The concept of the "equivalent dose" which takes into account the nature of radiations, is a refinement of the absorbed dose (*), the local dose introduced first in the history of radioprotection and nuclear medicine to estimate the energy deposited in the human body by radioactive or X-ray sources. The original unit for the 'absorbed dose was instituted in 1920. It was the 'rad' which has since been replaced by the gray, a unit worth 100 rads. In radiotherapies and nuclear therapies, it is the number of grays or milligrays absorbed by an organ or tissues that matters for doctors.
In the field of radioprotection, however, the concept of absorbed dose rather quickly proved to be inadequate. In was observed in the 1950s from exposiure involving neutrons and photons that a 'gray' of neutrons was ten times more carcinogenic than a 'gray' of photons. As a result, the absorbed dose of neutrons had to be multiplied by 10 to obtain a new, more helpful quantity, the 'equivalent dose'.
The equivalent dose is obtained by multiplying the absorbed dose for an organ or body part with a weighting factor - a number that takes into account the nature of the radiations (such as whether they are made up of photons, electrons, neutrons, alpha particles etc) and the severity of the biological damages it causes. The original equivalent dose unit was the rem which corresponds to the rad. The 'rem', the old unit for equivalent dose, has become the sievert, which has a value equal to 100 rems.
The weighting factors have empirical values which reflect the current knowledge in radioprotection and may be improved in the future. For instance, they do not consider a crucial factor, the dose rate, the number of grays absorbed per second. The less damaging particles are the less ionizing ones, photons and electrons.
In the field of nuclear diagnostics, where beta and gamma rays are used, the equivalent doses are therefore numerically identical to the absorbed doses, though the two have different units. In these situations, however, it has become more common to talk about the value in sieverts.
Another source of confusion is that the sievert serves as the unit for two different quantities: the equivalent dose, used to determine the dose absorbed by a specific organ or body part, and the effective dose, used to determine the effect on the whole body.
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