The Existence of a Neutron
A neutral particle as heavy as the proton hides inside every nucleus.
A strange radiation could fling protons out of wax, yet carried no electric charge at all — and explaining it forced physicists to admit a brand-new particle hiding in the heart of every atom.
The big idea
By 1920 the atom had a tiny, heavy, positive nucleus (Rutherford) circled by electrons. But the nucleus weighed about twice what its protons alone could account for, and no one knew why. James Chadwick found the missing piece: a particle with almost exactly a proton's mass but no electric charge — the neutron. Every nucleus is a tight pack of protons and neutrons.
Because it is electrically neutral, the neutron is the key that turns. Carrying no charge, it is not pushed back by a nucleus, so it can slip right in — which is exactly why neutrons, only a few years later, would be the tool used to split the atom.
How it came about
For over a decade Chadwick and his mentor Ernest Rutherford had suspected a neutral particle existed; Rutherford had even predicted it in a 1920 lecture. The decisive clues came from abroad. In Germany, Bothe and Becker noticed that beryllium hit by α-particles gave off a mysterious, very penetrating radiation. In Paris, Irène and Frédéric Joliot-Curie found that this radiation could knock protons out of paraffin wax — but explained it as a kind of ultra-energetic light, a γ-ray.
Chadwick read their paper and knew at once that light could not do this: a massless γ-ray flinging a heavy proton at that speed would need an absurd amount of energy. In a few intense weeks at the Cavendish Laboratory in early 1932, he measured how the radiation kicked not only hydrogen but nitrogen and other gases, and showed the numbers only made sense if it were a particle as heavy as a proton. He had found the neutron.
Why it mattered
The neutron completed the picture of the atom and at once explained isotopes — why the same element can come in slightly heavier and lighter versions (they simply carry different numbers of neutrons). And because it has no charge, the neutron became the perfect probe for reaching into the nucleus. Within seven years it had been used to discover nuclear fission; within thirteen, that discovery had built both reactors and the atomic bomb. Few single particles have changed history so quickly.
A way to picture it
Think of an invisible cue ball on a darkened pool table. You can't see the cue ball, but you can see the coloured balls it scatters. If it strikes a light ball (a proton) and then a heavy one (a nitrogen nucleus), and you measure how fast each flies off, you can work backward to the cue ball's weight — without ever seeing it. Chadwick's chargeless, invisible neutron was the cue ball; the recoiling protons and nitrogen nuclei were the coloured balls that gave its mass away.
Where it sits
This is the direct sequel to Rutherford's 1911 discovery of the nucleus — the particle Rutherford himself had guessed at in 1920. It set the stage for Fermi's 1934 theory of the weak force (which governs how a lone neutron decays) and for all the nuclear physics that followed. Its strangest children are the neutron stars — entire stellar cores crushed into a ball of neutrons — proposed just two years later and seen, as pulsars, in 1967.
Up to the present, all the evidence is in favour of the neutron, while the quantum hypothesis can only be upheld if the conservation of energy and momentum is relinquished at some point.
The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons.