High Quality Content by WIKIPEDIA articles! The gray (symbol: Gy) is the SI derived unit of absorbed dose, specific energy (imparted) and of kerma. Such energies are typically associated with ionising radiation such as X-rays or gamma particles or with other nuclear particles. It is defined as the absorption of one joule of such energy by one kilogram of matter. Unlike the pre-1971 roentgen, the gray has always been defined independently of any target material. The same beam of 1 roentgen would impart more grays to biological tissue than it does to air. The gray is sometimes used to measure beam kerma, in which case the reference target material must be defined explicitly. (Usually dry air at standard temperature and pressure.)