(/ˌbɛkəˈrɛl/; French: [ɑ̃ʁi bɛkʁɛl]; 15 December 1852 – 25 August 1908) was a French physicist, Nobel laureate, and the first person to discover evidence of radioactivity. For work in this field he, along with Marie Skłodowska-Curie and Pierre Curie, received the 1903 Nobel Prize in Physics. The SI unit for radioactivity, the becquerel (Bq), is named after him.
Describing them to the French Academy of Sciences on 27 February 1896, he said:
One wraps a Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative … One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts.
Later in his life in 1900, Becquerel measured the properties of Beta Particles, and he realized that they had the same measurements as high speed electrons leaving the nucleus. In 1901 Becquerel made the discovery that radioactivity could be used for medicine. Henri made this discovery when he left a piece of radium in his vest pocket and noticed that he had been burnt by it. This discovery led to the development of radiotherapy which is now used to treat cancer. Becquerel did not survive much longer after his discovery of radioactivity and died on 25 August 1908, at the age of 55, in Le Croisic, France. His death was caused by unknown causes, but was reported that “he had developed serious burns on his skin, likely from the handling of radioactive materials.”