In This Section
In the world today, radiation can be measured using two different systems of units, depending on the standards set by each country. The conventional system of units (including the Curie, Rad and Rem) was developed over the past 100 years. This system is still used by regulatory agencies in the United States because it is required by federal law. More recently, the System Internationale (SI) (including the Becquerel, Sievert and Gray) evolved as part of the metric system and is used to measure radiation by all other governments and scientists in the international community.
Depending on what aspect of radiation is being measured, different terms are applied:
- For the amount of radiation being given off by a radioactive material: the conventional unit is curie (Ci) and the SI unit becquerel (Bq).
- For radiation absorbed by a person: the conventional unit is rad and the SI unit gray (Gy).
- For biological risk of exposure to radiation: the conventional unit is rem and the SI unit sievert (Sv).
Japan and other countries use the international system, in which the unit for measuring radiation dose is the Sievert.
The conversions between Sievert and Rem are as follows:
1 microSievert (µSv) = 0.1 mrem
1 milliSievert (mSv) = 100 mrem
1 centiSievert (cSv) = 1 rem (1000 millirem)
1 Sievert (Sv) = 100 rem
The international unit “Gray” is used to measure absorbed radiation dose. The following conversion illustrates how the “Gray” translates into Rems:
1 Gray (Gy) = 100 Rads = 1 Sievert (Sv) = 100 Rem.
As a measure of levels of radioactivity, Japan and other countries use the unit “Becquerel,” whereas the United States uses the unit “Curie.” By definition, a Curie is an amount of radioactivity equivalent to a radioactive decay rate of 37 billion disintegrations per second. A Becquerel represents the amount of radioactivity equivalent to a decay rate of one disintegration per second.
Conversion factors between the two units are as follows:
1 Ci = 3.7×1010 Bq (37 billion Becquerels)
1 Ci = 37 GBq (37 GigaBecquerels)
1 µCi = 37,000 Bq
1 Bq = 2.70×10-11 Ci (less than one ten billionth of a Curie)
1 Bq = 2.70×10-5 µCi
1 GBq = 0.0270 Ci
For additional information on measuring radiation, visit the websites listed below.