A number of techniques which are used less commonly or have only recently been used to investigate archaeological materials need to be discussed. These include synchrotron-induced (SR) radiation, X-ray photoelectron spectroscopy (XPS), auger-electron spectroscopy (AES), particle-induced gamma emission (PIGE), time of flight secondary ion mass spectrometry (ToF-SIMS) and transmission electron microscopy (TEM).
Synchrotron radiation is produced by an instrument known as a synchrotron. The system consists of an accelerator, which produces high energy electrons, a booster accelerator and a storage ring. The technique produces intense radiation of all frequencies in the electromagnetic spectrum. The high intensity is accompanied by fine beam dimensions and this allows it to be used for a range of applications using a range of analytical techniques. For example SR can be used for SR-induced X-ray micro-diffraction, SR-induced X-ray fluorescence and SR-induced neutron diffraction. SR has been used to chemically characterize a range of materials including pigments, glass, luster glazes, the crystals in bones, ink, iron and phase transformations in pottery. Most recently a synchrotron source at Cornell has been used in conjunction with confocal X-ray fluorescence to analyze non-invasively separate layers of pigments in pictures. It has been possible to create a detailed depth profile through the layers of pigments. Studies have also been carried out using SR in order to investigate deterioration and in conservation science of both inorganic and organic materials. With the achievement of a one micron beam diameter at the new Diamond facility at Didcot in the UK, the time is ripe for a wide range of archaeological and conservation science applications using SR.
X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) both have roles to play in the investigation of archaeological materials. XPS essentially provides information about the chemical environment of the elemental species induced by a suitable radiation such as Al Ka or a monochroma-tized synchotron radiation (see above). The resulting information is useful because it can detect the presence of different bonding states for the same element. Photoelectrons have low kinetic energies and are absorbed if they are produced below a depth of 20-50 jA, so the technique provides information about surfaces. An X-ray photoelectron spectrum is a plot of binding energies versus the number of electrons detected. The different binding energies correspond to peaks in the spectrum. The technique has been used to investigate glass, pottery, stone, metals, dyes, pigments, paintings, paper, ink and stone. The degradation of materials can obviously benefit from being investigated by using XPS. Auger electrons are a kind of particle that is produced when a material is bombarded with electrons (such as from an electron gun). They also have very low kinetic energies and derive from the surface two or three atomic layers. Like XPS, Auger electron spectroscopy (AES) can be used for examining the chemical state of the atom from which they derive. The reason why this is possible is because Auger electrons are emitted from the outer orbitals of atoms. The outer orbitals are often involved in chemical bonding so the Auger electrons characterize the state of the atom.
With particle-induced gamma ray emission (PIGE) instead of the more commonly measured X-rays emissions being measured, gamma rays are measured instead. Typically this type of analysis is carried out at the same time at PIXE (see above) by bombarding the sample with protons. Although not commonly used (partly because accelerators are expensive) the technique, which is especially sensitive to the detection of light elements such as lithium and fluorine, can be used quantitatively.
Time-of flight secondary ion mass spectrometry (ToF-SIMS) is a technique which has only very recently been used to examine archaeological materials. SIMS involves sputtering a material with a beam of positively charged ions in an ultra high vacuum chamber. The fragments of the surface that are ejected (sputtered) range from atomic species to large ensembles of atoms (clusters). These secondary ions are extracted into a mass spectrometer for mass analysis. In the case of ToF-SIMS a short pulse (about 1 ns) of primary ions hits the surface and the resulting secondary ions are accelerated to a constant energy before entering a field-free drift tube. Ions of different mass must have different velocities so they are separated by flight-time in the drift tube (heavy ions travel more slowly). Their arrival time at a detector is registered and this produces an intensity versus time spectrum. The spectrum is built up from the addition of many such pulses. Both positively and negatively charged fragments are sputtered yielding both positive and negative ion mass spectra. The sputtered fragments, typically, can only escape from a shallow depth (about 95% of the signal comes from the top two monolayers of atoms). The technique is especially useful for detecting light elements such as boron and lithium and in principle it can be used in a quantitative mode. It is the only technique which can be used for both mapping crystallites and analyzing their impurities, down to ppb. Isotopic information is also produced. So far it has apparently only been used for the analysis of ancient opaque glass. Another sensitive technique which is used infrequently for examining archaeological materials is transmission electron microscopy (TEM). In this case a thin section of the sample is prepared and the electrons travel through the sample. It is especially useful for the detection of small crystallites in archaeological materials, identifying pigments and for the investigation of bone and tooth diagenesis.
See also: Archaeometry; Artifacts, Overview; Neutron Activation Analysis; Pottery Analysis: Chemical; Petrology and Thin-Section Analysis; Sampling Methods, Theory and Praxis; Stable Isotope Analysis; Vitreous Materials Analysis.
World History
