Definition
The gray and sievert units are both special names for the SI derived units of joules per kilogram (m2/s2 if expressed in base units), though they are not interchangeable.
- 1 Sv = 1 J/kg = 1 Gy
The gray is used with quantities of absorbed dose in any material, while the sievert is used with equivalent, effective, and committed dose in biological tissue. The latter quantities are weighted averages of absorbed dose designed to be more representative of the stochastic health effects of radiation, and use of the sievert implies that appropriate regulatory weighting factors have been applied to the original measurement.
For example, an absorbed dose of 1 mGy of alpha radiation gives an equivalent dose of 20 mSv because alpha has a radiation weighting factor of 20. This can appear to lead to a paradox whereby the energy of the incident radiation field is increased by a factor of 20 when sieverts are used, thereby violating the laws of Conservation of energy. However this is not the case, as it is the biological effect that is being expressed when using sieverts. Sieverts do not represent the actual physical energy imparted by the incident radiation, but an estimation of the biological effect.
Confusion can be caused as there are two different radiation quantities that can both be measured in the same units of J/kg; the sievert and the gray. To differentiate, consider an analogy where a fire can be put out with plain water or fire fighting foam. In either case, the flow rate could be measured in units of litre/sec, but the foam could be 5 or 20 times more effective than plain water, depending on the type of foam. One could choose to use litres per second (analogous to grays) to express the real flow rate of water or foam, and cubic decimetres per second (analogous to sieverts) to describe the equivalent flow of plain water that would produce the same effect as the foam.
The sievert and the gray are different names for the same unit of delivery, but they are used in different contexts.
Read more about this topic: Sievert
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