# External Beam IBA Measurements for Cultural Heritage
## 4 Pige  
In PIGE, when the energetic beam of ions strikes the sample, if the ion beam energy is enough, it can lead to a nuclear reaction with a sample nucleus. In some cases, a characteristic gamma-ray is promptly emitted. The reactions occur only for certain beam species (typically, protons) and sample isotopes (usually light elements, i.e., Li, Be, B, F, Na); this means that PIGE is highly specific to given isotopes. Because emitted gamma rays do not lose energy as they cross the sample, and they come from all the probed depth, PIGE also determines total amounts, not depth profiles. The typical reactions induced by protons and the energy of characteristic gamma rays used for PIGE analysis of light elements for cultural heritage studies are shown in Table 1.  
Given the high energy of the detected radiation, orders of magnitude higher than characteristic X-ray energy, the problem of absorption of the emitted radiation in the atmosphere in an external beam set-up is thus negligible, and the distance at which the PIGE detector is placed is not an issue, except for trying to maximise the subtended solid angle, hence the detection efficiency. The elements detected by PIGE are limited in number, but complementary to those detected by PIXE: in such a way PIGE technique is thus a  
Figure 3: On the left, comparison between PIXE spectra of a tintype at three different proton energies, namely, 2, 3, and 4 MeV; the reported beam energies refer to the effective values at the sample surface, after traversing the beam exit window and about 1 cm path in external atmosphere. The spectra are normalised by the proton beam charge accumulated during the measurements. On the right, the apparent composition ratios of the main metallic elements as a function of the different proton beam energies. perfect \"sidekick\" or a companion of PIXE for the analysis of light elements in cultural heritage samples (Figure 4). Quantitative information from PIGE spectra is typically obtained by comparing the gamma-ray yields from the sample to those from homologous reference standards, although free dedicated codes for standard-less PIGE analysis, such as PIGRECO [29] and ERYA [30], are now becoming available. The detectors typically employed for PIGE analysis are High-Purity Germanium (HPGe) detectors, because germanium is still the only material which can provide a combination of high-energy resolution and high-detection efficiency at the gamma-ray energy of interest. The drawback to using these detectors, though, has always been the need to cool them to cryogenic temperatures (about 77 K), using, traditionally, liquid nitrogen with a combination of a cryostat and a dewar. However, the modern electro-mechanical methods of achieving cryogenic temperatures have overcome the limits in convenience, practicality, and sustainability of a liquid-nitrogen-cooled device [31]. It has to be noted that less-expensive, compact, and lightweight inorganic scintillators such as Cerium-activated Lanthanum Bromide (LaBr\({}_{3}\)(Cr)) detectors have been used as well for PIGE, although the achieved energy resolution is about one order of magnitude worse. The benefit of PIXE-PIGE synergy can be elucidated in two case studies from the literature, namely, the identification of lapis lazuli pigment in paint layers [32] and the