The gray (Gy) is the derived unit of absorbed dose of ionizing radiation in the SI system of units. One gray (Gy) dose is equivalent to 1 joule of ionizing radiation energy absorbed per kilogram of organ or tissue weight. It is named after the British radiologist L. H. Gray (1905–1965). The rad is the older but still occasionally unit of absorbed dose. 1 Gy = 100 rad.