# External Beam IBA Measurements for Cultural Heritage
## 2 External Beam  
IBA techniques are typically performed in a vacuum chamber, for instance, in material science applications, but they can easily be performed with an external beam, i.e., with the ion beam extracted from the vacuum beam line of the accelerator into ambient pressure through a thin extraction window typically glued to the end part (exit nozzle) of the vacuum--the line from the accelerator [3]. The window material and its thickness are chosen in order to guarantee first resistance to the pressure gradient and resistance under beam irradiation. The window has then to be thin enough not to affect too much the beam properties (energy and lateral resolution) and to produce negligible background radiation when traversed by the beam. Whereas in the past 7.5-8 \(\upmu\)m thick polymeric films (Kapton, C\({}_{22}\)H\({}_{10}\)N\({}_{2}\)O\({}_{5}\) or Upilex, C\({}_{22}\)H\({}_{10}\)N\({}_{2}\)O\({}_{4}\)), or thin metallic films (Al, Zr) in the few micrometres range where used, nowadays, 50-500 nm thick inorganic membranes (Si\({}_{3}\)N\({}_{4}\)) are typically employed (the thinner membranes are used for beam extraction on nuclear microprobes [4]). The object to be analysed is then placed typically at about 1 cm distance from the window, with the possibility of flooding this path with He, for reducing beam energy loss, energy straggling, and lateral spread. For instance, the effect on 3 MeV protons passing through a 100 nm or 500 nm thick Si\({}_{3}\)N\({}_{4}\) membrane plus traversing in addition 1 cm of He before impinging on the sample, calculated with the SRIM-2008 code [5], is only an energy loss of 54 keV or 184 keV and an energy straggling of 6.4 keV or 12.4 keV, respectively.  
Working with an external beam set-up, especially for cultural heritage studies, as in many ion beam laboratories [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17], has the advantage of allowing fast and efficient measurements with direct analysis and easy positioning of object having any size and shape, without need of sampling, and no charging effects and heating from beam irradiation, thus strongly reducing the risk of damaging the object (truly \"not deliberately destructive\" measurements).  
Evidently, working with an external beam set-up has limitations and drawbacks such as beam energy loss, beam energy straggling, as discussed above, and increased beam lateral spread, the latter one limiting the physical size of an extracted nuclear microprobe. In addition, to reduce the increased energy straggling caused when heavier ions are passing through the exit window and the path in atmosphere, external beam measurements are typically limited to the use of a proton ion beam. Moreover, a flow of He gas is needed to reduce also the attenuation of low-energy X-ray photons in the air.  
In the following chapters, the basic principles of the principal IBA techniques applied to cultural heritage will be recalled, namely, particle induced X-ray emission (PIXE), particle induced Gamma-ray emission (PIGE), and Rutherford or elastic backscattering spectrometry (RBS/EBS), together with providing specific and practical details on how these techniques can be used in analysing cultural heritage objects with external beam set-ups.