IEER SDA Volume 4, Number 1

A Special Centerfold for Techno-Weenies

A Tool Kit on Natural and Man-made Radiation

This Centerfold gives figures for radioactivity commonly found around us due to natural sources and man-made sources (such as atmospheric nuclear weapons testing).

Natural radiation is ubiquitous; it is found in soil, water, and air. Types of natural radiation are described in the first table below; the second table summarizes natural radiation in sea water:

Types of Natural Radiation

Cosmic rays originate from the sun and outer space. They vary with altitude and latitude.

Cosmogenic radionuclides are produced from the interaction of cosmic rays with atoms in the atmosphere. The principal cosmogenic radionuclide is carbon-14.

Primordial radionuclides are found in the earth's crust. Examples are potassium-40 and the nuclides in the uranium and thorium decay chains.

Natural Radionuclide Concentrations in Sea Water

Radionuclides Concentration (picocuries/liter)
Potassium-40 300
Rubidium-87 2.8
Natural Uranium 2.2
Radium-226 0.03
Radon-222 0.1
Tritium 2.7
Sources: Eisenbud 1987; NCRP 1988; Benedict et al. 1981.

Levels of natural radiation are not uniform. In soils, for instance, the concentration of natural uranium varies from one place to another. And the radiation dose due to cosmic rays increases with altitude, since the rays filter through a smaller amount of atmosphere at higher altitudes. It is also important to note that human activity like uranium mining and milling may concentrate natural radioactive materials and/or make them a more mobile and accessible part of the environment.

The table entitled "Radionuclide Disposition Due to Fallout from Atmospheric Testing" shows the increase in radionuclides due to nuclear weapons testing. It includes radionuclides that occur naturally (tritium, for example), and radionuclides that do not occur naturally (strontium-90, for example).

How to use the Centerfold
The Centerfold can be used as a general reference, as a tool for understanding contamination at a given nuclear facility, or to help the reader determine whether unnatural radioactivity is present at a particular location.

In addition to concentrations and doses for natural radiation, the Centerfold includes typical ranges to give some idea of variations. These values are more useful for determining if there might be artificial radioactivity and/or very high levels of radioactivity at a given location.

For example, a typical value for uranium-238 is about 1 picocurie per gram (pCi/g) of soil, and a typical upper value for the range is 3.8 pCi/g. (A "picocurie" is a measurement of radioactivity. The prefix "pico" means "one trillionth," so one picocurie is a trillion times smaller than one curie.) While uranium-238 radioactivity levels of 10 picocuries per gram or more can occur in certain areas, values over three or four picocuries per gram are often indicative of man-made contamination. Sometimes even values as low as 2 picocuries per gram may be partly caused by human activity.

On the other hand, uranium ore with a uranium concentration of 0.2 percent has a specific activity of 1,300 picocuries per gram and such ores are sometimes found close to the earth's surface. (See Dr. Egghead for definition of specific activity.)

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Institute for Energy and Environmental Research

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Last updated: August, 1996