7.5.1 Units of radioactivity and radiation dose

The International System of Units (SI) unit of radioactivity is the Becquerel (Bq). 1 Bq is defined as the activity, A, for an amount of radionuclide in which one nucleus decays per second. 1 Bq = 1 spontaneous nuclear transformation per second.

The radiation dose resulting from ingestion of a radionuclide depends on a number of chemical and biological factors. These include:

• the fraction of the intake that is absorbed from the gut

• the organs or tissues to which the radionuclide may be transported and deposited

• the time that the radionuclide might remain in the organ or tissue before excretion.

The nature of the radiation emitted on decay and the sensitivity of the irradiated organs or tissues to radiation also needs to be considered.

The absorbed dose refers to how much energy is deposited in a specified tissue or organ of the human body. The equivalent dose is the product of the absorbed dose and a factor related to a particular type of radiation. The equivalent dose of radiation received by a person can be further quantified as the effective dose, which is the sum across the entire body, of the tissue or organ equivalent doses weighted to account for the sensitivity of each organ or tissue to radiation. The SI unit for effective dose is the sievert (Sv).

For monitoring purposes ‘doses’ are determined from the concentration of the radionuclide, which in the case of water is described in terms of Bq/L. This value is converted to an effective dose per year using a dose coefficient and the average annual consumption of water.