Lesson 3 Background Notes

Units for Measuring Radiation

Common Units – USA

These are the common units used in the United States in health physics.

Roentgen (R)

The roentgen is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. One roentgen is equal to depositing in dry air enough energy to cause 2.58E-4 coulombs per kg. It is a measure of the ionizations of the molecules in a mass of air. The main advantage of this unit is that it is easy to measure directly, but it is limited because it is only for deposition in air, and only for gamma and X-rays.

RAD (radiation absorbed dose)

The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of the different radiations.

Rem (roentgen equivalent man)

The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem, or mrem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of incident radiation.

Curie (Ci)

The curie is a unit used to measure radioactivity. One curie is that quantity of any radioactive material that will have 37,000,000,000 transformations in one second. Often radioactivity is expressed in smaller units, like thousandths (mCi), millionths ( μCi), or even billionths (nCi) of a curie.

Common Units – SI – International Standard

Note: These are the common units used throughout the world in health physics.

Gray (Gy)

The gray is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. The unit gray can be used for any type of radiation, but it does not describe the biological effects of the different radiations. Absorbed dose is often expressed in terms of hundredths of a gray, or centi-grays. One gray is equivalent to 100 rads.

Sievert (Sv)

The sievert is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of millionths of a sievert, or micro-sievert. To determine equivalent dose (Sv), you multiply absorbed dose (Gy) by a quality factor (Q) that is unique to the type of incident radiation. One sievert is equivalent to 100 rem.

Becquerel (Bq)

The becquerel is a unit used to measure radioactivity. One becquerel is that quantity of a radioactive material that will have 1 transformation in one second. Often radioactivity is expressed in larger units, like thousand (kBq), million (MBq), or even billion (GBq) becquerels. As a result of having one becquerel being equal to one transformation per second, there are 3.7 x 10¹⁰ Bq in one curie.

SI Prefixes

Many units are broken down into smaller units or expressed as mutiples, using standard metric prefixes. As examples, a kilobecquerel (kBq) is 1000 becquerels, a millirad (mrad) is 10^-3 rad, a microrem (μrem) is 10^-6 rem, a nanogram is 10^-9 grams, and a picocurie is a 10^-12 curies.