Canada and much of the world uses units from the Système International (SI) to measure radiation. The SI is based on the metric system. These units include the:
From the early 1900s, through to the 30s, until the 1960s, another set of units were used. They were called the:
The United States still uses these units. You may also find them in older documents.
|Old system to
to Old system
Table 1 footnotes
|The becquerel (Bq) replaces the curie (Ci).*
||The becquerel (Bq)Table 1 footnote * replaces the curie (Ci).
|The gray (Gy) replaces the rad (rad).
||The gray (Gy) replaces the rad (rad).
|The coulomb/kg (C/kg) replaces the roentgen (R).
||The coulomb/kg (C/kg) replaces the roentgen (R).
|The sievert (Sv) replaces the rem (rem).
||The sievert (Sv) replaces the rem (rem).
The becquerel (Bq) is named after the French physicist A.H. Becquerel. This unit measures radioactivity in a substance. It doesn't consider the type of radiation emitted or what its effects may be. One becquerel equals one nuclear disintegration per second. This is a very small unit, so multiples are often used. These include the:
The gray (Gy) was defined in 1975 in honor of English radiobiologist Louis H. Gray (1905-1965). This unit describes how much energy is 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. Measuring how many grays a substance receives in one hour tells us what the rate is. The gray is a very large dose of radiation. A more useful unit is the milligray (mGv). This is one-thousandth of a gray.
The sievert (Sv) is named after the Swedish physicist Rolf M. Sievert. The unit reflects the biological effects of the ionizing radiation absorbed. It is used to express 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). This is one-thousandth of a sievert.