# External Beam IBA Measurements for Cultural Heritage
## 3 Pixe
In the analysis of cultural heritage objects, instead, the ratio between total backscattered protons and X-rays is typically about 1/100 or better, thus the short-term effects are negligible, and it is not crucial the installation of the proton divertor. Nevertheless, the use of a magnetic proton deflector in front of a PIXE detector, also in the case of cultural heritage applications, can avoid unrecoverable damages to the detector chip from the long-term effects of backscattered proton irradiation, and the detector will last longer. If only one detector is available, then the overall intensity between the high- and the low-energy region of the X-ray spectrum can be effectively balanced using a \"funny filter\" [22], i.e., an absorbing foil (typically Mylar, C\({}_{10}\)H\({}_{8}\)O\({}_{4}\)) placed in front of the detector with a hole pierced so that a fraction of the X-ray within the detector solid angle is not attenuated. If the funny filter solution is adopted, it is advisable to choose a detector with a large active area (to increase the solid angle) and a thin entrance window (to enhance the transmission of low-energy X-rays). With respect to the X-ray detectors, the PIXE analyst has currently two choices: traditional Lithium-drifted Silicon (Si(Li)) detectors that dominated PIXE set-ups up until a decade ago, or the new and uncasingly developing Silicon Drift Detectors (SDDs). The SDDs were first proposed in the early 1980s as a position-sensitive semiconductor detector for high energy charged particles, based on a novel charge transport scheme where the field responsible for the charge transport is independent of the depletion field [23], and they are now routinely employed in X-ray spectroscopy because of the low capacitance of the collecting electrode (0.5-1 pF/cm\({}^{2}\)) and the low leakage current (1-2 nA/cm\({}^{2}\) at room temperature) resulting in improved energy resolution (<140 eV FHWM at Mn K\({}_{\alpha}\) line), yet at short shaping times (a few \(\upmu\)s or less), thus allowing the sustaining of high count rates (up to 100 kHz), and just with a Peltier cooling element. The only drawback of SDDs is the limited thickness of the active region, typically 450-500 \(\upmu\)m, although devices 1 mm or 2 mm thick are now available on the market, that reduce the intrinsic efficiency in the high-energy X-ray region, namely, above 20 keV. For instance, the intrinsic efficiency of a 450 \(\upmu\)m thick SDD at 20 and 30 keV is about 37% and 14%, respectively, to be compared to values of about 99% and 81%, respectively, for a 5 mm thick Si(Li) detector [24]. However, this intrinsic inefficiency can be compensated by increasing the effective covered solid angle by installing an array of multiple identical SDDs in similar geometry and summing up offline the spectra from each, as implemented, for instance, in the external beam PIXE set-up of the New AGLAE accelerator in Paris, France [25], or by using a single large area SDD, with an active area much larger than typically achieved by traditional Si(Li), as  
Figure 1: PIXE spectra of an emerald obtained with 3 MeV protons, collected simultaneously with a small area SDD for low-Z elements (Na-Ca) and a large area SDD with 450 mm Mylar foil as absorber for mid- and high-Z elements (Sc-Pb). The spectra are normalised by the proton beam charge accumulated during the measurement. performed, for instance, in the external beam PIXE set-up of the INFN LABEC ion beam laboratory in Florence, Italy [26]. A comparison of the typical performances of a large area SDD and a Si(Li) for PIXE spectroscopy in the analysis of mid and high Z elements in a glass sample is shown in Figure 2.