Radioactivity

Henri Becquerel observed the phenomenon in uranium salts in 1896, and the work of Marie and Pierre Curie established it as a property of the atom itself rather than of any chemical combination the atom happened to be in. Neither temperature, pressure nor chemical state alters the rate appreciably, because the process originates in the nucleus while chemistry involves only the outer electrons. Activity is measured in becquerels, one disintegration per second, with the older curie equal to 3.7 x 10^10 becquerels. The absorbed and biological doses received by matter are quantified separately, in grays and sieverts, since the damage caused depends on the type of radiation as well as the energy deposited. Naturally occurring radioactivity accounts for most human exposure.