how radioactivity was discovered

The Mechanism

Antoine Henri Becquerel was a professor at the Muséum National d'Histoire Naturelle in Paris and the third generation of his family to hold the chair — his grandfather Antoine César and father Alexandre Edmond had both studied phosphorescence and fluorescence, and the family cabinet contained decades' worth of glowing mineral specimens. In late February 1896 Becquerel was following up on a suggestion of the mathematician Henri Poincaré: perhaps the fluorescence Wilhelm Röntgen had photographed his hand's bones through eight weeks earlier was somehow connected to ordinary fluorescence. Becquerel took a sample of potassium uranyl double sulfate — a bright yellow-green crystalline uranium salt that fluoresced strongly under sunlight — placed it on a photographic plate wrapped in two sheets of thick black paper, put a copper Maltese cross between the salt and the plate, and set the whole assembly on a windowsill to let the sun charge the fluorescence. Two exposures on 24 and 26 February gave weak but real silhouettes of the cross once the plates were developed. Then Paris clouded over. For the next several days the sun did not come out, and Becquerel put the wrapped plate and the uranium salt into a dark drawer to wait. On 1 March 1896, tired of waiting, he developed the plate anyway — expecting a faint image or nothing. What came up in the fixer bath was a strong, sharp silhouette of the Maltese cross, as intense as the sunlight exposures had been. The uranium salt was emitting some kind of penetrating radiation on its own, without any external stimulation, indefinitely and without visible source. He presented the result to the French Academy of Sciences the next day — "Sur les radiations invisibles émises par les corps phosphorescents," *Comptes Rendus* 122 (1896) — and then, methodically, over the following weeks, showed that every uranium compound in the cabinet emitted the same rays whether or not it was phosphorescent, whether or not it had ever been exposed to light, and whether kept in the dark for weeks or months. The rays came from the uranium itself. Marie Skłodowska-Curie chose the phenomenon for her doctoral thesis two years later, coined the word "radioactivity," and by 1898 had isolated polonium and radium from tonnes of pitchblende by tracking the invisible emission through every chemical separation. Becquerel, Marie Curie, and Pierre Curie shared the 1903 Nobel Prize in Physics. It was the moment atoms stopped being indivisible: something inside them was breaking, spontaneously, and pouring out energy nobody could account for. Ernest Rutherford's transmutation experiments, the whole discipline of nuclear physics, and the Einstein mass-energy equivalence that had to bookkeep it — all of them are downstream of a plate that fogged in a Paris drawer for a week.