Effects of the behaviour of the proton-induced isotopes production on the analysis of ancient alloys

The present work concerns non-destructive ion beam analysis carried out by means of proton-beam-induced nuclear reactions in the 10–30 MeV energy range. We focused our attention on the yield distribution of the isotopes produced in the internal region of a metal alloy. This distribution, which defines the analytical region, displays, at incident energy of about 20 MeV in a bronze based alloy, a bell-shaped curve centred at about 600 μm with an average width of about 400 μm. By changing the incident proton energy it is possible to displace the above region in the interior of the metal body. It should be pointed out that if we neglect to take into account the correct behaviour of the isotope yield distribution in samples with surface inhomogeneities we can obtain erroneous analytical results. We describe some experiments based on proton activation analysis (PAA) carried out at the INFN-Laboratori Nazionali del Sud (LNS) in Catania where we verified the bell-shape of the radioactive isotope distribution. The resulting formulas are given for quantitative analysis.It is straightforward that, by choosing the appropriate incident proton energy, it is possible to confine the whole analytical region to the interior of the body, thus eliminating any influence of surface effects. We have called this particular approach deep proton activation analysis (DPAA). Examples of its use on archaeological artefacts are given.