Background radiation varies over a range of concentrations and exposure rates from a variety of
causes. The magnitude of variation can be significant over a short distance and also can vary in
the same place from time to time. The background variance can be from natural as well as
human activities. Understanding the characteristics of background, and the wide range of
background values encountered in the field is beneficial when designing and conducting surveys.
This is especially important because some of the current regulatory exclusion limits are set at a
concentration or exposure rate above background (EPA 1978). Risk-based cleanup levels of 10-4 to 10 -6 can be at or near background concentrations. Variation due to geology, chemical
and physical mobility and deposition, temporal, and human affects should all be considered.
It should be noted that background radiation levels were much higher in primordial times. "The background radiation field at the surface of the
Earth due to potassium, uranium, thorium, and decay series radionuclides in the continental crust was about 1.6 mGy/y at the time that life evolved and remained relatively constant for almost 2 billion years. The radiation dose rates from internal 40K have decreased steadily since life evolved
from about 5.5 mGy/y to about 0.70 mGy/y at present (Karam and Leslie 1999).
There are many excellent references that discuss background radiation. For more information, see:
Environmental Radioactivity from Natural, Industrial and Military Sources by Merrill Eisenbud and Tom Gesell, Academic Press, Inc. 4th Edition (Eisenbud and Gesell 1997).
NUREG-1501. Background as a Residual Radioactivity Criterion for Decommissioning. This has good information on fluctuations in background and how to measure background.
Numerous UNSCEAR Reports have data on background concentrations and exposures. The series is available for download here.
The 2008 Report, Annex B has recent worldwide data on natrual sources and enhanced sources of natural radioactivity.
There are two generic definitions for background:
1. naturally occurring concentrations of radionuclides that represent ambient conditions present in
the environment that are in no way influenced by human activity, or
2. concentrations of radionuclides from anthropogenic sources originating from non-site sources
(Gesell and Prichard 1975).
An example of this would be global fallout.
Components of Background
The following discussion is drawn largely from NCRP 94.
Four major components constitute "background sources" of radiation:
Most TENORM is associated with terrestrial sources, but the other types may interfere with
measuring levels of TENORM. Fig. 1. shows the background sources to the U.S. population by percentage.
The US Geological Survey has published updated maps of the US and Canda for uranium, thorium and potassium surface concentrations, as well as gamma absorbed dose rate, and cosmic dose rate (Duval et al. 2005).
Updated USGS maps. Background Radiation
Click on the thumbnail for Maps
Why are they sometimes considered background? Because they are ubiquitous in the environment as opposed to occurring locally.
Activities that have contributed to the dispersion of radionuclides in the environment:
- weapon tests and use
- accidents (Chernobyl)
- reactors (for this discussion, 14C)
Most anthropogenic radionuclides are short-lived, but some have half-lives of many years and are worthy of note, and are listed in Table 1:
Table 1. Examples of anthropogenic radionuclides with long half-lives
Also, the global inventory of 14C and 3H has been increased from human activities, and it is
sometimes necessary to measure these globally distributed radionuclides separately and to
distinguish them from locally produced sources. In addition, isotopes of Pu have been released
The variability of anthropogenic sources of radiation and radioactivity relates directly to the
population distribution and level of technology found in different areas around the world.
Deposition in an area depends upon wind and precipitation patterns (NRC 1994).
This type of background refers to both the primary energetic particles of extraterrestrial origin
that strike the earth's atmosphere and to the secondary particles generated by their interaction
with the atmosphere.
Primary radiation itself consists of two components, designated as galactic or solar depending on
Primary particles are attenuated in upper atmosphere. Reactions take place and generate
secondary particles. The cosmic radiation field at ground altitude (0 to 3 km) consists almost
entirely of secondary particles whose origins are almost exclusively galactic.
Annual external dose rates from cosmic rays depend slightly on latitude and strongly on altitude
(Table 2). The latitude effect is due to the charged-particle nature of the primary cosmic rays.
When they come near the earth, its magnetic field tends to deflect the rays away from the equator
and toward the poles (Gollnick 1988).
Table 2. Altitude Dependence of Cosmic Ray Dose
(dose equivalent; does not include the neutron component).
|Altitude, m (ft)
Cosmogenic radionuclides arise from the collision of highly energetic cosmic ray particles with
stable elements in the atmosphere and in the ground. The entire geosphere, the atmosphere, and
all parts of the earth that directly exchange material with the atmosphere contain cosmogenic
radionuclides. The major production of cosmogenic radionuclides results from the interaction of
cosmic rays with atmospheric gases.
The outermost layer of the earth's crust is another area where reactions with cosmic rays occur.
However, the rate at which they occur is several times smaller than the atmospheric component
because most of the cosmic rays are attenuated in the atmosphere. The result is that the
contribution to background dose is minimal.
The most important radionuclide produced is 14C. However, many others, such as 3H, 22Na, and
7Be, occur. Carbon-14 produced in the atmosphere is quickly oxidized to CO2. The equilibrium
concentrations of 14C in the atmosphere are controlled primarily by the exchange of CO2 between
the atmosphere and the ocean. The oceans are the major sink for removal of 14C from the
Most of the other cosmogenically produced radionuclides in the atmosphere are oxidized and
become attached to aerosol particles. These particles act as condensation nuclei for the formation
of cloud droplets and eventually coagulate to form precipitation. About 10 to 20% of
cosmogenically produced radionuclides are removed from the atmosphere by dry deposition on
the earth's surface.
Concentrations of cosmogenic radionuclides vary in the atmosphere with time and location.
Variations are day-to-day, seasonal, longitudal, and sunspot-cycle related. The concentrations of
some cosmogenic radionuclides, such as 3H, 14C, 22Na, and 37Ar, have increased during nuclear
tests. Reactors also generate 14C that eventually will be distributed in the atmosphere, but is
estimated to be two orders of magnitude lower than the natural concentration. The total effective
dose equivalent rate to the body produced by the primary cosmogenic radionuclides is just over
10 uSv (1 mrem/y), with essentially all of the dose arising from 14C.
The final component of background comes from radionuclides found in the earth. Several dozen
naturally occurring radionuclides have half-lives of at least the same order of magnitude as the
estimated age of the earth (4.5 109 y), and are assumed to represent a primordial inventory.
These primordial radionuclides are also what we are most concerned within the TENORM issue.
The primordials are usually divided into two groups:
- those that occur singly (non-series) and decay directly to a stable nuclide, and
- those that occur in decay chains (series) and decay to a stable isotope of lead
through a sequence of radionuclides of wide-ranging half-lives.
Two primary non-series radionuclides contribute to background dose, 40K and 87Rb.
Potassium-40 is a beta (87.3%) and gamma (10.67%) emitter and contributes to both internal and
external doses. It exists as a constant fraction of stable potassium (0.0117%). Its contribution to
external dose is variable, depending on its concentration in rocks and soil. Average concentration
is about 0.6 Bq/g (17 pCi/g) in crustal rock. Rubidium-87 is a pure beta emitter and is found in
crustal rock in concentrations of about 0.07 Bq/g (2 pCi/g). It is not an external hazard and is
rarely considered in dose calculations.
The remainder of the non-series radionuclides have combinations of half-lives, isotopic
abundance, and elemental abundance such that they have negligibly small specific activities and
are not significant in background calculations.
Potassium-40 is found in TENORM, particularly building materials (bricks, cinder blocks). It may
be necessary to determine background fractions separately from total concentrations.
Potassium is metabolically regulated by the body and is not controlled by intake.
There are three naturally occurring decay series, headed by the radionuclides 238U, 235U, and 232Th.
These series are commonly called the uranium series, the actinium series, and the thorium series
respectively. Table 3 lists components of the uranium and
thorium series, along with the non-series radionuclides. Generally, the actinium series does not
play a significant role in industrial TENORM due to its very low presence (1/6 of 238U) in the
If not subjected to chemical or physical separation, each of these series attains a state of secular
radioactive equilibrium. Technological enhancement of NORM as well as natural physical and
chemical reactions often interfere with this balance. Crustal concentrations of the heads of the
three series are extremely small (parts per million); the short-lived decay progeny are present in
such exceedingly minute concentrations that their behavior does not always follow chemical (mass
action) controls. There will be further discussion about this later.
Table 3. Principal Natural Radionuclide Decay Series
|Uranium-238 ||4.47 billion years ||alpha, x-rays|
|Thorium-234 ||24.1 days ||beta, gamma, x-rays|
|Protactinium-234m ||1.17 minutes ||beta, gamma|
|Uranium-234 ||245,000 years ||alpha, x-rays|
|Thorium-230 ||77,000 years ||alpha, x-rays|
|Radium-226 ||1600 years ||alpha, gamma|
|Radon-222 ||3.83 days ||alpha|
|Polonium-218 ||3.05 minutes ||alpha|
|Lead-214 ||26.8 minutes ||beta, gamma, x-rays|
|Bismuth-214 ||19.7 minutes ||beta, gamma|
|Polonium-214 ||164 microseconds||alpha|
|Lead-210 ||22.3 years ||beta, gamma, x-rays|
|Bismuth-210 ||5.01 days||beta|
|Polonium-210 ||138 days||alpha|
|Lead-206 ||stable|| |
| || || |
|Thorium-232 ||14.1 billion years ||alpha, x-rays|
|Radium-228 ||5.75 years ||beta|
|Actinium-228 ||6.13 hours ||beta, gamma, x-rays|
|Thorium-228 ||1.91 years ||alpha, gamma, x-rays|
|Radium-224 ||3.66 days ||alpha, gamma|
|Radon-220 ||55.6 seconds ||alpha|
|Polonium-216 ||0.15 seconds ||alpha|
|Lead-212 ||10.64 hours ||beta, gamma, x-rays|
|Bismuth-212 ||60.6 minutes ||alpha, beta, gamma, x-rays|
|Polonium-212 ||0.305 microseconds ||alpha|
|Thallium-208 ||3.07 minutes ||beta, gamma|
|Lead-208 ||stable|| |
| ||Non-Series Radionuclides|| |
|Potassium-40 ||1.28 billion years ||beta, gamma|
|Argon-40 ||stable|| |
|Calcium-40 ||stable |
|Rubidium-87 ||4.7 billion years ||beta|
|Strontium-87 ||stable|| |