Cover Image


View/Hide Left Panel

BOX 2-1

Units of Measurement

Specific Activity

The curie (Ci) is the traditional unit of radioactivity defined as the quantity of any radioactive nuclide in which the number of disintegrations per second is 3.700 × 1010. It is a concentration defined as the ratio of the amount of radioactivity divided by the mass or volume of radioactive substance. The International System unit of specific activity is the becquerel (Bq).

Absorbed Dose

The gray (Gy), formerly the rad, is the unit that describes the magnitude of absorbed radiation in terms of energy deposited in tissue. However, the amount of energy deposited in tissue does not account for differences in the biologic effects of different radiation types.

Dose Equivalent

The rem (roentgen-equivalent-man) is the traditional unit of measure that incorporates the relative biologic damage caused by different radiation types and deposition mechanisms. The International System unit for the biologically effective dose, dose equivalent, is the sievert (Sv).


Specific Activity

Absorbed Dose

Biologically Effective Dose


curie (Ci)

becquerel (Bq)

gray (Gy)

rad (old standard unit)


sievert (Sv)


1 Bq = 1transformation or disintegration per second = 2.7 × 10−11 Ci

1 Gy = 100 rad

1 mSv = 0.001 Sv

1 Sv = 100 rem

SOURCE: Adapted from ATSDR, 1999a,b.

emitted. Beta particles are high-energy electrons; the path length of a beta particle is up to 15 m in air and up to 1 cm in solids (ATSDR, 1999b). Gamma rays are electromagnetic ionizing radiation and constitute a radiation hazard even when present outside the body because they are highly penetrating.

Isotopes of uranium all have the same chemical properties because they all have the same number of protons, 92. However, variation in the number of neutrons gives the isotopes different radiologic properties. The radioactivity of isotopes can be compared by using specific activity, a measure of the number of nuclear transformations (disintegrations) per second per unit mass (see Box 2-1). The most abundant naturally occurring uranium isotope, 238U, has the lowest specific activity (1.24 × 104 Bq/g) (AEPI, 1995). The high specific activity of

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement