THE EXTERNAL PIXE SETUP FOR THE ANALYSIS OF MANUSCRIPTS AT THE FLORENCE-UNIVERSITY

The external beam PIXE setup of the 3 MV Van de Graaff accelerator in Florence is extensively used for the analysis of parchments, rulings, inks and miniature temperas of Medieval and Renaissance manuscripts. T he beam is collimated to about 200 mum and exits from the vacuum envir onment through a 8 mum Kapton window. Beam currents from 200 to 500 pA are used; they have been found to produce no visible damage to the ex amined materials. Two Si(Li) detectors are simultaneously used with di fferent geometries in order to cover the widest possible Z-range of th e detected elements: even Na is detected with good sensitivity. A robu st and versatile support has been constructed for safe and easy handli ng of the manuscripts. A laser alignment system allows accurate aiming at the detail of interest. A rotating chopper samples the beam at a f requency of the order of 1 Hz; X-rays produced on a Ni layer evaporate d on the graphite vane of the chopper are used for charge normalizatio n, since the weak current pulses would be unreliably measured in air d ue, in particular, to space-charge effects. An application is shortly presented, concerning the materials used for the ''colour'' rulings wh ich are found, starting from the 11th century, to delimit the writing field for the copyist.