Depth profiling the elemental composition with negative muons: Monte Carlo based tools for improved data analysis

Abstract

Gildings, patinas and alteration crusts are common features of many heritage artefacts, especially for metals. Their size depends on many factors, like the manufacturing method for gildings or the conservation state for alteration crusts: in some cases, it can be in the scale of the tens of microns. Such thickness would be difficult to investigate with classical non-destructive methods and would prevent getting information from the bulk of the sample. This work proposes an innovative approach for the study of multi-layered materials with the Muonic atom X-ray Emission Spectroscopy technique (μ-XES). Based on the detection of the high-energy X-rays emitted after the muon capture by the atom, this method is characterised by a remarkable penetration depth (from microns to cm). From the surface to the bulk, this technique can evaluate the variation of the elemental composition as a function of depth. The paper focuses on providing an improved interpretation of μ-XES data by coupling the analysis with the use of two Monte Carlo simulation software, GEANT4/ARBY and SRIM/TRIM. With these two software, it is possible to replicate the negative muon experiments and compare the experimental and simulated outputs to address the size of a given layer. To validate this approach, a set of standard gilded bronze and brass foils were measured at the ISIS Neutron and Muon source. From simulations, it was possible to evaluate the thickness of the superficial gold layer, with results in agreement with the preliminary SEM characterisation of the samples.