ASU Research E-Magazine: When Cultures Collide

Call it a collision of cultures at the subatomic scale.

From the modern world of high-tech particle physics, Dorothy Larson had access to a technique called proton-induced X-ray emission analysis (PIXE). From the prehistoric world of the Western Pueblo culture, the ASU graduate student in archaeology had 300 vessels to analyze. The vessels were once buried in a cemetery south of Eagar, Ariz.

Larson is the first person at ASU to use PIXE for large-scale analysis of materials. The result: She acquired some valuable data without harming the pots.

Using PIXE involves generating a narrow beam of protons in the vacuum of a particle accelerator with 2 million electron volts of energy. Traveling through air quickly strips the proton beam of half its energy as it exits the vacuum.

Still, the protons strike the clays and paints in the pots with enough energy to excite their atoms, which then give off X-rays at characteristic energy levels. “The method gives me data on the chemical composition of the different vessel parts,” Larson says.

The pots are from a site that contained approximately 100 burials that date to about A.D. 1050 to 1275. The site had to be excavated because it sat atop a toxic spill slated for cleanup.

Larson is looking for common elements in composition that might indicate the preferences of an individual social group or even an individual artist. A diversity of compositions would imply interaction between cultural groups. Similar composition would suggest that the group kept to itself.

In pottery making, Larson says that techniques tend to be somewhat conservative and get passed on from generation to generation. The PIXE analysis may prove especially helpful in documenting the chemical changes underlying a shift in paint technology. Western Pueblo pottery evolved over the years from a flat-color coating to a shiny glaze.

Without access to ASU’s Facility for Ion Beam Analysis of Materials, she would have had to do without the information. Other methods of determining chemical composition would have required damaging the pots.

Both the Hopi and the Zuni tribes trace their ancestry back to the Western Pueblo culture. The tribes have asked that the artifacts remain unharmed during analysis.

“It’s important to respect their wishes,” Larson says.

Larson had worked with the collection for a year before learning about the ion-beam facility from her adviser, Keith Kintigh, who had visited the laboratory during an open house. The facility is part of the ASU’s Center for Solid State Science and the Department of Physics and Astronomy.

Barry Wilkens, a research specialist at the facility, says he would like to attract student users from the life sciences, art and other disciplines. He wants students to learn the technique, learn how to take the data themselves, and see firsthand its potential and limitations.

“The biggest advantage we have is being able to work in air,” Wilkens explains. Sample preparation for vacuum analysis is time-consuming. “Nevertheless,” he adds, “there are some still real limitations to what you can do with things dug out of the ground.”

picture of Larson using the ion beam facility
Larson has a nontechnical background, but learned how to use the instrument and set up her experiment in one semester. By the end of her project she could run 100 samples in a day.

Although the archaeological applications of PIXE are nothing new, Larson says that the technique is not widely available. “It’s a real opportunity for archaeology here at ASU,” she says.