The new international system (si) unit of radiation dose, expressed as absorbed energy per unit mass of tissue. The si unit "gray" has replaced the older "rad" designation. (1 gy = 1 joule/kilogram = 100 rad). Gray can be used for any type of radiation (e.g., alpha, beta, neutron, gamma), but it does not describe the biological effects of different radiations. Biological effects of radiation are measured in units of "sievert" (or the older designation "rem"). Sievert is calculated as follows: gray multiplied by the "radiation weighting factor" (also known as the "quality factor") associated with a specific type of radiation.