Caratterizzazione qualitativa di antiche spade giapponesi attraverso imaging neutronico risolto in energia = Quantitative characterization of Japanese ancient swords through Energy-Resolved Neutron Imaging

The study of sword-forging techniques, and their time-evolution, represents one of the most interesting topics in the investigation of Japanese blade manufacturing techniques. In this work, we present novel results from a non-invasive approach to the study of five Japanese sword fragments pertaining to Koto (987-1596) and Shinto (1596-1781) times.The studied samples are large sword fragments, broken at approximately 10-20 cm from the tang, made available through the Stibbert Museum in Florence. All of them are signed and the authorship and attribution can be accurately identified. Even though an invasive approach could have been used, given the nature of the present samples, we have deliberately chosen to apply a non-invasive technique, in order to demonstrate the validity of neutron tomography methods. Thanks to their high penetration power, neutrons represent an almost unique probe for non invasive characterization of the bulk microscopic structure of massive metal objects. These techniques have been recently applied to the study of metal objects of archaeological and historical origin, giving detailed information on bulk properties (e.g. phase composition, texture, residual strain distribution) and allowing to obtain information on the manufacturing techniques, through identification of peculiar features related to these processes. On a different length scale, neutron tomography techniques can add useful information on the bulk conservation status and the inner structure (when present) of the artefact.We discuss, here, the results of a neutron imaging experiment, carried out at the ICON beamline operating at the neutron source SING (CH), applied the aforementioned sword fragments, using both, white and energy resolved, incident neutron beam.The tomographic reconstruction of the white beam images has permitted to identify some peculiar characteristics (e.g. slag inclusions, cracks, type of temper, alterations extending under the surface) related to the forging methods that were used by the different schools and traditions in Japan.The subsequent experimental investigation, using the energy resolved tomographic method gave us the possibility of increasing the image contrast for a selected phase, taking advantage of the abrupt change of the attenuation coefficients at the so-called Bragg cut-off. We have applied this method to maximize the ferrite contrast and to map the distribution of this phase in the bulk of the measured samples.