Units of radioactivity

For the quantitative measurement of different aspects of radioactivity, there are a substantial number of units of radioactivity. The number is relatively high because the aspects include the energy of a radiation source, as well as the effect of ionizing radiation in air, absorbed in arbitrary materials, and specific effects on biological systems. Doubling the number of units is the reality that they are defined both for the International System of Units (SI) and in "traditional" or "customary" units.

Property measured	SI unit	Other unit	Notes
Rate of emission from a source	Becquerel (Bq)	Curie (Ci) Counts per minute
Air ionization by radiation	G rays per hour (Gy/h)	Roentgen (R)	Used for X-rays and gamma radiation
Absorbed dose	Gray (Gy)	rad	1 Gray = 100 rad
Biological equivalent dose	Sievert (Sv)	rem

Radioactive source energy

1 Bq = 1 event of radiation emission per second. Since this is a very small unit, common measurements are:

• 1 kBq = 10³ Bq
• 1 MBq = 10⁶ Bq
• 1 GBq = 10⁹ Bq

The older unit, the Curie (Ci), is equal to 37 GBq. The Curie is a large unit, so common representations are

• 1 mCi = 10^-3Ci
• 1 uCi = 10^-6Ci
• 1 nCi = 10^-9Ci
• 1 pCi = 10^-12Ci

Linking the two sets of units,

1 Bq = 27 pCi

Neither the Bq nor the Ci measure the energy of release from the source, only its rate. The energy is specified in electronvolts.