Radiation Safety Program

Permits

Technical Guidance

Metric Prefixes and Units of Measure

Topic Areas

SI
Metric Prefixes That Expand a Unit
Metric Prefixes that Reduce a Unit
Exposure
Dose
Dose Equivalent
Activity
Summary List of Conversions

SI

SI stands for the French phrase, systeme internationale, which means, international system of weights and measures, previously known as the metric system. Apart from SI units, there are traditional or local units that many people prefer that are specific to world regions, or in specialized fields of study. These units have usually been developed outside of the SI scheme of measurements, but can be related with conversion factors and formulas.

Metric Prefixes That Expand a Unit

Kilo x 1,000 (thousand)

Mega x 1,000,000 (million)

Giga x 1,000,000,000 (billion)

Terra x 1,000,000,000,000 (trillion)

Metric Prefixes that Reduce a Unit

Milli x a thousandth

Micro x a millionth

Nano x a billionth

Pico x a trillionth

Exposure

Exposure is a measurement of ionization in air, of x-ray radiation and low to medium energy gamma radiation. Exposure cannot be measured for other kinds of radiation. The concept of exposure is considered scientifically obsolete; however, it is still often used. The term exposure is sometimes used imprecisely as a synonym for dose.

The röntgen (R) is the traditional unit of exposure, which represents the amount of radiation required to create 1 esu of charge in 1 cubic centimeter of dry air.

The coulomb per kilogram is the SI unit of ionizing radiation exposure, and is the amount of radiation required to create 1 coulomb of charge in 1 kilogram of matter.

1 röntgen = 0.0003 coulombs per kilogram.

Dose

Radiation energy deposited in matter is called “dose.” Geiger detectors and scintillation detectors can be calibrated to give measurements in dose-rate, when working with specific, known isotopes. There are also a number of dose measuring systems which are used in personnel dosimetry monitors or badges. Dose can also be mathematically calculated for any kind of radiation if the identity of the irradiating nuclide is known, and some idea of the amount of activity can be measured. The rad is the traditional unit of dose.

1 rad = 100 ergs per gram or 0.01 Joules per kilo-gram.

The gray (Gy) is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of matter.

1 gray = 100 rads.

1 rad = 10 milli-grays.

Dose Equivalent

Dose equivalent is a refinement of dose, accounting for the biological effect of radiation on human tissue. It is derived from assigning a weighting factor to measured dose. For gamma rays it is equal to the absorbed dose. For other kinds of radiation at various energies, and for various parts of the body, there can be weighting factors applied to the absorbed dose, to achieve the dose equivalent. These weighting factors are empirically derived. The rem is the traditional unit of dose equivalent.

1 rem = 100 ergs per gram or 0.01 Joules per killo-gram.

The sievert (Sv) is the SI unit of dose equivalent, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of matter.

1 sievert = 100 rems.

1 rem = 10 milli-sieverts

Activity

Activity is the rate of decay of radioactive material, observed as the emission of radiation particles (alpha, beta, photon, or neutron) from the atomic nucleus, indicating nuclear events. Activity is measured in units of :

becquerel (bq),
curie (ci), and
disintegrations per minute (DPM).

Some conversions factors:

1 becquerel = 1 nuclear disintegration / second.

1 curie = 3.7 x 10¹⁰ becquerel.

1 giga-becquerel = 27 milli-curies

1 DPM = 60 becquerel

1 micro-curie = 2,220,000 DPM

Summary List of Conversions

1 röntgen = 0.0003 coulombs per kilogram

1 rad = 100 ergs per gram or 0.01 Joules per kilo-gram

1 gray = 100 rads

1 rad = 10 milli-grays

1 rem = 100 ergs per gram or 0.01 Joules per killo-gram

1 sievert = 100 rems