Crystallographic analysis by time-of-flight neutron diffraction (ToF-ND) is the method that has been used most in Heritage Science studies carried out at ISIS since around 2001.
ToF-ND determines quantitatively the crystallographic parameters and phase content of a material. It is a similar form of analysis to conventional X-ray diffraction (XRD). Neutrons have much greater penetrating power than X-rays, so whereas XRD is useful for small powder samples and surface analysis, ToF-ND provides an average bulk analysis right through several centimetres of bronze or steel for example.
Any crystalline material - metal, pigments, rock, ceramics - can be analysed by neutron diffraction. A whole artefact can be placed in front of the neutron beam and analysed in air; no sampling or sample preparation is necessary. Analysis under vacuum or other environments (e.g. high or low temperature) is also possible. Measurements are made in live time, which can be useful for testing conservation materials & methods.
ToF-ND is completely non-invasive and does not mark, heat, or alter the object. There isn't any long-term activation as a result of the analysis; iron and steel artefacts can normally be handled immediately after measurement. Objects that contain a lot of silver or gold might be active for hours or days afterwards. Calculations of potential activation are made in advance depending on the particular object. It is a good idea to characterise objects by a common spectrographic method like X-ray fluorescence (XRF) before bringing them to ISIS. Objects should be photographed and X-rayed in advance too, to help in deciding exactly which points to analyse. With archaeological metal objects, it is often extremely useful to make a replica of the same major element composition and according to the presumed manufacturing method, and analyse that as a 'standard' to compare with the original object.
Multiple data points can be collected across an object. A typical measurement time per point is around 30 minutes. ToF-ND determines which metal, mineral, and intermetallic compounds are present, and how much of each phase. In metals, grain orientation can be seen, indicating manufacturing techniques like casting and hammering.
ToF-ND is not a spectrographic method, i.e. it does not determine a suite of elemental contents. But sometimes it is possible to calculate chemical concentrations, by quantifying discrete phases. For example, the tin content of bronze can be calculated, by analysing the shift in the copper alloy (CuSn) lattice parameters. Lead added to copper or bronze doesn't go into solid solution, but rather forms a separate phase. The elemental lead composition of an object can be derived by detecting all of this separate metallic lead phase. ToF-ND provides a genuinely non-destructive way to accurately determine bulk lead contents in copper alloy objects.
Corrosion products, both external and inside an object, are quantified too, provided they are present in sufficient volume. Each compound - for example, cuprite, ataca mite, and tin or lead oxides in bronze - forms a distinct mineral phase, so these are measured at the same time as the metal alloy. Corrosion minerals that have high crystalline symmetry (e.g. cuprite, which is cubic) can be measured quite quickly. But for instance the common copper corrosion product malachite, which has low symmetry, can in practice be difficult to detect in a reasonable time.
For more information on submitting a Heritage Science proposal at ISIS, or questions about specific objects, samples or archaeological artefacts you'd like to analyse, please contact Dr. Winfried Kockelmann (GEM instrument scientist; winfried.kockelmann@stfc.ac.uk) or Dr. Evelyne Godfrey (archaeological scientist; evelyne.godfrey@stfc.ac.uk).
