## 3 Application to Cultural Heritage Science
### Coinage Debasement  
Among all archaeological findings, coins are one of the best tools for historians to assess the fiscal health of the issuing state. Lots of studies have been carried out, but most of them, when non-destructive, could only probe the surface of the sample, or destructive techniques are used. Muonic X-ray spectroscopy, instead, can penetrate deep into materials and can give information about the bulk of the coin, especially when debasement is expected. Debasement is a reduction in the quality of the coin, where the surface is enriched in precious material, such as gold or silver, while the bulk contains less precious materials such as copper [33]. For example, towards the end of the Roman empire, silver coins witnessed a strong debasement: most of them were enriched on the surface to make the coin appear as pure silver, covering the debasement of the alloy. This was conducted by treating the coin with an organic acid that left a silver enriched surface by stripping the copper out of the alloy. In [PERSON] et al. [34], a [PERSON] coin was analysed by scanning the material with a different momentum, from 17 MeV/c to 35 MeV/c. Alongside the depth profiling, elemental analysis was performed. From the momentum scan, it is visible that at low momentum (surface or near-surface) only silver peaks are visible while going further into the sample, copper peaks are present (Figure 7a). The depth dependency is confirmed also by the elemental analysis, which clearly shows a surface enrichment of about 100 \(\upmu\)m thickness (Figure 7b). Coin debasement was not only mastered by the Romans: in [PERSON] et al. [21], a Japanese Tempo-koban gold coin (19 th Century) was analysed with a momentum scan from \(\sim\)6 MeV/c to \(\sim\)15 MeV/c. The coin was already known to have a gold enrichment on the surface from other analyses, but with a strong composition bias. With this muon scan, it was possible to determine a change in gold content from the surface to the core (Figure 8). This was determined by comparing the intensity ratio of Au/Ag. The results of the analysis clearly show a higher amount of gold around the surface of the coin. Gold then, decreases in the core, aligning to other measurement values (around 57%). As in the previous example, the results are consistent with those of the literature, using other analysis methods. Another study on Japanese coins was carried out on two Tempo-Tsuho coins [36]. In this case, muon spectroscopy was used to reveal the differences between the real coin and the counterfeit, which from eye inspection were very similar. The result of the analysis clearly shows a different composition of the two coins, as reported in Table 1. Here, the results are compared with XRF: the difference in the values could be due to the different spatial regions investigated with XRF, or from the contribution of oxide layers. Muon data can be more representative in this case: the two bronze coins seem to have the same appearance, but they have significantly different elemental compositions. Figure 7: **(a)** Momentum scan of the sample. At low momentum, copper is absent; (**b**) composition of the alloy. The data obtained from the analysis are in agreement with the literature [35]. The data was taken and adapted from Ref. [34]. In periods of better stability instead, debasement wasn't required. In [PERSON] et al. [37], a study on three Roman gold coins is presented. The paper compares the results from XRF analysis with the one from muonic X-ray emission. From XRF, the three coins show a high concentration of gold, almost full purity for two out of three. The issue of XRF, however, is that it only probes the first few microns of a material, so it is not representative of the alloy. Therefore, a measurement with muons is carried out. In this case, only two momenta were selected: 40 MeV/c for investigating the core of the coin and 18 MeV/c for the near-surface (Figure 9a,b).