Becquerel Discovers Radioactivity (1896): Uranium's Rays

Henri Becquerel’s discovery of radioactivity in Paris in early 1896 revealed that uranium gives off invisible, penetrating rays entirely on its own, without any outside source of energy. His report of 2 March 1896, describing photographic plates fogged in total darkness, opened the way to the work of the Curies and Ernest Rutherford and, in time, to nuclear physics as a whole.

Background: the excitement over X-rays

In late December 1895 Wilhelm Röntgen in Würzburg announced a new kind of ray that passed through flesh and blackened photographic plates (see Röntgen’s discovery of X-rays). The news spread across Europe within weeks. At the French Academy of Sciences on 20 January 1896, the mathematician Henri Poincaré pointed out that X-rays seemed to come from the glowing spot where cathode rays struck the glass of the tube, and asked whether other glowing, or phosphorescent, substances might give off similar rays.

Antoine Henri Becquerel (1852–1908) was well placed to answer. He held the chair of physics at the Muséum national d’histoire naturelle in Paris, the same post held before him by his father Edmond and his grandfather Antoine César. Edmond Becquerel had studied phosphorescence for decades, and the family laboratory owned a collection of luminous minerals and salts, including compounds of uranium.

The experiments of February and March 1896

Becquerel chose crystals of potassium uranyl sulfate, a uranium salt that glows after exposure to light. He wrapped photographic plates in thick black paper so that no light could reach them, placed the crystals on top, and left the arrangement in the sun. On development, the plates showed dark outlines of the crystals. He reported this to the Academy on 24 February 1896, still believing that sunlight made the salt phosphoresce and that the phosphorescence produced the penetrating rays.

He then prepared more plates, one with a small copper cross placed between salt and paper. On 26 and 27 February the sky over Paris was overcast, so he put the plates, with the uranium salt still on them, in a dark drawer. On 1 March he developed them anyway, expecting at most faint images. Instead the silhouettes were strong, including the shadow of the cross.

At the Academy on 2 March 1896 he reported that the effect did not need sunlight at all. Over the following weeks he showed that the rays persisted for months, that uranium compounds which did not phosphoresce also produced them, and in May 1896 that pure uranium metal gave an even stronger effect. The property belonged to the element uranium itself.

What Becquerel found, and what he missed

Becquerel also showed that the rays made air conduct electricity, so that a charged electroscope near uranium slowly lost its charge. This gave a way to measure the radiation, which Marie Curie would later refine into a precise method. Contemporaries spoke of “uranic rays” or “Becquerel rays”.

He did not, however, understand what the rays were. For a time he thought they could be reflected and refracted like light, an error later corrected. Interest faded during 1897, as X-rays seemed more useful and more spectacular. The deeper meaning, that atoms themselves were releasing energy, became clear only after others took up the work.

Key people

• Henri Becquerel was an experienced physicist who had worked on the polarisation of light and on infrared spectra before 1896. He was elected to the Academy of Sciences in 1889.
• Henri Poincaré raised the question about phosphorescence and X-rays that prompted the search.
• Marie Curie made uranium rays the subject of her doctoral research from late 1897, coined the word radioactivity in 1898 and, with Pierre Curie, found the new elements polonium and radium (see the Curies’ discovery of radium and polonium).

Consequences

In 1898 Marie Curie showed (as did Gerhard Carl Schmidt independently) that thorium was also radioactive, and the Curies found that some uranium ores were far more active than their uranium content explained. In 1899 Ernest Rutherford, then at McGill University, distinguished two kinds of radiation, which he called alpha and beta; Paul Villard identified a third, more penetrating kind in 1900, later named gamma rays. Becquerel himself showed in 1900 that beta rays were deflected by magnetic and electric fields like the electrons found by J. J. Thomson.

By 1902–1903 Rutherford and Frederick Soddy had concluded that radioactivity was the spontaneous transformation of one element into another, overturning the old idea that atoms were unchangeable. The study of radiation led directly to the nuclear model of the atom in 1911.

Myths and facts

Myth: Becquerel discovered radioactivity purely by accident. He was deliberately looking for rays from uranium. Chance, in the form of cloudy weather, led him to the test that showed his starting theory was wrong.

Myth: Nobody had noticed the effect before. In 1857–1858 Abel Niépce de Saint-Victor reported that uranium salts could darken photographic materials in the dark, but the observation was not followed up or understood. Becquerel’s systematic work made the effect a recognised phenomenon.

Timeline

• 1895, 28 December: Röntgen’s paper on X-rays is published in Würzburg.
• 1896, 20 January: Poincaré discusses X-rays and phosphorescence at the Academy of Sciences.
• 1896, 24 February: Becquerel reports images made by sun-exposed uranium salt through black paper.
• 1896, 26–27 February: Cloudy weather; plates and uranium salt are kept in a dark drawer.
• 1896, 2 March: Becquerel reports that the rays need no sunlight.
• 1896, May: He finds that uranium metal emits the rays strongly.
• 1898: Marie Curie introduces the term radioactivity.
• 1903, December: Becquerel shares the Nobel Prize in Physics with Pierre and Marie Curie.
• 1908, 25 August: Becquerel dies at Le Croisic, Brittany, aged 55.

Why it matters

Becquerel’s plates were the first evidence that matter contains a hidden store of energy released by atoms changing on their own. That insight underlies nuclear physics, radiocarbon dating, nuclear medicine and nuclear power, and the SI unit of radioactivity still bears his name.