Units of radioactivity

Relations among units

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.

Properly interpreting the units can be quite complex. A given quantum of ionizing radiation will have different effects not only due to the type of radiation, which is considered in the computation of the units of biological effect, sieverts and rems, but also due to the energy level within a radiation type. Fast, medium-speed and slow neutrons, for example, have different biological effects.

Activity of a source of radiation

As opposed to most of the other units, the SI unit becquerel (Si) and the common unit curie (Ci) deal with the activity of the source, not the effects on radioactivity reaching its destination. Linked with area or volume measurements, these units are useful in giving a quantitative measurement of contamination of areas of the ground or volumes of water. When the source contains multiple isotopes, as with a reactor accident, it is most useful to state activity of each isotope.

Basic units

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

Measuring contamination

Contamination of surfaces, such as the ground near a radiation release, is stated in Bq or Ci units per square metre. Contamination of water is expressed in Bg or Ci per liter.

Refining activity

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

Ionization of air

Roentgens measure the ionization of air from a radioactive source, but knowing that is less important than knowing other measurements.

Biologic effects

Radiation weighting (WR) factors

Radiation type	Energy level	Weighting factor (WR))
Gamma rays and x rays	All	1
Beta particles	All	1
Neutrons	< 10 KeV	5
	> 10 keV to 100 keV	10
	> 100 keV to 2 MeV	20
	> 2 MeV to 20 MeV	10
	> 20 MeV	5
Alpha particles	All	20

References

Summary of units

Property measured	SI unit	Other unit	Notes
Rate of emission from a source	becquerel (Bq)	curie (Ci) Counts (or disintegrations) per minute
Air ionization by radiation	grays 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