The sievert and the gray are both joules per kilogram, yet mean different things
On paper, the gray and the sievert are identical: one joule of energy per kilogram. But the gray records physical energy deposited in matter, while the sievert estimates the long-term cancer risk to a human body. Regulators insist on the separate names precisely so nobody mixes the two up.
Its name honours Rolf Maximilian Sievert, the Swede whose work in medical physics centred on measuring radiation doses and what they do to living tissue. It expresses stochastic risk, meaning the chance that ionising radiation will eventually cause cancer or genetic damage, rather than certain harm. That makes it central to dosimetry and radiation protection. Acute, certain tissue damage from very high doses is conventionally described with the gray instead.
Getting from one to the other requires weighting. The absorbed dose in grays is multiplied by factors for the type of radiation and the tissue involved, published by two bodies, the International Commission on Radiological Protection and the International Commission on Radiation Units and Measurements. The official SI definition multiplies absorbed dose by a dimensionless quality factor tied to how densely the radiation deposits energy along its path, a factor the SI leaves to ICRU recommendations. One sievert equals 100 rem, an older unit.
The ICRP puts one sievert at a 5.5 percent chance of eventually developing fatal cancer. That estimate rests on the linear no-threshold model, which assumes risk rises in proportion to dose even at low levels, and the model is disputed.
Measurement runs on two tracks. Dosimeters and instruments record operational quantities, the domain of the ICRU, while the ICRP models protection quantities such as effective and equivalent dose from the body's uptake and sensitivity, drawing on large epidemiological studies. The vocabulary can trip up experts: equivalent dose and dose equivalent are different things. Since 1990 the ICRP's protection quantities have used more complex models and dropped the phrase dose equivalent, which survives only in operational quantities. Joint proposals outlined in October 2015 would tidy this up by adding measures such as dose to the lens of the eye and dose to local skin.
Source: Sievert
