The Mechanism
In late 1931 Irène Joliot-Curie and Frédéric Joliot found that bombarding beryllium with alpha particles produced a strange, deeply penetrating radiation that could knock protons out of paraffin wax; they assumed it was high-energy gamma rays. At the Cavendish Laboratory in Cambridge, James Chadwick — whose mentor Ernest Rutherford had predicted a neutral particle back in his 1920 Bakerian Lecture — was convinced gamma rays could not carry enough momentum to eject fast protons without violating conservation of energy. Over roughly two weeks in February 1932 he re-ran the experiment, firing the beryllium radiation at hydrogen, nitrogen and other targets, measured the recoil energies, and showed the radiation had to be a neutral particle with almost exactly the mass of the proton. He announced it in a one-page letter, "Possible Existence of a Neutron," in *Nature* on 17 February 1932, followed by a full paper in the *Proceedings of the Royal Society A*. The neutron completed the picture of the atomic nucleus and made nuclear fission conceivable within the decade. Chadwick received the 1935 Nobel Prize in Physics.
Why It Matters
The surprising part is that the neutron was not spotted directly like a tiny dot in a detector. It was inferred from what other particles did after a collision. Even more striking, the first experimenters saw the right effect - protons being knocked loose - but misread the cause. Chadwick had to use recoil measurements and conservation of energy to show that ordinary gamma rays could not explain the results. The answer was a new kind of nuclear particle with no electric charge.
Wait — That's Not Quite Right
A common mistake is to think the neutron was discovered by simply seeing it in an instrument, as if it were a visible object. In reality, scientists discovered it by studying indirect clues from scattering and recoil. Another misunderstanding is that the Joliot-Curies were wrong in every way; their experiment was real and important, but they interpreted the penetrating radiation as gamma rays before Chadwick showed that a neutral particle fit the data better.
Vocabulary
- neutron
- proton
- alpha particles
- gamma rays
- beryllium
- paraffin wax
- recoil
- conservation of energy
- Cavendish Laboratory
- Ernest Rutherford
- Nature
- nuclear fission
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
Track a Hidden Particle by Its Push
Take a small bowl, a marble, a ping-pong ball, and a ruler. Set the bowl on a table and gently roll the marble toward it so it bumps another object, then watch how the second object moves. You are not seeing the push directly at first, only the motion after the collision, which is a bit like how Chadwick inferred the neutron from the recoil of protons.
Now repeat the idea with two different targets, such as a ping-pong ball and a heavier toy car. Notice how the same push can make different objects move different amounts. That is similar to how Chadwick compared recoil in hydrogen, nitrogen, and other targets to figure out the mass of the unknown radiation source.
If you want, draw three boxes labeled 'seen directly', 'seen by effect', and 'not enough evidence yet'. Put the marble in the second box and write one sentence about what clue it gives you. Adult supervision is helpful if you are working near breakable items or on a slippery floor.
marble, ping-pong ball, ruler, bowl or small cup, toy car or small object, paper and pencil, adult supervision for breakables and floor safety
Where this came from
- J. Chadwick, "Possible Existence of a Neutron," *Nature* 129, 312 (17 Feb 1932) — https://www.nature.com/articles/129312a0 ; APS News, "May 1932: Chadwick reports the discovery of the neutron" — https://www.aps.org/apsnews/2007/05/may-1932-chadwick-discovery-neutron
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