# External Beam IBA Measurements for Cultural Heritage
## 6 Beam Charge Measurements in External Set-Ups  
Quantitative analysis with IBA techniques relies on the accurate knowledge of the number of beam ions striking the sample. This can be normally accomplished by measuring the collected beam charge, integrating the beam electrical current during the measurements. When dealing with cultural heritage objects this cannot be achieved simply in a direct way, since most objects are insulating materials and, moreover, their thickness is such to completely stop the ion beam and to prevent the use of a Faraday cup simply installed behind the sample. Thus, indirect ways, based on the measurement of some effect induced by the incoming ion beam, and depending on its intensity, have to be used to normalise X-ray, gamma-ray, and particle yields during the measurements.  
Since the ion beam has to traverse an extraction window in external beam IBA measurements, the products of the interaction of the ions with the window material can be used profitably. The number of the beam particles impinging on the object to be analysed can then be measured indirectly by counting the number of Si X-rays produced by the beam in a Si\({}_{3}\)N\({}_{4}\) extraction window [44] or, alternatively, by counting the number of backscattered beam particles from Si and N in the extraction window (Figure 7) [45]. The method based on backscattered beam particles can be applied also if the extraction window is a polymeric material made of C, N, O, and H, such as Kapton or Upilex, since no detectable X-ray would be emitted.  
The beam charge deposited on the sample can be determined also using a beam chopper that periodically intersects the beam and samples its intensity, being either an in-vacuum system, for instance, measuring the Au RBS yield from rotating gold plated carbon blades placed before the exit nozzle [46], or an in-air system, for instance, measuring the Ni X-ray yield from a graphite vane covered with a thin nickel layer placed in the space between the extraction window and the sample [47].  
Figure 7: RBS spectrum of 2.5 MeV protons backscattered from Si and N in a 500 nm thick silicon nitride extraction window for beam normalization purposes. In the inset, a correlation between the area of the Si RBS peak and the integrated beam charge.  
It has to be noted that all these beam normalization methods rely on the installation and use of a dedicated detector, either an X-ray or a charged particle detector, followed also by its electronic data acquisition chain, thus a non-negligible investment is to be foreseen.  
Alternatively, the peak from elastic backscattering of protons on He when working under a helium flow (see Figures 5 and 6), or the peak area of the X-rays from the Ar present in air in in-air PIXE systems [48], can be used as normalization, but this is not as robust as the aforementioned methods. Actually, the number of Helium nuclei or Argon atoms encountered before the protons interact with the sample depends upon the surface topography, the surface roughness of the sample, and on accurate reproducible positioning of the sample at the same distance from the extraction window from one measurement to the other.