Background Radiation

Click Here for Graphic

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:

• 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 from fallout.

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.

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:

• 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.

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.

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