Background Radiation

Components of Background
Man-Made Sources
Cosmic Radiation
Cosmogenic Radiation
Terrestrial Radiation
Non-Series Radionuclides
Series Radionuclides

Background Radioactivity


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.


A good reference for the US is NCRP Report 160 Ionizing Radiation Exposure of the Population of the United States.(NCRP 2009)


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

Man-Made Sources


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:

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

Radionuclides

of Interest

Half-Life
137Cs 30 y
90Sr 28.1 y
85Kr 10.73 y

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 from fallout.

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).

Cosmic Radiation


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 origin.

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) Dose Rate, mrem/y Example
Sea level 31 Los Angeles
1,525 (5,000) 55 Denver
3,050 (10,000) 137 Leadville, Colo.
9,140 (30,000) 1900 Normal jetliner
15,240 (50,000) 8750 Concorde
24,340 (80,000) 12,200 Spy plane

Adapted from Gollnick 1988.



Cosmogenic Radiation


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 atmosphere.

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.



Terrestrial Radiation


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:



Non-Series Radionuclides


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.

Series Radionuclides

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 natural environment.

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

Nuclide Half-Life Major Radiations
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 microsecondsalpha
Lead-210 22.3 years beta, gamma, x-rays
Bismuth-210 5.01 daysbeta
Polonium-210 138 daysalpha
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

Source: NRC 1994


 

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