How is radiation measured?

Units of Radiation Measurements

The units of radiation measurement used in Canada and in much of the world are Système Internationale (SI) units, based on the metric system. These units include the becquerel, the gray, and the sievert, terms which are described below. In the 1960's the SI units of radiation replaced another set of units, which had been in use from the early twentieth century. The older units are still used in the United States and are found in some earlier documents; they are summarised in the table below.

Bequerel

The becquerel (Bq) is the unit used to measure radioactivity in a substance, without consideration for the type of radiation it is or what its effects may be. One becquerel is equal to one nuclear disintegration per second. Since this is a very small unit, other units are often used, including the kilobecquerel (kBq: thousand Bq), megabecquerel (MBq: million Bq), and gigabecquerel (GBq: thousand million Bq). The name recognises the scientific contributions of French physicist A-H. Becquerel.

Gray

The gray (Gy) is the unit used in describing the amount of energy absorbed by a substance from the radiation passing through it, or the absorbed dose. One gray corresponds to one joule of radiation energy absorbed by one kilogram of matter. The measurement of grays per hour received by a substance tells us the intensity of the radiation. This unit was named after the English radiobiologist Louis H. Gray.

Sievert

The sievert (Sv) is the unit that reflects the biological effects of the ionising radiation absorbed; it is used in expressing both the equivalent dose and the effective dose. The sievert is a very large dose of radiation; a more useful unit is the millisievert (mSv), which is one-thousandth of a sievert. The name honours Rolf M. Sievert, a Swedish physicist.

Person-sievert

The person-sievert (person-Sv) is the unit used to express the collective dose, a useful measure of the total radiation exposure resulting from a specific source or activity to an entire group of people.

Units of Measurements

This table provides a simple comparison between the terminology used in the Système Internationale and the old system.

What is being measured	International System	Old System	Conversion factors
Activity	Becquerel (Bq)	Curie (Ci)	1CI = 37 GBq
Radiation absorbed	Gray (Gy)	Rad (rad)	1rad = 10 mGy
Biological effect of radiation absorbed	Sievert (Sv)	Rem (rem)	1rem = 10 mSv