Characterization of metallic alloys samples using neutron-based techniques in the framework of the CHNet-BRONZE project
The aim of this research project is to use neutron-based analysis techniques to characterize objects of cultural interest, in particular ancient artefacts studied within the CHNet-BRONZE project. This project develops and calibrates in a quantitative way non-invasive techniques based on neutron absorption and scattering for analysis of historical and archaeological bronze and copper alloys artefacts; in particular it
addresses and combines different powerful bulk analysis: Bragg Edge Neutron Transmission analysis (BENT), Time-of-Flight Neutron Diffraction (ToF-ND) and Neutron Resonant capture Transmission Imaging (NRTI).
My research is focused on BENT analysis [1], which is an interesting technique providing a very accurate microstructural characterization of the sample. By interacting with the crystalline lattice of the materials, neutrons will give coherent elastic scattering until the Bragg condition is verified: from the Bragg's law nλ = 2dsinθ it can be seen that there is a maximum wavelength above which the Bragg scattering is
no longer possible, and at this value of λ an edge occurs in the spectrum of the intensity transmitted by the sample. Studying this spectrum it is possible to obtain interesting properties such as texture, microstructure, strain or defects present in the sample. These results are linked to the processes used to manufacture the studied object. For example, the position and intensity of the Bragg edge vary based on the
concentration of the alloy elements, while the texture can provide useful information on the pouring method of the metal inside the mold.
In order to make the analyses quantitative, reference samples made specifically for this purpose are first analysed. In particular, we want to study copper-based alloys with the addition of lead, zinc or tin in different concentrations. These studies will allow us to obtain calibration curves which depend only on the alloy material and not on the specific facility used for the measurements.
To validate the calibration curves, a case study of archaeological interest is chosen: bronze mirrors found in Locri, Calabria, and dating back to around the 5th century BC. The studies presented in this project will help answer questions regarding the mirrors production, such as whether they were obtained through a single casting or whether there are welds, or what are the concentrations of the raw materials used.
In the figures:
(1) Bragg edge transmission spectrum as a function of neutron wavelength, and crystalline structural information that can be obtained from the spectrum [2].
(2) One of the mirrors found in Locri [3].
References[1] J.R. Santisteban et al., Strain imaging by Bragg Edge Neutron Transmission, Nucl. Inst. Meth. A 481 (2002) 765 - 768 [2] Laboratory of Applied Neutron Beam Science and Hokkaido University Engineering, Faculty of Engineering. Bragg-edge and bragg-dip in neutron transmission spectrum. https://www.eng.hokudai.ac.jp/labo/QBMA/Bragg-edge/ method.html#3.[3] L. Costamagna e C. Sabbione, Una città in Magna Grecia Locri Epizefiri, Laruffa Editore, Reggio Calabria 1990 - fig. n. 97.
