Occupational Dose Evaluation in Support of the Development of
October, 2012
Rare Element Resources, Inc. Bear Lodge Project
8
1.3.3
Units of Radioactivity and Dose
The U.S. has not consistently adopted the International System of Units (SI) units for
radioactivity or radiation dose. The unit for radioactivity in the U.S. is the curie (Ci). The Ci is
based on the decay rate of 1 gram (g) of radium-226 and is equal to 3.7 x 10
10
radioactive decays
per second. The Ci is a large amount of radioactivity, consequently environmental levels of
NORM are often expressed in picocuries (1x10
-12
Ci), which are twelve orders of magnitude less
than a Ci. The amount of radioactivity is directly related to amount of the radionuclide and
inversely related to its half-life. The specific activity of a radionuclide relates the amount of
radioactivity per unit mass and can be expressed in units of Ci g
-1
. For example, the specific
activity of natural uranium is 7.1 x 10
-7
Ci g
-1
; therefore a concentration of 1 milligram (mg) per
kilogram (kg) of natural uranium in soil is equal to 710 picocuries (pCi) per kg of soil. Similarly,
the specific activity of natural thorium is 2.2 x 10
-7
Ci g
-1
; therefore 1 mg kg
-1
of natural thorium
in soil is equal to 220 pCi kg
-1
of soil. The activities of specific isotopes of uranium can be
determined by multiplying the radioactivity percent abundance in Table 1.1 by the activity of
natural uranium. For natural thorium, the activity is split equally between thorium-232 and
thorium-228 provided equilibrium (i.e., the radioactivity is the same) of the series radionuclides
exist.
The U.S. unit for dose equivalent is the rem and is equal to the absorbed dose in rad (100 ergs g
-
1
) multiplied by a radiation quality factor. The dose equivalent, hereafter referred to as “dose”, is
also often expressed as millirem [mrem (10
-3
rem)]. The risk of detrimental effects resulting from
a dose of radiation is assumed to be proportional to the dose (ICRP, 1990), although this has not
been demonstrated at low doses typical of routine occupational exposures to NORM.
1.3.4
Background Radiation Doses in the United States
Background radiation doses from natural sources, while variable, will always occur to some
degree. Sources of background radiation dose include the following:
Cosmic radiation from the sun and other sources,
Terrestrial gamma radiation from radionuclides present in earth,
Radon and its short-lived decay products in indoor and outdoor air, and
Radionuclides in the body from the air we breathe, the food we eat, and the water we
drink.
The global average background dose rate from naturally occurring sources is approximately 240
mrem y
-1
: some places in the world have dose rates of up to 1000 mrem y
-1
(UNSCEAR, 2000).
Table 1.2 lists the average annual dose from natural background radiation in the U.S. (NCRP,
2009).
