Answer to Question #661 Submitted to "Ask the Experts"

Q

How many naturally occuring radioactive elements exist and what are they?

A

Natural radioactivity originates from extraterrestrial sources as well as from radioactive elements in the earth's crust. About 340 nuclides have been found in nature, and more than 60 of these are radioactive. All elements having an atomic number greater than 80 possess radioactive isotopes, and all isotopes of elements heavier than number 83 are radioactive. The natural radioactivity of the earth includes three major categories. Primordial radionuclides have half-lives sufficiently long that they have survived since their creation. Secondary radionuclides are derived from radioactive decay of the primordials. Cosmogenic radionuclides are continuously produced by bombardment of stable nuclides by cosmic rays, primarily in the atmosphere. Some human-made radionuclides persist in the environment, but these are not "natural." A much larger number of radioactive isotopes than now exist were produced when the matter of which the universe is formed first came into being several billion years ago, but most of them have decayed out of existence. The primordial radionuclides which now exist are those that have half-lives at least comparable to the age of the universe. Radioisotopes with half-lives of less than about 10⁸ years have become undetectable in the 30 or so half-lives since their creation, whereas radionuclides with half-lives greater than 10¹⁰ years have decayed very little up to the present time. More detailed information on natural radionuclides, including lists of individual radionuclides, can be found in the book Environmental Radioactivity, Merril Eisenbud and Tom Gesell, Academic Press, San Diego, 1997, and on the Idaho State University sponsored Web site: Radioactivity in Nature. Tom Gesell