Measurement of Radiation
HOW ARE DIFFERENT AMOUNTS OF RADIATION EXPRESSED?
Many units have been used to express different amounts of radiation. The units often used follow:
These units relate to radiation exposure, radiation dose, or radiation protection. Other units (curie, becquerel) relate to radioactivity.
The rad, rem, roentgen, and curie are special radiation units. These units are older than the gray, sievert, and becquerel.
The International System of Units (or SI units) includes the gray, sievert, and becquerel:
The gray is the SI unit used for absorbed dose (see Section 2.2).
The sievert is the SI unit used for the dose equivalent and for the effective dose equivalent (see Section 2.4).
The becquerel is the SI unit used for radioactivity (see Section 2.3).
More information about the indicated units is provided in Sections 2.1 – 2.4.
2.1 Radiation exposure units
The roentgen describes the amount of x-rays or gamma rays to which a target (e.g., fly, mouse, rat, dog, human, cow, elephant, etc.) is exposed. The roentgen relates to the ability of x-rays and gamma rays to remove electrons from atoms in air. One roentgen corresponds to 2.58 x 10-4 coulombs per kilogram of air.
2.2 Radiation absorbed dose units
The radiation absorbed dose is important for describing radiation effects. The absorbed dose relates to how much radiation energy gets put into a given target mass (e.g., lung, eye, thyroid gland).
The absorbed dose has units of energy divided by mass (e.g., ergs per gram or joules per kilogram). It better measures harm to organs and tissue in the body than the exposure in R.
Different absorbed doses can arise in different organs or tissue of the body for the same exposure in R. Thus, if a person were exposed to 10 R of gamma rays, the eye, the thyroid, and the lung would have different absorbed doses. Special computer programs can calculate such doses.
Units of absorbed dose often used are the rad and gray (an SI unit).
The rad is a relatively old unit of absorbed dose. One rad corresponds to 100 ergs of radiation energy per gram of target substance. An exposure of a fly to 1 roentgen of gamma rays results in an absorbed dose of about one rad to the total body of the fly.
The gray unit represents 1 joule of radiation energy put into a kilogram mass. Thus, 1 gray equals 1 joule per kilogram.
The gray and rad apply to all types of ionizing radiation, unlike the roentgen unit, which only applies to x-rays and gamma rays.
Some useful conversion factors that relate to absorbed dose follow:
1 gray (Gy) = 100 rad
1 milligray (mGy) = 0.1 rad
1 rad = 1 centigray (cGy, 10-2 gray)
2.3 Radioactivity Units
Radioactivity arises from the disintegration of unstable atoms and is expressed in units like the becquerel (Bq) and curie (Ci).
One becquerel (Bq) corresponds to 1 disintegration (transformation) per second.
One curie represents 37,000,000,000 (i.e., 3.7 x 1010) disintegrations per second. A curie is a very large amount of radioactivity.
Some useful conversions follow:
1 megacurie (MCi) = 106 curies (Ci).
1 kilocurie (kCi) = 103 curies (Ci).
1 millicurie (mCi) = 10-3 curies (Ci).
1 microcurie (µCi) = 10-6 curies (Ci).
1 nanocurie (nCi) = 10-9 curies (Ci).
1 picocurie (pCi) = 10-12 curies (Ci).
1 femtocurie (fCi) = 10-15 curies (Ci).
1 microcurie (µCi) = 37,000 becquerels (Bq)
1 nanocurie (nCi) = 37 becquerels (Bq)
1 picocurie (pCi) = 0.037 becquerels (Bq)