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SI units of ionizing radiation

SI Brochure, from Section 2.2.2

In the field of ionizing radiation, the SI unit of activity is designated the becquerel rather than the reciprocal second, and the SI units of absorbed dose and dose equivalent are designated the gray and the sievert, respectively, rather than the joule per kilogram. The special names becquerel, gray, and sievert were specifically introduced because of the dangers to human health that might arise from mistakes involving the units reciprocal second and joule per kilogram, in case the latter units were incorrectly taken to identify the different quantities involved.

Derived quantity SI coherent derived unit (a)
Name Symbol Expressed
in terms of
other SI units
in terms of
SI base units
activity referred to a radionuclide (c) becquerel (b) Bq   s–1
absorbed dose, specific energy (imparted), kerma gray Gy J/kg m2 s–2
dose equivalent,
ambient dose equivalent,
directional dose equivalent,
personal dose equivalent
sievert (d) Sv J/kg m2 s–2

(a) The SI prefixes may be used with any of the special names and symbols, but when this is done the resulting unit will no longer be coherent.
(b) The becquerel is used only for stochastic processes in activity referred to a radionuclide.
(c) Activity referred to a radionuclide is sometimes incorrectly called radioactivity.
(d) See CIPM Recommendation 2 (CI-2002) on the use of the sievert.

[ from Table 3 ]
SI Brochure

photo of Henri Becquerel

Henri Becquerel's early work was concerned with the polarization of light, the phenomenon of phosphorescence and the absorption of light by crystals (his doctoral thesis). He was elected a member of the French Académie des Sciences in 1889. For his discovery of natural radioactivity in 1896, Henri Becquerel was awarded half of the Nobel Prize for Physics in 1903, the other half being awarded to Pierre and Marie Curie for their study of the Becquerel radiation. This is why the General Conference on Weights and Measures (CGPM) of 1975 (Resolution 8) decided to honour Henri Becquerel by adopting the special name of becquerel, Bq, for the SI derived unit of activity. This proposal had been made by the International Commission for Radiation Units and Measurements (ICRU) and accepted by the Consultative Committee for Units (CCU) as Recommendation U 1 (1974), the earlier non-SI unit having been named after the Curies.

For more biographical information see:

Hal Gray worked at the Cavendish Laboratory, Cambridge, UK with Rutherford (1927-1932) on the absorption of gamma rays in matter. This research resulted in the Bragg-Gray principle, the application of which enabled the measurement of energy imparted and absorbed dose. That is why the gray was proposed as the special name for the SI derived unit associated with these quantities by the International Commission for Radiation Units and Measurements (ICRU) of which Hal Gray was a former Vice-Chairman.

The Consultative Committee for Units (CCU) accepted this proposal in 1974 (Recommendation U 1 (1974)). Subsequently, the 15th General Conference on Weights and Measures (CGPM) adopted the special name gray, Gy, to be included in the SI in 1975 (Resolution 9). Indeed, Gray's name is also commemorated in the Gray Laboratory for radiobiology research based at Mount Vernon Hospital in the UK (where he worked from 1933), in the Gray Trust that sponsors a biennial Gray Conference, and in the Gray Medal awarded by the ICRU. For biographies see ICRU News June 1997, the Royal Society Biographical Memoirs, and the websites of the Gray Laboratory and the Gray Cancer Institute.

Rolf Sievert developed the Sievert chamber for measuring radiation dose and the Sievert integral for the calculation of exposure at a point P. He was one of the first members of both the International Commission for Radiological Protection (ICRP) and the International Commission for Radiation Units and Measurements (ICRU). As a pioneer in radiation protection, he was responsible for instigating Sweden's first radiation protection law passed in 1941 and developing the plans for the Swedish Radiation Safety Authority, the SSM. Rolf Sievert devoted much of his life to radiation protection issues and was elected Chairman of the ICRP from 1956 to 1962.

In his honour, the General Conference of Weights and Measures (CGPM) in Resolution 5 of 1979 adopted the sievert, Sv, as the special name for the SI derived unit for the radiation protection quantity, dose equivalent. This had been proposed by the ICRP and the ICRU and accepted by the Consultative Committee for Units (CCU) as Recommendation U 1 (1978). For a biography see the Karolinska Institute.

  September 1910

The Radiology Congress met in Brussels and established the International Radioactivity Commission (Commission Internationale de Radioactivité). Mme M. Curie was asked to prepare an International Radium Standard.

  18 March 1912

A. Debierne addressed a letter to Ch.-Éd. Guillaume (then Deputy Director of the BIPM) asking him if the BIPM would be willing to keep the Radium Standard.

  12 May 1912

S. Meyer, secretary of the Commission Internationale des Étalons de Radium, thanked R. Benoît, Director of the BIPM, for agreeing to keep the standard.

  21 February 1913

The standard was deposited "in the lower division of the safe located in the first vault".

  1913 to 1935

Eight times in total the international standard was taken out for various comparisons at the Laboratoire Curie and brought back to the vault at the BIPM. After being used for the last time it was kept at the Laboratoire Curie.


O. Hönigschmid prepared twenty radium standards from pitchblende, originating from High Katanga, containing a high percentage of radium free of mesothorium. Among them, standard No. 5430, with a mass almost identical to that of the "Curie standard", was assigned to France and handed over to the Laboratoire Curie.

  21 April 1939

Standard No. 5430 was deposited at the BIPM.

  30 May 1940

The standard was transferred to a place outside Paris for safe keeping.

  October 1948

At the 9th CGPM the Soviet delegation proposed to "organize comparisons of national standards of radium with international standards of radium and to maintain these at the Bureau International".

  July 1953

The Joint Commission for Radioactivity (Commission mixte de radioactivité), meeting in Stockholm, entrusted standard No. 5430 to the Director of the Radium Institute (Institut du Radium).

  3 December 1959

The Faculty of Sciences of Paris, to which the Radium Institute belonged, decided to entrust the BIPM with the radium standard No. 5430. This standard remained the "property of the University of Paris".

  October 1960

The 11th CGPM authorized the BIPM to maintain radium standard No. 5430.

  9 March 1961

The University of Paris donated the standard to the BIPM and an international comparison of radium standards was organized in 1963. Later, primary measurement methods and the introduction of the SI definition for activity replaced the need to hold such standards.

  22 January 1993

Having kept the artefact safely at the BIPM for a further thirty-two years, the radium standard 5430 was finally disposed of as radioactive waste, by the French authorities, following international safety recommendations.