Radiation Units


Measuring radioactivity

The nuclei of certain atoms are unstable and spontaneously disintegrate emitting radiation (alpha-particles, beta-particles or gamma rays). This changes the nature of the nucleus, and so the atom transforms (decays) into a different type of atom. The radioactivity of a given amount of material is the number of nuclear decays that take place per unit of time.

Radioactivity units used:

  • becquerel (Bq) is the SI unit of radioactivity equal to one nuclear decay per second. 1 Bq = 1 / s
  • Therefore, an amount of material that produces one nuclear decay per second, is said to have a radioactivity of 1
  • Bq exabecquerel (Ebq) = 1018 Bq

Measuring exposure to radioactivity

Ionizing radiation (such as alpha, beta and gamma radiation) is a very high-energy form of electromagnetic radiation, and can strip electrons from the atoms in the material through which it passes. This may damage human cells, causing death to some cells and modifying others. Dose is a measure of the amount of energy from ionizing radiation deposited in a specified material.

Absorbed dose is the absorbed energy per unit mass.

Unit of absorbed dose used:

  • gray (Gy) is the SI unit of absorbed dose, which is a joule per kilogram (J kg–1)

Effective dose is the absorbed dose multiplied by a factor that takes into consideration the type of radiation and the susceptibility of various organs and tissues to development of a severe radiation-induced cancer or genetic effect. Moreover, it applies equally to external and internal exposure and to uniform or non-uniform irradiation.

Units of effective dose used:

  • sievert (Sv) is the SI unit of effective dose. Its units are J kg–1.
  • millisievert (mSv) 1 mSv = 0.001 Sv

Source & © Further information on the International System of Units (SI) is provided by the International Bureau of Weights and Measures (BIPM) 

Related publication:
Chernobyl homeChernobyl Nuclear Accident
Other Figures & Tables on this publication:

Figure 1. Surface-ground deposition of 137Cs throughout Europe as a result of the Chernobyl accident (De Cort et al. 1998)

Figure 2. Pathways of exposure to man from environmental releases of radioactive materials

Figure 3. Incidence rate of thyroid cancer in children and adolescents exposed to 131I as a result of the Chernobyl accident (Jacob et al., )

Figure 4. Prevalence at birth of congenital malformations in 4 oblasts of Belarus with high and low levels of radio nuclide contamination (Lazjuk et al., 1999)

Figure 5. Reduction with time of 137Cs activity concentration in milk produced in private and collective farms of the Rovno region of Ukraine with a comparison to the temporary permissible level (TPL)

Figure 6. Averaged 137Cs activity concentrations in fish from Kyiv reservoir (UHMI 2004)

Figure 7. Changes with time in the use of Prussian blue in the CIS countries (IAEA, )

Figure 10. What worries you most today?

Where is Chernobyl located?

Administrative regions surrounding the Chernobyl reactor

Table: Summary of average accumulated doses to affected populations from Chernobyl fallout

Table: Chernobyl-related construction, 1986-2000

Figure 1. Surface-ground deposition of 137Cs throughout Europe as a result of the Chernobyl accident (De Cort et al. 1998)

Major radioactive substances released by the Chernobyl accident

Radiation Units