EXPLANATION OF TERMS
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) = 10^18 Bq
petabecquerel (PBq) = 10^15 Bq
terabecquerel (TBq) = 10^12 Bq
gigabecquerel (GBq) = 10^9 Bq
megabecquerel (MBq) = 10^6 Bq
kilobecquerel (kBq) = 10^3 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 (Sv) = 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
Acute radiation syndrome
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 texLate 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.
