Chadwick and the Discovery of the Neutron

In February 1932, a letter only a page long appeared in the journal Nature, titled “Possible Existence of a Neutron”. Its author, the English physicist James Chadwick, had worked almost non-stop for about ten days to solve a puzzle that had baffled some of the best scientists in Europe. The particle he identified completed the basic picture of the atom and, within a few years, would lead to nuclear fission, nuclear power and the atomic bomb. This is the story of how it was found.

A missing particle

By 1920, physicists knew about two subatomic particles: the negative electron and the positive proton. Rutherford had shown that an atom’s positive charge and most of its mass sit in a tiny nucleus.

But the numbers didn’t add up. A helium nucleus has a charge of +2, which suggests 2 protons, but a mass about four times that of a proton. Where did the extra mass come from?

In a lecture in 1920, Ernest Rutherford suggested that the nucleus might contain a neutral particle with about the same mass as a proton: perhaps a proton and an electron bound tightly together. He called it a “neutron”. But a particle with no charge would be very hard to detect, because charged-particle detectors of the time relied on the particles ionising the gas or material they passed through.

Chadwick’s long road

James Chadwick was born in 1891 near Manchester and studied under Rutherford. In 1913, he went to Berlin to work with Hans Geiger. When the First World War broke out in 1914, he was interned as a British citizen in a camp at Ruhleben, a former racecourse near Berlin, where he spent four years. Remarkably, he and other scientist prisoners set up a small laboratory in a horse stable, experimenting with whatever materials they could find, including a brand of radioactive toothpaste.

After the war, Chadwick rejoined Rutherford, who by then led the Cavendish Laboratory in Cambridge. For more than a decade, Chadwick searched for the neutron, trying various ways to knock it out of nuclei or detect its effects, without success.

The beryllium puzzle

The breakthrough began elsewhere.

• 1930: In Germany, Walther Bothe and Herbert Becker bombarded the light metal beryllium with alpha particles from polonium. It emitted a very penetrating radiation that passed through several centimetres of lead. They assumed it was high-energy gamma rays.
• January 1932: In Paris, Irène Joliot-Curie (daughter of Marie Curie) and her husband Frédéric Joliot placed a block of paraffin wax, which is rich in hydrogen, in the path of this radiation. They found that it knocked protons out of the wax at very high speeds. They still interpreted the radiation as gamma rays, knocking protons out rather as X-rays knock electrons out of atoms.

When Chadwick read their report, he was immediately doubtful, and so was Rutherford. For a gamma ray photon to give a heavy proton that much energy, it would need an enormous energy, far more than was reasonable. And the collision process they proposed would, by the known physics, be extremely unlikely.

Ten days of experiments

Chadwick set to work, reportedly averaging only a few hours’ sleep a night. He used a polonium source and beryllium target, and directed the mysterious radiation at different materials, measuring the recoil of the atoms that were struck:

• Hydrogen (from paraffin wax): protons were knocked out with high energy.
• Nitrogen, helium, lithium and other light elements: their nuclei recoiled too, with different energies.

He reasoned like this: if the radiation consisted of particles with mass, then by comparing how much energy they gave to nuclei of different known masses, he could use the laws of conservation of energy and momentum, just as for colliding snooker balls, to work out the mass of the incoming particle.

The results fitted beautifully with a neutral particle whose mass was almost the same as a proton’s. A gamma ray explanation didn’t fit at all.

The reaction producing the particles was:

⁹₄Be + ⁴₂He → ¹²₆C + ¹₀n

Chadwick estimated the neutron’s mass as between 1.005 and 1.008 times the mass of a hydrogen atom. The modern value is about 1.0087 u, slightly heavier than a proton. See the neutron and isotope notation.

Why did it penetrate so easily? Because, with no charge, it isn’t slowed by electrical interactions with electrons. It only interacts when it collides almost directly with a nucleus.

Recognition

Chadwick’s short letter to Nature was published on 27 February 1932. He received the Nobel Prize in Physics in 1935. The Joliot-Curies, who had come so close, won the Nobel Prize in Chemistry the same year for their discovery of artificial radioactivity.

At first, Chadwick thought of the neutron as a proton and electron combined, as Rutherford had imagined. Within a couple of years, physicists concluded that it was a particle in its own right. Today we know it’s made of three quarks (one up and two down), and that a free neutron decays in about 15 minutes into a proton, an electron and an antineutrino.

What the neutron explained

The discovery instantly cleared up several puzzles:

• Mass numbers: nuclei contain protons and neutrons; mass number = protons + neutrons. See mass number explained.
• Isotopes: atoms of the same element with different masses simply have different numbers of neutrons. See isotopes, ions, isomers and allotropes.
• Nuclear stability: neutrons add strong-force attraction without electrical repulsion, holding larger nuclei together. See inside the atomic nucleus.

From the neutron to fission

Because neutrons have no charge, they aren’t repelled by nuclei. They became the perfect tool for probing and transforming nuclei:

• 1934: Enrico Fermi in Rome bombarded element after element with neutrons, producing new radioactive isotopes, and discovered that slow neutrons are especially effective.
• 1938: Otto Hahn and Fritz Strassmann found barium among the products of neutron bombardment of uranium.
• 1939: Lise Meitner and Otto Frisch explained this as nuclear fission: the uranium nucleus had split in two, releasing enormous energy and more neutrons.

Within a decade of Chadwick’s letter, the first nuclear reactor had run (1942), and within thirteen years, nuclear weapons had been used. Chadwick himself led the British scientific mission to the Manhattan Project. See nuclear fission vs fusion.

Lessons from the story

• Interpretation matters. The Joliot-Curies had the key data first, but interpreted it through the accepted idea of gamma rays.
• Conservation laws are powerful tools. Chadwick identified an invisible particle purely from how it made other particles recoil.
• Prepared minds. Chadwick had been looking for the neutron for over a decade, so he recognised it immediately when the evidence appeared.

Key takeaways

• Rutherford proposed a neutral nuclear particle in 1920 to explain why nuclei are heavier than their protons alone.
• Radiation from beryllium bombarded with alpha particles was first thought to be gamma rays.
• In 1932, Chadwick showed, using recoil measurements and conservation of energy and momentum, that it was a neutral particle with about a proton’s mass: the neutron.
• The neutron explained isotopes, mass numbers and nuclear stability.
• Neutrons’ lack of charge made them ideal for inducing nuclear reactions, leading to the discovery of fission within seven years.