The British physicist and radiologist and founder of radiobiology Louis Harold Gray (1905-1965) introduced the unit Rad (acronym for radiation absorbed dose) in the 1930s, which was renamed Gray (Gy) after him in 1978. One gray is a mass-specific quantity and corresponds to the energy of one joule absorbed by one kilogram of body weight. Acute whole-body exposures in excess of four Gy are usually fatal to humans.