Neutron

In February 1932, James Chadwick published a paper with one of history's most cautious titles: "The Possible Existence of a Neutron." After just two weeks of experiments at Cambridge's Cavendish Laboratory, he had identified a particle that Ernest Rutherford predicted twelve years earlier—one that would unlock nuclear fission, chain reactions, and the atomic age.

Rutherford discovered the proton in 1919 by bombarding nitrogen with alpha particles. In 1920, he proposed a neutral particle must exist inside nuclei—helium had atomic number 2 but mass number 4. Something besides protons accounted for extra mass. But without charge, a neutral particle would pass through matter like a ghost. No ionization trails. No magnetic deflection. No way to see it directly.

In 1930, German physicists Walther Bothe and Herbert Becker bombarded beryllium with alpha particles from polonium, detecting penetrating neutral radiation. They assumed it was gamma rays. In January 1932, Irène Joliot-Curie and Frédéric Joliot in Paris added paraffin wax—the radiation knocked out high-energy protons. They attributed this to Compton scattering.

When their paper reached Cambridge, Rutherford erupted: "I don't believe it." The numbers didn't work—ejecting protons via photon scattering would require impossibly energetic gamma rays. But if the radiation consisted of neutral particles with proton mass, collision mechanics worked perfectly.

Chadwick's apparatus was simple: polonium and beryllium aimed at various targets. He bombarded paraffin, helium, nitrogen, lithium, measuring ejected particle energies. A massive neutral particle colliding with nuclei would behave like billiard balls. When energies matched this pattern and radiation couldn't be deflected magnetically, the conclusion was inescapable. The nuclear reaction: ⁹Be + ⁴He → ¹²C + ¹n.

The neutron's neutral charge became its most valuable feature—it penetrated nuclei without electromagnetic repulsion. In 1934, Fermi bombarded uranium with neutrons. In December 1938, Hahn and Strassmann achieved fission. On December 2, 1942, exactly ten years after Chadwick's discovery, Fermi achieved the first chain reaction. The invisible particle had become nuclear power's foundation.