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 be am setup are discussed, together with advantages and limitations in IB A applications. As far as exit windows are concerned, presently the be st choice seems to be 8 mum Kapton(R) foils. They can last for over on e week of beam irradiation under standard conditions and give rise any way to no sudden rupture. Aluminized Mylar(R) windows can indeed be ob tained in thinner foils, but their resistance under beam bombardment i s much poorer. Other possible choices for the window material, which a re shortly discussed, are nickel and zirconium foils. In a helium atmo sphere, Si(Li) detectors with very thin Be windows (8 mum), used for P IXE 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 extern al 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 adopt ed are either external rotating choppers, on which the yield of beam-i nduced interactions is sampled, or in-vacuum particle detectors monito ring the RBS spectrum of the exit window itself. The possibility of ex tracting microbeams as small as 10 mum, e.g. for geological applicatio ns, or diffused beams of some mm2, e.g. for environmental applications , is also shortly discussed. In the final part of the paper, some exam ples are given of recent external PIXE-RBS applications to the analysi s of paints and inks of ancient manuscripts. Attributions of miniature s to different artists, tentatively suggested by art-historians, have been strengthened by the IBA measurements. These have shown in some ca ses that the sources of supply of the raw material were different even though the kind of pigment was the same. From the RBS spectra, prelim inary data concerning ink absorption within parchment or paper - which may affect the quantitative determination of ink composition - have b een obtained.