sent to Washington, in an improvised code, was: the Italian navigator has landed in the New World.

Tomlin Hawthorne

Fusion is the opposite. Two very light nuclei, pushed together hard enough, merge into a heavier one, and that releases energy too. It's what powers the Sun and every star.

Tomlin Hawthorne

It seems odd that both splitting and merging release energy. Here's the resolution. The most tightly bound nuclei, the most stable arrangement of protons and neutrons there is, are in the middle of the range, around iron. Anything much heavier than iron can release energy by breaking up toward it. Anything much lighter can release energy by fusing toward it. Iron is the bottom of the valley. Everything rolls downhill to iron. Remember that. In six episodes' time it'll decide how stars die.

Tomlin Hawthorne

I promised to come back to beta decay.

Tomlin Hawthorne

A neutron turns into a proton, and an electron flies out. The glue doesn't do that. Electricity doesn't do that. It needs yet another force, a fourth one, far feebler than the others, with an even shorter reach. Physicists, having used up strong, called it the weak force. It's the only thing in nature that can turn one kind of particle into another.

Tomlin Hawthorne

And it brought a crisis. When physicists measured the energy of the electrons coming out, it didn't add up. Sometimes the electron had a lot of energy. Sometimes hardly any. The nucleus was losing a fixed amount each time, and the electron was carrying off a varying share. The rest had vanished.

Tomlin Hawthorne

Energy, not conserved. Some people were ready to accept that. Niels Bohr, for one, was prepared to give up the law.

Tomlin Hawthorne

In December 1930, Wolfgang Pauli, whom we met last week, proposed a way out. He did it in a letter to a conference he wasn't attending. It began: Dear Radioactive Ladies and Gentlemen. He apologised for not coming in person. He was needed, he explained, at a ball in Zurich.

Tomlin Hawthorne

His idea was that a second particle comes out with the electron, and shares the energy. A particle with no electric charge, and little or no mass, that passes through matter as though it weren't there, which is why nobody had seen it. He called it a desperate remedy. And he said to a friend: I've done a terrible thing. I've postulated a particle that cannot be detected.

Tomlin Hawthorne

It was later named the neutrino, Italian for the little neutral one. It turned out to be real. It turned out to be one of the commonest things in the universe. And in four episodes' time it'll be the first crack in our best theory of matter.

Tomlin Hawthorne

The myth of the week is that radiation is catching. That something exposed to it becomes radioactive itself. In general it doesn't. An X-ray doesn't make you radioactive. Food that's been sterilised with gamma rays isn't radioactive. Being radioactive means containing unstable nuclei. Being irradiated just means being hit.

Tomlin Hawthorne

And a second one, about half-life. People hear half-life of thirty years and think: so after sixty years it's gone. No. After sixty, a quarter's left.

Tomlin Hawthorne

And the question to take away. This one's squarely in the social sciences.

Tomlin Hawthorne

By the numbers, deaths per unit of energy produced, nuclear power is among the safest sources we have. Safer than coal by a factor of hundreds, once you count air pollution and mining. And yet it's far more feared.

Tomlin Hawthorne

In 1987 a psychologist, Paul Slovic, published a famous paper on how people perceive risk. He found that public fear doesn't track the death toll. It tracks two other things. Dread: is the harm catastrophic, uncontrollable, involuntary, fatal? And the unknown: is it invisible, new, poorly understood, delayed? Nuclear power scores at the top on both. Driving a car scores low on both, and kills over a million people a year.

Tomlin Hawthorne

So here's the question. Is that irrational? Should a sensible society overrule those feelings with arithmetic? Or do dread and unfamiliarity capture something real, about consent and control and trust, that a body count leaves out?

Tomlin Hawthorne

Your challenge. A hundred coins. If you haven't got a hundred, fifty will do, or dried beans with a mark on one side.

Tomlin Hawthorne

Shake them in a box and tip them out. Take away every one that's come up heads. Those have decayed. Count what's left and write it down. Shake the survivors, tip, remove the heads, count. Keep going until there's nothing left.

Tomlin Hawthorne

Plot the numbers, if you like. You'll get a curve that drops steeply and then tails off. A hundred, fifty or so, twenty-five or so, twelve, six.

Tomlin Hawthorne

That's the decay curve of every radioactive substance there is. And you'll have noticed that no coin knew when its turn was coming. That, with different numbers, is how we know the age of the Earth.

Tomlin Hawthorne

Next time. In 1928 a painfully silent young Englishman writes down an equation for the electron. It has an extra solution he didn't ask for. It turns out to describe a kind of matter nobody had imagined. And there's some of it coming out of the bananas in your kitchen.