Chernobyl: Explanation of terms
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#### Activity

Quantity of a radionuclide. It describes the rate at which spontaneous nuclear transformations (i.e., radioactive decay) occur in it. It is measured in becquerels (Bq), where 1 Bq equals one nuclear transformation per second.

Several multiples of the becquerel (Bq) are used throughout the text. They are the following:

exabecquerel (EBq) = 1018 Bq

petabecquerel (PBq) = 1015 Bq

terabecquerel (TBq) = 1012 Bq

gigabecquerel (GBq) = 109 Bq

megabecquerel (MBq) = 106 Bq

kilobecquerel (kBq) = 103 Bq

#### Collective dose

Total dose over a population group exposed to a given source. It is represented by the product of the average dose to the individuals in the group by the number of persons comprising the group. It is measured in person-sieverts (person-Sv).

#### Dose

A general term denoting a quantity of radiation. Depending on its application it can be qualified as "absorbed dose", "equivalent dose" and "effective dose".

Absorbed dose

Quantity of energy imparted by radiation to a unit mass of matter such as tissue. Absorbed dose is measured in grays (Gy), where 1 Gy equals 1 joule of energy absorbed per kilogramme of matter. One gray produces a different intensity of biological effects on tissue depending on the type of radiation (alpha, beta, gamma, neutrons). One common submultiple of the gray, the milligray, is often used. One milligray (mGy) is equal to 10-3 Gy.

Effective dose

Weighted sum of the "equivalent doses" to the various organs and tissues multiplied by weighting factors reflecting the differing sensitivities of organs and tissues to radiation. The weighting factor for each organ or tissue expresses the fractional contribution of the risk of death or serious genetic defect from irradiation of that organ or tissue to the total risk from uniform irradiation of the whole body. Effective dose is measured in sieverts (Sv). Some submultiples of the sievert are used throughout the text. They are the following:

millisievert (mSv) = 10-3 Sv

microsievert (µSv) = 10-6 Sv

Equivalent dose

Quantity obtained by multiplying the "absorbed dose" in an organ (e.g., thyroid) or tissue by a factor representing the different effectiveness of the various types of radiation in causing harm to the organ or tissue. This factor, whose value varies between 1 and 20 depending on the type of radiation, has been introduced in order to allow grouping or comparing biological effects due to different radiations. Equivalent dose is measured in sieverts (Sv). One sievert produces the same biological effect, irrespective of the type of radiation.

#### Health effects

A clinical scenario characterized by a complex of acute deterministic effects affecting various organs and body functions in the irradiated person.

Deterministic effects (also called acute health effects)

Early deleterious radiation effects on living tissues (e.g., body, organ or tissue death, cataracts), which generally occur only above a threshold of dose and whose severity depends on the level of dose absorbed. They become generally evident within a short time from the irradiation (hours, days or weeks, depending on the dose received). Throughout the text the doses producing Deterministic effects are expressed in grays (Gy).

Genetic effects (also called hereditary effects)

Stochastic effect which occur in the progeny of the exposed person.

Stochastic effects (also called late health effects)

Late deleterious radiation effects (e.g., leukaemia, tumours) whose severity is independent of dose and whose probability of occuring is assumed to be proportional to the dose received. It is also assumed that there is no threshold dose below which stochastic effects will not occur. The stochastic effects occur, therefore, at doses lower than those producing deterministic effects and may manifest themselves after a long time (years, decades) from the irradiation. Throughout the text the doses producing stochastic effects are expressed in sieverts (Sv).

#### Intervention level

The value of a quantity (dose, activity concentration) which, if exceeded or predicted to be exceeded in case of an accident, may require the application of a given protective action.

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