Gray (unit)

The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter.

It is used as a measure of absorbed dose, specific energy (imparted), and kerma (an acronym for kinetic energy released per unit mass). It is a physical quantity, and does not take into account any biological context. Unlike the pre-1971 non-SI roentgen unit of radiation exposure, the gray when used for absorbed dose is defined independently of any target material. However, when measuring kerma the reference target material must be defined explicitly, usually as dry air at standard temperature and pressure.

The corresponding cgs unit, the rad (equivalent to 0.01 Gy), remains common in the United States, though "strongly discouraged" in the style guide for U.S. National Institute of Standards and Technology authors.

The gray was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue. It was adopted as part of the International System of Units in 1975.

One gray is the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter.

The CIPM says that "in order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose D and the name sievert instead of joules per kilogram for the unit of dose equivalent H".

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