What Represents the Quantity of Radiation?

Many different terms are used in scientific as well as popular literature to represent the quantity of radiation. This is partly because the United States generally uses conventional units such as curies, while other countries use SI or metric units such as becquerels. More significantly, different terms are in use to represent radiation observed under different conditions: a quantity of radiation emitted, a quantity of radiation absorbed (exposure or absorbed dose) and a quantity of radiation harmful to a person (equivalent dose).

Radiation Emitted

• Marie Curie and her husband Pierre were pioneers in working with radium.

  The traditional term used in measuring sources of radiation is a curie, abbreviated Ci. The term dates from the pioneering work of Marie and Pierre Curie and their work with radium. One curie describes the amount of radioactive decay per second in one gram of radium. The decay consists of about 37 billion disintegrations. The metric term is a becquerel, abbreviated Bq. A becquerel is a single disintegration, so one 1 Ci = 37 billion Bq.

Radiation Absorbed

• A Geiger countyer is used to detect the presence of gamma rays.

  Distance and shielding from a source of radiation will affect the quantity of radiation that would be absorbed, also known as radiation exposure. The conventional term used in measuring exposure is the rad, from "radiation absorbed dose." The metric equivalent is the gray, abbreviated Gy. One gray is equal to 100 rads. Another term, roentgen (R), is sometimes used to measure radiation exposure, but it only applies to gamma rays and X-rays, and only in air. Geiger counters detect gamma rays, and may report the amount in roentgens or counts/clicks per minute (CPM).

Equivalent Dose

• Chest X-rays typically produce a radiation dosage of 0.001 Sv.

  Different types of radiation have different effects on human tissue. To compensate for this, there are formulas to convert the radiation absorbed or exposure into a quantity that expresses the equivalent dose. The term used for these figures is the rem, from "roentgen equivalent man." The metric equivalent is the sievert (Sv). A typical chest X-ray produces an equivalent dose of 0.001 Sv. The average dose for Chernobyl evacuees was 0.03 Sv.

Prefixes

• Prefixes are often used with measurement terms. For example, since a curie is a very large amount of radiation, the amounts used in nuclear medicine are typically expressed as millicuries (mCi) or microcuries (µCi). Terms that refer to relatively small amounts, such as the becquerel, are modified with prefixes familiar to computer users, such as kilo-, mega- and giga-.