equivalent dose
Equivalent dose (symbol HT) is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionizing radiation. In quantitative terms, equivalent dose is less fundamental than absorbed dose, but it is more biologically significant. Equivalent dose is measured using the sievert but rem is still commonly used (1 Sv = 100 rem).
Equivalent dose (HT) is calculated by multiplying the absorbed dose to the organ or tissue (DT) with the radiation weighting factor, wR. This factor is dependent on the type and energy of the incident radiation. The value of wR is 1 for x-rays, gamma rays and beta particles, but higher for protons (wR = 5), neutrons (wR is between 5 and 20 depending on energy), alpha particles and heavy fragments (wR = 20) etc.
The numerical values given above are valid for legal EU regulations for calculating equivalent dose in an organ or tissue. In 2007, International Commission on Radiological Protection (ICRP) published a new set of radiation weighting factors (ICRP Publ. 103: The 2007 Recommendations of the International Commission on Radiological Protection) as below:
- photon, all energies = 1
- electron, muons, all energies = 1
- protons, charged pions = 2
- alpha particles, fission fragments, heavy ions = 20
- neutrons = continuous function of neutron energy calculated using 3 complex equations depending on energy
Permissible dose
- occupational exposure: 20 mSv/year (effective dose)
- lens of eye: 20 mSv/year (updated in ICRP 2011 - previously 150 mSv/year)
- skin (average dose over 1 cm of the most highly irradiated area of skin): 500 mSv/year
- hands and feet: 500 mSv/year
- females of reproductive capacity: 13 mSv/any three month period (additional restriction is to protect a recently conceived foetus within a female who may be unaware of her pregnancy)
- fetus (during declared term of pregnancy): 1 mSv/over declared term of pregnancy
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