James Chadwick

Who Weighed a Particle With No Charge · Physicist · 1891–1974

The reticent Cavendish physicist who read a Paris result, saw immediately that the momentum could not balance, and in about a fortnight proved the existence of the neutron.

Who was Chadwick?

James Chadwick was born near Manchester in 1891 to a cotton-spinner's family, and joined Rutherford's physics class because he had queued at the wrong desk during enrolment and was too shy to say so. He went to Berlin to work under Hans Geiger, and was interned there for the whole of the First World War in a stable at a racecourse, doing what physics he could with improvised apparatus. Rutherford had argued for years that the nucleus must contain a neutral particle of roughly the proton's mass; nobody could find it, because a particle with no charge leaves no trail. In early 1932 Irène and Frédéric Joliot-Curie in Paris reported a strange penetrating radiation from beryllium that could knock protons out of paraffin wax, and identified it as gamma radiation. Chadwick read the paper and knew at once that gamma rays could not do that — the momentum arithmetic does not work. In roughly a fortnight of almost continuous work he fired the beryllium radiation at several different targets, measured the recoil velocities, and showed that the numbers only balanced if the radiation was a neutral particle of about the mass of a proton. He had found the neutron. Because it carries no charge it is not repelled by a nucleus, which is exactly why it can penetrate one — and that is why fission became possible seven years later. He won the Nobel Prize in 1935 and led the British Mission to the Manhattan Project, work he was never comfortable about afterwards.

Major achievements

• Discovered the neutron in February 1932, by measuring recoils rather than tracks
• Showed that the Paris interpretation as gamma radiation could not balance momentum
• Awarded the Nobel Prize in Physics in 1935
• Established the neutron's mass as close to the proton's
• Led the British Mission to the Manhattan Project

Life in brief

• 1891 — Born near Manchester: To a cotton-spinner's family.
• 1908 — Joins Rutherford's class: By queueing at the wrong desk and being too shy to say so.
• 1914 — Interned in Berlin: Four years in a stable at a racecourse.
• 1932 — The Paris paper: Beryllium radiation knocking protons out of wax, called gamma rays.
• 1932 — The neutron: A fortnight of recoil measurements; the momentum only balances for a neutral particle.
• 1935 — Nobel Prize in Physics: For the discovery of the neutron.
• 1943 — The British Mission: Leads it to the Manhattan Project.
• 1974 — Dies in Cambridge: Having said very little about any of it.

Explore the life of Chadwick — five chapters

1. The Wrong Desk — Too shy to say so, and a physicist by accident
2. Four Years in a Stable — Interned at a racecourse, doing physics with toothpaste
3. Looking for a Particle That Leaves No Trail — Twelve years of failure
4. A Fortnight in February — Reading somebody else's data harder than they had
5. Not of Any Use to Anyone — The Manhattan Project, and a man who said very little

Read the five-chapter life of James Chadwick →

Where Chadwick appears in your course

James Chadwick has a genuine claim on 1 lesson of the Pearson Edexcel International GCSE science course built into Incandio:

• Atomic Structure and Isotopes — Physics: The neutron is half of statement 7.2 and it was the last of the three particles to be found, because everything physicists had for detecting particles worked by detecting CHARGE — and a neutron has none, so it leaves no track and triggers nothing. Chadwick found it in 1932 by an entirely indirect route: he fired an unidentified radiation from beryllium at various targets and measured how fast the nuclei it struck recoiled, then showed the numbers only balanced if the thing doing the striking was neutral and about as heavy as a proton. He is the person to ask how you establish the existence and the mass of something you cannot detect directly.

Begin

• Start a conversation with Chadwick — a dramatised, historically grounded AI portrayal
• Read the Historical Brief
• Explore the life of Chadwick — five chapters
• Find your exam topics · Meet all 208 figures