The Incorporation of Laser Ablation for Sampling into a Proton X-ray Emission System
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The Incorporation of Laser Ablation for Sampling into a Proton X-ray Emission System Meg Abraham, Peter Northover and Geoff Grime Department of Materials University of Oxford Parks Rd, Oxford UK ABSTRACT The 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. INTRODUCTION The scanning proton microprobe (SPM) is used as a tool for the analysis of art and archaeological objects at a number of laboratories around the world [1]. The proton induced Xray emission (PIXE), generated from collisions of focused high-energy protons with all elements above He, yields X-rays for elemental analysis. The external beam of the SPM (fig.1) offers the advantage that the object is not placed in the vacuum system and thus any size object can be accommodated in this scheme. The MeV ions will travel over a cm through air maintaining a resolution of typically 0.25 mm. Further, X-ray scatter is reduced by bringing the beam into a He shroud. Typically, in the Oxford set-up, two SiLi X-ray detectors are used. The first is a thin vacuum window detector modified with a magnet shield to prevent damage to the detector from backscattered protons. This detector has improved resolution for the low Z number elements. The second is a standard window SiLi detector. The system is also fitted with a Rutherford Backscatter detector and a gamma ray detector. The external beam facility (fig 2) has been modified such that the sample can be rotated into the beam line of a Nd:YAG laser in order to allow for ablation of small amounts of corrosion from the surface. The region of the surface removed by the laser is typically less than 500µm in diameter and so by comparison with other sampling techniques, the effect on the appearance of an object is minimal. Analysis of a spot this size requires a focused proton beam. The aim is to get accurate trace element data for the underlying metal without sampling or aggressive cleaning and additionally to get new information from the corrosion products. The laser ablation system consists of a pulsed Nd:YAG laser operating at a maximum power of 1 J cm-2. The system has second and third harmonic crystals that permit operation at the fundamental wavelength of 1064 nm as well as 532 nm and 355 nm. The q-swi
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