Radiation units are confusing for three or four reasons.
- There are different units depending on whether you’re measuring how much radiation is being emitted or measuring how much is being received.
- There are different ways of quantifying the amount of radiation received depending on whether you’re doing physics or biology.
- For each of these measurements there are traditional units and SI units.
If you’re not familiar with scientific units, a fourth source of confusion is the prefixes for various powers of 10: milli-, micro-, etc.
The amount of radioactivity emitted by a source is measured in Becquerels or Curies. The SI unit the becquerel (Bq), one decay per second. The traditional unit Curie (Ci) is 3.7 × 1010 Bq and is about the radioactivity of a gram of radium.
The amount of radiation received by a source is measured in grays or rads. The SI unit Gray (Gy) corresponds to one joule of energy absorbed by one kilogram of matter. The traditional unit rad is 0.01 Gy.
The biological effect of radiation is measured in Sieverts or rems. Biologically effective dose is the amount of radiation received multiplied by the relative biological effectiveness (RBE) of the type of radiation source. For x-rays, the RBE is 1. For alpha rays, the RBE is 20. The SI unit of effective dose is the Sievert (Sv), which corresponds to one Gy of x-rays. A rem is 0.01 Sv.
Another unit of effect is the banana equivalent dose. A banana is 0.0001 mSv, or roughly the effective dose of radiation from eating a banana.