# External Beam IBA Measurements for Cultural Heritage
## 3 Pixe  
In PIXE, an energetic beam of ions, usually protons, is directed towards the sample; as the ions cross the sample, they ionize the sample atoms, which emit characteristic X-rays, i.e., each atomic species emits X-rays with known energies. The minimum energy of detected X-rays is typically about 1 keV, thus all elements from Na to U are simultaneously detected; no information on organic elements (C, N and O) is obtained. Because X-rays do not lose energy as they cross the sample, but they can be eventually only attenuated, and they come from all the probed depth, PIXE determines total amounts of elements, not depth profiles. Among IBA techniques, PIXE is indeed unrivalled and the mostly used for the non-destructive analysis of cultural heritage objects [18] since it is highly sensitive over a broad range of elements, and it can be performed with external beams while maintaining the object in atmosphere. In this case, a He flow should be maintained in front of the X-ray detector to avoid attenuation of the low-energies X-rays in normal atmosphere, since 1 cm of air absorbs almost completely 1 keV X-rays, while the transmission is 99% in 1 cm of He. For the interpretation of PIXE spectra, the most used tool is the Guelph PIXE software package GUPIXWin v3.0.3 [19], widely applied and extensively tested in many publications through the use of certified reference materials. Concerning the PIXE detector set-up, a more homogeneous efficiency and sensitivity over all the range of detected elements is achieved if a two-detectors set-up is adopted. These two detectors complement each other in terms of the efficiency curve and the covered solid angle, one dedicated to the low-Z element detection and the other to medium-high-Z element detection. This solution helps mitigating the intrinsic unbalance in the physics of X-ray production in PIXE, which favours the low- and medium-Z elements. This unbalance is a problem, for instance, when detecting low or trace quantities of high-Z elements in presence of a lower-Z matrix: if the count rate due to the low-energy X-rays emitted by low-Z elements is kept within tolerable limits, the total number of counts of X-rays from medium- and high-Z elements will result too small in reasonable measurement times. Thus, one detector is chosen with a large active area to cover the widest possible solid angle, thus increasing the detection efficiency regardless of the X-ray energy but is prevented from obtaining a large count rate from low-energy X-rays by shielding its surface with an absorber of appropriate thickness that absorbs the low-energy X-rays. This detector is thus optimized only for medium- and high-Z elements. The second detector, with no absorber and minimum dead layers, for instance, with a thin entrance window, but with a small active area and covering a much smaller solid angle, is then used to detect simultaneously the low energy X-ray emitted by the low-Z elements (Figure 1). Note that PIXE detectors with a thin entrance window used in presence of a large backscattered protons flux from the sample can suffer short-term effects and exhibit a worsening of the energy resolution under beam irradiation [20]. This quality loss of the acquired spectra is due to the deposit of MeV energy from the backscattered protons reaching the detector chip, leading to the saturation of the front-end electronics and to the increase of the noise level [21]. To divert these backscattered protons without substantial limitations to the detector intrinsic efficiency at low X-ray energies, a deflector made of permanent magnets is mounted in front of the detector. The use of a proton magnetic deflector is mandatory, for instance, in the ion beam analysis of aerosol particles samples collected on filters, where the ratio between total backscattered protons (from the filter) and X-rays (from aerosol particles) is typically about 1/10 or worse.