James Chadwick

Chapter 1 · The Wrong Desk

Too shy to say so, and a physicist by accident

1891 – 1913 · Bollington · Manchester

James Chadwick was born at Bollington near Macclesfield in October 1891, the son of a *cotton spinner* who later ran a laundry. The family had very little money, and Chadwick was brought up largely by his grandparents.

He won a scholarship to Manchester Grammar School and then a place at the *University of Manchester*, intending to read *mathematics*.

At enrolment he joined the wrong queue and found himself being interviewed for *physics*. He realised the mistake and — by his own account — was *too shy to say so*, so he enrolled in physics instead.

That is how he came into Ernest Rutherford's department.

He found the first-year physics lectures poor and considered leaving. He stayed, did well, and by his final year was doing research under Rutherford directly, measuring the intensity of gamma radiation.

He was, by all accounts, extremely quiet. He said little, wrote sparely, and was noted for understating his results.

In 1913 he won a scholarship that required him to work at an institution abroad, and Rutherford sent him to *Berlin*, to work under *Hans Geiger* — who had built the counter that made the gold-foil experiment possible and who was, by then, running his own laboratory.

Geiger's group had a *counter*, which is far superior to counting scintillations in the dark by eye. With it, Chadwick found that the beta particles from a radioactive source do *not* come out with a single energy but with a *continuous spectrum* of energies.

That was a genuinely disturbing result — it appeared to violate conservation of energy — and it was only resolved in 1930 when Pauli proposed the *neutrino*, an undetected particle carrying off the missing energy.

“I am afraid neutrons will not be of any use to any one.”

— James Chadwick, on the practical prospects of his discovery

Why this matters

Chadwick's finding that beta particles come out with a continuous range of energies appeared to violate conservation of energy and led eventually to the neutrino.

You have the wrong queue and the Berlin counter. What would you ask him?

Ask Chadwick

• “Did you really join physics because you were too shy to say so?”
• “What is wrong with a continuous beta spectrum?”
• “What did a counter give you that eyes could not?”
• “Why did you nearly leave physics in your first year?”
• “What did Rutherford see in you?”

Chapter 2 · Four Years in a Stable

Interned at a racecourse, doing physics with toothpaste

1914 – 1918 · Ruhleben · Berlin

Chadwick was still in Berlin in August 1914 when war broke out. Geiger, who was called up, advised him to leave immediately. He delayed — partly, it seems, because he did not want to abandon an experiment — and was arrested as an *enemy alien*.

He spent the next *four years* interned at *Ruhleben*, a civilian internment camp set up in a *racecourse* outside Berlin. The inmates were housed in *stables* and in lofts above them, six men to a stall.

He was twenty-two when he went in and twenty-six when he came out.

Ruhleben was, by the standards of the war, not brutal — but it was cold, badly fed, overcrowded and interminable, and Chadwick's health suffered permanently. He had digestive problems for the rest of his life.

The internees organised themselves remarkably: they ran a camp school, a library, a theatre, a magazine and a scientific society.

Chadwick and a fellow internee, Charles Ellis — a young army officer who had been travelling in Germany and who Chadwick converted into a physicist during the internment — set up a *laboratory* in the camp. They obtained materials by whatever means were available: radioactive toothpaste, which was sold at the time as a whitening product and which contained thorium; tinfoil; borrowed glassware.

With this they did real experiments on the ionisation of gases and published afterwards.

Ellis went on to become a professor of physics and Chadwick's collaborator.

Chadwick came home in November 1918 with no money, no position and no prospects, and went to Rutherford, who found him a small teaching post at Manchester and then took him to *Cambridge* in 1919 when he took over the Cavendish.

Why this matters

Chadwick lost four years of his career to a racecourse stable, and did physics there with radioactive toothpaste and borrowed glass.

You have the stable, the toothpaste and the four years. What is your question?

Ask Chadwick

• “What was Ruhleben actually like?”
• “How do you do physics in a camp?”
• “Why did you not leave Berlin when Geiger told you to?”
• “What did four years cost you?”
• “How did you turn an army officer into a physicist?”

Chapter 3 · Looking for a Particle That Leaves No Trail

Twelve years of failure

1920 – 1932 · Cambridge

In 1920 Rutherford gave a lecture proposing that the nucleus must contain, besides protons, a *neutral particle* of roughly the proton's mass.

His reasoning was arithmetic. Helium has a charge of two and a mass of about four. Nitrogen has a charge of seven and a mass of about fourteen. Every nucleus heavier than hydrogen weighs roughly *twice* what its charge suggests. Something with mass and no charge must be there.

Rutherford's own picture — a proton and an electron bound very tightly together — turned out to be wrong, but the requirement was right.

Chadwick spent *twelve years* looking for it.

And the difficulty is fundamental. Every method of detecting a particle in 1920 depends on its *electric charge*. A cloud chamber shows a track because a charged particle ionises the vapour it passes through. A Geiger counter registers ionisation. A scintillation screen flashes when a charged particle strikes it. An electric or magnetic field deflects a charged particle so it can be identified.

A particle with *no charge* does none of this. It passes through matter leaving no ionisation, no track, no flash and no deflection. It is, by every available means, *invisible*.

So the only way to detect it is *indirectly*: let it hit something, and observe what the something does.

Chadwick tried many approaches over the years — bombarding elements, looking for anomalies, searching for evidence of a neutral emission — and found nothing.

He later said the problem was always at the back of his mind, and that he simply kept coming back to it.

Why this matters

Every detection method of the period depended on electric charge, so a neutral particle could only be found by observing what it did to something else.

You have twelve years and an invisible target. What would you ask?

Ask Chadwick

• “Why must there be a neutral particle in the nucleus?”
• “Why can't you detect an uncharged particle directly?”
• “What did you actually try over twelve years?”
• “How do you keep working on something that keeps failing?”
• “Was Rutherford's proton-electron picture ever plausible?”

Chapter 4 · A Fortnight in February

Reading somebody else's data harder than they had

January – February 1932 · Cambridge · Paris

In early 1932 *Irène* and *Frédéric Joliot-Curie* in Paris published a striking result.

Bombarding *beryllium* with alpha particles produced a very penetrating radiation. When that radiation was directed at *paraffin wax*, it knocked *protons* out of it with considerable energy — protons that could be detected easily, because they are charged.

The Joliot-Curies interpreted the penetrating radiation as *gamma rays*: high-energy electromagnetic radiation.

Chadwick read the paper and, by his own account, knew almost immediately that it could not be right.

The objection is *momentum*. Gamma rays are massless. For a massless photon to knock a proton out of wax with the observed energy, the photon would need an energy of about fifty million electron-volts — far more than any known nuclear process could supply, and inconsistent with everything else about the reaction.

The arithmetic simply does not balance.

But it balances *perfectly* if the penetrating radiation consists of *neutral particles of about the proton's mass*. A billiard ball transfers its momentum efficiently to another billiard ball of the same mass; a photon does not.

Chadwick worked for about a *fortnight*, almost continuously, sleeping very little.

His method was elegant. He fired the beryllium radiation not only at hydrogen in paraffin but at *nitrogen*, and at other elements, and measured the *recoil velocities* of each in a small ionisation chamber.

If you know the recoil velocity of two different target nuclei of known mass struck by the same projectile, you have two equations, and you can solve for the projectile's *mass* and *velocity*.

The mass came out very close to that of the proton.

He published a letter to *Nature* on 17 February 1932 titled *Possible Existence of a Neutron* — characteristically understated — and the full paper in May.

He received the *Nobel Prize in Physics* in 1935.

Why this matters

Chadwick found the neutron by measuring how fast two different target nuclei recoiled, which gives two equations and lets you solve for the projectile's mass.

You have the Paris paper and the fortnight of recoils. What is your question?

Ask Chadwick

• “Why can't gamma rays knock protons out of wax like that?”
• “How do recoil velocities give you a mass?”
• “Why did the Joliot-Curies miss it?”
• “What was that fortnight like?”
• “Why title the paper 'Possible Existence'?”

Chapter 5 · Not of Any Use to Anyone

The Manhattan Project, and a man who said very little

1932 – 1974 · Cambridge · Liverpool · Los Alamos

Chadwick reportedly remarked that neutrons would not be of any use to anyone.

They are the single most useful particle in nuclear physics, for exactly the reason that made them hard to find.

A charged particle approaching a nucleus is *repelled* by it — both are positive — and must be accelerated to enormous energy to get close. A *neutron* carries no charge, so nothing repels it. It can drift into a nucleus at low energy and be absorbed.

That is why neutron bombardment works where proton bombardment struggles. *Enrico Fermi* began systematically bombarding elements with neutrons in 1934, and found that *slow* neutrons are absorbed even more readily than fast ones.

In December 1938 Otto Hahn and Fritz Strassmann bombarded uranium with neutrons and found *barium* — an element roughly half uranium's size. *Lise Meitner* and *Otto Frisch* explained it as *fission* in January 1939, and noted that fission also releases further *neutrons*, which can cause further fissions.

That is a *chain reaction*, and it depends entirely on the particle Chadwick found.

Chadwick moved to *Liverpool* in 1935 to build a cyclotron. In 1940 he was a principal author, with Rudolf Peierls and Otto Frisch, of the assessment that a uranium bomb was feasible with a critical mass of a few kilograms — the *MAUD Report*, which persuaded the British and then the American governments.

From 1943 he led the *British Mission* to the *Manhattan Project*, and was the only British scientist with full access to every part of it. He got on well with General Groves, which most scientists did not.

He said very little about it afterwards. What is recorded suggests he was deeply uneasy: he told an interviewer that he had realised a nuclear bomb was not only possible but *inevitable*, and that he then had to take sleeping pills, and that they were the only remedy — and he had gone on taking them for twenty-eight years.

He was knighted in 1945, became Master of Gonville and Caius College, and retired to North Wales.

He died at Cambridge in July 1974, aged eighty-two.

Why this matters

A neutron is not repelled by a nucleus, which is exactly why it was invisible to every detector and exactly why it made fission possible.

You have the useless particle and the twenty-eight years of sleeping pills. What would you ask?

Ask Chadwick

• “Why is an uncharged particle so useful for hitting a nucleus?”
• “What did you mean about the sleeping pills?”
• “What was in the MAUD Report?”
• “Why did you say so little afterwards?”
• “Would you have found it if you had known?”

What Chadwick changed

The neutron completed the picture of the nucleus and made neutron-induced fission possible, so reactors, medical isotopes and nuclear weapons all descend from a fortnight's work in 1932. Neutron scattering is now a standard tool across physics, chemistry and biology for determining the structure of materials.

A debate that continues

Irène and Frédéric Joliot-Curie produced the beryllium result and drew the wrong conclusion from it, and the credit question between the Paris and Cambridge groups was genuinely sore; Chadwick also did not predict the neutron — Rutherford did — and searched unsuccessfully for twelve years.

Keep exploring — ask Chadwick

• “What else is invisible because of how we detect things?”
• “Should you publish what a weapon could be made from?”
• “How do you read somebody else's data properly?”

Related lives

• Ernest Rutherford — Who Found the Atom Was Nearly Empty
• Lise Meitner — Who Did the Arithmetic in the Snow
• Marie Curie — Two-Time Nobel Laureate
• J. Robert Oppenheimer — Father of the Atomic Bomb

Related themes

The neutron and the nucleus · Nuclear fission · Detecting particles