Roentgen equivalent man has been defined since 1976 as equal to 0.01 sievert, which is the more commonly used SI unit outside the United States. A number of earlier definitions going back to 1945 were derived from the roentgen unit, which was named after Wilhelm Röntgen, a German scientist who discovered X-rays. The acronym is now a misleading historical artifact, since 1 roentgen actually deposits about 0.96 rem in soft biological tissue, when all weighting factors equal unity. Older units of rem following other definitions are up to 17% smaller than the modern rem. One rem carries with it a 0.05% chance of
Roentgen (unit) 1962 it was redefined. The CCU decided to define the SI unit of absorbed radiation in terms of energy per unit mass, which in MKS units was J/kg. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was equal to 100 rad. The definition of the roentgen had had the attraction of being relatively simple to define for photons in air, but the gray is independent of the primary ionizing radiation type, and can be used for both kerma and