Neutron-based analytical methods for the non-invasive characterisation of iron artefacts

1. Neutron-based analytical methods for the non-invasive characterisation of iron artefacts Evelyne Godfrey and Winfried Kockelmann [E.Godfrey@rl.ac.uk] STFC Rutherford Appleton Laboratory, ISIS Neutron Facility, Harwell, OX11 0QX, UK Neutron-based methods can be used to characterise and assess the conservation state of iron artefacts. Time-of-flight neutron diffraction (TOF-ND) is a quantitative ‘bulk’ method that measures right through the thickness of an object, giving an average result. An entire object can be put in front of the neutron beam, in air, without any sample preparation. A far greater number of objects can thus be analysed than would ever be possible by traditional metallographic (i.e. destructive sampling) techniques. a a Fig.1. Forged bloomery steel standard 0.9wt%C over first 2.5mm in from thin end of wedge; 0.67%C over first 5mm in b 0.29wt%C over first 2.5mm in from thin end of wedge; 0.2%C over first 5mm in b c a 0.25wt%C over first 2.5mm in from thin end of wedge; 0.23%C over first 5mm in b c d c Ferrite; 3% of volume is wüstite; 11% of vol. isfayalite over first 5mm in from thin end d d Fig.4. Phases measured by TOF-ND Fig.2. TOF-ND plots of forged bloomery steel standards Figs.3a-d. Reflected light micrographs of bloomery standards: top to bottom: over 0.8wt%C; ca. 0.2wt%C; ca. 0.1wt%C; ferrite with large area of fayalite & wüstite slag. All high in phosphorus Non-invasive neutron-based methods Chemical Analysis PGAA : Prompt Gamma Activation Analysis: based on thermal and cold neutron capture; prompt gamma rays; high sensitivity for some light elements (H, K , Cl); minimum volume analysed 2 x 2 x 2 mm NRCA : Neutron Resonant Capture Analysis: based on epithermal neutron capture; prompt gamma rays; good sensitivity for some heavy elements (Au, As, Ag, Sb, Sn, Zn); minimum vol. analysed with neutron resonance transmission (NRT) method is 2 x 2 mm Crystallography and microstructure Neutron Diffraction : based on elastic scattering of thermal neutrons by periodic, long-range ordered (crystalline) or non-periodic, short-range ordered (glass) arrangements of atoms; phase and structure analysis, texture analysis, microstructure analysis; minimum vol. analysed ca. 1 x 1 x 0.5 mm; residual stress analysis; Bragg-Edge transmission for mapping of strains and phases, based on Bragg scattering; minimum vol. analysed 2 x 2 mm Imaging Neutron Radiography/Tomography : real space imaging based on capture and scattering of thermal and cold neutrons; internal view of objects with a spatial resolution down to 100 microns; attenuation of neutron beam passing through object gives contrast for different elements; high sensitivity for some light elements (e.g. H); contrast by variation of neutrons energies; also phase contrast radiography, based on neutron refraction