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For twelve years physicists suspected that atomic nuclei held a neutral particle, and for twelve years nobody could find it. James Chadwick did it in a few weeks in 1932, using nothing more advanced than the momentum and energy equations from your Class 11 collisions chapter. This post covers who he was, the puzzle he solved, the calculation he made with his own numbers, and what students can learn from his way of working.
Who was James Chadwick?
His Nobel biography records that he was born on 20 October 1891, entered Manchester University in 1908 and graduated in physics in 1911. He then worked under Ernest Rutherford on radioactivity. In 1913 he went to Berlin to work with Hans Geiger, and during the First World War he was interned at Ruhleben. In 1919 he returned to work with Rutherford, who had moved to the Cavendish Laboratory in Cambridge. He was elected a Fellow of the Royal Society in 1927.
He received the 1935 Nobel Prize in Physics "for the discovery of the neutron". He died on 24 July 1974.
Why did anyone expect a neutral particle?
In his Nobel lecture, Chadwick credits the first suggestion of the neutron as we know it to Rutherford in 1920: a proton and electron bound so closely that the result had almost no external field and could "move freely through matter". Chadwick says he made several attempts to detect such particles during the 1920s, without success.
The break came from others' puzzling results. Walther Bothe and Herbert Becker found that beryllium bombarded with alpha particles gave off a very penetrating radiation, which they took to be gamma rays. Then Irène and Frédéric Joliot-Curie showed that this radiation knocked protons out of paraffin wax at very high speeds. Chadwick saw that this was very hard to explain if the radiation was light, and began testing it at once.
How did Chadwick prove it was a particle?
He reported first in a short letter, Possible existence of a neutron, in Nature on 27 February 1932, then in a full paper, The existence of a neutron, in the Proceedings of the Royal Society that June.
The argument in his Nobel lecture runs like this. Suppose the radiation is a particle of mass M moving at up to speed V. In a head-on elastic collision with a nucleus of mass m at rest, the nucleus gets the largest possible speed, u = 2MV/(M + m). So:
For hydrogen (m = 1): U_H = 2MV/(M + 1).
For nitrogen (m = 14): U_N = 2MV/(M + 14).
Divide one by the other and the unknown V cancels: U_H/U_N = (M + 14)/(M + 1).
Worked example with Chadwick's numbers
From the ranges of the recoiling nuclei, his lecture gives U_H ≈ 3.7 × 10⁹ cm/s for protons knocked out of paraffin wax and U_N ≈ 4.7 × 10⁸ cm/s for nitrogen nuclei in an expansion chamber.
Ratio: 3.7 × 10⁹ / 4.7 × 10⁸ ≈ 7.87.
Set (M + 14)/(M + 1) = 7.87, so M + 14 = 7.87M + 7.87.
Collect terms: 14 − 7.87 = 7.87M − M, so 6.13 = 6.87M.
M ≈ 0.89, which Chadwick quotes as about 0.9.
So the particle has about the mass of a proton. It also passed through 10 to 20 cm of lead, which a proton of that speed could never do. Penetration of that kind depends on charge, so the particle had to be neutral. Chadwick later used the break-up of the deuteron to get a more exact mass of 1.0085 atomic mass units, against 1.0081 for the hydrogen atom.
Mistakes to avoid when you try this:
Using the wrong collision formula. u = 2MV/(M + m) is the speed of the struck target in a head-on elastic collision, not the speed of the incoming particle afterwards.
Inverting the ratio. The lighter target recoils faster, so U_H/U_N must be greater than 1.
Algebra slips. Gather the M terms on one side before dividing.
Our guide to collisions and momentum problems covers the elastic collision results this relies on. If you want to check each step of a problem like this, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What happened next?
His Nobel biography notes that, being uncharged, the neutron is not repelled by the nucleus and can reach and split even heavy nuclei. That opened the way to nuclear fission. From 1943 to 1946 Chadwick led the British Mission to the Manhattan Project in the United States. He was knighted in 1945.
What can a student take from how he worked?
Keep an idea alive for years. He looked for the neutron through the 1920s before the right clue came.
Take other people's puzzles seriously. The Joliot-Curie result did not fit the gamma-ray explanation, and he acted on that mismatch.
Remove unknowns with ratios. One division turned an impossible measurement into a solvable equation.
Check with a second line of evidence. Mass from collisions, neutrality from penetration: two independent tests made the case.
For the background to Chadwick's work, read about Rutherford's gold foil experiment, and for where neutrons appear in exams, our guide to nuclear binding energy problems.
Try it: if helium nuclei (m = 4) had recoiled at 2.0 × 10⁹ cm/s, what mass would U_H/U_He give?
