Non-destructive measurements for characterisation of materials and datation of Corona Ferrea of Monza

Non-destructive analyses of Corona Ferrea of Monza, a late Roman or Longobard origin, were performed using energy dispersive-XRF portable instrumentation. To irradiate the internal surfaces of the six gold plates which make up the Crown we employed the radioactive isotope americium-241 as the x-radiation source, while to probe the other parts (approximately 200 separate points were studied) we used various types of x-ray tubes equipped with glass capillary to focus the x-rays on single small spots were used. It was not possible to use monochromatic exciting radiation when analysing the Monza Crown; furthermore, none of its surfaces proved to be flat. This meant that the secondary, concentration-dependent x-ray emission from copper could not be calculated, neither was it possible to calculate the influence of surface irregularities on x- ray intensity. We overcame these difficulties by a method that involved calculating the ratios: copper line intensity to gold line intensity (ICu/IAu) and silver line intensity to gold line intensity (IAg/Au). We then compared these ratios to the same ratios determined in standard samples of gold alloy whose compositions were accurately known and similar to that of the Crown. In this way the secondary excitation effect of copper was allowed for. The method depends upon the ratio of the intensities of two x-ray emission lines from a metal alloy being relatively insensitive to the geometry of irradiation