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Informative irradiation
Q: What do wrinkle-resistant fabrics, cow livers, contaminated dirt, ancient pottery, and Native American copper artifacts have in common?

A:They have all been tested in a nuclear reactor to identify their trace elements. The technique, called neutron activation analysis, works like this:

You zap the sample with energized neutrons from the reactor.
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Some atoms take up a neutron, becoming radioactive isotopes of the same element (isotopes are varieties of an element with different numbers of neutrons).
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The isotopes decay by sending out gamma rays whose energy is characteristic of the isotope. That identifies the elements present.
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From the intensity of the gamma rays, scientists determine the concentration of the elements.
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The blue light in this nuclear reactor is Cerenkov radiation, emitted by high-speed charged particles in the cooling water.

Below, Richard Cashwell analyzes gamma-ray emissions from irradiated samples.Photos David Tenenbaum.
©1999, The Why Files.

glowingThe technique actually predates the invention of the nuclear reactor, says Richard Cashwell, who directs the University of Wisconsin-Madison nuclear reactor. In recent years, he says, advances in gamma ray detection have improved accuracy and versatility. Even in a sample less than 1 milliliter in size, many elements can be measured to parts per million or parts per billion.

Once the exact proportions of important trace elements are known, samples can be compared to existing ones, and to each other. Using neutron activation analysis, Cashwell has aided an effort by archeologist George Rapp of the University of Minnesota at Duluth. Rapp has been tracking copper artifacts found at North American archeological sites to specific copper mines.

The work, Rapp says, shows that copper was traded over hundreds of miles, but not over the thousands of miles once thought. For years, archeologists thought that copper in pre-Columbian North America came from Michigan's Upper Peninsula, but it's now known that the element, used in knives and other tools, was also dug from mines in the Northern Appalachians and elsewhere. The Why Files reported that copper was traded more widely in the Arctic.

nuke manNeutron activation, and other ways of measuring the concentration of elements, have other benefits for analyzing art and craft. They can determine the origins of clays used in ceramics and reveal techniques of ancient metal working. If an object's elemental profile indicates that it came from a certain ore body, any differences between the profiles of the object and the ore may reflect the smelting temperature, Cashwell says.

Beyond art and artifacts, neutron activation can also detect contaminants in soil and oyster tissue, measure the flow of drugs in animals and the presence of trace elements in bullets, and, yes, test how that anti-wrinkle treatment sticks to fabric. "You name it, and we've irradiated it," says Cashwell.

No word yet on how well that anti-wrinkle stuff sticks...

You can test my Leonardos, but you can't trash them...


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©1999, University of Wisconsin, Board of Regents.