Proton-Induced X-Ray (PIXE) Spectrometry in Archaeology

 

PIXE spectrometry is a powerful analytical tool that is used extensively in research as diverse as the study of the mobility of environmental pollutants, the tracking of trace element build-up during the onset of health disorders, and the identification of variability of ink recipes used in the printing of medieval bibles. Penn Museum and the Bartol Research Institute of the University of Delaware have collaborated over the past two decades, to refine PIXE spectrometry for application to the characterization of ancient materials, in part to track the chronological development of early copper-, steel, and glass-making, and in part to document novel features of early goldworking practices, such as gilding and soldering.

Left: Dr. Charles P. Swann adjusts the proton beam alignment on the PIXE unit at the Bartol Research Institute of the University of Delaware.


PRACTICAL ASPECTS OF PIXE SPECTROMETRY

When a material is bombarded by protons, several kinds of atomic reactions occur, among them the displacement of electrons out of their orbits in individual atoms. As each disturbed atom recovers its stability, it emits a group of x-rays which in their energies characterize the element from which they originate, and in their intensities are proportional to the concentration of that element. The composition of the material overall is thus represented by a multi-peaked spectra that provide simultaneously the constituents of the "recipe" for an artifact's production-e.g., how much tin was added to copper to make bronze-and the minor and trace elements which may be indicate a metal's provenance or explain a glass's source of coloration.

A number of features, practical and theoretical, make PIXE spectrometry particularly valuable in the study of ancient artifacts:

. It is essentially non-destructive, usually requiring only that the artifact's surface be lightly abraded, to remove corrosion or grime.
. For well established reasons that relate to the way protons interact with matter, it is very sensitive in the measurement of trace elements, often with detection limits that are an order-of-magnitude better than those obtainable by other analytical techniques of materials analysis, such as x-ray fluorescence (XRF) and scanning electron microscopy (SEM/EDAX).
. When operated in a "microbeam mode" (i.e., with a spatial resolution of about a twentieth of millimeter), it can be used to study individual parts of an complex artifact such as a item of highly decorated gold jewelry, an ore of mixed minerals, or a remnant of smelting slag.
. Since the artifact does not have to be put into any kind of special vacuum chamber during analysis, it is a convenient way to gather data about an artifact, such as a bronze statuette or a gold plaque, before it is included in an exhibition display.

A limited number of reprints are available on the technical aspects of PIXE Spectrometry as applied to archaeological materials, including:
Equipment Refinement
Detection Limits for Copper
Detection Limits for Glass

Some of the publictions that have resulted from this collaboration are abstracted below. For questions on content and/or availability of reprints, please contact Stuart Fleming via email.

Distinguishing Between Native and Smelted Coppers using PIXE Spectrometry: A Case History from Early Colonial America

Metal-forming Practices among the Seventeenth Century Illinois, 1640-1682

Confounding the Conquistadors: Tumbaga's Spurious Luster

Roman Mosaic Glass: A Study of Production Processes, using PIXE Spectrometry

Roman Influences on Metalworking at the Titelberg (Luxembourg): Compositional Studies using PIXE Spectrometry

Recent Applications of PIXE Spectrometry in Archaeology: Observations on the Early Development of Copper Metallurgy in the Old World

Recent Applications of PIXE Spectrometry in Archaeology: Characterization of Bronzes with Special Consideration of the Influence of Corrosion Processes on Data Reliability

Recent Applications of PIXE Spectrometry in Archaeology: Characterization of Chinese Pottery Exported to the Islamic World

A Bronze Figure of Tutankhamun: Technical Studies

Penn Museum | 3260 South Street, Philadelphia, PA 19104 | www.penn.museum