Achieving High Spatial Resolution in Elemental Mapping of Metal Samples from Archaeological Contexts

ABSTRACTImproving the characterisation of archaeological artifacts brings a need to understand better the relationships between composition, structure and properties. With archaeological material there is also a requirement to consider the effects of ageing and environmental interactions in altering the original structure and composition, both in the bulk and at the surface. However, curatorial constraints and, frequently, the condition of the objects preclude the sampling methods required for the most powerful means of structural analysis of materials, the high resolution transmission electron microscope. The samples normally available are small bulk samples and we must find other means of maximising spatial resolution in microchemical and microstructural analysis of both bulk and surface regions of the samples. This paper describes ways in which this is being achieved using the scanning proton microprobe (SPM) with both particle induced X-ray emission (PIXE) and Rutherford back scattered proton (RBS) spectra at resolutions down to ca. 1:m, electron probe microanalysis (EPMA) at 250-300nm, and scanning Auger microscopy (SAM) at resolutions of 10-20nm, but only from the surface layers of atoms in a sample. Examples will be given which demonstrate the contribution that each instrument can make, and that new and useful information is obtained each time resolution is increased. They will also show that structural features can be identified which are invisible to other microscopies. It will also be shown how modern PC-based software has greatly enhanced the mapping capability of all instruments.

The Incorporation of Laser Ablation for Sampling into a Proton X-ray Emission System

ABSTRACTThe variation with depth of the composition of corrosion layers on buried metal objects can provide the archaeologist with valuable information relating to the burial conditions of the object. In some cases these layers can be very thick and so, normally destructive methods such as sampling are used to characterise the metal and corrosion layers when possible. We have developed a system for the use of a microfocused high power pulsed Nd:YAG laser to ablate the corrosion layer in a series of controlled steps while monitoring the composition of the exposed surface using focused PIXE and RBS in the external beam facility of the Oxford Scanning Proton Microprobe. With proper manipulation of the data we are able to calculate the values for trace elements at various depths in the corrosion layer. It is hoped that this system will lead to a better understanding of corrosion properties.