APPENDIX 1 VALUES AND UNITS USED IN RADIATION PROTECTION
  1 The main values used in radiation protection
It is impossible to apply radiation protection rules without metrology, as the most important exposure indicators for radiation protection are the doses received by man. Transposition of Council directive No 96/29/Euratom of 13 May 1996 laying down the basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionising radiation enabled the definitions of the main values used in radiation protection to be updated (appendix 13-7, regulatory part of the Public Health Code).
Activity and becquerel

Activity (A): the activity A of an amount of a radionuclide in a particular energy state at a given time is the quotient of dN by dt, where dN is the expectation value of the number of spontaneous nuclear transitions with emission of ionising radiation from that energy state in the time interval dt.

                            dN
                     A = ——
                            dt

The unit of activity of a radioactive source is the becquerel (Bq).
 
Absorbed dose and gray

Absorbed dose (D): energy absorbed per unit mass

                            dE
                     D = ——
                            dm
where
dE is the mean energy communicated by the ionising radiation to the matter in a volume element;
dm is the mass of the matter in this volume element.
The term "absorbed dose" designates the mean dose received by a tissue or an organ.

The unit of absorbed dose is the gray (Gy).

The absorbed dose D represents the quantity of energy absorbed per unit mass of tissue. 1 gray (Gy) corresponds to the absorption of 1 joule per kilogram. This quantity designates the mean dose absorbed by a tissue, organ or the whole body. However, the absorbed dose cannot be directly used in radiation protection because it does not take account of the fact that the biological effects of the energy intake depend on a number of parameters:
• the quality of the radiation, in other words how it loses its energy in the micro-volumes along its path. This depends on its nature, whether electromagnetic (X or gamma rays) or electrically charged or uncharged particle (alpha, beta or neutrons);
• the characteristics of the organ or tissue into which the energy is taken, as not all tissues have the same sensitivity to radiation;
• the dose rate, that is the inclusion of the time factor in the energy intake.

A large number of experiments have analysed the importance of each of these factors with regard to the biological effects of irradiation. To manage all the doses received by an individual, equivalent dose must be used which take account of these exposure parameters. Weighting factors are thus applied to the "absorbed dose" when one wishes to define the "equivalent dose" which takes account of the nature of the radiation and the "effective dose" which concerns the whole body.
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