Units of radioactivity

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Revision as of 15:45, 6 April 2011 by imported>Howard C. Berkowitz (→‎Activity of a source of radiation)
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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.





Activity of a source of radiation

As opposed to most of the other units, the SI unit becquerel[1] (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

Absorption in materials

Biologic effects

Qualifiers

Decay reactions

References

  1. While the abbreviations of units are capitalized, the spelled-out names are not, to avoid confusion with the person they honor.
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