The units of radioactivity

• The Becquerel

The radioactivity of a sample can be quantified by the number of radioactive nuclei that decay per second. The unit of measure of radioactivity is the Becquerel (Bq), corresponding to the decay of one nucleus per second. The activity of radioactive sources is often expressed in multiples of the Becquerel (kBq, Mbq, Gbq, Tbq, etc.) while the activity of samples collected in the environment is expressed in Bq, mBq or μBq. Radioactivity is often expressed per unit volume (e.g. Bq/l or Bq/m³), per unit mass (e.g. Bq/kg) or per unit area (Bq/m²). The old unit of measure for radioactivity was the Curie (Ci), defined as the activity of one gram of radium (1Ci = 37 billion Bq).

These radioactivity units reflect the rate of decay of a radioactive sample but not the related energy or the effect on human beings.

• The Gray (Gy)

The unit of measure of the absorbed doses of radiation is the Gray (Gy), corresponding to the energy imparted to a unit mass of matter. It can therefore be used to express the quantity of radiation absorbed by an organism or an object exposed to a given source of radiation. The Gray was replaced by the rad in 1986 (1 Gy = 100 rads = 1 Joule per kg of irradiated matter).

• The Sievert

The effects of radioactivity on living organisms or on inert matter are not directly related to the Becquerel, for several reasons. First, the energy released by the decay of an atom varies from one radionuclide species to another. Second, the radiation emitted by different radionuclide species is often of a different nature. Third, the different types of radiation do not necessarily affect the organism in the same manner.

Certain body tissues and organs are more sensitive to radiation than others. To take this into account, the dose equivalent is multiplied by a weighting factor intended to reflect the radiosensitivity of each tissue or organ, leading to an effective dose. The advantage of this system is that all types of human exposure to ionizing radiation can be placed on the same scale of risk. The value of the effective dose is generally very small and is therefore often expressed in milliSieverts (mSv).

The relationship between these three units can be represented schematically by a child throwing snowballs at another child. The number of snowballs thrown can be compared to the number of decays emitted by a radioactive source, i.e. its activity in Becquerels. The number of snowballs hitting the other child represents the absorbed dose (in Grays). The marks left on the body, depending on the weighs of the snowballs and the sensitivity of the parts of the body hit, are the effects produced and can be compared to the effective dose (in Sieverts).