Equivalent dose is a crucial concept in radiological protection, developed to provide a measure of the biological effect of radiation on tissues and organs, taking into account the type of incident radiation. Unlike absorbed dose, which measures the energy deposited per unit mass, equivalent dose characterises the potential for harm from that deposited energy, acknowledging that different types of radiation produce different degrees of biological damage for the same absorbed dose.
The International Commission on Radiological Protection (ICRP) defines equivalent dose, , to a tissue or organ (T) as the absorbed dose, , due to radiation type (R), weighted by a radiation weighting factor, . This can be expressed by the following summation:
Where:
The radiation weighting factors () are established by international bodies like the ICRP and are dependent on the type and energy of the radiation. For example, alpha particles have a much higher value than photons or electrons because they deposit their energy more densely, causing more concentrated damage. Typical values include:
These factors are crucial for accurately assessing the risk of stochastic effects, such as cancer and hereditary effects, which are probabilistic in nature and whose likelihood increases with dose. They are distinct from quality factors used historically in some contexts.
The standard international unit for equivalent dose is the Sievert (Sv). One Sievert is equivalent to one Joule per kilogram (J/kg), but it is specifically used for equivalent dose to distinguish it from absorbed dose (measured in Grays, Gy) and to emphasise the biological weighting. In older systems, the unit 'rem' (roentgen equivalent man) was used, where 1 Sv = 100 rem.
Equivalent dose provides a more biologically relevant measure than absorbed dose alone for assessing the potential for harm to human health. It is particularly important for:
It is important to note that equivalent dose is a protection quantity and cannot be measured directly. It is calculated from measurable physical quantities like absorbed dose and the applicable radiation weighting factors. Further details on the definitions and values can be found in ICRP publications, particularly ICRP Publication 103, which provides the current recommendations.