ADVANTAGES AND LIMITATIONS OF EXTERNAL BEAMS IN APPLICATIONS TO ARTS AND ARCHAEOLOGY, GEOLOGY AND ENVIRONMENTAL-PROBLEMS

The physical and technological problems associated with an external beam setup are discussed, together with advantages and limitations in IBA applications. As far as exit windows are concerned, presently the best choice seems to be 8 mum Kapton(R) foils. They can last for over one week of beam irradiation under standard conditions and give rise anyway to no sudden rupture. Aluminized Mylar(R) windows can indeed be obtained in thinner foils, but their resistance under beam bombardment is much poorer. Other possible choices for the window material, which are shortly discussed, are nickel and zirconium foils. In a helium atmosphere, Si(Li) detectors with very thin Be windows (8 mum), used for PIXE analysis, have undergone the problem of gas permeation inside the cryostat, but they always recovered to their original condition with a simple pump and bake procedure. Particle detectors we used for external RBS analysis are cheap, standard silicon-junction, which have shown no significant problem of performance deterioration even after weeks of use. The difficulty of a correct current measurement when operating with an external beam is pointed out. Solutions which have been adopted are either external rotating choppers, on which the yield of beam-induced interactions is sampled, or in-vacuum particle detectors monitoring the RBS spectrum of the exit window itself. The possibility of extracting microbeams as small as 10 mum, e.g. for geological applications, or diffused beams of some mm2, e.g. for environmental applications, is also shortly discussed. In the final part of the paper, some examples are given of recent external PIXE-RBS applications to the analysis of paints and inks of ancient manuscripts. Attributions of miniatures to different artists, tentatively suggested by art-historians, have been strengthened by the IBA measurements. These have shown in some cases that the sources of supply of the raw material were different even though the kind of pigment was the same. From the RBS spectra, preliminary data concerning ink absorption within parchment or paper - which may affect the quantitative determination of ink composition - have been obtained.