Quantitative Neutron Resonance Transmission Imaging for archaeometallurgy: calibration using binary bronze alloys within the CHNet_BRONZE Project

Traditional metallurgical studies of archaeological bronzes often rely on invasive methods, such as metallography or SEM analysis on cross-sections. On the other hand, neutron-based techniques offer unmatched advantages for the non-invasive study of precious artefacts. The CHNet_BRONZE Project aims to develop a complete non-invasive quantitative protocol exploiting neutron absorption and scattering methods available at the ISIS Neutron and Muon Source (U.K.). Their transition to fully quantitative tools optimised for historical copper-based alloys is the core of the project. Neutron Resonance Transmission Imaging (NRTI) has previously demonstrated its potential for elemental identification and semi-quantitative analysis. However, its application as a robust quantitative imaging tool for complex copper-based alloys is still limited by the lack of dedicated calibration strategies and by matrix-dependent effects. In this work, we present a systematic calibration approach for NRTI based on a set of custom-engineered binary Cu-Sn alloys (3–18 wt% Sn), specifically designed to reproduce the compositional range of archaeological bronzes. Our results demonstrate that tin provides a more sensitive and monotonic response to concentration variations, whereas copper resonance dip intensities remain relatively stable, reflecting its role as the matrix reference. This behaviour is also reflected in the selective mapping of the two elements carried out by selecting the resonances: the absorption contrast varies perceptibly for tin depending on its concentration, while it remains almost constant for copper. Therefore, the calibration on binary Cu-Sn alloys has been made using the Sn resonance around 111 eV and shows a good agreement for binary systems. Deviations observed for a multi-component reference alloy highlight the role of matrix effects and the need for further refinement of the method. Overall, this work provides a quantitative baseline for NRTI and represents a key step towards its application as a non-destructive tool for 2D compositional mapping in complex archaeological bronzes, where identifying compositional variations is key to understanding ancient manufacturing processing.