Gy

Gy, symbol Gy, is the SI unit of absorbed dose of ionizing radiation. It quantifies the amount of energy deposited by radiation in a kilogram of matter, defined as one joule of energy per kilogram. The gray is named after Louis Harold Gray, a British physicist who contributed to radiobiology. It is used to express the energy delivered to tissues in medical, industrial, and research contexts.

The gray measures physical energy deposition rather than biological effect. The sievert is the corresponding unit used to express potential biological risk, taking into account radiation type and tissue sensitivity. To relate the two, the absorbed dose in grays can be converted to an equivalent dose in sieverts by applying a radiation weighting factor (D_eq in Sv = D in Gy × w_R). The effective dose further accounts for tissue-specific sensitivities with tissue-weighting factors, providing a single value for overall risk.

In practice, the gray is used with dosimeters to monitor dose to patients and workers in radiology, nuclear medicine, and radiotherapy. Submultiples such as the centigray (cGy) and milligray (mGy) are commonly used for more practical measurements. Typical medical procedures deliver doses spanning from fractions of a gray in some imaging studies to several tens of grays across a radiotherapy course. Natural background radiation contributes only a small, chronic dose to the body, highlighting the difference between physical dose and projected biological impact.