S.I. Quantities and Units in Radiation Safety and Conversion Factors
The main quantities of interest to users of this manual are absorbed dose, dose-equivalent, exposure and activity. These are defined as follows:
1. Absorbed dose (usually referred to as dose) is a physical quantity that represents the energy imparted by the radiation to unit mass of any absorbing material. The 'old' unit of dose is the rad, defined as an energy absorption of 100 ergs per gram of material. The S.I. (Système International) unit of dose is the gray (symbol: Gy), where
- 1 Gy = 1 joule per kilogram, or
- 1 Gy = 100 rads
2. Dose equivalent (DE) may be regarded as an expression of dose in terms of its biological effect. Dose-equivalent takes account of the fact that, for a given absorbed dose such as 1 gray, a radiation of one type and/or energy may give rise to a greater biological effect than a radiation of another type and/or energy.
Dose equivalent = dose X quality factor (Q)
where the Q depends on the radiation concerned. For most radiation used in medicine Q is 1.0 so that DE is numerically equal to dose. The 'old' unit of DE is the rem, where
1 rad = Q rems
For example, an absorbed dose of 1 rad of neutrons, for which the quality factor is 10, gives rise to a dose-equivalent of 10 rems.
The S.I. unit of DE is the sievert (symbol: Sv) where
1 Gy = Q sieverts, or
1 Sv = 100 rems
Other relationships of interest are:
1 mSv = 100 mrem
10 µSv = 1 mrem
50 mSv = 5 rems
3. Exposure is a quantity that expresses the ability of radiation to ionise air and thereby create electric charges that can be collected and measured. The unit of exposure is the roentgen (symbol: R), which is the quantity of X- or (-rays which gives rise to 1 electrostatic unit of charge per cc of air at standard temperature and pressure, i.e. 0°C and 101 kilopascals or 760 mm Hg. The roentgen is now defined in terms of the S.I. unit of charge, the coulomb (symbol: C) as follows:
- 1 R = 2.58 X 10-4 Ckg-1 of air, or
- 1 Ckg-1 of air = 3876 R
The concept of exposure is used mainly in diagnostic radiology, where we speak of an "entrance exposure of 1 R at the patient's skin" (not to be confused with an "exposure of 0.1 sec" meaning that the tube is activated for 0.1 second).
Numerically, 1 R is equivalent to a dose of 0.87 rad for most radiation's used in medicine, but for purposes of radiation safety it is sufficient to assume that the roentgen and rad are numerically equal. However, the roentgen is not defined, and therefore cannot be used, for neutrons, charged particles, or photons of energy in excess of about 2 MeV.
4. Activity is the transformation (disintegration) rate of a radioactive substance. The curie (symbol: Ci) corresponds to a transformation rate of 3.7 X1010 disintegration per second, the microcurie (µCi) to 37,000 dis/sec. The S.I. unit of activity is the becquerel (symbol: Bq), where
- 1 Bq = 1 disintegration per second
- 37,000 Bq = 1 µCi
- 3.7 X 107 Bq = 1 mCi
- 3.7 X 1010 Bq = 1 Ci
5. Prefixes are used to denote units which are larger or smaller than the basic unit. The preferred set of prefixes in the International System uses a factor of 1000 (103) between a prefix and the next one, above or below (the use of the prefix centi for 10-2 is an exception to this rule):
1 gigabecqurel (GBq) = 109 Bq = 1,000,00,000 Bq = 27 millicuries (mCi)
Note that (with the exception of kilo, k) the symbols for prefixes denoting factors greater than 1 are written in upper case, whereas those for factors less than 1 are written in lower case.