Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionising radiation. It is equal to the energy deposited per unit mass of a medium, and so has the unit joules (J) per kilogram (kg), with the adopted name of gray (Gy) where 1 Gy = 1 J.kg^-1.

The absorbed dose is not a good indicator of the likely biological effect. 1 Gy of alpha radiation would be much more biologically damaging than 1 Gy of photon radiation for example. Appropriate weighting factors can be applied reflecting the different relative biological effects to find the equivalent dose.

The risk of stochastic effects due to radiation exposure for the population can be quantified using the effective dose, which is a weighted average of the equivalent dose to each organ depending upon its radiosensitivity.

Other related values include:

• absorbed dose rate (Gy.s^-1): amount of radiation delivered over a time period
• rad: the international unit of absorbed dose pre-1980 where 1 Gy = 100 rad
• kerma (Gy): kinetic energy released per unit mass