# External Beam IBA Measurements for Cultural Heritage
## 5 RBS/EBS  
In RBS, an energetic beam of ions, usually protons or alpha particles, is directed towards the sample. As it hits a sample nucleus head-on, it is scattered back: the energy of the backscattered ions depends on the sample atomic species; thus, measuring it the sample atomic species is determined. All elements with an atomic mass A higher than the atomic mass of the ion beam are detected. Thus, for instance, by using a proton beam, all the elements heavier than protons (from deuterons on) can be detected. If there is more than one element in the sample, for instance, two, for simplicity, then two separate signals are observed: the heavier the sample atom, the higher the backscattered energy. As the ion beam crosses the sample, it slowly loses energy: ions which are backscattered at a certain depth arrive to the detector with lower energy than if they were backscattered at the surface, hence the energy scale can be transformed into a depth scale. The backscattered particle spectra can then simultaneously provide information on elemental composition, depth distributions, and on sample thickness. Since the mass resolution degrades as the ratio of the target nuclei mass to the ion beam mass increases, in particular, when a proton beam is used, RBS can be used efficiently to determine those light elements such as C, N, and O that are difficult or impossible to determine using PIXE or PIGE (Figure 5). RBS is often referred to also as EBS, since EBS is the natural extension of RBS for higher ion beam energies when the elastic scattering cross section cannot be considered purely Rutherford anymore [35]; when using protons as bombarding ion beams, as typical in cultural heritage IBA applications, this condition holds true for elastic backscattering on nuclei up to Zn when the proton energy is 3 MeV. Of course, the availability of experimental or theoretically evaluated elastic scattering cross sections for few MeV energy protons on such elements [36] is mandatory for the practical applicability of EBS. Quantitative information from RBS or EBS spectra are obtained from the results of simulation of the experimental spectra using dedicated codes, one of the widely used being SIMNRA [37]. It has to be noted that some of the intrinsic limitations when working with extracted beams, such as increased beam energy straggling and lateral spread, are making external beam RBS analysis less attractive than in an in-vacuum set-up, because these could worsen spectroscopic characteristics such as mass and depth resolution. The use of a He flow is thus welcomed also in this case, as is external beam PIXE, to mitigate these effects. Working under a helium flow produces a very distinctive feature in the particle spectra from external-beam RBS measurements, namely, the appearance of a peak due to elastic scattering of protons on He that does not belong to the sample itself, but it is a \"parasitic\" element, contained in the first layer that the protons encounter before interacting with the sample. A proper design of the external beam RBS detection system can further reduce beam energy straggling and lateral spread effects. The charged particle detector, typically a Silicon surface barrier detector or a Si PIN diode, can be placed, for instance, inside a long aluminium case, maintained at a pressure of at least below 1 mbar (accomplished by a low vacuum pump such as a membrane pump) and closed at the end facing the incoming backscattered ions by a very thin window (a few \(\upmu\)m thick Mylar foil or a few hundreds of nm thick silicon nitride membrane). In this way, the detector can be kept at the proper distance from the sample. The longer the distance the better the definition of the scattering angle and the less the angular spread, at the same time reducing the path that the backscattered ions have to travel in atmosphere at ambient pressure.