# External Beam IBA Measurements for Cultural Heritage
## 5 RBS/EBS
In Table 2, the total energy resolution FWHM, achievable in external beam RBS with 3 MeV protons, is shown, including partial contributions to the total energy resolution due to the different elements in the set-up, starting from a detector with an energy resolution of 13 keV FWHM. The values in Table 2 refer to two different ideal set-ups, using extracted standard, millimetre-sized beams or microbeams. The ultra-thin window for beam extraction and the short external path covered by the beam particles typically adopted in external microbeams allow obtaining a final total energy resolution for the external microbeam RBS system not very different from the in-vacuum RBS case, where only the detector energy resolution would contribute, whereas, in external standard millibeams, the achieved energy resolution is about 50% worse. Nonetheless, the obtained energy resolution values still allow valuable standard external RBS measurements, in particular, for light elements such as C, N, and O, which are difficult or impossible to determine with PXE or PIGE. Figure 5: Experimental (open squares) and simulated (full line) RBS spectra obtained with 3 MeV protons. On the left, lead-white paint layer covered with about 15 \(\upmu\)m thick varnish. On the right, 525 nm thick gold leaf laid on wood by a hole clay. The partial spectra of the detected elements (C, O, Fe, Pb, and Au), and the contribution of He from the helium flowing are shown as dashed lines. Spectra from simultaneous RBS and PIXE measurements are, nevertheless, often analysed separately to obtain complementary information, for instance, using RBS to obtain the composition of a low-Z matrix as input data for PIXE analysis of trace metals in that matrix [38]. In samples with a layered structure particle and X-ray spectra from PIXE and RBS, measurements can indeed be handled and analysed self-consistently and their synergic use permits to derive detailed data about composition and elemental depth distribution of the analysed material [39]. This approach is often referred to as \"Total IBA\" [40] or \"Global IBA\" [41]. As an example, we report here on external beam EBS-PIXE analysis of a presumed oil gilding on brass sample using a 3 MeV proton beam (Figure 6). \begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{2}{c}{**External Millibear**} & \multicolumn{2}{c}{**External Microbeam**} \\  & **Thickness** & **Energy Straggling** & **Thickness** & **Energy Straggling** \\ \hline Extraction window, Si\({}_{3}\)N\({}_{4}\) & 500 nm & 7 keV & 100 nm & 3 keV \\ Window-sample path, He & 10 mm & 8 keV & 3 mm & 4 keV \\ Sample-detector path, He & 10 mm & 8 keV & 3 mm & 4 keV \\ Detector case window, Mylar & 1 μm & 7 keV & 1 μm & 7 keV \\ Detector dead-layer, Si & 310 nm & 5 keV & 310 nm & 5 keV \\ \hline Detector energy resolution & & 13 keV & & 13 keV \\ \hline Total energy resolution & & 21 keV & & 16 keV \\ \hline \end{tabular}
\end{table} 
Table 2: Total energy resolution FWHM, achievable in external beam RBS with 3 MeV protons, according to two different ideal set-ups, namely, using extracted standard or millimetre-sized beams and microbeams. The partial contributions to the total energy resolution, apart from detector energy resolution, due to the different main elements in the set-ups, calculated with the SRIM-2008 code [5], are indicated. Figure 6: External beam IBA measurements of a presumed oil gilding on brass with 3 MeV protons. (**a**) Experimental (red symbols) and simulated (blue line) EBS spectra. (**b**) Comparison of experimental (red) and fitted (light blue) X-ray peak areas from PIXE, for main and trace elements. (**c**) Fitted structure of the sample showing the elemental depth distribution. The EBS and PIXE spectra were analysed and fitted in a self-consistent approach using the DataFurnace code [42; 43], and the sample actually revealed to be a thin copper foil as \"gilding\" (Cu 94% Zn 2% Fe 1%, 510 ug/cm\({}^{2}\) thick) applied with an oil-based size or mordant (H 40-49% C 41-49% O 10-11%, 1.0 mg/cm\({}^{2}\)) on brass substrate (Cu 65% Zn 35%).