Radioactivity measurement units

Measuring radioactivity: the becquerel

The becquerel (Bq) measures the activity of the radioactive source, meaning the number of atoms which, within a particular time frame, transform and emit radiation.

1 Bq = 1 emission of radiation per second.

This is a very small unit, and multiples are often used:

• 1 MBq = 1 mega becquerel = 1,000,000 Bq
• 1 GBq = 1 giga becquerel = 1,000,000,000 Bq
• 1 TBq = 1 tera becquerel = 1,000,000,000,000 Bq

The radioactivity of an environment, a material or a foodstuff is given in Becquerels per kilogram or per liter.

Measuring the absorbed dose: the Gray

The gray (Gy) measures the absorbed dose, meaning the energy transferred to the material by ionizing radiation upon encountering it.

1 Gy = 1 joule per kilogram

Sub-multiples are often used:

• 1 mGy = 1 milligray = 0.001 Gy
• 1 μGy = 1 microgray = 0.000001 Gy 
• 1 nGy = 1 nanogray = 0.000000001 Gy

Measuring the biological effect: the sievert

The sievert (Sv) evaluates the effects of ionizing radiation on living material. At equal doses, the effects of radioactivity on living tissue depends on the type of radiation (alpha, beta, gamma, etc.), on the organ concerned and naturally on the length of exposure.

Contrary to the becquerel, the sievert is a very large unit, and we often use sub-multiples:

• 1 mSv = 1 millisievert = 0.001 Sv
• 1 µSv = 1 microsievert = 0.000001 Sv