Gray : unit of deposited energy

The unit dose of energy deposit by radiation is the gray (Gy) or milligray (mGy). One gray corresponds to an energy deposit of 1 joule per kilogram of matter. Unlike the sievert (the biological dose unit), the gray does not distinguish if the deposit occurs in inert or living matter. This unit is much used in radiation therapy (it measures the dose to the tumour), or to measure the exposure of an organ in an accident or a diagnosis. The gray now supplants an old unit, the rad : 1 gray = 100 rad)