Becquerel

Henri Becquerel came from a dynasty of French physicists: his grandfather Antoine César Becquerel had done foundational work in electrochemistry, and his father Alexandre-Edmond Becquerel had studied fluorescence—the property of certain substances to absorb light and re-emit it. Henri, the third generation, was professor of physics at the Muséum National d'Histoire Naturelle in Paris, working in his father's laboratory using his father's equipment and surrounded by his father's specimens.

In January 1896, Wilhelm Röntgen announced the discovery of X-rays, and the announcement electrified European physics. Becquerel, whose family specialty was fluorescence, hypothesized that fluorescent materials might emit X-rays when excited by sunlight—he believed the phenomena might be related. He set up an experiment: he placed uranium potassium sulfate crystals (a fluorescent material) on top of photographic plates wrapped in black paper, intending to expose them to sunlight and then develop the plates to see if X-ray-like radiation had penetrated the paper.

The Paris winter of February 1896 intervened. The sky was overcast for several days. Becquerel, unable to expose his crystals to sunlight, put the whole setup—unexposed crystals on wrapped plates—in a dark desk drawer and waited for better weather. On March 1, apparently expecting to find faint impressions, he developed the plates anyway. The impressions were not faint—they were sharp, dark silhouettes of the crystals. The uranium salts had exposed the plates without any sunlight at all. Something was coming out of the uranium itself, without any external energy source, without any trigger. He had stumbled upon radioactivity.

Becquerel published his findings in March 1896 and continued investigating the mysterious rays—'Becquerel rays,' as they were initially called—for two years. Marie and Pierre Curie then took up the phenomenon, coined the term 'radioactivity,' and discovered polonium and radium. Marie Curie, Henri Becquerel, and Pierre Curie shared the 1903 Nobel Prize in Physics. The SI unit of radioactivity—one nuclear disintegration per second—was named the becquerel in 1975. It replaced the curie (named for Marie and Pierre) as the standard unit. A cloudy week in Paris, and a physicist too impatient to wait, changed the course of modern physics.