## 3 EM Techniques for CH Diagnosis
### Particle-Induced X-Ray Emission (PIXE) in CH Applications  
PIXE may be considered the brother of NAA. Suitable for elemental analysis, the working principle is basically the same as that of NAA: the object is irradiated with photons or high-energy particles (electrons, protons, \(\alpha\)-particles, deuterons, heavy ions), which induce the sample to emit characteristic X-rays. The beam path and sample are generally under vacuum. Like NAA, PIXE also requires calibration and accelerators (although portable instruments for in situ PIXE analysis are available [116]), but its extremely high sensitivity--on the order of parts per million (ppm)--and its high spatial resolution make this technique much more attractive than NAA.  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Definition/Meaning** \\ \hline Detection limit & The lowest concentration of the analyte to be detected \\ Irradiation level & The energy of irradiation per unit area. \\ Waiting time & The time needed for isotopes to become active \\ \hline \end{tabular}
\end{table}
Table 10: NAA’s most relevant parameters and their meaning 1.  
\begin{table}
\begin{tabular}{c c} \hline
**Advantages** & **Limits** \\ \hline Suitable for bulk specimens & Limited availability \\ High penetration power (entire sample & In practice, a nuclear reactor is needed \\ volume may be analyzed) & \\ \hline \end{tabular}
\end{table}
Table 11: NAA advantages and limits for CH applications 1.  
An example of NAA application to CH is the analysis of a fragment of a Roman fresco from La Villa della Piscina di Centoelle in Rome, which revealed the presence and quantities of Fe, Mg, Al, Na, and Cl. The analysis was conducted at the Frascati Neutron Generator (FNG).  
Practical Application of PIXE to CH  
Maybe not everybody knows that underneath the Louvre Museum, there has been AGLAE since 1988. AGLAE stands for _Accelerateur Grand Louvre d'analyse elementaire_; it can accelerate protons and \(\alpha\)-particles and performs PIXE analysis [117; 118]. The PIXE facility at AGLAE is already mentioned in a study from 1990, only two years after the inauguration of AGLAE [119]. In that study, it is reported how the PIXE facility of the Louvre made it possible to determine the major, minor, and trace elemental composition of paint pigments.  
According to [103], PIXE is often used for the study of organic heritage materials; therefore, there is a certain number of studies addressing the trade-off between the information obtained and the safety of the analysis. In [120], it is instead stated that since PIXE is suitable only for elements heavier than sodium, it is not successful on pure organic materials. However, it turns out to be a good approach in cases where organic materials contain inorganic inclusions, such as inks and pigments on parchment and paper. In [121], [PERSON] et al. present the application of the PIXE technique (using AGLAE facilities) together with PIGE (Particle-Induced Gamma Emission) and RBS (Rutherford Backscattering Spectrometry) to examine rock paintings and lustrous ceramics. The processing of each pixel in the PIXE image produced a quantitative map of both major and trace elements.  
The combination of PIXE and PIGE techniques has also been applied in the restoration of the four north windows of Sainte-Chapelle in Paris [122]. Thanks to the complete set of elemental chemical concentrations analyzed by PIXE and PIGE, it was confirmed that the majority of the glass was original. The PIXE technique has also been used to assign a likely origin to a set of Mycenaean glass. Micro-PIXE measurements have been used to generate a consistent dataset, focusing not on major or minor elements but on trace element concentrations [59].