Henri Becquerel

The Discovery of Radioactivity (Henri Becquerel)

The discovery of a form of radiation that could pass through solid matter fired the imagination of a generation of scientists who rushed to study this phenomenon. Roentgen had noted that cathode-ray tubes emitted x-rays at the spot that emits light when the cathode rays hit the glass walls of the tube. This observation caught the attention of the French physicist Henri Becquerel.

Becquerel decided to investigate the connection between x-rays and the fluorescence of the glass walls of the cathode-ray tube. He knew that salts of uranium, such as potassium uranyl sulfate [K₂UO₂(SO₄)₂ 2 H₂O] emit light when exposed to the UV radiation in sunlight. He therefore wrapped a photographic plate in black paper, placed crystals of this salt on top of the plate, and exposed the crystals to sunlight. When the plates were developed, black spots were found beneath the crystals, suggesting that some form of radiation had been emitted by the uranium salts that passed through the paper and fogged the photographic plate.

Becquerel found the same results, however, when the crystals and photographic plate were prepared and kept in the dark. He also noted that much better images were obtained with pure uranium metal, which did not fluoresce when exposed to sunlight. The radiation given off by uranium metal and its compounds apparently had nothing to do with whether they were exposed to sunlight. It soon became evident that a new form of radiation had been discovered, for which Marie and Pierre Curie suggested the name radioactivity.